SCI - part 1

Question 1

Understand and explain each of the ten ‘McKay Principles’.

Answer 1

1. “There is no single definition of life which helps us in our quest for extraterrestrial Life”
   
   • Life is a material system that is subject to
     
     • reproduction
     • mutation
     • natural selection
   • Above is defective as fossils, biomarkers, dormant cells, and single individual organisms may not demonstrate Darwinian evolution BUT would all be of great interest to Astrobiologists
2. In searching for extant (existing) or extinct life, the most useful guide is the list of important ecological requirements:
   
   • ENERGY
     
     • support life
   • CARBON
     
     • foundation of biochem
   • LIQUID WATER
     
     • limiting factor
   • OTHER ELEMENTS
     
     • N (nitrogen, P (phosphorous), S (sulfur)
3. Life is composed of and generates carbon-based (organic) compounds
   
   • ~181 different molecules/radicals have been discovered in interstellar environments
   • But C has a unique ability to make a vast variety of long chains and ring compounds associated with living systems
4. Organic matter of biological origin may be differentiated from abiotic matter as life “preferentially selects and uses a few specific organic molecules”
   
   • This high level of selectivity is probably a general feature
   • icrobes dominate planeture of life,
   • Preferential selection of organic compounds typically occurs as a result of:
     
     • Thermodynamic efficiency
     • The specificity of enzymatic reactions
     • Chirality = natural selection (left vs. rt hand)
5. Small microbial life is more probable than multicellular forms
   
   • For most of the Earth’s history, life was microscopic
   • Today, microbes dominate the planet
   • Extremophiles: Thermophiles, halophiles, radioresistant (survive hostile env)
6. Metabolism and movement are the only two ways to determine if something is alive
   
   • Some minimal level of metabolism is required to keep an organism alive
   • Dormant life forms (seeds, spores, etc.) are alive but we can only verify this by reactivating them as vegetative cells, which metabolize or move
   • Metabolism and movement are the only two indicators of the living state BUT can also be abiotic, (Fire, virus)
7. Fossils recognized on the basis of morphological or organic evidence provide strong evidence of past life
   
   • complementary indications of biological origin are necessary to make the case for life, -stromatolites - geochemical
8. Fossils tell us essentially nothing about the biochemical or genetic nature of the organisms they record
   
   • The search for life must attempt to seek the following charact of any alien organism:
     
     • Biochemical
     • Genetic
     • Ecological (possibly)
9. Photosynthetic life can create global-scale effects in a planet’s environment, which can be observed as biomarkers across space
   
   • O3 produced photochemically from photosynthesized O2 is another type of biomarker
10. Intelligent life can generate signals and artifacts detectable across space
    
    • SETI
    • Advanced technology

The ten ‘McKay Principles’ help search for life in our Solar System as no other global biospheres or intelligent beings are currently known. The search for life is focused on microbial ecosystems.

Question 2

Explain why Mars, Europa and Enceladus are of special interest to the international Astrobiology research community.

Answer 2

Of the ecological requirements listed earlier (Table 1), liquid water is the most restrictive but…

• There is direct evidence that water flowed on an infant Mars and contemporary subsurface liquid water may exist
• Europa
  
  • May have a water ocean beneath its surface layer of ice. Internal heating generated by a molten core as well as tidal flexing.
• Mars
  
  • Possible subsurface reservoirs and some evidence of minor seasonal deposits of liquid water
• Enceladus
  
  • Cold geysers of liquid water and ice apparently fueled by sub-surface liquid reservoirs (2006).
  • volcanism and tidal heating

Question 3

Explain and differentiate between the geological terms: mineral,
rock,androck texture.

Answer 3

• Mineral:
  
  • Solid material of well-defined composition, formed by natural processes.
  • Arranged in a regular pattern
  • Minerals are crystalline.
• Rock:
  
  • Naturally-formed, solid assemblage of mineral grains
  • Mineral grains may be fragments of crystals or intact crystals
  • Contains several mineral types
  • Classified according to grain arrangement and size
• Rock Texture:
  
  • Shape of the grains in rock
  • Size
  • Relationship between them define the texture

Question 4

Explain the main characteristics of the three, main rock types:
sedimentary, igneous and metamorphic.

Answer 4

• Sedimentary:
  
  • Fragmental texture i.e. grains are cemented together with spaces between individual grains (weather, water, wind)
  • sandstone
• Igneous:
  
  • Formed by solidification of liquid magma
  • Interlocking, intergrown, crystalline texture with randomly orientated crystals
• Metamorphic:
  
  • Heated to a few hundred degrees C and/or been subjected to high pressure
  • intergrown texture
  • crystals aligned or arranged in parallel bands.

Question 5

Explain how most lunar geological features (mountains, craters, maria, rilles, etc.) formed and contrast this to how most geological features formed on Earth.

Answer 5

Lunar geological features are formed as a result of meteorite bombardment and volcanism.

• Mountains: (highlands)
  
  • meteorite bombardment
  • light-colored, heavily-cratered, relatively high
• Craters:
  
  • meteorite bombardment
  • peaks in craters from volcanism
• Maria: (low lands)
  
  • formed when lava erupted onto the surface and filled low areas
• Rilles:
  
  • channels where lava has flowed

vs Earth =

• The Earth is much more geologically active and, in addition to volcanism and early meteorite bombardment features, has been sculpted by:
  
  • Plate-tectonic activity
  • Strong atmospheric and oceanic effects

Question 6

Explain why lunar dust has been described as the “No. 1 environmental hazard on the Moon”

Answer 6

Question 7

Explain what the discovery of abundant anorthosite rocks in the lunar highlands revealed to Geologists

Answer 7

• Anorthosite rocks abundant in lunar highlands
• Anorthosites are plutonic igneous rocks i.e. a rock formed by slow crystallization
• composed almost entirely of one silicate mineral: plagioclase feldspar (alumino silicates with variable Na and Ca)
• Just one mineral in a rock suggests that it formed due to floating or sinking in a magma (molten rock below surface)
• Hence, it is believed that the Moon was surrounded by a huge ocean of magma soon after it formed.

Question 8

Explain the Magma Ocean Concept

Answer 8

• When Moon formed it was enveloped by a layer of molten rock (magma) hundreds of km thick
• As magma crystallized, the denser minerals sank while the less dense ones (such a feldspar) floated
• Hence, the Moon developed an ancient anorthosite crust
• The dense minerals (olivine and pyroxene) later re-melted to produce the basalts that compose the maria.

Question 9

Explain why it is unlikely that water ever flowed on the lunar surface

Answer 9

• No evidence has been found
• All Earth rocks contain some evidence of water
• Apollo 15 collected regolith samples from Hadley Rille. A “rille” is a river-like channel where lava flowed during the eruption of mare basalts
• All samples collected showed no mineralogical/geochemical evidence to suggest that water formed these features
• The Moon (apart from possible polar ice) is “as dry as a bone”.

Question 10

Explain why some 26% of the near side of the Moon’s surface is basalt unlike 2% for the far side.

Answer 10

Distribution of Basalt

• Most basalt in either hemisphere is found in areas of lowest elevation, particularly in the very large impact basins
• Basalt is not evenly distributed on the lunar surface. Nearly 26% of the near side of the Moon is basalt and only 2% of the far side is basalt.
  
  • Question: Why do we observe this striking difference? What does it tell us about the Moon’s early history?
  • Answer: There are several theories, which attempt to explain this difference. One theory suggests that the early Moon had an unstable orbit (e.g. one month perhaps 29 days; another month perhaps 30 days, etc.) and, hence, tidal heating was probably more significant than it is today. The near-side of the Moon was therefore slightly warmer than the far-side (perhaps a few degrees). This slight difference in mean temperature was probably responsible for much of the early volcanism, much of which was associated with the release of lava via cracks in the giant impact basins.

Question 11

Explain the Ejected Ring Theory of lunar formation and why it remains the most credible theory of how the Moon formed

Answer 11

Hartmann and Davis (1975) and Cameron and Ward (1976): “The Ejected Ring Theory”.

1. A projectile as large as Mars collides with the “nearly- completed” Earth 4.5-4.6 Billion years ago;
2. Metallic core of projectile gets added to Earth’s core (predicted by computer simulation);
3. Much of rocky mantles of Earth and projectile vaporized;
4. Some of this vaporized debris orbits Earth and, eventually, accretes to form the Moon.

Question 12

Explain how Geologists can differentiate between lunar rocks and rock from Earth.

Answer 12

• Moon rocks are usually >3 billion years old (unlike terrestrial rocks)
• Lunar rocks don’t have minerals associated with the presence of water e.g. amphiboles, an important group of rock-forming silicates, including hornblende [Ca₂(Mg, Fe, Al)₅ (Al, Si)₈O₂₂(OH)₂ ], the commonest
• Lunar rocks are depleted in volatile elements e.g. Na, Z, Pb, Hg, etc.
• Look for enrichment of certain elements e.g. the Moon’s KREEP rocks (potassium, Rare Earth elements, phosphorous)

Question 13

Define what an Exoplanet (Extrasolar Planet ) is.

Answer 13

An Extrasolar Planet (Exoplanet) is one which
orbits a star – or remnant of a star (???) –
beyond our Solar System

defined as Planets :

• With masses below the limiting mass for thermonuclear fusion of D, i.e. ~13 MJ , (where MJ is mass of Jupiter) for objects of solar metallicity*
• which orbit stars or star remenants

Question 14

List and explain five different techniques employed by Astronomers to detect astrophysical bodies.

Answer 14

Extrasolar Planets can be detected if they interact with EM radiation or other matter nearby

1. reflected radiation (Moon)
2. emitted radiation (Sun)
3. absorbed or occulted radiation/Transit Method (Occultation Method) (solar eclipse)
4. refracted radiation (gravitational lens)
   
   • uses the relativistic phenomenon called gravitational lensing (traversing a curved region of space-time)
5. radial velocity method (bodies gravitating a common center of mass)
   
   • measures the star’s variable velocity along the line of sight

Question 15

Reflected radiation

Answer 15

• Detecting Exoplanets via reflected radiation is challenging. Why ?
  
  • To do so, a telescope must:
    
    • Collect enough light to detect the planet
    • Be able to individually resolve the star and planet
  • Both of these requirements dictate:
    
    • Large (telescope) aperture
    • Long integration times
    • Good seeing conditions
    • Ultimately, we need to calculate the brightness of Extrasolar Planets to assess if we can detect them via their reflected radiation

Question 16

Emitted radiation

Answer 16

• Recall: the problem using reflected light to identify Extrasolar Planets is the enormous brightness ratio between the star and planet
• Infrared (IR) radiation emitted by the planet is more promising as brightness ratio is much smaller
• In the case of the Jupiter-like planet around a nearby star, the IR brightness ratio can be reduced to one million to one, or better
• IR observations of Extrasolar Planets atmospheres is difficult (why ?) but allow detection of biosignature gases, e.g.
  
  • H2O, O3
  • CH4
  • etc.

Question 17

Absorbed or occulted radiation

Answer 17

• A planet can, potentially, be detected by
occultation when a star’s apparent brightness
(magnitude) is reduced as the planet passes in
front of it
• An occultation of Venus produces ~0.1%
decrease in the apparent brightness of the Sun
• For a Jupiter-sized planet around a Sun-like
star, ~1% decrease in the apparent brightness
would occur due to occultation

Question 18

Refracted radiation

Answer 18

• Light can be bent (refracted) passing from one medium to another
• Light can also be refracted close to massive bodies (stars, planets)
• General Relativity (1915) characterizes this light as “traversing a curved region of space-time”
• Relativistic phenomenon called gravitational lensing
• Gravitational micro-lensing causes a background star to brighten and fade over weeks when a foreground star (lens) passes in front of it
• The presence of an Extrasolar planet may be detected as a multi- day peak on top of the star’s micro-lensing time profile (see Slide #21)

Question 19

Effects on the motion of nearby
matter (Radial Velocity Method and
Astrometry)

Answer 19

• In the 1950s, Otto Struve (1897-1963) proposed that the presence of a massive planet could influence the orbit of a star
• The continuous changes in radial velocity of the star (“wobble”) could be detected via shifts in spectral lines
• Doppler spectroscopy (Doppler Detection Method) or Radial Velocity Method measures a star’s variable velocity along a line of sight
• This technique (circa 900 detections) along with the transit method (3,126 detections) are the most successful metods of Exoplanetary detection

Question 20

Transit Method (Occultation Method)

Answer 20

• Transit Method (Occultation Method) only possible when planet’s orbit takes it between the star and the Earth
• A light-curve (right) shows Astronomers how brightness varies with time
• Size of the dip allows an estimate of the radius of the planet (see box right)
• A knowledge of the mass from radial velocity (Doppler spectroscopy) data permits an estimate of density (terrestrial, Gas Giant, etc.)

Question 21

List and briefly state the characteristics of Earth’s four, thermal (atmospheric) layers.

Answer 21

1. Troposphere:
   
   1. 80% mass of atmosphere.
   2. Strong IR heating.
   3. Temp. decrease ~6.5ºC/km
2. Stratosphere:
   
   1. Very dry.
   2. Heating via UV absorption by ozone defines stratosphere
3. Mesosphere:
   
   1. Cool region.
   2. Temperature decreases with height
4. Thermosphere:
   
   • temperature increases with height due to UV/Xray photodissociation and photoionization of N2 and O2

Question 22

Explain what Earth’s primordial atmosphere was like and why it changed radically.

Answer 22

Primordial Atmosphere

• Earth’s earliest atmosphere rich in CO₂ and H₂O
• CO₂ fractional concentration 103 times present value, i.e. ~38%
• Minor amounts of N₂ and sulfates

So, what happened to change this ?

• Most atmospheric H₂O condensed to form primordial oceans
• Most atmospheric CO₂ dissolved in oceans (this is also a problem today)
• Some dissolved CO₂ combined with metal (Ca, Mg) ions to form carbonate minerals such as CaCO₃
• Inert atmospheric N₂ became enriched over geological time periods
• Much O₂ came from plants (photosynthesis) and cyanobacteria
• Plants and animals appeared 500 Ma ago: O₂ concentrations have increased from ~1% to ~21% over the last ~600 Ma
• O₂ and O₃ are important biomarkers

Question 23

Explain the underlying physics of Earth’s greenhouse effect and the role of anthropogenic gases (CO₂, CH₄, etc.).

Answer 23

• Earth’s atmosphere is quite transparent to incoming, visible radiation and opaque to outgoing IR radiation on account of absorbent greenhouse gases (H₂O, CO₂, CH₄, N₂O, halocarbons, etc.), cloud droplets and aerosols
• Heated ground radiates IR radiation, some of which is reflected or re-radiated (as thermal radiation) by the absorbent greenhouse gases. Result = net heating effect

Question 24

Explain why some atmospheric gases are greenhouse gases, while others are not.

Answer 24

• Molecules oscillate via their bonds
• Diatomic molecules, e.g. CO, possess a dipole moment if their electrons are unevenly distributed
• The size of the dipole moment depends on the size of the partial charges (δ+, δ-), the atoms, and on the distance between them
• Generally, heteronuclear, diatomic molecules (like CO) are IR active while homonuclear diatomic atoms (like N₂) are not
• In the case of polyatomic molecules such as H₂O, the two bond dipoles cancel each other out
• BUT the bending and stretching modes result in different dipole modes
• Hence, the overall H₂O molecule has an electric dipole moment and is IR active
• Being IR active means that molecules – such as the greenhouse gases – can absorb or emit photons in the infrared part of the spectrum
• Tropospheric H₂O molecules, for example, absorb IR radiation emitted from the Earth’s surface
• This gives the H₂O molecules more thermal energy, i.e. they become warmer

Question 25

Give one example of paleo-climatic (ancient climate) data, which shows the dramatic increase in anthropogenic gases in Earth’s atmosphere since the advent of the Industrial Revolution.

Answer 25

• Paleoclimatic data from many sources, e.g. ice cores in Greenland and Antarctica down to 3 km
• Ice contains ancient air bubbles with trace gases (CO₂, CH₄, etc.).
• Analyses of bubbles shows atmospheric concentration of CH₄ was constant until 1700s but has increased dramatically in the last century
• CH₄ currently ~1893*-1762** ppb

Question 26

Explain the origins, and negative effects, of acid rain and photochemical smog.

Answer 26

Acid rain refers to rain drops with pH lower than
unpolluted rain drops, i.e. circa ≤5.6*

Acid rain also refers to acid snow, fog and dew

• Causes:
  
  • Reactions of industrial emission gases, NOx** and SO₂
  • Reactions of NOx and hydrocarbons from
    biomass burning
    
    • NOx and SO₂ converted to HNO₃ and H₂SO₄ via a chain of reactions, e.g. involving OH (hydroxylradical)
• Effects:
  
  • Fresh water ecosystems and biodiversity:
    
    • Low pH values toxic to fish eggs especially, which impacts food-chain, e.g. birds
    • Low pH leaches soil of nutrients/minerals (e.g Mg, K, Ca) while concentrating toxic Al
      
      • Toxic Al finds its way into local lakes and rivers
    • Some microbes in soils cannot survive acidic conditions
  • Forests:
    
    • Soils leached of nutrients and enriched in toxic Al
    • Acids strip waxy coating of leaves, inhibiting photosynthesis
    • Above effects make trees vulnerable to disease and exposure
  • Other effects:
    
    • Acidic damage to ornamental stones and carvings, e.g. CaCO₃ (limestone, marble)

Question 27

Explain the underlying physics and chemistry relating to the ozone hole over Antarctica, and why ozone destruction is so intensive during (Antarctic) Spring.

Answer 27

• Insoluble CFC gases diffuse into the stratosphere from the troposphere and react catalytically with microscopic particles to release reactive Cl (and other halogen atoms)
• Cl atoms catalyses conversion of O3 to O2: just one Cl atom can destroy 103-105 O3 molecules
  
  • anthropogenic, stratospheric ozone destruction via a two-step, catalytic reaction:
    
    • Cl + O3 → ClO + O2
    • ClO + O → Cl + O2
• Over Antarctica, there’s a polar vortex much of the year*, i.e. a cyclone system isolating frigid air
• During the sunless Winter at 15-25 km (
• PSCs composed of tiny crystals of H2O, NOx, HCl, ClNO3, etc.
• Crystal surfaces are the locations where CFC residues react catalytically to produce more active forms of Cl and Br (e.g. Cl2), which accumulates during the Winter
• Antarctic Spring brings sunlight (UV photons), which photodissociates Cl2 generating a large reservoir of Cl atoms
• Reactive Cl now available to complete the twostep, catalytic reaction.