Chapter 4 Flashcards
1
Q
What is environment and what factors affect an organism?
A
When I am thinking about the environment of a plant, or any type of organism for that matter, I am thinking about the many factors that might influence that organism. These factors can be both living and non-living components of the environment. For example, you can imagine that a caterpillar feeding on a leaf is an important factor for some plants, as are conditions such as temperature and sunlight. We call living factors biotic, and non-living factors abiotic, and you will learn more about these in this topic.
2
Q
Mars distance and radius.
A
Mars lies at 2.27 × 10^8 km from the Sun (and 7.74 × 10^7 km from Earth).
It has a radius of 3390 km, a little over half that of Earth (6371 km).
3
Q
What is physiology?
A
The study of how an organism functions.
4
Q
First spacecraft to conduct a successful flyby of another planet.
A
Our understanding of Mars changed dramatically in 1964 when Mariner 4 became the first spacecraft to conduct a successful flyby of another planet and return images of the planet from space.
5
Q
Mars water features, a history.
A
‘Prior to the Mariner missions, a lot of fiction was written, and believed, about Mars. Giovanni Schiaparelli made excellent telescope observations, but he named the lines he saw and drew “canali”, Italian for a water feature, either natural or artificially made. Once Percival Lowell translated that to canal, indicating something artificial, some people believed that there could be a civilisation on Mars, just as advanced as ours. Others thought there might at least be some water and life there, or at least some rivers and some water. Mariner 4 wiped out that idea, and it took us a while to get it back. The reason was that the Mariner 4 images were at such low resolution that the water-cut channels could not be seen in them. Only later Mariner missions showed them, with their higher resolution images.’
6
Q
Two Mars rovers inc. longest lasting one.
A
Two stars of the Mars exploration team were the twin rovers Spirit and Opportunity. Both advanced our knowledge of the planet’s surface many times over, but Opportunity in particular is renowned as the longest lasting rover, having operated for about 15 years before communication with it was lost in 2018.
7
Q
Current Mars missions.
A
At the time of writing (June 2021), NASA’s Mars 2020 mission is now operational, with the Perseverance rover having successfully landed on Mars on 18 February 2021. China’s Tianwen-1 mission also delivered their first rover to the planet’s surface – the Zhurong rover – on 14 May 2021. NASA’s Mars Science Laboratory (MSL) mission is also still in full swing after landing in 2012, with the Curiosity rover busy travelling across the surface of Mars taking pictures, collecting and analysing samples, and sending data back to her operators on Earth, on a regular basis. Susanne is one of the scientists working on this mission. We will meet another Open University Mars scientist in Part 2, talking about the ExoMars mission that successfully delivered the ESA-Roscosmos Trace Gas Orbiter into Mars’ orbit in 2016.
8
Q
Google Earth Mars part 1.
A
https://www.google.com/mars/#lat=[enter latitude]&lon=[enter longitude]&zoom=[enter zoom factor]
where the zoom factor is a value between 1 and 8, with 1 being no zoom, and 8 being the maximum zoom in to the location.
For example, if you wanted to go to the location at latitude 17.86 and longitude –133.67, and zoom in by a factor of 6 on this point, you would enter:
https://www.google.com/mars/#lat=17.86&lon=-133.67&zoom=6
Note: you may need to reload your page after entering the location in the search bar to get taken to the location.
Try this now: go to the location at latitude 27.157 and longitude –419.934 with zoom factor 5. You should be directed to the location shown in Figure 1.5.
9
Q
Google Earth Mars part 2.
A
Do activity.
10
Q
Is Mars similar to Earth?
A
So, we can see that Mars is a rocky body, and that the general landscape or terrain on Mars can be directly compared with parts of Earth in terms of topographical features (e.g. mountains, valleys, different slopes, areas in full light or shadow, etc.) and changes in geology (e.g. different rock types and sediments in different regions).
11
Q
Two types of planets.
A
Earth and Mars, together with Mercury and Venus, are often described as the terrestrial planets, an expression that means Earth-like, and refers to them being made predominantly of solid rock. It separates them from the gas planets (Jupiter, Saturn, Uranus and Neptune), which are composed mostly of some combination of hydrogen, helium and water existing in various physical states, but some of which may also have solid interiors.
12
Q
Astronomical Unit.
A
When considering the distances of the planets from the Sun in our standard distance units, we inevitably find that we are using huge numbers. For example, the Earth lies approximately 150 000 000 000 m (1.5 × 1011 m, or 150 million km) from the (centre of the) Sun. It is much more convenient to define the Sun-to-Earth distance as one astronomical unit (AU). Thus Earth is at 1.0 AU, Jupiter at 5.2 AU and Neptune at 30.0 AU from the Sun.
13
Q
Mars day and year compared to Earth.
A
Earth and Mars have days of similar length. The length of a solar day on Mars (called a sol) is 24 hrs 40 minutes. However, they take different lengths of time to complete one revolution of the Sun. One full revolution of Mars around the Sun, a Mars year, takes approximately 687 Earth days.
14
Q
A
15
Q
What is kinetic energy and when does it increase?
A
Kinetic energy is the energy of a body or a system with respect to the motion of the body or of the particles in the system.
The mean kinetic energy of the constituent molecules of a substance will increase when the temperature of that substance increases.
16
Q
What is the lowest possible temperature
A
When the substance cools, the mean kinetic energy of the molecules decreases and they move more slowly. Eventually a point would be reached at which the molecules have no kinetic energy, and so no further cooling could occur. The temperature at which this would happen is known as absolute zero, which is the lowest temperature possible. On the Celsius scale, this temperature has a value of −273.15 °C.
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Q
A
18
Q
Kelvin.
A
For many scientific purposes, it makes sense to define a temperature scale for which zero on the scale is absolute zero. (On such a scale, negative temperatures are impossible as you cannot get any lower than zero.) The scale with this property, which is widely used by scientists, is known as the absolute temperature scale; it is also known as the kelvin scale, named after the British physicist and engineer William Thomson, Lord Kelvin (1824–1907). The unit of temperature on this scale is called the kelvin (K). A change of one kelvin is the same as a change of one degree Celsius, so there are 100 kelvin between the normal freezing and boiling temperatures of water. The absolute and Celsius scales are compared in Figure 1.8. To convert degrees Celsius into kelvin, you just add 273.15 to the Celsius temperature. Thus the normal freezing temperature of water (0 °C) is 273.15 kelvin, or 273.15 K.
19
Q
Capital letter exceptions.
A
The temperature scale named after Anders Celsius is usually known as the Celsius scale, not the celsius scale. You may also have heard of the scale for the capsaicin content of chili peppers, which is called the Scoville scale, after Wilbur Scoville.
20
Q
Mars and microbes.
A
So, martian surface temperatures vary from lows of about −140 °C (at the winter polar caps) to highs of up to 35 °C (in summer at the equator ).
Microbes grow -10°C to 120°C.
21
Q
Mars atmosphere - history.
A
It has been known for over 150 years that Mars has an atmosphere, initially thought to be like Earth’s, then compared to the exosphere of the Moon (an almost non-existent layer of gas surrounds the Moon; the Moon has no true atmosphere). By the 1960s, when the first spacecraft missions flew by Mars, the presence of carbon dioxide and water had already been detected in the martian atmosphere. Nevertheless, little was known at that time of the detailed composition of the martian atmosphere and even less was known about its physical properties, such as temperature and pressure. Estimates of the atmospheric pressure on Mars, for example, were too high. Since the early 1960s, our knowledge of the atmosphere of Mars has expanded enormously, due to information from spacecraft and more sophisticated Earth- and space-based telescopes.
22
Q
Mars v Earth atmosphere.
A
Although Earth and Mars are both terrestrial planets, their atmospheres appear, at least on first inspection, to display more differences than similarities.
The first difference is that the atmospheric pressure on Mars is only 6.5 millibars (mb or mbar – either is fine) whereas the atmospheric pressure on Earth is, on average, 1000 mb. The total mass of Earth’s atmosphere is 5.2 × 10^18 kg, compared to 2.3 × 10^16 kg for Mars.
The major constituents of each atmosphere are shown in Figure 1.9. The compositions given here are those at the surfaces, where the atmospheres are most dense.
The interaction of energy from the Sun with an atmosphere at higher altitudes leads to chemical reactions that convert some of the molecules into different species. For example, part of the oxygen component of the Earth’s atmosphere is converted into ozone at higher altitudes. Consequently, the composition of the atmospheres changes with altitude.
23
Q
Mars atmosphere constituents.
A
Mars
Carbon dioxide: 96% Argon: 2% Nitrogen: 2% Oxygen: 0.15%
Earth
Nitrogen: 78%, Oxygen: 21%, Argon: 1%, Water (variable).
24
Q
Mars v Earth water and methane
A
Although it is not immediately striking in the image above, one important difference in the composition of the atmosphere of the Earth compared with that of Mars is the Earth’s larger water content. The tiny amount of water present in the martian atmosphere is too small to be shown on this diagram. Such tiny amounts are known as traces; there are also traces of free oxygen, carbon monoxide and methane.
The possible presence of methane on Mars has generated excitement, because it could be evidence of current geological, or even biological, processes. On Earth, methane is only formed as a by-product of life, or through the interaction of water with rock at high temperatures. However, determining the source of methane on Mars has proven difficult. The ExoMars Trace Gas Orbiter has been scanning the martian atmosphere for methane since 2018, but it has not yet seen any evidence for the ‘spikes’ in methane abundance that earlier measurements suggested. This could mean that, if it is present, it remains close to the ground and is not detectable by the orbiting spacecraft.
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Greenhouse effect.
The thickness of a planet’s atmosphere affects the planet’s temperature: so too does the composition of the atmosphere. The surface of a planet absorbs much of the solar radiation landing on it, and this is then emitted back to space as infrared (IR) radiation. An atmosphere around a planet serves as an insulating blanket that traps some of this infrared radiation before it escapes, resulting in a warming effect. The ability of the atmosphere to trap heat is known as the greenhouse effect, as it is similar to being in a warm greenhouse (although the process is not the same; the greenhouse glass is not itself a particularly good heat insulator, but it reduces air circulation which would otherwise act to disperse the heat).
26
Greenhouse gases.
Only particular gases act to trap heat in the atmosphere, and these are called greenhouse gases. Therefore the extent of the greenhouse effect on a planet depends on the composition of the atmosphere, and on the amount of these gases present.
You may have said carbon dioxide (CO2) or possibly methane (CH4). Even though CO2 is the most prevalent anthropogenic greenhouse gas, the major greenhouse gas in the Earth’s atmosphere, in terms of abundance, is actually water vapour. Other greenhouse gases include nitrous oxide (N2O), ozone (O3) and chlorofluorocarbons (CFCs).
Even though the martian atmosphere is predominantly composed of a greenhouse gas, it is not dense enough to trap sufficient heat to create a significant warming effect.
27
Polar ice caps
Mars has two permanent polar ice caps (Figures 1.1, 1.10 and 1.11). The northern polar cap has a diameter of about 1000 km during the northern Mars summer, and contains about 1.6 million cubic km (km3) of ice, which if spread evenly on the cap would be 2 km thick. (This compares to a volume of 2.85 million km3 for the Greenland ice sheet.)
The southern polar cap has a diameter of 350 km and a thickness of 3 km. It contains a similar volume of ice to that of the northern polar ice cap, for the equivalent season.
28
Dry ice.
The caps at both poles consist primarily of water ice. During a pole’s winter, it lies in continuous darkness, and the intense cold causes as much 25–30% of the carbon dioxide in the atmosphere to freeze into CO2 ice, also known as ‘dry ice’.
Frozen carbon dioxide accumulates as a comparatively thin layer about 1 m thick on the north cap in the northern winter only, while the south cap has a permanent dry ice cover about 8 m thick.
When summer comes, and the pole is again exposed to sunlight, the ice sublimates (Box 1.2), creating enormous winds that sweep off the poles as fast as 400 km per hour. Thus the size of the polar caps varies markedly between summer and winter on Mars (Figure 1.12).
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Sublimation ice and dry ice.
As a general rule, and under most familiar conditions, if you heat a solid it will turn to liquid, and more heat needs to be applied to turn it to vapour. Melting frozen water and then heating it to its boiling point is the most familiar example. However, under appropriate conditions, solids can be transformed directly to vapour without melting first. ‘Dry ice’, the substance used on stage to generate clouds of mist, is frozen carbon dioxide, and at the atmospheric pressure on Earth this turns directly to vapour without melting. As you may recall from Topic 1, this process is called sublimation, and the solid carbon dioxide is said to ‘sublimate’ when it turns to vapour.
The factor that determines whether a solid will melt or sublime is the pressure of the vapour of the particular substance surrounding the solid. If you make a snowman in a very dry climate, it is possible to wake up the next morning and find absolutely no trace of it, except perhaps the carrot and sticks from his nose and arms. In a more humid climate, with a higher water content in the air, the snowman will melt and form a puddle before that in turn evaporates and dries up.
Carbon dioxide is stable in liquid form only when confined by carbon dioxide gas (vapour) at a pressure of several times the Earth’s total atmospheric pressure. You can see this if you look at the phase diagrams for water and carbon dioxide in Figure 1.13.
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Phase diagram for water and carbon dioxide.
The phase diagrams for carbon dioxide and water are both marked on this diagram.
The triple point of carbon dioxide, or the intersection of the solid, liquid and gas curves, is at the top of the graph and towards the middle at approximately 5 bars and 210 K. The line that separates the gas phase of carbon dioxide from the solid phase rises from ten to the minus four bars and 120 K, in a convex-upwards curve, through the triple point and beyond. The gas phase is on the higher temperature side of the line. The line separating the gas phase from the liquid phase rises vertically (i.e. at a constant temperature) from the triple point. The liquid phase is on the higher temperature side of the line.
The triple point for water, or the intersection of the solid, liquid and gas curves, is half way up the graph and towards the right at just below 10 to the minus two bars and at 273 K. The line that separates gaseous water from the solid phase rises from ten to the minus seven bars and 180 K, in a convex-upwards curve, through the triple point and beyond. The gas phase is on the higher temperature side of the line. The line separating the gas phase from the liquid phase rises vertically (i.e. at a constant temperature) from the triple point. The liquid phase is on the higher temperature side of the line.
As well as the two phase diagrams, a rectangle representing the range of surface temperatures on Mars has been drawn on the graph. The short vertical sides of the rectangle representing the extremes of temperature are at approximately 140 K and 295 K, and the longer horizontal sides of the rectangle are at 0.8 times ten to the minus three bars and 1.2 times ten to the minus three bars. Within this rectangle, a small area between approximately 210 K and 235 K has been coloured red to indicate the normal temperature range.
31
Can carbon dioxide exist as a liquid under the normal conditions on the martian surface?
No, the much lower atmospheric pressure (100 times lower!) means that liquid carbon dioxide certainly cannot exist there.
In contrast to the Earth, when the frost and ice of the martian polar caps are heated up they do not melt to produce liquid water or CO2, but turn straight to a gas or vapour.
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Polar cap substance exchange.
There is a continual seasonal exchange of carbon dioxide and water between the polar caps via the atmosphere (Figure 1.14). So, not surprisingly, the Viking missions observed relatively large amounts of water in the atmosphere close to the north polar cap of Mars, especially during summer when the cap sublimates. Rather less enhancement of water was observed at the south polar cap in its summer.
Figure 1.14 During summer in the martian northern hemisphere, energy from the Sun (solar radiation) warms the frozen carbon dioxide (dry ice) and water-ice in the northern polar cap. These materials sublimate and migrate to the southern pole where they form condensed substances.
These seasonal actions transport large amounts of dust and water vapour, giving rise to Earth-like frost and large cirrus clouds. Clouds of water-ice were photographed by the Opportunity rover in 2004 (Figure 1.15).
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35
1.6 activity 1.2
Complete it!
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Schiaparelli map.
Figure 1.16 Schiaparelli’s map of the whole of the planet Mars, with its un-doubled dark lines, observed during the six oppositions of 1877–1888 [translated from the Figure caption in French, above]. Note that Schiaparelli indicated the presence of ice caps.
Figure 1.16 shows a hand-drawn map, of Mars about double the width compared to its height. The map area is divided into a grid pattern by horizontal and vertical lines, with a thicker horizontal line in the middle marking the equator. The top quarter of the map is mostly featureless and is labelled “Mare Austraale”. There are three circles in that area, named Thyle I and Thyle II, and Argyre II. Towards the bottom of the map, the drawing becomes more complex. The bottom half of the map is very different. There many random criss-crossing lines, no large circles, and only one small, shaded area in the bottom quarter. There are many labels on the map, including Hesperia and Arabia.
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Hubble telescope.
Figure 1.17 is a photograph of the Hubble Space Telescope. The telescope is a long metallic tube with flat ends. Attached around the middle of the tube, proximately half its length, are rectangular shapes, which are solar panels. Halfway along the tube, a thin metal pole extends, which has a triangular shape on its end. This is an antenna. At one of the of the tube is a white, metallic, round shape, which is a door to the telescope aperture.
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Types of radiation.
Visible light from the Sun is just one small part of the spectrum of solar radiation. And the Sun is not the only body that emits radiation; any object that gives out heat (including you) will emit radiation waves.
Most sunscreens and sunglasses will have information on the amount of protection they provide against ultraviolet (UV) radiation
Radios and mobile phones make use of the radio wave part of the spectrum
remote controls use infrared radiation
microwave ovens use microwaves to cook
airport security scanners use X-rays
a nuclear reactor produces gamma radiation.
39
What do these form.
All of these types of radiation are collectively referred to as the electromagnetic spectrum (or EM spectrum for short) and are illustrated in Figure 1.18.
This diagram is arranged in four layers.
The top layer consists of six small images, reading from left to right as follows: a radiation warning sign labelled gamma rays; an x-ray of an hand labelled x-rays; a pair of sunglasses labelled ultraviolet; a flame labelled infrared; a microwave oven labelled microwaves; a mobile phone labelled radio waves.
The second layer consists of a single, grey, wavy line. On the left of the diagram, underneath the label ‘gamma rays’, the waves are tightly bunched together. As the line progresses towards the right of the diagram, the waves become more spread out. The height of the waves does not change across the page.
The central layer consists of a grey, open-ended bar representing the extent of the electromagnetic spectrum. Above the left hand end of the bar is an arrow, pointing left, labelled increasing frequency. At the right hand end of the bar is an arrow, pointing right, labelled increasing wavelength. Reading from left to right, the bar contains the words gamma rays, x-rays, ultraviolet, visible, infrared, microwaves, radio waves. Each word sits on a white background, the width of which indicates the position of that form of radiation within the spectrum. The visible part of the spectrum is so narrow that the word has had to be written vertically.
In the lowest layer of the diagram, the visible spectrum has been expanded. A bar contains the colours of the rainbow; from left to right violet, blue, green, yellow, orange, red. The left-hand, purple end of the bar is labelled 400 nanometres and seven point five times ten to the fourteen hertz (7.5 × 1014 Hz). The right-hand, red end of the bar is labelled 700 nanometres and four point three times ten to the fourteen hertz (4.3 × 1014 Hz).
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Properties of waves.
wavelength, which is the distance from the top of the crest of the wave to the top of the next crest (of the same wave);
frequency, which measures the number of wave crests that pass a fixed point in a given time.
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IR and UV wavelength compared w visible light.
As the figure shows, the infrared radiation has longer wavelengths than visible light and ultraviolet radiation has shorter wavelengths than visible light.
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Gamma rays.
gamma rays: Gamma rays have an extremely short wavelength, <0.01 nanometres (or <1 x 10-11 m) or the size of an atomic nucleus. A Gamma Ray Spectrometer (GRS) can detect the gamma rays produced when cosmic rays interact with the nuclei of different chemical elements, and on the Mars Odyssey mission was used to map the distribution of 20 common elements including C, Si, Fe and Mg. The GRS map of hydrogen was used to infer the presence and distribution of water.
Gamma rays are also used in medicine to take images of internal organs, for example in PET scanners, but they are extremely harmful to life due to their ionising property. They create charged radicals in the materials they pass through, including DNA (creating mutations) and other cellular components. The amount of radiation used to carry out medical scanning is therefore very carefully controlled.
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X-rays.
X-rays: This graphic shows results from the first analysis of martian soil by the Chemistry and Mineralogy (CheMin) experiment on NASA's Curiosity rover. The soil sample, taken from a wind-blown deposit called 'Rocknest' within Gale Crater, where the rover landed, is similar to volcanic soils in Hawaii.
By directing an X-ray beam at a sample and recording how X-rays are scattered by the sample at an atomic level, the instrument provided the first definitive identification and quantification of minerals on Mars. Each mineral has a unique pattern of rings, or 'fingerprint', revealing its presence. The colours in the graphic represent the intensity of the X-rays, with red being the most intense. The image reveals the presence of crystalline feldspar, pyroxenes and olivine mixed with some amorphous (non-crystalline) material.
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Ultraviolet.
ultraviolet: The Imaging Ultraviolet Spectrograph (IUVS) on the MAVEN orbiter looked at the hydrogen and oxygen coronas of Mars, where the outer fringe of the thin atmosphere meets space. In this region, atoms that were once a part of carbon dioxide or water molecules near the surface can escape to space. These molecules control the climate, so following them allows us to understand the history of Mars over the last four billion years and to track the change from a warm and wet climate to the cold, dry climate we see today.
46
Visible light.
Mars was 80 million km from Earth. The Hubble image reveals details as small as 30 to 50 km across.
The orange area in the centre of the image is Arabia Terra, a vast upland region in northern Mars that covers about 4500 km. The landscape is densely cratered and heavily eroded, indicating that it could be among the oldest terrains on the planet. The large, dark region at the far right is Syrtis Major Planitia, one of the first features identified on the surface of the planet by seventeenth-century observers. Late-afternoon clouds surround its summit in this view. The long dark features known as Sinus Sabaeus (to the east) and Sinus Meridiani (to the west) are dark bedrock and fine-grained sand deposits ground down from ancient lava flows and other volcanic features.
An extended blanket of clouds can be seen over the southern polar cap. The icy northern polar cap has receded to a comparatively small size because it is now late summer in the northern hemisphere. Hubble photographed a wispy afternoon lateral cloud extending for at least 1600 km at mid-northern latitudes. Early morning clouds and haze extend along the western limb.
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Infrared.
infrared (IR): During the martian day, the Sun heats the surface. Surface minerals such as carbonates, silicates, hydroxides, sulfates, hydrothermal silica, oxides and phosphates radiate this heat back to space in characteristic ways and this is detected by instruments on orbiting spacecraft, such as THEMIS on Mars Odyssey. The radiation detected is represented by false-colour images, such as the one below, to identify places of potential geological interest, including where water may have once flown and interacted with the rock.
THEMIS is determining the distribution of minerals on the surface of Mars and helping scientists understand how the mineralogy of the planet relates to the landforms. In this image of Gale crater, home to NASA's Curiosity Mars rover, windblown dust appears pale pink and olivine-rich basalt looks purple. The bright pink on Gale's floor appears due to a mix of basaltic sand and windblown dust. The blue at the summit of Gale's central mound, Mount Sharp, probably comes from local materials exposed there. The typical average martian surface soil looks greyish-green. Scientists use false-colour images such as these to identify places of potential geologic interest. The diameter of the crater is 96 miles (154 km). North is up.
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Microwaves.
microwaves: In 2009, a team from the University of Michigan and the Jet Propulsion Laboratory showed that faint, erratic microwave emissions from Mars coincided with the appearance of a dust storm in images taken by an orbiting spacecraft. The researchers developed a special detector capable of distinguishing faint, high-frequency microwaves from the more intense low-frequency microwaves that come from many sources on Earth and in space. The upper image, taken by the Mars Global Surveyor Spacecraft, shows the dust storm in question appearing as a white cloud, while the lower image displays measurements of the microwave signals over time, with the red colours corresponding to the strongest detections. The researchers suggested that electrical discharges within the dust storm, which have been observed in dust storms on Earth, could be responsible for these signals. If confirmed, microwaves could be used to track future extreme weather events on Mars.
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Radio waves.
radio waves: MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) is an instrument onboard the orbiting Mars Express spacecraft. It uses radio waves to search for liquid or frozen water up to 5 km below the surface of Mars. The instrument emits a ‘chirp’ of radio waves that are reflected off material such as rock, ice and water on, or under, Mars’ surface. Water, in particular, is highly conductive and produces a very strong radar return signal. The image here shows the strength of the return signal for a region near Mars’ south polar ice cap. The brightest colours show the most reflective regions and correspond to what mission scientists believe to be a body of liquid water, hiding beneath 1.5 km of ice. Follow-up observations with MARSIS in 2020 provided further support for the existence of stable liquid water below the ice at Mars’ south pole. Similar “subglacial” lakes are observed in Antarctica using radar sounding techniques, however, temperatures are much lower beneath Mars’ south polar cap than under Antarctica’s ice sheet. The water under Mars’ south pole is believed to be kept from freezing on Mars because of its high salinity and the pressure of the overlying ice.
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Was there water on Mars.
We know from the exploration that we have done that there are no large bodies of liquid water on the surface of Mars today, but there is plenty of frozen water, and evidence for there having been significant liquid water in the past. The evidence that water has been an important feature of the martian geological history includes:
vast river networks of deep valleys and channels that extend thousands of kilometres
large lake basins and outflow channels caused by floods
sedimentary rocks and minerals on the martian surface, formed as a result of weathering processes, including action by flowing water
rounded pebbles found in now dry river beds indicating past flowing water strong enough to tumble and round the pebbles over time.
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What was the evidence that indicated that Mars once had much more water?
Infrared telescopes on Earth have revealed the ratio of normal to heavy water molecules in the atmosphere at different locations and seasons on Mars. Heavy water molecules contain a heavy isotope of hydrogen called deuterium, which remains trapped in the martian water cycle, while normal hydrogen is preferentially evaporated and lost to space. The researchers found that water in the polar icecaps is highly enriched in deuterium, indicating that Mars has lost a tremendous quantity of ‘normal hydrogen’, and hence a lot of water.
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Mars ocean.
It was estimated to cover 20% of the planet’s surface area, and be up to one mile deep. As Mars lost its atmosphere over billions of years, it lost the pressure and heat needed to keep water liquid, so the water evaporated, causing the ocean to shrink. The Curiosity mission estimate was the ocean lasted 1.5 billion years, long enough to allow the emergence of life, but this new evidence suggests it could have been even longer than that.
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Water on Mars now.
In the year 2000, high-resolution images from the Mars Orbiter Camera on board Mars Global Surveyor showed gullies several metres deep and hundreds of metres long running down the internal slopes of craters. It was suggested that they were carved by water that had escaped from underground storage. Such small and sharp features had to be young. They could still have been thousands of years old but annual changes were soon noticed in a few gullies which appeared to suggest that they were still active today.
In 2008 the lander Phoenix actually saw water on Mars. When it scraped away at the dirt, it found water-ice a few centimetres down, but more excitingly droplets that could hardly be anything other than water were seen to form on the lander’s legs.
(Rothery, 2015)
In late September 2015, NASA announced that patterns of dark stripes (called Recurring Slope Lineae) were observed to be advancing and receding on a seasonal basis down the steep slopes of some mountains and craters. These were proposed to be the result of liquid brines flowing downslope in summer and then evaporating. Images of the stripes suggested that these potential brines are extremely salty, containing magnesium perchlorate, magnesium chlorate and sodium perchlorate. These salts, which have been detected elsewhere on the martian surface, can act just like antifreeze, allowing the water to remain in a liquid state even at temperatures well below 0 °C. What’s more, perchlorate salts are capable of drawing moisture from the atmosphere to become a briny liquid. However, other teams have suggested that Recurring Slope Lineae can form through entirely dry means, and their true origins are still debated.
Regardless of the origin of the Recurring Slope Lineae, salts such as perchlorates may allow liquid water to exist in other regions on Mars. In 2018, NASA reported the possible presence of a 20 km-wide lake underneath the southern polar ice cap. Another study in 2020 provided evidence for more lakes hidden under the ice. These lakes are thought to resist freezing because of the presence of salts.
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Part 1 summary.
There are many similarities between Earth and Mars, from the length of one day (equivalent to about 24 hours), the presence of four seasons (caused by both planets being tilted on their axes), and the general physical terrain (with the topography and landscape of Mars looking very similar to parts of Earth). However, you should also have found from Activity 1.2 that there are also some extreme physical differences between the two planets. Any one of lower solar intensity, high radiation levels, thin atmosphere, extremely low average temperature and extremely low atmospheric pressure would all be detrimental to the life found on Earth. If life does exist on Mars, it would need to be able to cope with such extreme physical conditions.
The key concepts and principles you have studied in this part are:
the similarities and differences between two of the planets in our Solar System, Earth and Mars
Both planets have similar landscapes and terrain, and a day length of about 24 hours.
Earth has liquid water now, Mars had liquid water a long time ago, so life may have been possible.
Conditions are extreme on Mars now, with high radiation levels, extreme temperatures and very low atmospheric pressures.
temperature scales
Mars has much greater temperature extremes and gets much colder than Earth.
the electromagnetic spectrum
Solar radiation hitting Mars ranges from short (nm) to long (m) wavelengths.
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Four tenets of life.
Growth, reproduction, metabolism and responses to stimuli.
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If we were trying to ‘build’ a living, self-sustaining organism, what would be required?
We would require materials to build the structure (something equivalent to cells) of the organism, a transport mechanism to move these materials to the required locations, and energy to perform the construction. In addition, the environment in which the organism is to live must provide stable conditions so that the organism can survive and grow.
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How many elements for life? Are they on Mars?
Life on Earth relies mainly on four elements – hydrogen, oxygen, carbon and nitrogen – together with smaller amounts of two other elements, sulfur and phosphorus.
These six elements, all present on Mars, are found in a wide variety of organic combinations and, on Earth, each combination has its own role in maintaining and perpetuating living systems.
To fully appreciate how living systems operate we must begin to think of these elements in terms of the molecules in which they are contained.
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Molecules in a bacterium, types and abundance.
See pic.
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After water, what types of molecules make up living system. Are they small or large. What are they comprised of.
Except for water, most of the molecules in a living system are large organic molecules.
These can be subdivided into four different types: lipids, carbohydrates, proteins and nucleic acids.
These large molecules are usually the products of combining many individual organic molecules called monomers (from the Greek for ‘single parts’) to create polymers (from the Greek for ‘many parts’).
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Lipids (fats and oils)
Lipids, such as fats and oils, are a diverse group of organic compounds occurring in living organisms, which are insoluble in water but soluble in organic solvents.
They include compounds such as fatty acids, oils, waxes and steroids, and have a wide range of functions in living organisms.
Fats and oils are a convenient means of storing food energy in plants and animals.
Lipids are able to group together in a flexible manner that enables them to play a critical role in cell membranes, which form part of the outer limit of cells.
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Carbs
Carbohydrates
Carbohydrates are a group of organic compounds occurring in foods and living tissues, and they perform many vital roles in living organisms.
They contain only carbon, hydrogen and oxygen, usually in the ratio 1:2:1.
The simplest carbohydrates are the sugars, including glucose and sucrose, which are essential intermediates in the conversion of food into energy.
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Proteins & amino acids.
Proteins (from the Greek proteios or ‘primary’) are the most complex large molecules found in living systems.
Proteins are perhaps the most important of life’s chemicals and have an enormous number of different roles. For example, they provide structure (e.g. in human fingernails and hair) and act as catalysts (e.g. aiding digestion in our stomachs). (Note: a catalyst is a substance that increases the rate of a reaction but that is not itself used up in the reaction; proteins with catalytic properties are called enzymes.)
There are around 100,000 types of protein, each made up of smaller components called amino acids. Of the approximate 500 known amino acids, only 20 are found in proteins of living systems but they are combined in an enormous variety of ways. There may be 50 to 1000 amino acid side chains branching from the protein backbone, and the order of these determines the shape and, in turn, the protein’s function. Amino acids can be regarded as the building blocks of life.
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Catalyst.
A catalyst is a substance that increases the rate of a reaction but that is not itself used up in the reaction; proteins with catalytic properties are called enzymes.
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Nucleic acids.
Nucleic acids are the largest of the biological molecules and exist as a collection of individual nucleotides linked together in long linear polymers. Individual nucleotides contain:
a sugar molecule
molecules containing phosphorous and oxygen, known as phosphate groups
a nitrogen-containing molecule known as a (nitrogen-containing) base.
The most famous large organic molecule is DNA (deoxyribonucleic acid), which stores the genetic code, the instructions for building proteins, for all types of life on Earth.
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The cell structure as it pertains to development of life.
Many different molecules must be in close association for living systems to operate. This is because, in chemistry, the rate of a reaction generally increases with the concentration of the molecules reacting. Yet what is there to stop molecules simply drifting off in solution and bringing a halt to the chemistry of life? The answer lies in the structure of the cell. As you saw in Topic 2, in its simplest form, a cell is a small bag of molecules that is separated from the outside world (Figure 2.1). The cell membrane is built of lipids and proteins and surrounds the cell contents. The cell membrane restricts the movement or flow of molecules into and out of the cell and thereby protects the cell’s contents.
Figure 2.1 is a photograph of a cell taken through a microscope. The photograph is in grey scale. The cell is an elongated oval shape, arranged with the long axis horizontal. The edge of the cell is defined and appears as a thick dark grey band. The outer edge of this is labelled cell wall. The inner edge of this is labelled cell membrane. The area within the cell wall is mottled with various shades of grey, and there are two brighter patches. One is labelled DNA. The grey mottled pattern is labelled cytosol.
So cells provide an environment in which biochemical processes can occur and genetic information can be stored. Cells are the basic structural unit of all present-day organisms on the Earth, but they vary in number, shape, size and function. For instance, bacteria are single-celled organisms whereas humans contain many billions of cells.
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Diffusion v osmosis.
While both processes involve the movement of molecules from a volume of high concentration to a volume of lower concentration, osmosis relates only to the movement of water molecules across a membrane.
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Elements fundamental to the development of living organisms must be able to interact with one another, and that occurs most readily in the presence of water. Water has been called the universal solvent because it performs this function so well.
Few other solvents can match the abilities of water to facilitate life. Water exists as a liquid in a temperature range that is not too cold to sustain biochemical reactions and not too hot to stop many organic bonds from forming.
As we saw in Table 2.1 above, water molecules (H2O) are the major component of bacteria; more generally, water accounts for 70% of the mass of living tissue.
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Solvent.
A solvent is a liquid that can dissolve another substance or substances to form a solution.
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Ten most common elements in human body.
See pic.
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What helps Earth sustain life.
Two properties distinguish the Earth from other planets in our Solar System: liquid water covers much of its surface and the planetary environment maintains the water in its liquid state (think back to the phase diagram in Figure 1.11). This enables Earth to support abundant life on its surface.
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Where does energy to maintain life come from?
Energy is required to power the processes that maintain life. In humans, this energy comes from food. The energy is stored mainly in carbohydrates, but proteins and lipids also release energy when broken down.
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Catabolism.
process whereby chemical compounds are broken down to release energy
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Source of energy on Earth.
The most abundant source of energy on the surface of the Earth is sunlight, which provides over 1000 watts (W) of power for every square metre at the distance of the Earth. (Note that the watt is the SI unit of power.) Even with a significant fraction reflected from clouds (as was explained in Topic 3), it still dominates the energy budget at the Earth’s surface.
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Photoautotrophs.
Organisms that use light as their primary source of energy are known as photoautotrophs (from photo, light; auto, self; troph, nourishment).
How is the energy from sunlight converted into complex organic molecules in plants?
Plants convert carbon dioxide, water and other nutrients into more complex organic molecules by photosynthesis, effectively converting the energy of sunlight into chemical energy stored in these molecules.
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Chemoautotrophs.
Organisms that cannot make energy by photosynthesis but instead obtain energy from the oxidation of simple inorganic molecules are known as chemoautotrophs (from chemo, chemical; auto, self; troph, nourishment). The reactions that release this energy, whether organic or inorganic, are all similar at a basic level. Essentially, charged particles (protons or electrons) are transferred across a membrane, creating a gradient of concentration and a separation of electrical charge, thus forming a tiny battery. For example, some deep-ocean bacteria meet their energy needs by stealing electrons from the iron dissolved in the surrounding water, in a chemical process called oxidation (this is the same chemical reaction that forms rust). There are also manganese-oxidising bacteria that live inside volcanic rocks.
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Autotrophs v heterotrophs.
Organisms also require a carbon source to grow. Those organisms that can produce complex organic molecules, such as sugars or proteins, from simple carbon compounds (such as carbon dioxide) are autotrophs. All plants and some bacteria are autotrophs.
Organisms that instead break down other complex organic molecules are called heterotrophs. All animals, some fungi and most bacteria are heterotrophs.
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Biosignature.
Scientists use the term ‘biological signature’, or ‘biosignature’ for short, for any evidence that indicates present or past life.
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Examples of biosignatures.
Figure 2.2 Examples of biosignatures on Earth: (a) stromatolites, or fossilised mounds of colony-dwelling bacteria. (b) A trace fossil, showing evidence for the movement of animal life across the sediment. This one is called Climactichnites, probably trackways from a slug-like animal. (c) Coal, a concentrated accumulation of plant material preserved in sedimentary sequences, such as here at Hartley Bay, Northumberland. (d) An example of flint, a silica nodule found in chalk deposits. The biological origins of flint are still debated. A flint nodule can take on a shape of a biological structure, and the silica may come from the remains of the animals such as sponges, but they can also form non-biologically. (e) Model of methane, a gas formed as a by-product of animal digestive processes.
Figure 2.2 consists of five parts: a to e.
Part a is a close up photograph of a cliff face showing the rock formation. There are horizontal beds of rock, but in the centre of the image os a bulbous shape. This is the fossilised mound of a stromatolite.
Part b is a close up photograph of a rock. It shows a series of tramline-like patterns that criss-cross one another. These are trackways from an animal.
Part c is a close up image of a cliff face showing the rock formation. There are horizontal beds. The bottom beds are black and the top beds are white. The black beds are coal.
Part d is a close up photograph of a rock face. The rock is mainly white except for a grey, bulbous shape in the centre of the photograph. This shape is flint, and the white rock is chalk.
Part e is an illustration of a molecule, showing the atoms as balls and the bonds between them as sticks. The structure has a carbon atom at the centre, which is bonded to four hydrogen atoms. The hydrogen atoms are depicted as slightly smaller balls to the carbon atoms.
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Fossils and microfossils.
Life forms preserved in the geological record are known as fossils. When they are too small to be seen with the naked eye they are called microfossils.
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Since we know that microorganisms, such as bacteria, can leave behind microfossils – physical traces of their existence in the rock record on Earth – these might also be one potential line of evidence for life on ancient Mars.
Microfossils form when the organic matter that a microorganism was composed of is replaced by inorganic minerals, in a manner that preserves the original shape of the organism, cell, or colony of cells. Some examples of bacterial microfossils from the Bitter Springs Chert in Australia are shown in Figure 2.3. These are around 850 million years old but, despite their age, can still be clearly recognised as bacteria since they share many of the same features as modern bacteria.
It has also been suggested that microfossils may be found in much older rocks on Earth (even up to 3.5 billion years old), but with increasing age comes increasing uncertainty, and often these claims are very controversial. Fossils become more degraded and less recognisable with age, and it becomes much more difficult to determine whether they were formed through biological or non-biological processes. This is a cautionary tale when searching for microfossils in samples from Mars.
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Filamentous cyanobacteria.
Fossils of 850 million year old filamentous cyanobacteria from the Bitter Springs Chert in Australia. This type of bacteria generates its energy through photosynthesis and is still common today. The scale bars are given in units of micrometres (a micrometre being a thousandth of a millimetre) (Wacey, Elioart and Saunders, 2019).
Figure 2.3 contains six parts: a to e. All are photographs taken using electron microscopes and all show long, thin tubular, worm-like features. In part a, the feature is brown on a white, mottled background. A red square is drawn around part of the tube, showing how the tube is segmented into many smaller parts. The scale bar indicates the tube is 5 micrometers in diameter and approximately 100 micrometers long. Parts b, c and d are magnified photographs of the segment of tube highlighted in part a. Parts b and c are brown, with the tube walls and segments in bright orange. Part d is blue, with the tube walls and segments in red. At the end of the tube the tube walls are green. Red represents carbon G, blue represents quartz, and green represents carbonate. Part e is a grey scale photograph of a cross section of the tube, showing it is circular. Part f is a grey scale photograph of the tube, showing it many segments. Each segment is approximately 5 micrometers in length.
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Allan Hills 84001
Back in 1996, scientists became very excited when it was announced that scanning electron microscope photographs of a martian meteorite called Allan Hills (ALH for short) 84001 had revealed an object that looked like a little worm (Figure 2.4). It was microscopic, and was interpreted as a fossilised bacterium.
Described image
Figure 2.4 The photograph of martian meteorite ALH 84001 that caused all the excitement. The object that looks like a worm is about 0.4 micrometres long.
The photo shows a dark surface with a large number of light patches on it. Most of the light patches take the form of small blobs but some are larger more irregular shapes. There is one longer, slightly curved, tube-shaped structure in the centre of the photo. The tube is about twice as long as any of the other shapes, and appears to be divided along its length into several even segments.
Figure 2.4 The photograph of martian meteorite ALH 84001 that caused all the excitement. The object that looks like a worm is about 0.4 ...
Other possibilities were quickly suggested. Was it contamination from the laboratory in which it was being studied? Was it contamination from its flight into Earth’s atmosphere and onto Earth’s surface? (It is believed that ALH 84001 was blasted off from the surface of Mars by a meteorite impact about 17 million years ago and fell on Earth roughly 13 000 years ago, so it had quite a long time in which to be contaminated.) Or, since the photo does make it look like the ‘creature’ is properly embedded in the meteorite, rather than superficial detritus, was it perhaps a mineral structure that had no organic origin at all?
Most scientists now lean towards the latter explanation. Perhaps they have learned to be wary, for the story of a contaminated meteorite is not a new one.
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Textural fabrics in sediments.
As well as becoming fossilised directly, microorganisms can affect the structure of sediments in their immediate environment, leading to features that might serve as biosignatures. Stromatolites are finely layered rocks (Figures 2.2a and 2.5) produced in shallow marine environments by the trapping of sediments by colonies of cyanobacterial cells, forming either flat or mound-like microbial mats.
Modern stromatolites in Shark Bay, Western Australia. The flat, rounded mounds are up to about 1 m across, and around 30 cm high.
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Geological factors in identifying stromatolites.
The oldest stromatolites (around 3300 to 3400 million years old) have been found in at least two locations, one at Strelley Pool in Western Australia, and the other in South Africa (the Buck Reef Chert). As with microfossils, great care must be taken in interpreting features as stromatolites because, as with the features in the slightly older Apex Chert in Western Australia, there have been several instances where characteristics initially interpreted as being stromatolites were subsequently reinterpreted as being of non-biological origin.
Some scientists have suggested that the shallow lakes present on early Mars may have been ideal conditions in which stromatolites, or other types of fossilised microbial mats, could form and be preserved. The NASA Perseverance Rover is equipped with fine-scale imagers to search for these types of structures within samples of martian rocks.
However, in order to be convinced about the biological origin of a feature, it is clear that relying solely on shape is not enough. The geological environment must also be considered, i.e. were the rocks originally igneous or sedimentary?
Note: Igneous rocks are those which have been formed from cooling and solidifying molten materials (magma or lava); sedimentary rocks are those formed by the deposition and subsequent cementation of that material at the Earth's surface and/or within bodies of water.
In the case of Strelley Pool and Buck Reef, the host rocks seem clearly to have been sedimentary, laid down in shallow seas, and thus appropriate for the formation of stromatolites. So it looks as if these first traces of life on Earth occurred at least around 3400 Ma ago.
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Trace fossils.
Another textural clue to the presence of life is the trace fossil. Trace fossils are fossilised evidence of the movement or behaviour of an organism: this might include the tracks or burrows left by an animal when walking or feeding; or fossilised faeces. However, trace fossils like those in Figure 2.2b suggest the presence of a moderately complex organism – and so are less likely to be found on Mars.
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Clue: biologically produced organic matter
Organic compounds (molecules built mainly of carbon and hydrogen) are another potential biosignature that we might search for on Mars. However, while all life is composed of organic matter, not all organic matter is associated with life. Indeed, non-biological organic compounds have been discovered in many locations in the Solar System, including asteroids, comets, and the moons of other planets. It is therefore crucial to distinguish between organic matter of biological origin, and organic matter of non-biological origin.
On Earth, finding peat or coal is proof of the presence of large, vegetated wetland environments in the past. This is because peat is the accumulation of large quantities of partially decayed plant material, and coal is, essentially, cooked and buried peat. Crude oil (or petroleum) is generated from the decay of large quantities of organic material such as plankton and algae.
It is generally assumed that life on Mars is highly unlikely to have attained either the level of complexity, or population density, such that bulk deposits of solid waste products would be produced (either pre- or post-mortem).
Much smaller-scale traces of organic materials – at the molecular level – have been found on Mars. The rover Curiosity drilled into mudstones in Gale crater (Figure 2.6) that are similar to clays found in dried-up lake beds on Earth, meaning that if life has ever existed on Mars, we might expect to find evidence of it preserved here.
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SAM organic chemicals analysis.
Figure 2.7 comes from the Sample Analysis at Mars (SAM) instrument on board the rover, and illustrates the detection of martian organic molecules detected when samples of powder – collected from two mudstone targets called ‘Cumberland’ and 'Mojave' – were heated. These long-chain carbon molecules were the first definitive detection of martian organic material on the surface of Mars, even though Curiosity had identified organic compounds previously. On those occasions, the molecules were at such low amounts that they might instead have been contamination. The organic molecules in Figure 2.6 were present in much higher amounts and must have come from the rocks themselves.
Described image
Figure 2.7 Results from the SAM analysis of organic chemicals from surface sediments on Mars from the Cumberland and Mojave drill hole (Eigenbrode et al., 2018).
Figure 2.7 contains three parts: a to c. All three are line graphs, arranged to share some axes, with c across the bottom, and a and b positioned side by side above. Overall, the figure is an x-y line graph. The x-axis is retention time in seconds and the axes go from 0 at the origin to 1000, in increments of 100 seconds. The y-axis is MS response in counts per second. Part a is labelled SAM and has an x-axis that covers retention times from 0 to 300 seconds on the x-axis, and a y-axis that covers MS response from 0 to 6 x 104 counts per second. Two lines are drawn - a green solid line, labelled Cumberland 7 and an orange dotted line, labelled Cumberland blank. The Cumberland blank has a line that fluctuates between 0 and 1 count per second, except a single distinct peak of 2 counts per second at a retention time of 175 seconds. The Cumberland 7 line also fluctuates between 0 and 1 count per second. It also has a distinct peak of 3.3 counts per second at 175 seconds retention time, plus a slightly smaller peak at 190 seconds retention time. This is labelled methylsulfide. There is also a much larger peak of 4.5 counts per second at 220 seconds retention time, labelled dimethylsulfide.
Part b is labelled SAM and has an x-axis that covers retention times from 300 to 1000 seconds on the x-axis, and a y-axis that covers MS response from 0 to 6 x 103 counts per second. Two lines are drawn - a green blue line, labelled Mojave and an orange dotted line, labelled Cumberland blank. The Cumberland blank has a line that is stable throughout at approximately 0.5 counts per second. The Mojave line fluctuates between 0 and 2 counts per second and has several distinct peaks. These are at 370 seconds retention time, labelled thiophene, 550 seconds retention time, labelled 2-methly thiophene, 570 seconds retention time labelled 3-methyl thiophene and at 630 seconds retention time, labelled 2,2,2-trifluoro-N-methly-acetamide. This last peak extends beyond the top of the graph, so is truncated.
Part c is labelled Laboratory breadboard and has an x-axis covers retention times from 0 to 1000 seconds on the x-axis, and a y-axis that covers MS response from 0 to 4 x 107 counts per second, but is in reverse with 0 at the top and 4 x 107 at the origin. A single purple line is shown. This is stable across all retention times at 0.3 counts per second, except for troughs at 220 seconds retention time, 230 seconds retention time, 350 seconds retention time, 370 seconds retention time, labelled thiophene, 550 seconds retention time, labelled 2-methly thiophene, 570 seconds retention time labelled 3-methyl thiophene and at 730 seconds retention time, labelled 2,5-dimethylthiophene.
Figure 2.7 Results from the SAM analysis of organic chemicals from surface sediments on Mars from the Cumberland and Mojave drill hole ...
The way that the SAM instrument heats the sample means that any organic molecules present are broken down into smaller fragments, which are represented on the graphs as peaks. The upper graphs on Figure 2.7 show data from the samples, drilled from the two locations in the mudstone in Gale Crater. The lower graph shows data from an experimental sample run in a replica of the SAM instrument on Earth, at NASA’s Goddard Space Flight Center (the Laboratory Breadboard). Peaks in the upper graphs show that the martian samples contained the same molecules as those in the experimental sample, giving confidence to the identification of the molecules on Mars.
The molecules shown in Figure 2.7 are typical of ancient biological organic matter on Earth, such as coal. Although exciting, this alone is not evidence for life on Mars, since many other non-biological processes might generate the same compounds. However, it shows that organic material can be preserved for billions of years in mudstones on Mars. New missions such as NASA’s Perseverance Rover and ESA’s ExoMars Rosalind Franklin Rover will be able to search for such material.
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Clue: atmospheric constituents.
Gases in a planet’s atmosphere can also serve as biosignatures. On Earth, the presence of oxygen is entirely a consequence of photosynthesis, and would quickly disappear if it was not constantly replenished. Similarly, methane (which has the molecular formula CH4), a trace component of Earth’s atmosphere, is produced by microorganisms in the digestive tracts of animals and in deep sediments.
The joint Roscosmos/European Space Agency’s ExoMars Trace Gas Orbiter was built to hunt for gases such as methane in the martian atmosphere, which could signal the presence of life. On Earth, without biological processes, methane would be quickly lost from the atmosphere. So, finding methane on Mars could be a very strong biosignature.
The Trace Gas Orbiter began science operations in the martian orbit in May 2017. To date, it has not detected any methane in the atmosphere of Mars. This disagrees with the positive detection of methane by other missions, such as the Curiosity Rover, leading some scientists to suggest that methane might be present close to the surface and then quickly broken down before it can disperse and be detected in the atmosphere. Measurements by both missions are ongoing.
(Note also that the absence of methane is not evidence that life does not exist!)
The Open University's Dr Manish Patel co-leads the team looking after the methane-observing instrument on board, known as NOMAD. Watch Video 2.1 to hear what he and The Open University's Professor Stephen Lewis have to say about their roles in the mission, and why they feel it is important to monitor gases in the martian atmosphere.
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ExoMars.
As ExoMars is a live mission, this information is only current at the time of writing! If you want to know more and stay updated, then more information can be found on the European Space Agency website: ExoMars: Has life ever existed on Mars?
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Clue: isotopes
Another tool that scientists use to search for evidence of ancient life on Earth is the measurement of isotopes. You will remember from Topic 1 that isotopes are atoms of the same element with different numbers of neutrons. This means that they have different masses, with some isotopes of the same element being heavier and others lighter.
You will learn more about isotopes in Topic 9, but, for now, all you need to know is that some are stable, meaning that they do not decay over time, and these are very useful in the search for evidence of life.
Biological metabolisms tend to prefer lighter isotopes, meaning that their waste products end up lighter than if the same chemicals were produced by a non-biological process. If more lighter isotopes of certain elements are found this could suggest the presence of life. However, measuring isotopes on the surface of Mars is challenging. The Curiosity Rover has performed some measurements of stable isotopes of sulfur in Gale crater, but the results were ambiguous. Returning samples to Earth will allow much more sophisticated methods to be employed in laboratories.
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Abiogenesis.
Which came first, the chicken or the egg? Clearly the true origin of life must start from something ‘abiotic’, or non-living. Early scientists believed in spontaneous generation of life, from a spirit in the air, based on their observations of maggots multiplying from dead meat and mould cultures appearing on bread, but Louis Pasteur discredited these ideas when he showed that sterile environments do not grow mould.
Looking up abiogenesis (the idea that the earliest life forms on Earth developed from non-living matter) on Wikipedia gives a bewilderingly long list of possible mechanisms and situations for the origin of life: from the production of complex organic compounds during the birth of a star; via the formation of proto-cells in freshwater pools on the flanks of volcanoes; to the suggestion that concentrations of radioactive minerals on sandy beaches provided the necessary energy for living organisms.
Two things seem clear. First, it is possible, in a laboratory, to start with inorganic reactants and recreate chemical reactions that then build the molecules that make up life (amino acids, lipids, etc.). We will look at this a little later on. Second, it is possible to think of places on the early Earth, or beyond, where the necessary ingredients would come together with sufficient energy for those reactions to take place.
In addition, working backwards from what we know of life today, and from the fossil record, scientists are also trying to identify the genetic make-up for the simplest viable organism.
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Earth was formed about 4.5 billion years ago (Figure 2.8), and is maybe one third as old as the Universe itself. To begin with it was a very hot place; mostly molten rock. As it cooled, it formed first a solid crust, then an early atmosphere, and then its oceans.
The span of time before reliable records of life (i.e. any fossils) can be found is known as the Hadean Eon: by convention it began with the formation of the planet and ended 4.0 billion years ago. Between 4.1 and 3.8 billion years ago, the Moon and the other inner planets (Mercury, Mars, and presumably Earth and Venus) were deeply cratered by the so-called ‘Late Heavy Bombardment’ of asteroids and comets.
Described image
This diagram is based round a circle with a hole in the centre. In the central hole, there is a grey arrow indicating that the information should be read in a clockwise direction from the top (12 o’clock). 12 o’clock represents both the beginning of the history of the Earth, and the present day.
From 12 o’clock until about 1.30, the circle is coloured red-pink. This segment is labelled Hadean. It starts at approximately 4.6 Ga and ends at 4.0 Ga. The next segment is coloured bright purple, is labelled Archean, and runs until 5.30 on the clock, or 2.5 Ga. Next, the Proterozoic is dark blue, and runs to 10.30 or 541 Ma. The Paleozoic is mid-blue, and runs from 541 Ma to 252 Ma, or approximately 11.20 on the clock. The Mesozoic is coloured green and runs to 66 Ma or nearly 11.50 on the clock. The last ten minutes or so represent the Cenozoic, coloured pale green.
At the top of the circle, a label says 4550 Ma: Formation of the Earth. Several additional points are labelled on the outer edge of the circle. The first is the formation of the moon at 4527 Ma, about ten past 12. The second is the end of the Late Heavy Bombardment at circa 4000 Ma (or 1.30). This second point is also labelled ‘first life’. Next is the earliest start of photosynthesis, at c. 3200 Ma, or about 3.30. Next, the atmosphere becomes oxygen rich, at c. 2300 Ma, or 6.00. Next, the first vertebrate land animals, at c. 380 Ma, or 11.00.
A series of coloured lines partially wrap around the outer edge of the circle, in a clockwise direction, and almost all continuing to the present day, at 12 o’clock. The closest to the circle is dark purple, and starts at 1.30. It is labelled prokaryotic life. The next is blue, and starts just after 6.30, and is labelled eukaryotic life. The next is turquoise, and starts at 9.00, and is labelled 1.2 Ga multicellular algae. The next is green, starts at 10.00, and is labelled 900 Ma multicellular animals. The next is yellow, starts at 10.50, and is labelled 420 Ma animals on land. The next is orange, starts at just after 10.50, and is labelled 420 Ma vascular plants on land. The line representing non-avian dinosaurs runs from 230 to 66 Ma, or sometime between 11.30 and 11.50. (Note that this line finished before 12, because these dinosaurs are now extinct.) Finally there is a very short section of line, barely visible, from just seconds before 12 o’clock, which is labelled 2.8 Ma genus Homo and 0.2 Ma Homo sapiens.
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Possible microbial mat fossils (similar to stromatolites) have been found in 3.48 billion-year-old sandstone from Western Australia. The presence of biogenic graphite (a form of carbon) has been taken as evidence for life in 3.7 billion-year-old rocks from Akilia Island in south-western Greenland. And by 3300 Ma, rounded tubular cells are known to have existed, fossilised in a pyrite-bearing sandstone at Strelley Pool (also in Western Australia). However, it’s important to note that these possible occurrences of ancient fossilised life remain highly controversial.
The damaging environmental effects of the Late Heavy Bombardment may have repeatedly sterilised the surface of the planet, but sub-surface environments may have been a safe haven for the earliest life-forms (Figure 2.9). If the deep marine hydrothermal setting we examine below provides a suitable site for the origin of life, then abiogenesis could have happened as early as 4.0 to 4.2 Ga, whereas if it occurred at the surface of the Earth, abiogenesis could only have occurred between 3.7 and 4.0 Ga. As new evidence comes to light, geochemists now think that life likely existed on Earth at least 4.1 billion years ago – 300 million years earlier than previous research suggested.
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Figure 2.9 A timeline for life on Earth and Mars.
Figure 2.9 shows a timeline with the most recent events to the left hand side and the oldest events to the right hand side. The timeline is illustrated with text and photographs or illustrations. The timeline shows the following events:
200 million years (since the formation of the Solar System) - the oldest known terrestrial rocks. This is accompanied by an image of a zircon crystal.
500 million years - orbital migration of the gas giants. This is accompanied by an illustration of planets of various sizes.
500-600 million years - Late Heavy Bombardment. This is accompanied by an illustration of a planet being bombarded by objects.
800 million years - first life forms on Earth. This is accompanied by a photograph of a rock overlain with an illustration of a cellular creature.
1.2 billion years - liquid water on Earth. This is accompanied by an image of the Earth from space, showing the full globe.
1.5 billion years - oxygenation of Earth’s atmosphere. This is accompanied by an image of the Moon appearing over the edge of the Earth, taken from space. The Moon is partially obscured by the haze of the Earth’s atmosphere.
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single-celled organisms (e.g. bacteria). When ‘life elsewhere in the Solar System’ is discussed, this certainly does not mean entities that we could talk to, or otherwise communicate with. It doesn’t even mean to imply life forms that would be easily visible, or display obvious movement over distances visible to the naked eye. It probably means the simplest form of life that we can imagine.
One of the points emphasised in Topic 2 was the remarkable uniformity that exists at the cellular level between organisms. This uniformity suggests strongly that all life that survives on Earth today evolved from a single ancestral stock – the last universal common ancestor (LUCA; although ‘last’ is often omitted). This is the ‘root’ of the ‘tree of life’, which shows the relationship between the three domains of organisms: Bacteria, Archaea and Eukarya (Figure 2.10).
Described image
Figure 2.10 The universal phylogenetic tree showing the three domains (branches) of life; all branches shown have living representatives. Halophiles live in very salty environments such as the Dead Sea. Methanogens release methane. Thermoacidophiles live in very hot and acid places. For clarity, some branches have been left unlabelled.
The figure shows a tree-like shape, with the trunk at the bottom of the figure.
The branch to the left of the trunk is labelled ‘Bacteria’. The branch divides into 6 smaller branches. The first smaller branch on the left has been coloured blue, and is one of several to be labelled ‘thermophiles’. The third smaller branch on the left has been labelled ‘cyanobacteria’.
The branch on the right soon divides into two smaller branches, the first labelled ‘Archaea’, the other ‘Eukarya’.
The Archaea branch again splits, with two small branches on the left being coloured blue and labelled both ‘thermophiles’ and ‘thermo-acidophiles’. The right-hand branch continues with 4 small branches to the left and one to the right. Two of the small branches on the left are coloured blue, and labelled both ‘thermophiles’ and ‘methanogens’. One of the other small branches on the left is just labelled ‘methanogens’ and the small branch on the right is labelled ‘halophiles’.
The Eukarya branch divides several times, and at its tip are three small branches labelled ‘animals’, ‘fungi’ and ‘plants’.
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Some characteristics of living organisms are universal and others occur only in certain types of organism. Choose four of the characteristics listed below that are common to all living organisms, and which must therefore have been present in the LUCA.
1. are composed of cells that use oxygen to produce ATP during respiration
2. have a genetic code that can be replicated and that utilises nucleotides
3. are composed of one or more cells, each of which is enclosed in a membrane
4. are composed of cells that can grow and reproduce
5. are composed of cells that have chloroplasts and carry out photosynthesis
6. use an external source of energy
7. are composed of cells that contain mitochondria, the organelles where aerobic cell respiration occurs
8. are composed of cells that have a cell wall external to the cell membrane.
Yes, that's correct.
Answer
The four characteristics are 2, 3, 4 and 6, so these must have been present in the LUCA.
None of the other characteristics is truly universal. In particular,
(1) there are many bacteria that respire anaerobically and do not use oxygen in the process
(5) only autotrophic eukaryotes (plants and algae) contain chloroplasts
(7) only eukaryotes (not prokaryotes) have mitochondria
(8) animal cells and certain bacteria do not have a cell wall external to the cell membrane.
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Defining the nature of the LUCA can be taken further. For example, it is reasonable to assume that our earliest ancestors had the simplest possible type of cellular organisation.
Would this early life have been unicellular or multicellular, a prokaryote or a eukaryote?
It would have been unicellular, because this is the simpler type, and a prokaryote. Eukaryotic cells acquired organelles such as mitochondria and chloroplasts (the organelles in which photosynthesis takes place in plants) by taking on board prokaryotic partners – so they must be regarded as more complex or advanced.
On Earth there is evidence for life originating at least 3700 Ma ago. The great antiquity of the first single-celled organisms should be contrasted with the length of time that human beings have been in existence. In fact our own species, Homo sapiens, has only been around for about 0.2 Ma. If you imagine that all of geological time (4600 million years) is scaled to a 24-hour day, this is equivalent to only about the last 3 seconds!
Why might the fact that unicellular prokaryotes were the first living organisms on Earth be relevant to the question ‘Is (or was) there life on Mars?’
If life evolved elsewhere in the Solar System, it might well have originated in a broadly similar manner to that on Earth – whether it evolved at a similar time or not. Life-forms on Mars might therefore be expected to share characteristics with terrestrial bacteria.
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Orgin of life : atmosphere
In the early 1950s, a PhD student called Stanley Miller (studying under his supervisor Harold Urey) carried out an experiment (known as the Miller-Urey experiment) that has influenced theories on the origins of life for decades. Miller took mixtures of gases (water, ammonia, methane and hydrogen) inside a glass vessel and passed an electric current through them. Eventually, a tar-like deposit formed on the inside surface of the glass vessel that was found to be a mixture of more complex organic species, including amino acids. These had been built up from the simple gas molecules present at the start of the experiment.
From these experiments, it was assumed that the first steps in chemical evolution must have taken place in the early atmosphere, which (in the 1950s) was thought to contain substantial amounts of ammonia, hydrogen and methane. In such an atmosphere, ultraviolet radiation from the Sun and electrical discharges from lightning can provide the energy necessary to produce a wide array of simple organic building blocks, such as amino acids. These could then have entered the oceans in rain and accumulated there.
Unfortunately for this early hypothesis, we know now that the Earth’s early atmosphere was probably not made of ammonia, hydrogen and methane, but is more likely to have been predominantly nitrogen and carbon dioxide (rather like that on Mars now!). So the hypothesis that the first stages of chemical evolution took place in the atmosphere, and then rained down onto the Earth’s surface, had to be discarded.
However, the Miller-Urey experiment is still significant in theories of the origin of life, and it has been adapted many times to simulate other environments where life, or the molecules needed for life, might have originated, such as in volcanic gas plumes and in space.
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Origin of life: warm little pond
In a letter written in 1871 to botanist Joseph Hooker, Charles Darwin wrote, with some vision:
It is often said that all the conditions for the first production of a living organism are present, which could ever have been present. But if (and Oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed.
Was it more likely that life got going in water at the surface? The main reasoning behind surface origin theories was that the base of the terrestrial food chain required photoautotrophs, i.e. sunlight was necessary for life to survive.
Why was sunlight considered necessary for life to survive?
The sunlight would act as an energy source for the anabolic processes necessary to sustain life.
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Pond problem and solution.
The problem associated with a surface origin of life is the build-up of complex molecules from simple starting materials. First, direct UV radiation from the Sun is more likely to break molecules down rather than build them up. (We will look at the effects of radiation on living organisms in the next part of this topic.) Furthermore, if life were to get going in an ocean (or even a pond), then the concentration of starting materials would have to be very high to give them a chance to interact.
One solution would be if there were clay minerals present, which often form in the presence of water. The surfaces of clay minerals can act as catalysts for the polymerisation (growth) of larger molecules from smaller molecules and as a substrate on which molecules could aggregate. Clay minerals also have a layered structure, and it has also been proposed that organic molecules could become trapped and preserved between their layers, protecting them from UV radiation and concentrating them for further reactions.
It is possible that reactions on clay surfaces produced a rich mixture of long-chain organic molecules. These might include:
phospholipids (lipids linked to phosphate groups), which could spontaneously form a membrane-like structure over the mixture and segregate it more firmly from the surrounding medium
amino acids that could link together to form short proteins
nucleotides that could join up to form nucleotide chains.
This list of molecules is still far from being a living cell, but they are the building blocks necessary for life.
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Hydrothermal vent.
In parallel with the understanding of the role that clays could have played in the origin of life came the discovery in the 1970s of hydrothermal vents, and the theory that life might be based on chemical, rather than photosynthetic, energy.
Hydrothermal vents (which include ‘black smokers’ and ‘white smokers’) occur along ocean floor mid-ocean ridges where molten rock wells up and forms new oceanic crust (more on this in Topic 7). First discovered in the 1970s, these vents are hot springs where super-heated water (up to 350–400 °C in the case of black smokers, lower for white smokers), rich in hydrogen (H2), methane (CH4) and hydrogen sulfide (H2S), shoots up from the sea floor (Figure 2.11a). In a black smoker, where the hot water meets the cold oxygen-rich bottom water, there is an instant chemical reaction and sulfides precipitate out from the water, colouring it black. The sulfides build up rapidly to form ‘chimneys’ reaching heights of several tens of metres.
Discovery of hydrothermal vents revealed that, despite the depth and darkness, parts of the ocean floor are home to an unusual collection of animals such as clams, mussels and tubeworms (Figure 2.11b), feeding on the bacteria and archaea that flourish in these very hot conditions.
(a) An underwater photograph of a surface covered in irregular spiky structures, coloured in white, yellow–green and blue, with smoke-like clouds of a dark grey liquid billowing out from three openings in the surface. There is no scale.
(b) The photograph shows a close-up of the spiky structures seen in (a). Here they can be seen as a collection of white or pale blue straight tubes, most of which end in a gently curving reddish-purple structure. In between these can be seen a few sea-shells. Again, there is no scale.
The sketch shows five of these structures. This colony of tube worms are attached, in a clump, at the base to the surface of a rock, near a vent. The white shell part of the tube worm has a label 'bacteria within'. The reddish body of the tube worm is extruding from the white structure, and, in detail, is divided at the top, suggesting the presence of a ‘mouth’ of sorts. Arrows indicate that sulfides are coming out of the vent, and passing by the ‘mouths’ of the tube worms. Arrows also indicate that CO2, O2, H2O and H2S are taken in by the tube worms.
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Ph
See pic.
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Origins of life video.
02:40 min, pause the video:
As explained, chemoautotrophs need a chemically powered source of energy, a little bit like a battery, powered by a flow of ionic charge from one side of the ‘battery’ to the other. Nick Lane and his group are attempting to recreate conditions in which this differential charge can be generated, and they suggest that the structure of the hydrothermal vent towers provides a barrier across which the charge can build up, but will still flow. Note that he uses the term ‘proton’. As you know, a proton is one of two particles found in the nucleus of an atom. However, in this context, he is referring to a positively charged hydrogen ion (H+). It is called a proton because it is a hydrogen atom, consisting of one proton in its nucleus, that has lost an electron, so it has a positive charge.
At 02:57 min, pause the video:
Hydrogen gas dissolved in the fluids rising to the surface at these vents provides a source of H+ ions while the seawater on the outside will contain negatively charged ions. The two fluids will try to equilibrate, driving a flow of charge across the membrane, and providing the energy needed to drive the chemical reactions that build large organic molecules (and, in theory at least, could power those molecules into life).
At 03:19, pause the video:
Once the set-up conditions have been defined, the experiment is left to run for a while, and then samples are collected from the solution to see if any organic molecules have been created.
At 03:42, pause the video:
Formaldehydes are small organic molecules, with the basic formula CH2O but with many variants on that basic formula. They are formed during the metabolism of amino acids in humans, and are common in living organisms. They are building blocks in the synthesis of other more complex organic compounds.
At 04:43, pause the video:
The experiment has gone well so far: using geochemical techniques and the right starting conditions, some of the organic molecules will concentrate many times over.
At 05:46, pause the video:
Lipids are molecules that have a hydrophilic (water-loving) end and a hydrophobic (water-fearing) end. In a surrounding water environment, these molecules will preferentially reorganise themselves and self-assemble into 'vesicles', or fluid-fulled sacs, pointing their hydrophobic ends inwards away from the water and their hydrophilic ends outwards into the water. This allows for chemical separation and containment – the basis for a simple cell.
At 06:35, pause the video:
You will learn more about exoplanets, planets orbiting stars other than the Sun, in Part 4. You will learn more about light waves and spectroscopy in Topic 8.
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Panspermia.
Another theory is that of panspermia, in which life had an extraterrestrial origin and reached Earth from space, carried by comets, asteroids or dust particles. Although the majority view now favours chemical evolution, some scientists have an open mind on the question and some important molecular building blocks of life are known to be formed in space.
The astronomer Sir Fred Hoyle (1915–2001) resolutely maintained that an extraterrestrial origin for life must be the case, however, because it was just too unlikely that chemical evolution could have led to life on Earth in the time available. He knew that traces of life have been found in rocks around 3850 Ma old. Given that Earth formed 4600 Ma ago, that only left 750 Ma years to progress from a molten Earth to an inhabited Earth (even though by bacteria).
It is now thought that the first traces of life are in rocks 3400 Ma old, a period of 1200 Ma since the formation of the Earth. However, it is still not known how rapidly chemical evolution occurred. Just because no fossil (chemical or biological) traces of life before 3400 Ma ago have been found, it does not necessarily mean that life did not exist before that time – it may just mean that it wasn't preserved or hasn’t been found yet.
Listen to the following audio sequence (extract from ‘The meteorite and the hidden hoax’, 2016), taken from the end of the episode of Science Stories that we started listening to earlier. Here, Professor Monica Grady, from the Open University, is talking to presenter Philip Ball about a group of meteorites known as carbonaceous chondrites. As their name implies, they are comprised of carbon-bearing compounds. In the first sentence of the clip, they mention one particular such meteorite known as Orgueil.
Audio player: Audio 2.2
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Audio 2.2 The meteorite and the hidden hoax. (2:34 min)
Does Professor Monica Grady allow, or rule out, the possibility of panspermia?
We have serendipitously discovered that some forms of life on Earth can live in space when we send it there – so we need to be incredibly careful that we don’t start a new colony of Earth-life on Mars (or elsewhere) unintentionally. However, that in itself proves that life can successfully travel through space, so perhaps the idea of life beyond Earth is not so far-fetched after all.
So, although it is still a minority view, consideration is given by some scientists to a possible extraterrestrial origin for life on Earth. But note that panspermia does not solve the problem of how life began – only where...
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Analogue sites for Mars.
In this part, we examine how life on Earth responds to extreme environmental conditions. After all, what we might consider an extreme environment on Earth may be closer to conditions found on Mars; scientists talk about some extreme environments on Earth being ‘analogue sites’ for Mars.
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Why study extremophiles.
Studying the organisms that inhabit extreme environments on Earth teaches us about the boundaries of life (the conditions under which life is possible), and therefore helps us to recognise which environments beyond Earth may be capable of supporting life.
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Extremophile.
Organisms that have adapted to grow best under different extreme conditions are known as extremophiles (Video 3.1). The number of known extreme-loving organisms from across a variety of environments is increasing all the time, confirming that many forms of life can exist, and indeed thrive, under conditions that would be deadly to humans.
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Extremotolerant.
Organisms that can tolerate exposure to extremes, but grow best when conditions are more mild, are known as extremotolerant.
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Polyextremophiles.
Organisms that can survive multiple extremes are known as polyextremophiles.
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Are environments unchanging and how do organisms respond.
The physical and chemical conditions of any environment (excepting perhaps those in a controlled laboratory) are never static, and vary both temporally (i.e. over time) and spatially (i.e. over a space or distance).
Organisms that are fixed in place in their environment (e.g. plants) have to be able to cope with any temporal variations that occur, whereas organisms that can move within their environment (e.g. animals), can reduce the impact of both spatial and temporal variations by moving from one location to another at different times (e.g. operating above and below ground, moving from a shady to sunny area, moving to an area richer in food, searching for mates, migrating on a seasonal basis, etc.).
In general, all organisms have adapted to operate within a certain range of conditions within their natural environment, and different organisms have adapted to survive within different ranges of variation depending on the precise environment in which they live.
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Tolerance range.
The range of conditions over which an organism is adapted to live is called the tolerance range. This can be represented graphically by plotting a biological factor associated with the organism (e.g. growth rate) against variable environmental factors (e.g. temperature). Figure 3.1 shows an example: the response of growth rate to changes in temperature for five different species of microbes (labelled A to E).
Each of the shapes has a growth rate that rises fairly rapidly (but some more so than others) with increasing temperature. This then reaches a peak, representing the highest growth rate for that particular microbe, after which the growth rate decreases rapidly with increasing temperature.
The x-axis of this graph is labelled temperature in degrees C, and it is divided into ten degree intervals from minus ten at the origin to 120 degrees at the right of the graph. The y-axis is labelled growth rate, and it does not have any units or divisions, so the top of the axis is instead marked with an arrow to illustrate the direction of increase is upwards. The total height of the graph is about 4 and a half centimetres on the image.
Curves are drawn on the graph to illustrate the growth rate for each of five microbe species, A, B, C, D and E, at different temperatures.
The curve for Microbe A starts at minus five degrees, rises to a rounded peak of 1.2 centimetres at plus four degrees, and tails off again to disappear at about 12 degrees. The curve is roughly symmetrical.
The curve for Microbe B appears at plus eight degrees, rises gradually to a rounded peak of 2.7 cm at 39 degrees, and then drops sharply to disappear at 47 degrees.
The curve for Microbe C appears at 41 degrees, rises less gradually to a rounded peak of 3.5 cm at 60 degrees, and then drops sharply to disappear at 67 degrees.
The curve for Microbe D appears at 65 degrees, rises (more gradually than C but less so than B) to a rounded peak of 3.0 cm at 88 degrees, and then drops away to disappear at 96 degrees.
The curve for Microbe E appears at 90 degrees, rises to a rounded peak of 3.0 cm at 106 degrees, and then drops away to disappear at 114 degrees. The shape of the curve for E is very similar to that for C.
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Optimum temperature.
The temperature at which each species grows and reproduces most rapidly is called the optimum temperature. At temperatures above and below this optimum but relatively close to it, the microbe will survive, but its growth rate will be reduced.
The ends of each curve represent the temperature limits for each microbe; in other words, each of the five microbes will not grow (and may not survive) at temperatures higher or lower than the ends of their curve.
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What other than growth rate can be affected by what.
Similar responses can also be shown for other environmental conditions (both physical and chemical, e.g. amount of radiation, salinity, pH, etc.) against any biological processes (e.g. reproductive success, respiration rates), for any type of organism. In each case, the organism will have its own set of response functions that define its limits of survival.
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Why.
Looking back at Figure 3.1 (repeated below), you will notice that most of the curves are a similar shape. As the temperature at which each microbe species is living rises above the optimum temperature, there is a steep drop in the growth rate of the microbes. This is largely a result of the denaturing of the microbes’ enzymes. Once the enzymes are denatured, the microbes soon die, so the growth rate drops very steeply to zero. Lowering the temperature of the microbes below the optimum value does not denature the enzymes but simply causes all the reactions in the cell to proceed more slowly. Thus, the microbes are not killed immediately, but their growth rate is reduced, so the curve falls more gently at temperatures below the optimum value. They do eventually die, probably because their cell membranes become too stiff for normal functioning of the cells.
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Temperature
Temperature presents a range of challenges to living organisms. The structural breakdown of cells caused by the formation of ice crystals in sensitive plants can be readily witnessed in those parts of the world that experience cold winters or even just the occasional frosty night (Figure 3.2 - frostbitten aloe)
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Psychrophiles. Water damage.
On Earth, cold environments are actually more common than hot environments. The Earth’s oceans maintain an average temperature of 1–3 °C. However, large areas of the Earth’s surface are permanently frozen or are unfrozen for only a few weeks in summer. Some of these frozen environments support life in the form of microbes known as psychrophiles (from the Greek ‘psukhros’ for cold and ‘phile’ for loving). Representatives of all major groups of organism are known from environments with temperatures just below 0 °C. Freezing in liquid nitrogen at a temperature of –196 °C can preserve many microbes successfully. However, the lowest recorded temperature for active microbial communities is substantially higher, at –18 °C.
Liquid water is both a solvent for life and an important reactant or product in many biological processes. Does water expand or contract when it freezes? Why have the fleshy leaves shrunk in the case of the aloe in Figure 3.2?
As you discovered in Topic 1, because ice is less dense than water it occupies a greater volume. When water freezes the resulting ice crystals can rip cell membranes apart, and in Figure 3.2 the plant has thawed, but the cell structure could not be restored. The freezing of water inside cells is almost always lethal (though we will look at some exceptions below).
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Sodium acetate experiment.
Sodium acetate seed crystallises in sodium acetate supersaturated solution.
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Biological antifreeze.
Two principal adaptations have evolved to deal with temperatures below the freezing point of water: the protection of cells by preventing ice formation or, if ice does form, protection of the cells during thawing.
One way that organisms prevent ice forming is to accumulate compounds that can depress the normal freezing point of water, i.e. accumulate a sort of anti-freeze. (You may be aware that concentrated alcohols such as vodka freeze at lower temperatures than plain water. Animals don’t use vodka, but other chemicals have a similar effect.) Other animals coat the miniature ice crystals with a layer of proteins as soon as they form, preventing them from growing any larger. High concentrations of such anti-freeze compounds can enable the survival of some animals to temperatures as low as −60 °C.
Salts dissolved in water in the natural environment can also act as an anti-freeze, meaning liquid water can exist even at very low temperatures (for example, see Figure 3.3). This is particularly relevant for Mars, where abundant salts have been detected yet temperatures are usually well below 0 °C (sometimes as low as -120 °C). Whilst scientists have pointed out that this could lead to briny liquids on the present-day surface of Mars, any organism would need to have evolved psychrophilic adaptations in order to survive within them.
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Lost Hammer brine spring.
Lost Hammer’ brine spring on Axel Heiberg Island in the Canadian Arctic. The white material filling most of the image is salt, deposited by the spring over many years. The spring waters are at around -5 °C all year round (and do not freeze thanks to the high levels of salt), but microorganisms have been found here, adapted to life in the freezing temperatures.
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Effect of heat on cell membrane.
At the other extreme, high temperatures result in the structural breakdown of biological molecules such as proteins and nucleic acids. Temperatures of 100 °C will disrupt the structural integrity of most cell membranes to the extent that they leak important cellular constituents.
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Opposite of psychrophiles.
Luckily, life on Earth has adapted to a surprising range of temperatures (Figure 3.4). Although the majority of organisms grow best at moderate temperatures of between 15 °C and 50 °C (the mesophiles in Figure 3.4), the temperature preferences of other organisms range from hyperthermophiles (able to reproduce at temperatures >80 °C) to psychrophiles where maximum growth occurs at temperatures <15 °C.
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Temperature adaptations.
On the left-hand side of the image, three types of organism are listed, and brackets are used to indicate the temperatures at which these are known to exist: mesophiles between 20 degrees and 50 degrees; thermophiles between 50 and 80 degrees; and hyperthermophiles between 80 degrees and 120 degrees.
On the right-hand side of the image, some of the major groups of organisms are listed, and leader lines are used to indicate the approximate point on the temperature scale at which these organisms thrive. In the following list the organism is followed by the indicated temperature.
Heterotrophic bacteria: 85 degrees
Cyanobacteria and photosynthetic bacteria: 80 degrees
Fungi, algae and protozoa: 55 degrees
Mosses: 45 degrees
Most plants, and insects: 37 degrees
Fish, fungi, algae, protozoa, bacteria, archaea: between minus five and 30 degrees
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Hyperthermophiles.
No complex animals or plants are known that can tolerate temperatures above 50 °C for prolonged periods of time, though microbial thermophiles that are content at temperatures up to 60 °C have been known for a long time. True extremophiles, those able to flourish in greater heat, were first discovered in the 1960s during a study of microbial life in hot springs and other waters of Yellowstone National Park in the USA. To date, more than 50 species of hyperthermophiles have been isolated, the most resistant of which grow in the walls of hydrothermal vents (such as black smokers) where they reproduce in an environment of about 105 °C; indeed many hyperthermophilic organisms won’t grow at all at temperatures below 90 °C.
So how have organisms adapted to these high temperatures? Thermophilic and hyperthermophilic organisms appear to have adapted by having DNA and proteins that are better able to cope with higher temperatures, and more stable cell membranes.
What is the upper temperature limit for life? Do ‘super-hyperthermophiles’ capable of growth at 200 °C or 300 °C exist? At present we do not know, although it seems likely that the upper limit is about 140 °C, as above this temperature, proteins and nucleic acids break down. The consequent loss in the integrity of DNA and other essential molecules would probably prevent reproduction.
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We know that Mars is currently a rather cold planet relative to Earth, and has frozen conditions at the poles. Pschyrophiles might therefore have an advantage. Why might the ability to survive at high temperatures be relevant to finding life on Mars?
There are places on Mars where high temperatures existed at various times in its past, at least for periods of hundreds to thousands of years and quite probably much longer. It has certainly had live volcanoes, and high temperatures are also generated during impact events.
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Radiation effects.
Radiation is energy in the form of waves or particles, such as electromagnetic radiation (e.g. gamma rays, X-rays, ultraviolet radiation, visible light, or infrared radiation – see Part 1). Very high levels of radiation do not occur naturally on Earth. We know too that Mars lacks the protection afforded the Earth’s surface by its atmosphere, and in particular the Earth’s ozone layer absorbs significant amount of ultraviolet radiation.
The effects of radiation on living organisms have been well studied as a result of research on the use of radiation in medicine and on the consequences of human activity ranging from warfare to space travel. Indeed, the level of radiation on the martian surface resulting from cosmic rays (high energy atomic or subatomic particles travelling through space) is a major concern for space agencies contemplating future exploration of the planet by astronauts. Radiation in the ultraviolet region of the electromagnetic spectrum can cause serious damage to DNA in a number of ways, including by changing the composition of its bases, adding or removing bases or even breaking the double helix.
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Deinococcus radiodurans.
One organism known to withstand exceptional levels of radiation, and which probably qualifies as a radiation extremophile, is the bacterium Deinococcus radiodurans (Figure 3.5), first discovered in 1956. It can withstand exceptionally high doses of ultraviolet and gamma radiation, which is thought to be because of the bacterium's ability to repair any damaged DNA.
Described image
Figure 3.5 A transmission electron micrograph (TEM) of D. radiodurans, acquired in the laboratory of Michael Daly, Uniformed Services University, Bethesda, MD, USA.
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Mars radiation.
NASA have estimated the danger posed to an astronaut visiting Mars by calculating the average number of times per year each cell nucleus in the human body would be hit by a cosmic-ray particle. Figure 3.6 is an attempt to correlate this risk with their potential landing location.
The top part of this figure, part (a), is made up of two circles representing opposite global projections of Mars. The one on the left shows the south pole, near the bottom of the image. The Hellas impact basin is situated in the lower left quadrant of the image. The one on the right shows the north pole near the top of the image, and the Tharsis volcanoes situated in the centre of the image, close to the equator of the globe. (So the Hellas basin is out of sight, around the back of this globe.) The globes are coloured to show the risk to human health generated as a result of the estimated average number of times per year each cell nucleus in the human body would be hit on Mars by a cosmic-ray particle. The range is given in a key, shown as a coloured bar above the two globes. The scale runs from a moderate risk level shown in dark blue, to a high-risk level colour-coded red. Very low risk areas are almost black. The main features are coloured as follows: On the left-hand image, the Hellas basin is very dark blue. Surrounding the basin there is a fairly rapid transition through green, moderately high risk, to yellows and reds. At the top right of the globe the colours return to green and then pale to medium blues again. The south pole is fairly dark red. On the right-hand image, the north pole is in an area coloured dark blue. Moving south, some areas are blue or pale blue as far as the equator. Near the volcanoes, the colours rise rapidly through green to yellow to red and even browns, indicating a very high risk zone. The tops of the volcanoes are white: this colour is not on the scale. The bottom of the globe is all red. There is one other noticeable feature shown in blue; this is a linear feature extending to the right of the volcanoes, close to the equator.
The lower part of the figure, part (b), is a pair of circles illustrating false-colour topographic maps of the southern (left image) and northern (right image) hemispheres of Mars. The scale for this part of the diagram is again a coloured bar from blue to red, but this time the dark blue represents areas with relative altitudes of 10 km below the average for Mars, green about average, and red to white represents areas with altitudes of 5 to 10 kilometres above average for Mars. The views in part (b) do not exactly match the views shown in part (a), but they are at least similar, so that the Hellas basin is still at the lower left of the left-hand circle, and the volcanoes are slightly centre-left of the right-hand circle. The colours for a particular feature in part (b) of the diagram are usually the same as the colours for the same area in part (a) of the diagram. For example, the Hellas basin is dark blue in part (b) also, and the linear feature in part (a) is also blue in part (b). The area near the south pole is coloured with reds and yellows in both parts of the figure, and the area surrounding the north pole is coloured blue in both parts.
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Comparing Figures 3.6a and b, how does the risk posed by cosmic ray radiation relate to the topography of Mars?
Radiation levels are highest in areas of high elevation, e.g. the southern highlands of Mars, and lowest in areas of low elevation, e.g. the northern lowlands or the Hellas impact basin (the large round dark area in Figure 3.5).
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What might be responsible for the apparent relationship between radiation levels and elevation on Mars?
The lower areas have more atmosphere above them to block out some of the radiation. Earth’s thick atmosphere shields us from most cosmic radiation, but Mars has a much thinner atmosphere than Earth does.
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How else could organisms escape the high radiation levels experienced at the surface?
Some organisms bury themselves deep underground. Both thick ice cover and a layer of rock would give satisfactory levels of protection from the damaging UV.
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Dessication.
You saw in Topic 1 that the high melting point of ice and boiling point of water, and the wide range of temperatures over which it remains liquid, make water an essential solvent for life. Water limitation therefore represents a particularly extreme environment for life. Some organisms (see Table 3.2) can tolerate extreme desiccation by entering a state of apparent suspended animation, characterised by little water within their cells and a cessation of biological activity. This is well documented in organisms such as bacteria, yeasts, fungi, plants and animals associated with environments where the water essential for active life is often transient and sporadic. When the water disappears, these organisms appear to be dead for periods of days, weeks, or even years until moisture returns, at which point they ‘come back to life’ and resume their normal activities. (Think back to Topic 2: how do you know when something is alive?)
One example of such an organism is Deinococcus radiodurans (which you also met in Section 3.3), whose extraordinary resistance to radiation is thought to be a consequence of its evolutionary adaptations to cope with extreme desiccation.
One of the cutest examples of a desiccation-tolerant organism is the tardigrade. (You encountered one, briefly, in the extremophiles video at the start of this topic.) Tardigrade means ‘slow stepper’ and – unlike D. radiodurans – this microscopic beast is a multicellular creature with almost recognisable features such as a head and legs. A common name for the tardigrade is the water-bear.
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Anhydrobiosis in the tardigrade.
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Pressure.
Terrestrial plants and animals at the Earth’s surface have evolved at normal atmospheric pressure. However, pressure increases with depth in the oceans, so marine organisms may have to deal with much higher pressures. Atmospheric pressure also decreases with altitude, so that by 10 km above sea-level, it is around one-quarter of the atmospheric pressure at sea-level.
Pressure presents problems to life because it forces volume changes. When pressure increases, the molecules in cell membranes pack more tightly, restricting the flow of essential fluids through the cell membrane. When external pressure decreases, fluids expand and cause swelling, which may be felt as pain in joints, or headaches. Many humans are sensitive even to daily weather-related changes in atmospheric pressure, although the full range we would normally experience is just a few tens of millibars. Undersea divers and high-altitude climbers can develop more serious conditions such as decompression sickness (the ‘bends’), and pulmonary or cerebral oedema (fluid build-up).
Organisms that can tolerate high pressures have often adapted the compositions of their cell membranes to improve the flow of essential fluids and nutrients. Pressure-loving piezophiles have been recovered from the Earth’s deepest sea-floor, the Mariana Trench, where they thrive at pressures of more than 1100 times the pressure at the Earth’s surface (Figure 3.8).
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Gravity
The strength of the gravitational force will also have an effect on an organism. Until recently, all organisms on Earth have lived with a gravitational force of 9.81 N kg−1 (with very minor variations depending on distance from the centre of the Earth). The advent of space exploration means that humans have had to deal with a range of different gravity regimes, from the huge forces experienced during launch to ‘microgravity’ environments on board the International Space Station (ISS) (Figure 3.9).
Figure 3.9 (a) The International Space Station. (b) Spiders in space proving that they can build normal webs in microgravity conditions.
Although most research on microgravity has been concerned with the effects on human health, studies on board the ISS have demonstrated that gravity plays an important role in a variety of biological processes. Scientists now believe that there are conditions in which the weightless environment influences the cellular machinery, resulting in specific changes to cell membranes and the reproduction of microbes.
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pH
As we noted in Figure 2.13 in Part 2 (repeated below), pH is a numerical scale that runs from 0 to 14 and indicates the acidity or alkalinity of a solution. A solution is acidic if the pH is less than 7 and alkaline if it is greater than 7. Pure water has a pH of 7 and is neutral. Biological processes tend to occur towards the middle range of the pH scale, because pH values in the natural environment fall within this range (e.g. the pH of seawater is approximately 8.2).
Described image
Figure 2.13 repeated from Part 2
Figure 2.13 repeated from Part 2
In the second of the alternative practical activities associated with this part of the topic, you can make your own pH indicator solution to measure the pH of various solutions found around the home. Full details are available from the Practical activities tab for this topic.
Some extremophiles are known to prefer highly acidic or alkaline conditions, e.g. the acidophiles and alkaliphiles, respectively. Acidophiles thrive in the rare acidic habitats that have a pH of between 0.7 and 4, and alkaliphiles favour alkaline habitats with a pH of between 8 and 12.5.
Highly acidic environments can occur naturally within rocks as a result of the passage of water through the rock, for example at some hot springs. However, acidophiles are not able to tolerate a significant increase in acidity inside their cells, where it would destroy important molecules such as DNA. Thus, they survive by keeping the acid out. But the defensive molecules that provide this protection, as well as others that come into contact with the environment, must be able to operate in extreme acidity. Indeed, enzymes have been isolated from acidophiles that are able to work at a pH of less than 1 (vinegar has a pH of about 4).
Alkaliphiles live in highly alkaline soils and in so-called soda lakes, such as those found in Egypt, the Rift Valley of Africa and the western USA. Above a pH of 8 or so, certain molecules, notably those made of RNA, break down. Consequently, alkaliphiles, like acidophiles, have found a way to maintain neutrality inside their cells.
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Probiotics and stomach acid
The supermarkets are full of probiotic drinks saying they are full of the ‘friendly bacteria’ (microbes) that are needed for a healthy gut. However, the stomach is a very acidic environment, caused by cells in the stomach wall producing hydrochloric acid.
What effect will the acid in the stomach have on any microbes in these drinks or on food being eaten?
An acidic environment is likely to kill many of the microbes (specifically all the neutrophiles and alkaliphiles) ingested with the food. This is not necessarily a bad thing, because some of these microbes could otherwise cause illness.
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Helicobacter pylori.
There is, however, a bacterial species called Helicobacter pylori (Figure 3.10), which is an acidophile that can live very successfully in the human stomach. Helicobacter pylori possesses four to six flagellae which allow it to burrow through the mucous layer of the stomach and reach the stomach lining, where it takes up residence. It is able to detect the local pH, which is somewhat higher (nearer to neutral) in the stomach lining, and if it senses that it is being carried nearer to the low pH (acidic) contents of the stomach, it uses its flagella to burrow deeper again.
Described image
Figure 3.10 Helicobacter pylori, with flagellae.
Figure 3.10 Helicobacter pylori, with flagellae.
To survive the low pH, H. pylori produces an enzyme called urease, which breaks down a compound in the stomach called urea, along with water, to produce carbon dioxide (CO2) and ammonia (NH3).
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Smokers.
Observations of the deep oceans in the late 1970s discovered hydrothermal vents in the Pacific Ocean (called ‘black smokers’) on the ocean floor, pumping out highly acidic (pH 2–4), hot water at temperatures of 60 °C to 400 °C, and at high pressures. (In comparison, the ambient seawater temperature at the ocean floor is ~2 °C.)
In 2000, very different fields of hydrothermal vents (called 'white smokers') were discovered in the Mid-Atlantic, venting highly alkaline fluids with a pH of 10–11. Further investigations revealed that hydrothermal vents emitting acidic fluids were being driven by volcanic heat below the sea floor, while the highly alkaline vents were driven by reactions between the seawater and deep crustal rocks making up the adjacent seafloor, giving them the distinctive black and white 'smoke' colours.
Despite the very different fluids being emitted, both types of hydrothermal vent were associated with numerous acidophiles or alkaliphiles, with the microbes using minerals and chemical compounds emitted from the vents as sources of energy to sustain life and boost growth. These microbes represented the first step in the food chain, supporting an abundance of other acid- and alkaline-loving life, located within the area immediately adjacent to the vent (Figure 3.11).
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Salinity.
Salts are typically water-soluble crystalline compounds containing either a metallic element (e.g. sodium, magnesium) or ammonium (NH4+) in combination with elements such as chlorine or groups of elements such as nitrates and carbonates.
Salinity is a measure of the total quantity of dissolved salts in water. Sodium chloride, which is common table salt, is one such salt, but others are composed of elements such as magnesium and calcium (e.g. magnesium chloride and calcium chloride). Organisms can live within a range of salinities, from distilled water to saturated salt solutions. Halophiles are organisms that require high concentrations of salt to live. Their optimal sodium chloride concentrations for growth range from twice to nearly ten times the salt concentration of seawater. They are found in habitats like the Great Salt Lake in Utah, USA, the Dead Sea in Jordan, salterns (evaporation basins for extracting salt from seawater) and salt marshes (e.g. Figure 3.12). Some high-salinity environments are also extremely alkaline, as the weathering of sodium carbonate and certain other salts can produce alkaline solutions (Figure 3.13). Not surprisingly, microbes in those environments are adapted to both high alkalinity and high salinity and are therefore considered polyextremophiles.
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Examples of salinity.
Figure 3.12 (a) and (b): Images of the salt flats of the Great Rann of Kachchh (also known as the Great Rann of Kutch), a salt marsh in the Thar Desert in Gujarat. The white material covering the surface in both images are salts. These locations undergo an annual cycle of desiccation and flooding, with the desiccation resulting in the deposition of salts as the water evaporates. The microbes that survive in this environment have adaptations to high salt concentrations that fluctuate over the course of a year.
Figure 3.13 (a) and (b) Images of Lake Natron, a salt and soda lake in the Arusha Region of northern Tanzania. Note the pink colouration caused by halophilic organisms. (The flamingos that feed off these creatures are also famously pink.)
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Mol.
In chemistry, one common unit of measurement used to express amounts of a substance is the mole. The concept of a mole takes a little getting used to but, once you realise that it is simply a way of counting without the inconvenience of using a very large number, you will appreciate its usefulness. There are a few more than 602 200 000 000 000 000 000 000 or 6.022 × 1023 particles (such as molecules or atoms) in a mole (the symbol for which is mol).
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Avogadro’s number.
The number 6.022 × 1023 is called Avogadro’s number. The word 'mole’ describes this fixed number of particles, in the same way that the word ‘dozen’ is used to indicate that there are 12 of something. A dozen eggs is a set of 12 eggs; a mole of eggs would contain 6.022 × 1023 eggs. Note that both a dozen and a mole are dimensionless quantities – you have to add the dimension (eggs, or particles) yourself.
Just as you can have a dozen eggs and a dozen tractors, and these two sets would have entirely different masses, so the mass of a mole will depend on what you are counting. The mass of a mole of a substance is obtained by adding together the relative atomic masses (RAM) of all of the atoms in the formula unit, and following this by the unit of mass, g (grams).
So, for example, the relative molecular mass (RMM) of methane is calculated as:
RMM (CH4) = RAM C + 4 RAM H
Hence the mass of one mole of CH4 is 16.043 g or approximately 16 g.
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Molar mass.
The molar mass of a substance is the mass of that substance, measured in grams per mole, and it is derived directly from the relative molecular mass.
So the molar mass of methane is 16.043 g mol−1 or approximately 16 grams per mole.
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find the number of moles of water in 100 g
See pic
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Concentration.
The amount of a certain substance held in a given volume of a solution is known as the concentration of the solution.
The other key quantity required for a concentration is the volume of solvent in which the chemical is dissolved. A litre (l) is commonly used for the unit of volume.
One litre of average seawater contains about 35 grams of salt, so we say seawater has a salt concentration (or salinity) of 35 g l−1 (35 grams per litre).
Assume that sea salt is entirely NaCl (and ignore other components such as magnesium and calcium salts for now). Can you convert this into a concentration in moles per litre (mol l−1)?
Mass of substance = 35 g.
Molar mass of NaCl, calculated above = 58.440 g mol−1.
We need to divide the mass of the substance by the molar mass:
So, one litre of seawater contains 0.60 moles of NaCl. Therefore, the concentration of NaCl in seawater is 0.60 mol l−1.
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Adaptations to increased salinity
Halophilic microbes have evolved two ways of coping with the dehydration that would naturally occur in saline conditions. Some of their energy is used to make extra molecules (including sugars and amino acids) in their cells so that, overall, the concentration inside the cell is equal to or greater than in the salty water outside. This allows osmosis to work in the microbe’s favour, preventing water from being drawn out of the cell, and therefore avoiding dehydration. The potential for water to move from one area to another because of osmosis or another driving force (e.g. gravity or mechanical pressure) is known as water potential, as it refers to the potential energy of the water (relative to pure water). You met potential energy previously in Topic 2.
The second approach used by some halophiles is to selectively accumulate sufficient potassium ions (K+) to make the concentration of dissolved molecules similar inside the cell to the saline water outside. However, as potassium ions are similar to the sodium ions (Na+) of salt itself, significant modifications have first to be made to ensure the enzyme systems and the other proteins in the cells can function properly.
Saline conditions also affect the cell membranes. To counteract this, some halophiles have different fats in their membranes that are only stable in saline conditions.
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effects of changing the chemical environment within or surrounding a cell.
See pic.
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Salinity swapsies
What would happen to a halophilic microbe if it was placed in freshwater?
This is the condition shown in the top left-hand image of Figure 3.16. The inside of the microbe would have a much higher concentration of dissolved molecules than the outside water, and so water would rush into the cells. The microbe cell would then be forced to expand and after a few minutes, it might burst.
What would happen to a freshwater plant if it was placed in salt-water?
This is the condition shown in the lower right-hand image of Figure 3.16. The inside of the plant cells would have a much lower concentration of dissolved molecules than the outside water, and so water would rush out of the cells. The plant cells would then be forced to contract and the structure of the cell walls would start to be damaged.
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Geothermal hot springs
Geothermal hot springs occur where volcanic processes bring magma close to the surface and into contact with groundwater.
Hot springs and geysers are commonly associated with geothermal areas such as those of New Zealand (Figure 3.17). They are characterised by steam and hot water, and can be low pH (acidic) or high pH (alkaline). They can also contain high levels of toxic metals such as mercury. They are, nonetheless, environments that sustain a remarkably diverse range of life. The range of colours visible in Figure 3.17 reflects different algal populations growing around the Waiotapu hot springs in New Zealand.
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Why might hot-spring environments be important in the search for evidence of past life on Mars?
Mars has been volcanically active in the geological past (remember the volcanoes you saw on your tour in Part 1?), so the heat from volcanism may have resulted in the production of environments on or near the surface of Mars where hot water may have existed.
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What sort of organisms might we find in hot-spring environments?
The relatively high temperatures would favour thermophiles or hyperthermophiles, while the alkalinity of the water might favour alkaliphiles.
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Hot spring in Waiotapu Park
Hot spring in Waiotapu Park, Rotorua in North Island, New Zealand. The various colours around the edge are due to microbial mats formed by organisms that thrive in different temperature and pH environments.
Figure 3.17 is a photograph of a hot spring. The spring itself appears as a flat open body of water, like a lake. Steam is emitted from the lake. The ground around the lake is orange, white and grey.
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Desserts.
Deserts are environments on Earth that are extremely dry and can be either hot or cold. Water is always the limiting factor in such ecosystems. The Atacama Desert in Chile is one of the hottest and driest areas, while the coldest and driest places on Earth are the so-called dry valleys of Antarctica (Figure 3.18), where there is no ice cover and very little water and the temperatures are extremely low – almost Mars-like. The primary inhabitants of both kinds of desert ecosystems are bacteria, algae and fungi that live on or a few millimetres below the surfaces of rocks. Organisms that have adapted to living on or beneath the surfaces of rocks are referred to as endoliths (from the Greek endon, meaning ‘within’ and lithos, ‘stone’).
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Antarctica.
Figure 3.18 Satellite view of the McMurdo dry valleys in Antarctica, one of the most extreme environments on Earth. The dry valleys are large ice-free regions with average temperatures of around –20 °C. It never rains in the dry valleys, and only occasionally snows (equivalent to 10–20 mm of rainfall per year).
At first glance the dry valleys of Antarctica appear lifeless, yet organisms have been recovered from within the rocks and minerals, and algae and bacteria have been found in the occasional lakes that form from melting glaciers in the summer months.
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Sahara.
Figure 3.19 (a) and (b): Images of the Saharan desert in Western Sahara. This region experiences extreme summer heat (~45 °C) and rarely goes below ~30 °C in the summer months.
Figure 3.19 consists of two parts: a and b. Both are photographs of the Sahara desert taken on sunny days. Part a is a photograph showing fine grained golden sand in the foreground. There are some tufts of vegetation, and there are ripples in the sand. The desert extends to the horizon, with flat topped mountains in the far distance. The horizon is horizontal and the sky is blue with fluffy white clouds. Part b is a photograph showing a golden desert. The sand has a roughly texture, and is not as smooth as in photograph a. There is no vegetation. The desert extends to the horizon, with flat topped mountains in the far distance. The horizon is horizontal and the sky is blue with fluffy white clouds.
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Subsurface environments.
As you saw in Section 1, conditions on Mars today are harsh, so it would be hard for life to survive. However, one or two of the organisms we’ve examined in this section could hypothetically withstand one or more of the martian extremes, though they would need some protection. Mars, for the most part, is dry and frigid. It receives only 43% as much energy from the Sun as the Earth, but the thin carbon dioxide-rich atmosphere absorbs little of the harmful ultraviolet radiation.
The search for present-day life on Mars is focused on the possibility of life existing below the surface.
The plausibility of subsurface life on other planets has been enhanced by the discovery on Earth of subsurface lithotrophic microbial ecosystems. These microbes are not reliant on sunlight to survive. Instead, they appear to thrive on chemical energy in basalt, a rock common to Earth and Mars, but one that contains little of the normal nutrients that feed microbes. These microbes were found in groundwater samples taken more than 1000 m below the surface of the Columbia Basin basalt rocks in the western USA and appear to exist on a diet of mostly hydrogen. The basalt connection suggests that it might be possible for microbes to exist in the martian subsurface. This does not mean there is life on Mars, but if such organisms can exist here, then, in theory, they could also exist on Mars.
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Of the various extremophiles and extremotolerant organisms covered in this section, which do you think might prove most resistant to the arid and exposed conditions on the martian surface?
One of the toughest is Deinococcus radiodurans, which has evolved to cope with high-radiation conditions and desiccation. However, to grow and reproduce, even Deinococcus would require a source of liquid water eventually.
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Activity to complete!!!
Activity 3.2 Extremophiles in Mars-like environments
Timing: You should allow around 30 minutes for this activity.
Now that you know some more about the incredible range of organisms and their survival mechanisms, we can revisit the landscapes that you viewed in Activity 1.4. Choose an Earth environment that might be most similar to one of the sites you looked at on Google Mars: we suggest you choose either a rocky desert (e.g. Figure 3.20a or 3.20b) or an ice-field (e.g. Figure 3.20c).
Described image
Maximise for Described image imageMaximise
Figure 3.20 (a) Red dune fields in the Great Sandy Desert in Australia. (b) A rocky desert in Jordan. (c) An aerial view of the North Pole.
Figure 3.20 (a) Red dune fields in the Great Sandy Desert in Australia. (b) A rocky desert in Jordan. (c) An aerial view of the North ...
Start by identifying some organisms that might be found living within your chosen environment.
Select one organism, and in no more than a few sentences, post a summary description on your tutor group forum that explains:
what environment you chose to investigate
what organism you selected
some of the main physical and chemical factors that are directly affecting this organism in your chosen environment
and finally, whether the environment represents the natural habitat of your chosen organism.
Compare your summary description outlining what you have identified, observed and described with examples from other students. Hopefully your tutor group will have come up with a few different organisms for each environment. As you do this, consider the following points:
Which summary description(s) did you find most informative?
What did you like most about this description (e.g. overall structure of the summary, the style of writing, clear descriptions, etc.)?
Having read other students’ contributions, is there anything you could do to your summary description to improve it further?
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Activity : table!!
See pic and fill out your Word doc.
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Activity 4.1 Decide for yourself!
Timing: Spend no more than 20 minutes on this activity.
Think about the evidence that either supports or rejects the proposal that ‘Life could exist on Mars’. Divide the points into two lists.
Imagine that you have been asked to speak at a debate on the subject. Based on your lists, decide on which side of the debate you would choose to speak.
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What makes exoplanet habitable?
Throughout this topic, we have consistently asserted the presence of liquid water as being the most important requirement for life to exist. The first thing we consider when discussing the potential for life on other planets or planetary bodies is whether there is any water present on that body and, in turn, whether the water is in liquid form.
Important physical parameters governing whether liquid water can exist at a planet’s surface include the distance between the planet and its host star, the mass of the star and the star’s intensity. Together, these factors determine the type and intensity of radiation (in the form of light) received at the planet’s surface. These properties will be covered in more detail in Topic 10.
Together, these physical environmental factors (distance from star, mass of star, and star’s intensity) form the basis of the concept of the ‘habitable zone’, the region of space where it is possible for liquid water to exist at planetary surfaces. In our own Solar System, Earth is positioned in the habitable zone, which is also sometimes called ‘the Goldilocks zone’ in reference to the fairytale Goldilocks where the perfect bowl of porridge to eat was neither too hot or too cold, but ‘just right’.
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CHZ
Figure 4.1 (a) The current habitable zone, shown in grey, for our Solar System. (b) The solar energy output of any star changes over the lifetime of the star. As the energy levels of our Sun have increased through time, the position of the habitable zone has moved further away. The continuously habitable zone is marked CHZ.
(a) The top part of this image shows the Sun at the centre of a disc formed by four concentric rings illustrating the orbits of Mercury, Venus, Earth and Mars. The orbits are to scale, not completely equidistant but none-the-less fairly evenly spaced. The upper part of the image is superimposed by a grey zone which represents current habitable zone for our Solar System. The grey zone is ring shaped, and does not cover the Sun itself or the orbits of Mercury or Venus, but starts just outside the orbit of Venus and ends a little way outside the orbit of Mars. The lower part of this image shows the four planets as they appear to a viewer from space: for example, Mercury is the smallest, and speckled blue; Earth has continents and oceans and clouds.
(b) This image also shows a sun at the centre of four circles. The distance between the inner circle and the third circle from the centre is labelled HZ at time t0. The distance between the second and third circles from the centre is labelled CHZ (for continuously habitable zone). The distance between the second circle from the centre and the outer circle is labelled HZ at time t1. This illustrates the fact that the habitable zone gets further away from a sun as the sun gets older and emits more energy.
Note: No scale is given here: refer to Figure 1.4 for further information on orbital spacing.
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Earth is not the only planet in this zone. Venus and Mars are located at its edges. As well as distance from the Sun, what other physical factor controls the temperature on Earth?
The thick and dense, greenhouse gas-rich atmosphere that encircles the Earth is the real reason why the planet has an average surface temperature of 15 °C. The atmosphere protects the surface of the Earth from some of the most hazardous radiation for life and also acts as a thermal blanket retaining some of the infrared radiation reflected and emitted from the Earth’s surface. Without the atmosphere, Earth would have an average surface temperature of about 0 °C.
Another important factor that affects the temperature at the planet surface is the proportion of radiation reflected back to space – the albedo. The Earth has an albedo of 0.31 (i.e. 31% of radiation that hits the Earth’s surface is reflected back into space and does not play a part in warming the surface). Lighter coloured surfaces reflect more radiation than darker surfaces.
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Although the Earth’s average albedo is 0.31, this value varies across the surface of our planet. Which regions or surfaces on the Earth would you expect to have a higher albedo and which have a lower albedo?
White snow and ice reflect a lot of radiation, so the polar regions and snow-covered landscapes (and clouds) have a higher albedo. The dark colour of liquid water and forest canopies mean that oceans and the land surfaces preferentially absorb more radiation, resulting in a low albedo.
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! Activity 4.4 Finding the right exoplanet !
Timing: Allow 40 minutes to 1 hour to complete this activity.
In this final activity in Topic 4, you are going to investigate the various physical properties that determine whether a planet lies within the habitable zone for a given star.
You will use a web-based interactive activity to explore the effects of changing:
the size of the star
the distance of the planet from the star
the planet’s albedo
the concentration of greenhouse gases in the planet’s atmosphere.
and then use values from the Open Exoplanet database to find a known planet which might harbour life. To help you get started, watch Video 4.6.
Video player: Video 4.6
Progress controller
Progress controller
Timer 00:00 / 09:32
Video 4.6 How to use the web-based tools in Activity 4.3. (9:32 min)
The two links you will need for this activity are the Planet Temperature Calculator website and the Open Exoplanet Catalogue.
Once you have watched Video 4.6, use the two websites to find another planet (or planets) that could be habitable. Choose any values you like for the albedo and greenhouse strength of your planet(s) – because your guess is as good as ours!
Now write a message on your tutor group forum giving the name of the planet(s) you chose, the values for the four conditions, and the output average temperature for your planet(s) under those conditions. How many potentially habitable planets has your tutor group found?
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Part 4 summary
Conditions on Mars in the past appear to be compatible with what is required for the development of life. About 4.5 billion years ago, ~20% of the surface was covered by a large ocean and network of lakes and rivers, and the planet was enveloped with a thick, insulating atmosphere. Over geological time, this atmosphere, the oceans, rivers and lakes have all been lost to space, frozen as ground ice, or locked away into water-bearing minerals, leaving Mars a drier and much colder planet today.
Results from spacecraft and martian meteorites have not yet supplied unambiguous evidence for a martian biosphere, and so the question of life on Mars remains, so far, unresolved. The current physical and chemical environmental conditions on Mars are not immediately conducive to any form of life on Earth (with the possible exception of the tardigrades, although even they would be in a permanently dormant phase on Mars and might not survive for long).
Weighing up all the scientific evidence, it would be reasonable to state that the answer to the question ‘Is there life on Mars?’ is ‘not at present, as far as can be discerned’. However, there have been times in Mars’ past where the environment could have been suitable for life and, with new discoveries are happening every day, who knows what the future may bring?
The key concepts and principles you have studied in this part are:
habitable zones, and the search for life beyond the Earth
The habitable zone is defined as the range of orbital distances around a star where liquid water may be able to exist on the surface of a planet.
Exoplanets located in the habitable zone of other solar systems could potentially support life.
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Part 3 summary
All life exists with a range of physical and chemical conditions (such as temperature, pH, salinity, pressure and radiation) to which they are best suited, known as their tolerance range.
By studying organisms that can live in extreme environments, insight can be gained about the limits of life on Earth and therefore the environments that we could consider habitable on other bodies in the Solar System. By understanding how extremophiles and extremotolerant organisms can survive in some of the harshest environments on Earth, we should be able to better understand how life might be able to withstand the extreme conditions associated with extraterrestrial environments.
The key concepts and principles you have studied in this part are:
the effects of extreme physical conditions, such as temperature, pressure and radiation levels, on life
Life can exist on Earth in extreme conditions (temperatures below 15 °C and above 100 °C).
Life can exist at high pressures as found in the deep sea.
Deinococcus radiodurans is known to be able to tolerate high radiation levels such as would be found on Mars.
Some organisms can tolerate extreme desiccation and, when water is available, rehydrate and recover.
the effects of extreme chemical conditions, such as salinity and pH, on life.
Different bacteria can withstand very high acidity or alkalinity.
Some bacteria can grow in conditions of high salt.
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Part 2 summary
2.7 Summary of Part 2 Mars and the origin of life
The key concepts and principles you have studied in this part are:
an introduction to scientific theories regarding the origin of life on Earth
An early theory that the chemical precursors of life were produced in the atmosphere is now discarded.
Life could have started in a hydrothermal vent in the absence of oxygen.
Panspermia is the theory that life on Earth had an extraterrestrial origin.
some of the evidence for life, past and present, that scientists might look for in the search for life elsewhere.
Evidence includes:
microfossils
textural fabrics
organic material
atmospheric constituents such as methane
isotope biosignatures.
In the next section, we will go on to look at organisms on Earth that live in conditions normally considered hostile to life, and show how an understanding of their adaptations to those conditions is informing scientists in the search for life on other planets.
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Stromatolite
Stromatolites are dome-shaped due to the layered growth patterns of microbial mats, where the curved surface maximized sunlight exposure for photosynthesis. These mats trap sediment, which then mineralizes, forming layered structures that grow vertically to maintain sunlight access.
Here's a more detailed explanation:
Microbial Growth:
Stromatolites are formed by microbial mats, primarily cyanobacteria, that grow in shallow, sunlit waters.
Sunlight Capture:
The dome shape of stromatolites allows them to maximize sunlight exposure, crucial for photosynthesis.
Layering:
As the microbial mats grow, they trap and mineralize sediments, creating thin, alternating layers.
Vertical Growth:
The upward growth of the stromatolite allows the surface to remain exposed to sunlight, ensuring continued photosynthesis.
Environmental Factors:
The specific shape and distribution of stromatolites are also influenced by factors like water depth, currents, and the type of substrate.
