Nov. 29th - Atmospheric Processes & Mars

Question 1

Without the greenhouse effect, Earth’s surface would be…

Answer 1

too cold for liquid water to flow and for life to flourish

Question 2

How does the greenhouse effect warm a planet?

Answer 2

• The energy that warms a planet comes from sunlight, and in particular from visible light.
• Some of this visible light is reflected back to space, and the rest is absorbed by the surface
• The absorbed energy must ultimately be returned to space, but planetary surfaces are too cool to emit visible light.
• Instead, planetary surface temperatures are in the range in which they emit mostly infrared light.
  * The greenhouse effect works by temporarily “trapping” some of this infrared light, slowing its return to space.

Question 3

The greenhouse effect occurs only when an atmosphere contains…

Answer 3

gases—called greenhouse gases—that can absorb the infrared light.

Answer 4

Once a greenhouse gas molecule absorbs the energy of an infrared photon, it quickly releases the energy by emitting a new infrared photon.

However, the new photon will be emitted in some random direction that is unlikely to be the same direction from which the original photon came. This photon can then be absorbed by another greenhouse gas molecule, which does the same thing.

The net result is that greenhouse gases tend to slow the escape of infrared radiation from the lower atmosphere, while their molecular motions heat the surrounding air.

In this way, the greenhouse effect makes the surface and the lower atmosphere warmer than they would be from sunlight alone. The more greenhouse gases present, the greater the degree of surface warming.

Question 5

Note that the greenhouse effect by itself does not alter a planet’s overall energy balance:

Answer 5

As long as the strength of the greenhouse effect hasn’t changed, the total amount of energy that a planet receives from the Sun will be precisely balanced with the amount of energy it returns to space through reflection and radiation. If it were not, the planet would either heat up (if it received more energy than it returned) or cool down (if it returned more energy than it received).

Question 6

There are two major and closely related lines of evidence for the greenhouse effect:

Answer 6

1. The greenhouse effect can be directly measured in the laboratory.
2. Validating the laboratory measurements with real worlds.

Question 7

There are two major and closely related lines of evidence:

First, the greenhouse effect can be directly measured in the laboratory

Answer 7

Although the actual setups are somewhat more complex, the basic idea is simply to put a gas (such as carbon dioxide) in a tube, shine light of different wavelengths (such as visible light and infrared light) at it, and measure how much of that light passes through and how much is absorbed.

Question 8

There are two major and closely related lines of evidence:

The second line of evidence comes from validating the laboratory measurements with real worlds.

Answer 8

Models that predict planetary temperatures without taking into account the greenhouse effect (“no greenhouse” temperatures) do not match actual temperatures for worlds with greenhouse gases.

In contrast, when the models include the greenhouse effect as measured in the laboratory, the predictions precisely match the actual temperatures for every planet in our solar system, as well as for Titan, the only moon with a substantial atmosphere.

This match of predicted and actual planetary temperatures represents confirmation of the laboratory measurements of the greenhouse effect, and explains why there is essentially no scientific doubt about the mechanism of the greenhouse effect.

Question 9

We can better appreciate the importance of the greenhouse effect by comparing …

Answer 9

each planet’s average surface temperature—or global average temperature—with and without it.

Question 10

A terrestrial planet’s interior heat has very little effect on its surface temperature, so sunlight is the only significant energy source for the surface

Therefore, without the greenhouse effect, a planet’s global average surface temperature would depend on only two things:

Answer 10

The planet’s distance from the Sun, which determines the amount of energy received from sunlight. The closer a planet is to the Sun, the greater the intensity of the incoming sunlight.

The planet’s overall reflectivity, which determines the relative proportions of incoming sunlight that the planet reflects and absorbs. The higher the reflectivity, the less light absorbed and the cooler the planet.

Question 11

Both distance from the Sun and reflectivity have been measured for all the terrestrial worlds. With a little mathematics, these measurements can be used to calculate the…

Answer 11

“no greenhouse” temperature that each world would have without greenhouse gases

Question 12

The “no greenhouse” temperatures for Mercury and the Moon:

Answer 12

lie between their actual day and night temperatures, since they have little atmosphere and hence no greenhouse effect.

Question 13

As we should expect from the mechanism of the greenhouse effect, planets with greenhouse gases have…

Answer 13

higher temperatures than they would otherwise, and the more greenhouse gas, the higher the temperature.

Question 14

“No greenhouse” Temperatures

Mars

Answer 14

Mars has a carbon dioxide atmosphere, but its low pressure tells us that the greenhouse effect should be quite weak.

As a result, Mars has a global average temperature only 6°C higher than its “no greenhouse” temperature.

Question 15

“No greenhouse” Temperatures

Venus

Answer 15

At the other extreme, Venus’s thick atmosphere of carbon dioxide creates a greenhouse effect that bakes its surface to a temperature more than 500°C hotter than it would be otherwise.

Question 16

“No greenhouse” Temperatures

We can also see why the greenhouse effect is so important to life on Earth

Answer 16

Without the greenhouse effect, our planet’s global average temperature would be a chilly −16°C (+3°F), well below the freezing point of water. With it, the global average temperature is about 15°C (59°F), or about 31°C warmer than the “no greenhouse” temperature.

This greenhouse warming is even more remarkable when you realize that it is caused by gases, such as water vapor and carbon dioxide, that are only trace constituents of Earth’s atmosphere.

Question 17

MATCH INSIGHT 10.1

Question 18

The greenhouse effect can warm a planet’s surface and lower atmosphere, but other processes affect the temperature at…WHAT?

Answer 18

higher altitudes

Question 19

The way in which temperature varies with altitude determines what is often called…

Answer 19

atomspheric structure

Question 20

Earth’s atmospheric structure has four basic layers:

Answer 20

1. Troposphere
2. Stratosphere
3. Thermosphere
4. Exosphere

Question 21

Troposphere

Answer 21

the lowest layer, in which temperature drops with altitude (something you’ve probably noticed if you’ve ever climbed a mountain).

Question 22

Stratosphere

Answer 22

begins where the temperature stops dropping and instead begins to rise with altitude. High in the stratosphere, the temperature falls again.

Question 23

Thermosphere

Answer 23

begins where the temperature again starts to rise at high altitude.

Question 24

Exosphere

Answer 24

the uppermost region, in which the atmosphere gradually fades away into space.

Question 25

The key to understanding atmospheric structure lies in interactions between…

Answer 25

atmospheric gases and energy from the Sun

Question 26

The key to understanding atmospheric structure

Although visible light dominates the solar spectrum, the Sun also emits significant amounts of ultraviolet light and x-rays.

X RAYS & IONIZATION:

Answer 26

X-rays have enough energy to ionize (knock electrons from) almost any atom or molecule. They can therefore be absorbed by virtually all atmospheric gases.

Question 27

The key to understanding atmospheric structure

Although visible light dominates the solar spectrum, the Sun also emits significant amounts of ultraviolet light and x-rays.

ULTRAVIOLET PHOTONS & IONIZATION:

Answer 27

Ultraviolet photons generally do not have enough energy to cause ionization, but they can sometimes break molecules apart.

For example, ultraviolet photons can split water (H2O) molecules and are even more likely to be absorbed by weakly bonded molecules, such as ozone (O3), which split apart in the process.

Question 28

The key to understanding atmospheric structure

Although visible light dominates the solar spectrum, the Sun also emits significant amounts of ultraviolet light and x-rays.

VISIBLE-LIGHT PHOTONS & IONIZATION:

Answer 28

Visible-light photons generally pass through atmospheric gases without being absorbed, but some are scattered so that their direction change

Question 29

The key to understanding atmospheric structure

Although visible light dominates the solar spectrum, the Sun also emits significant amounts of ultraviolet light and x-rays.

INFRARED PHOTONS & IONIZATION:

Answer 29

can be absorbed by greenhouse gases, which are molecules that easily begin rotating and vibrating.

Question 30

Most visible sunlight is either absorbed by the surface or reflected to space, but a small amount of the visible light is scattered by atmospheric molecules.

This scattering has two important effects:

Answer 30

1. Scattering makes the daytime sky bright, which is why we can’t see stars in the daytime
2. Scattering explains the colors of our sky

Question 31

Visible Light: Warming the Surface and Coloring the Sky

Scattering makes the daytime sky bright

Answer 31

• Reason why we can’t see stars in the daytime.
• Without scattering, sunlight would travel only in perfectly straight lines, which means we’d see the Sun against an otherwise black sky, just as it appears on the Moon.
• Scattering also prevents shadows on Earth from being pitch black. On the Moon, shadows receive little scattered sunlight and are extremely cold and dark.

Question 32

Visible Light: Warming the Surface and Coloring the Sky

Scattering explains the colors of our sky

Answer 32

Visible light consists of all the colors of the rainbow, but not all the colors are scattered equally.

Gas molecules scatter blue light (shorter wavelength and higher energy) so much more effectively than red light (longer wavelength and lower energy) that, for practical purposes, we can imagine that only the blue light gets scattered. When the Sun is overhead, this scattered blue light reaches our eyes from all directions, so the sky appears blue.

At sunset or sunrise, when sunlight passes through more air on its way to our eyes, so much of the blue light is scattered away that we are left primarily with** red light** to color the sky.

Question 33

Infrared Light and the Troposphere

The surface returns the energy it absorbs by radiating in the infrared, and greenhouse gases can then absorb this infrared light and warm the troposphere.

Because the infrared light comes from the surface, more is absorbed closer to the surface than at higher altitudes, which is why…

Answer 33

the temperature drops with altitude in the troposphere. (The relatively small amount of infrared light coming from the Sun does not have a significant effect on the atmosphere.)

Question 34

Infrared Light and the Troposphere

Why is the troposphere the only layer of the atmosphere with storms?

Answer 34

The drop in temperature with altitude, combined with the relatively high density of air

The primary cause of storms is the churning of air by convection, in which warm air rises and cool air falls. Recall that convection occurs only when there is strong heating from below. **In the troposphere, heating from the ground can therefore drive convection. **

Question 35

Ultraviolet Light and the Stratosphere

Above the troposphere, the air density is too low for greenhouse gases to have much effect

Answer 35

infrared light from below can travel unhindered through higher layers of the atmosphere and into space.

Question 36

Ultraviolet Light and the Stratosphere

Heating from below therefore has little effect on the stratosphere.

Instead, the primary source of heating in the stratosphere is the absorption of…

Answer 36

…solar ultraviolet light, which on Earth is absorbed primarily by ozone (O3)

Question 37

Ultraviolet Light and the Stratosphere

Why does temperature tend to increase with altitude as we go upward from the base of the stratosphere?

Answer 37

• Most of this ultraviolet absorption and heating occurs at moderately high altitudes in the stratosphere
• This temperature structure prevents convection in the lower stratosphere, because heat cannot rise if the air above is hotter. The lack of convection makes the air relatively stagnant and stratified (layered), with layers of warm air overlying cooler air; this stratification explains the name stratosphere.

Question 38

Ultraviolet Light and the Stratosphere

Lack of convection on weather - Stratosphere

Answer 38

• Means that the stratosphere has essentially no weather and no rain.
• Pollutants that reach the stratosphere, including the ozone-destroying chemicals known as chlorofluorocarbons (CFCs), remain there for decades.

Question 39

X-Rays and the Thermosphere

The density of gas in the exosphere is too low for it to absorb significant amounts of these x-rays (absorbed by the first gases they encounter as they enter the atmosphere), so most x-rays are absorbed in the…

Answer 39

Thermosphere
* The absorbed energy makes temperatures quite high in the thermosphere (thermos is Greek for “hot”), but you wouldn’t feel much heat because the density and pressure are so low
* Virtually no x-rays penetrate beneath the thermosphere

Question 40

X-Rays and the Thermosphere

Solar x-rays also ionize a small but important fraction of the thermosphere’s gas.

The portion of the thermosphere that contains most of the ionized gas is called the

Answer 40

Ionosphere
* Very important to radio communication, because it reflects most radio broadcasts back to Earth’s surface
* Without this reflection, radio communication would work only between locations in sight of each other.

Question 41

The Exosphere

Answer 41

The extremely low-density gas that forms the gradual and fuzzy boundary between the atmosphere and space

• Gas density in the exosphere is so low that collisions between atoms or molecules are very rare, although the high temperature means that gas particles move quite rapidly.
• Lightweight atoms and molecules sometimes reach escape velocity and fly off into space.

Question 42

Magnetospheres and the Solar Wind

One other important type of energy coming from the Sun:

Answer 42

the low-density flow of subatomic charged particles called the solar wind

Question 43

Magnetospheres and the Solar Wind

On the Moon and Mercury:

Answer 43

solar wind particles hit the surface, where they can blast atoms free

Question 44

Magnetospheres and the Solar Wind

On Venus and Mars:

Answer 44

solar wind particles can strip away atmospheric gas

Question 45

Magnetospheres and the Solar Wind

Earth:

Answer 45

Earth’s strong magnetic field creates a magnetosphere that acts like a protective bubble surrounding our planet, deflecting most solar wind particles around it

Question 46

Magnetospheres and the Solar Wind

The magnetosphere still allows a few solar wind particles to get through, especially near the magnetic poles.

Once inside the magnetosphere, these particles move along magnetic field lines, collecting in…

Answer 46

…charged particle belts that encircle our planet.

• The high energies of the particles in these belts can be hazardous to spacecraft and astronauts passing through them.

Question 47

The Aurora lights are caused by…

Answer 47

Charged particles trapped in the magnetosphere

• Variations in the solar wind can rattle and shake the magnetosphere and give energy to particles trapped there.
• If a trapped particle gains enough energy, it can follow the magnetic field all the way down to Earth’s atmosphere, where it collides with atmospheric atoms and molecules.
• These collisions cause the atoms and molecules to radiate and produce the moving lights of the aurora.
• Because the charged particles follow the magnetic field, auroras are most common near the magnetic poles and are best viewed at high latitudes

Question 48

What factors can cause long-term climate change?

Scientists have identified four major factors that can lead to long-term climate change on the terrestrial worlds:

Answer 48

**1. Solar brightening: **The Sun has grown gradually brighter with time, increasing the amount of solar energy reaching the planets.
2. Changes in axis tilt: The tilt of a planet’s axis may change over long periods of time (Small gravitational tugs from moons, other planets, and the Sun), making seasons more (warm)/less (cold) extremes
3. Changes in reflectivity: An increase in a planet’s reflectivity means a decrease in the amount of sunlight it absorbs, and vice versa (can be affected by dust particles, valled aerosols)
4. Changes in greenhouse gas abundance: More greenhouse gases tend to make a planet warmer, and less make it cooler.

Question 49

How does a planet gain or lose atmospheric gases?

Sources of Atmospheric Gas: Outgassing

Answer 49

Volcanic outgassing = primary source of gases for the atmospheres of Venus, Earth, and Mars. (Recall that the terrestrial worlds were built primarily of metal and rock, but impacts of ice-rich planetesimals (from beyond the frost line) brought in water and gas that became trapped in their interiors during accretion)

Question 50

How does a planet gain or lose atmospheric gases?

Studies of volcanic eruptions show that the most common gases released by outgassing are…

Answer 50

• Water (H2O)
• Carbon dioxide (CO2)
• Nitrogen (N2)
• Sulfur-bearing gases (H2S and SO2).

Question 51

How does a planet gain or lose atmospheric gases?

Sources of Atmospheric Gas: Vaporization

Answer 51

After outgassing creates an atmosphere, some atmospheric gases may condense to become surface liquids or ices.

The subsequent vaporization of these surface liquids (evaporation) and ices (sublimation) therefore represents a secondary source of atmospheric gas.

For example, if a planet warms, the rates of vaporization will increase, adding gas to the atmosphere.

Question 52

How does a planet gain or lose atmospheric gases?

Sources of Atmospheric Gas: Surface Ejection

Answer 52

The tiny impacts of micrometeorites, solar wind particles, and high-energy solar photons can knock individual atoms or molecules free from the surface.

This surface ejection process explains the small amounts of gas that surround the Moon and Mercury.

It is not a source process for planets that already have substantial atmospheres, because the atmospheres prevent small particles and high-energy solar photons from reaching the surface

Question 53

Losses of Atmospheric Gas

Condensation

Answer 53

The condensation of gases that then fall as rain, hail, or snow is essentially the reverse of the release of gas by vaporization.

On Mars, for example, it is cold enough for carbon dioxide to condense into dry ice (frozen carbon dioxide), especially at the poles.

Question 54

Losses of Atmospheric Gas

Chemical Reactions

Answer 54

Some chemical reactions incorporate gas into surface metal or rock.

Rusting is a familiar example: Iron rusts when it reacts with oxygen, thereby removing oxygen from the atmosphere and incorporating it into the metal.

Question 55

Losses of Atmospheric Gas

Solar Wind Stripping

Answer 55

For any world without a protective magnetosphere, particles from the solar wind can gradually strip away gas particles into space.

Question 56

Losses of Atmospheric Gas

Thermal Escape

Answer 56

If an atom or a molecule of gas in a planet’s exosphere achieves escape velocity, it will fly off into space.

The relative importance of thermal escape on any world depends on its size, distance from the Sun, and atmospheric composition.

In general, more thermal escape will occur if a planet is small (so that it has a low escape velocity) or close to the Sun (which makes it hotter, so that atoms and molecules of atmospheric gas are moving faster).

Lightweight gases, such as hydrogen and helium, escape more easily than heavier gases, such as carbon dioxide, nitrogen, and oxygen

Question 57

What is Mars like today?

Answer 57

• The present-day surface of Mars looks much like deserts or volcanic plains on Earth
• However, its low atmospheric pressure explains why liquid water is unstable on the martian surface

Question 58

What is Mars like today?

Mars’ Atmosphere

Answer 58

• Made mostly of carbon dioxide, but the total amount of gas is so small that it creates only a weak greenhouse effect
• The temperature is usually well below freezing, with a global average of about -50 degrees
• The lack of oxygen means that Mars lacks an ozone layer, so much of the Sun’s damaging ultraviolet radiation passes unhindered to the surface.

Question 59

What is Mars like today?

Mars’ Axis Tilt

Answer 59

• Mars has an axis tilt similar to that of Earth, which means it undergoes seasonal changes.
• However, while axis tilt is the only important influence on Earth’s seasons, Mars’s seasons are also affected by its orbit

Question 60

What is Mars like today?

How are seasons impacted by orbit?

Answer 60

Mars’s more elliptical orbit puts it significantly closer to the Sun during southern hemisphere summer (and farther from the Sun during southern hemisphere winter), giving its southern hemisphere more extreme seasons—that is, shorter, warmer summers and longer, colder winters—than its northern hemisphere.

Temperatures at the winter pole drop so low (about ) that carbon dioxide condenses into “dry ice” at the winter polar cap. Meanwhile, frozen carbon dioxide at the summer pole vaporizes into carbon dioxide gas, and by the peak of summer only a residual cap of water ice remains

Question 61

What is Mars like today?

Martian Winds

Answer 61

The strong winds associated with the seasonal cycling of carbon dioxide gas can initiate huge dust storms, particularly when the more extreme summer approaches in the southern hemisphere

At times, the martian surface becomes almost completely obscured by airborne dust. As the dust settles out, it can change the surface appearance over vast areas (for example, by covering dark regions with brighter dust); such changes fooled astronomers of the past into thinking they were seeing seasonal changes in vegetation. (can also produce “dust devils”)

Question 62

What is Mars like today?

Martian Winds & The Sky

Answer 62

Martian winds and dust storms leave Mars with perpetually dusty air, which helps explain the colors of the martian sky.

The air on Mars is so thin that, without suspended dust, the sky would be essentially black even in daytime.

However, light scattered by the suspended dust tends to give the sky a yellow-brown color. Different hues can occur as the amount of suspended dust varies, and in the mornings and evenings.

Question 63

What is Mars like today?

Water Ice on Mars

Answer 63

Although there is no liquid water on Mars’s surface today, **there is a fair amount of water ice. **

The polar caps are made mostly of water ice, overlaid with a thin layer (at most a few meters thick) of carbon dioxide ice

In sum, if all the water ice now known on Mars melted, it could cover the entire planet to an average depth of at least about 20 to 30 meters.

Question 64

How has Mars’s climate differed in the past?

Mars’s climate appears to have undergone at least two types of long-term climate change:

Answer 64

1. Changes that recur over time due to a changing axis tilt
2. **An even longer-term change **that transformed Mars from a much warmer, wetter planet to the cold desert we see today.

Question 65

How has Mars’s climate differed in the past?

Mars Climate and Axis Tilt: Theoretical calculations suggest that Mars’s axis tilt should vary far more than Earth’s.

This extreme variation arises for two reasons:

Answer 65

1. Jupiter’s gravity has a greater effect on the axis of Mars than on that of Earth, because Mars’s orbit is closer to Jupiter’s orbit.
2. Earth’s axis is stabilized by the gravity of our relatively large Moon, but Mars’s two tiny moons (Phobos and Deimos) are too small to offer any stabilizing influence on its axis.

Question 66

How has Mars’s climate differed in the past?

When Mars’ axis is highly tilted…

Answer 66

• The summer pole becomes much warmer, allowing substantial amounts of water ice to vaporize, along with carbon dioxide, into the atmosphere.
• The pressure therefore increases, and Mars becomes warmer as the greenhouse effect strengthens—although probably not by enough to allow liquid water to become stable at the surface

Question 67

How has Mars’s climate differed in the past?

When Mars’s axis tilt is small…

Answer 67

• The poles may stay in a perpetual deep freeze for tens of thousands of years.
• With more carbon dioxide frozen at the poles, the atmosphere becomes thinner, lowering the pressure and **weakening the greenhouse effect, thereby cooling the entire planet. **

Question 68

How has Mars’s climate differed in the past?

Longer-Term Climate Change: Water Previously on Mars

Answer 68

• Mars had at least some periods when water flowed on its surface and rain could fall, but only before about 3 billion years ago
• Because these conditions could have existed only if temperatures were warm enough to keep water from freezing and the atmospheric pressure was high enough for liquid water to be stable, we conclude that Mars once must have had a much thicker atmosphere, with a much stronger greenhouse effect.

Question 69

How has Mars’s climate differed in the past?

Scientists continue to debate the full extent of the warm and wet periods on Mars.

Part of this debate revolves around models of the past and present martian climate. In particular, models indicate that…

Answer 69

If present-day Mars had as much carbon dioxide and water as we expect it to have had in the distant past, its greenhouse effect would make it warm enough to allow the water to be liquid, and the pressure would be great enough for the water to remain stable.

However, because the Sun was dimmer in the distant past, even more greenhouse warming would have been needed to allow for liquid water when Mars was young

Question 70

Why did Mars change?

Loss of atmospheric gas

Answer 70

Mars must somehow have lost most of the carbon dioxide gas that once filled its atmosphere.

• This loss would have weakened the greenhouse effect until the planet essentially froze over.
• Some of the carbon dioxide condensed and became part of the polar caps.
• Studies indicate that some may also be chemically bound into carbonate rocks (rocks rich in carbon and oxygen).
• But the bulk of the gas was probably lost to space.

Question 71

Why did Mars change?

Leading Hypothesis As to Why Mars Lost Atmospheric Gas

Answer 71

• Early in its history, Mars probably had molten, convecting metals in its core, much like Earth today.
• The combination of this convecting metal with Mars’s rotation should have produced a magnetic field and a protective magnetosphere.
• However, the magnetic field would have weakened as the small planet cooled and core convection ceased, leaving atmospheric gases vulnerable to being stripped into space by solar wind particles.
• More specifically, the hypothesis suggests that carbon dioxide molecules were dissociated into carbon and oxygen atoms by sunlight or chemical processes, and the resulting atoms were then stripped away by the solar wind.

Question 72

Why did Mars change?

Loss of Water

Answer 72

Because Mars lacks an ultraviolet-absorbing stratosphere, atmospheric water molecules would have been easily broken apart by ultraviolet photons. The hydrogen atoms that broke away from the water molecules would have been lost rapidly to space through thermal escape.

In fact, MAVEN has shown that water loss is enhanced in southern summer, when Mars’s proximity to the Sun warms the planet and drives dust storms. This is because a warmer, dustier atmosphere allows water molecules to reach higher in the atmosphere where they can be broken apart.

Once the hydrogen atoms were lost, the water molecules could not be made whole again. Initially, oxygen from the water molecules would have remained in the atmosphere, but over time this oxygen was lost, too. Some was probably stripped away by the solar wind, and the rest was drawn out of the atmosphere through chemical reactions with surface rock. This process literally rusted the martian rocks, giving the “red planet” its distinctive tint.

Question 73

Why did Mars change?

Sediments deposited at earlier times have minerals suggesting that they formed in purer water than sediments deposited later. This mystery may have a simple solution relating to…

Answer 73

…the volcanoes that dot the martian surface.

Volcanic eruptions should have released sulfur-bearing gases, and these gases can dissolve in water and thereby make the water more acidic, which allows more salts to dissolve. Martian water therefore would have become saltier and more acidic with time simply because more sulfur-bearing gases had been released by volcanoes

Question 74

Why did Mars change?

Size as the Critical Factor

Answer 74

In summary, Mars’s fate was probably sealed by its relatively small size:
* It was big enough for volcanism and outgassing to release water and atmospheric gas early in its history, but too small to maintain the internal heat needed to keep this water and gas.
* As Mars’s interior cooled, its volcanoes quieted and released far less gas, while its relatively weak gravity and the loss of its magnetic field allowed existing gas to be stripped away to space.

Question 75

Why did Mars change?

Mars’s distance from the Sun helped seal its fate:

Answer 75

Even with its small size, Mars might still have some flowing water if it were significantly closer to the Sun, where the extra warmth could melt the water that remains frozen underground and at the polar caps.