Transcript Search for Life in the Universe

Search for Life in the Universe
Chapter 7
Mars
(Part 2)
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Outline
• Search for Life
– Viking Experiments
– Beyond Viking
• Martian Meteorites
– Meteorites from Mars?
– ALH84001
– Implications for the Origin of Life
• Future Mars Explorations
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Next Decade
Preventing Contamination
Human Exploration
Terraforming Mars
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Viking Experiments (1)
• Carbon assimilation
– CO2 and CO from Earth (14C instead of 12C) released above
Martian soil, with and w/o water
– 14C found in the soil
– Same results at 175C (350F)
 Geochemical, not biological process
• Gas exchange
– Mix Earth “food” with Martian soil
– Look for H2, N2, O2, CH4 & CO2
– O2 released
– Released also in the dark (not photosynthesis)
– Released also after soil sterilized
– O2 also released when soil exposed only to H2O
 Non-biological process
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Viking Experiments (2)
• Labeled release
– 14C and 35S put in nutrients fed to the soil
– Abundances measured in the gas above
– Control experiment with soil first heated to 50C
(122F) and 160C (320F): no release
• Gas chromatograph/mass spectrometer
– Heat soil to 500C (930F)  break up living
organisms and organic molecules
– Look for products with a mass spectrometer
– Upper limit: few parts per billion of soil organic
• Conclusion: no life
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Beyond Viking
• Site in hindsight
– No liquid water
– No volcanic activity
• Developing understanding
– Space missions have to be planned well in advance
– Promising locations: result of mission, not known in advance
– Extremophiles just beginning to be discovered at the time
• Control of landers
– Round-trip light travel time to Mars: 525 minutes
– Autopilot: both for landing and robotic travel on Mars
– Flat v. mountainous terrain: safer but less interesting
• Human control
– Humans on the spot, or at least bring back rocks
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Meteorites from Mars?
• Differentiating meteorites from rocks
– Element abundances different from Earth
– Isotopic abundances different from Earth
• How did they come here?
– Impacts can throw off small rocks
– They then orbit around the Sun until they hit Earth
– Estimated Martian shower: ~109 tons
• Evidence for Martian origin
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Volcanic rock
Young: < 1 byr old
Earth origin ruled out by 16O:17O:18O ratio
Trapped gas: same composition as Martian atmosphere
measured by Viking
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ALH84001
• 4.5 byr ago: solidified from molten rock in the southern
highlands of Mars
• 4.54.0 byr ago: affected by nearby impacts, but not
launched into space
• 3.9 byr ago: infiltrated by water  carbonate grains
• 16 myr ago: launched into space (cosmic ray evidence)
• 13,000 yr ago: landed in Antarctica and protected by
atmosphere (age from decay of isotopes produced by
cosmic rays)
• 1984: collected
• 1993: recognized as Martian
• 1996: possible evidence of life discovered
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Evidence for Life
• Layered structure
– Alternating layers rich in Mg, Fe, and Ca
– Non-equilibrium process: on Earth doable by biological activity
• Polycyclic aromatic hydrocarbons (PAHs)
– On Earth commonly produced by decay of dead organisms
– Exist on other meteorites, although in lower quantity
• Magnetite (Fe3O4) chains
– Sizes/shapes of grains that on Earth are made only by bacteria
• Rod-shapes structures
– Look like bacteria, but 100 times smaller than normal
– Such nanobacteria exist on Earth
– Nanobacteria appear to contain DNA
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Counterevidence for Life
• Nonbiological explanations
– Layered carbonate: pulses of hot water with different elements
dissolved in them
– Chemical, not biological PAHs
– Magnetite chains:
• Imaging artifacts?
• Biology sole source?
– Rod shapes: bacteria unique explanation?
• Contamination on Earth
– Careful procedures: contamination not due to humans
– Control samples: no contamination
– Higher abundance of organic material deep in the rock
– But: living bacteria found in the rock
 PAH argument weakened, but not other arguments
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Implications for the Origin of Life
• Transport of life from planet to planet
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Need to survive blastoff
Need to survive re-entry
Need to survive in space
1:10,000 meteorites travel less than 10 yr
• Implications to origin of life
– Substantial rock exchange between Earth, Mars, and
Venus (no evidence yet)
– OK, so who gave life to whom?
– Who had life first?
– Whichever, Earth had life pretty soon after it formed
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Next Decade
• Logistics
– Viking braked with rocket  need fuel
– Pathfinder bounced with air bags
 Landing position much less certain
 Danger of obstacles greater
– Aerobreaking: circularize orbits around Mars using friction with
its atmosphere
– Optimal alignment every 26 months  biennial missions
• Planned missions
– US missions: alternating orbiters and landers
– Additional European and Japanese missions
– Planning time: 34 yr, so lessons applied to mission after next
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Preventing Contamination
• Contamination of Mars
– Don’t introduce microbes to Mars and then deduce that you
found life there
– What if Earth microbes wipe out indigenous Martian life?
– International treaty requires strict safety measures
– More cause for concern near polar caps or volcanoes than in
open terrain
• Contamination of Earth
– Danger to life on Earth not so severe because microbes affect
life similar to what they already know
– Example: HIV thought to have jumped from chimpanzees to
humans
– We know how to handle very dangerous microbes that are
always fatal, e.g., Ebola virus
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Human Exploration
• Advantages:
– Decision making
– Adaptation and improvisation
• Disadvantages:
– Cost
– Risk to astronauts
– Time to develop program: multiple robotic missions
may end up being faster
– Contamination of Mars
• And then there is politics…
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Terraforming Mars
• Terraforming: new idea to change Mars to make it more
livable by humans
• Main mechanism: improve greenhouse effect
• Chlorofluorocarbons (CFCs): help the greenhouse effect
artificially, at least initially, until polar CO2 sublimes
• UV breaks up CFCs  need constant production
• Need 106 times Earth production for ~106 years to make
enough CFCs
• Ethical questions exist, but humans are unlikely to take
them into consideration if there is a dire
social/economic/political pressure at home
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