Lecture #38: Exobiology II
The Main Point
• Exobiology II
– Life in Extreme Environments
– Searching for Life in
Our Solar System
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Life on Earth exists in extreme environments,
providing some support for NASA’s ongoing
search for life elsewhere in our solar system
More exotic "niches"
on Earth
Mars
Europa
Titan? Elsewhere?
• Reading: Continue Chapters 24.124.1-24.3
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Life on Earth
Extremophiles
• Life developed early on the Earth
• Conditions have not always been ideal...
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Changing atmospheric chemistry
Large-scale variations in climate
Active geology
Impacts
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Evidence of the diversity of life is
provided by groups of microorganisms knows as extremophiles
(lovers of extreme conditions)
These life forms occupy niches of
– Extreme temperature
– Extreme acidity
– Extreme salinity
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Greatest range: prokaryotes
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Substantial range: eukaryotes
– Simple, single-celled organisms
• The result of life's adaptability to these
variations is a dizzying array of diversity
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– More complex, nucleated, and/or
multicellular organisms
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Nealson (1997)
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Life in Extreme Environments!
Life in the Solar System
• From permafrost to hot springs
• From battery acid to salty lakes
• Deep under the ocean
• The enormous range of diversity and ruggedness
of life on Earth has only recently been recognized
• The idea of simple life beyond Earth is not as
crazy as it used to be!
• We can make a "short list" of places to look:
– Life relies on geothermal energy
• Deep under the ground
– Mars
– Europa
– Titan
– Life using geochemical energy
• Some organisms have even
survived long-duration exposure
to vacuum and radiation in space
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• And there are probably more that we could add...
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Evidence for Life on
Mars from a meteorite?
Life on Mars?
• We saw that Mars preserves clues that its
climate may once have been very different...
• And that there is still a substantial (?)
inventory of water at or near the surface...
• And that there is evidence for fairly recent
volcanism and/or geothermal heat sources...
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• Recall (Lec. 23) that a small number (~34) of
meteorites are thought to have come from Mars
• Special one: ALH84001
• Found in Antarctica in 1984
• Thought to be a sample of ancient Martian crust:
radiometric age around 3.5 billion years
• Cosmic ray exposure indicates ejection from Mars
around 15 million to 20 million years ago
• Outer chemical evidence indicates that it fell to
Earth about 13,000 years ago
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Evidence for Life on
Mars from a meteorite?
But much skepticism!
• Four pieces of evidence presented by scientists that
ALH84001 preserves signs of past life on Mars:
• Is the rock from Mars?
• Was it contaminated by Earth life while sitting in
Antarctica for 13,000+ years?
• There have been abiologic explanations proposed
for each piece of "biologic" evidence
• No "controls" on some new methods used
• "Extraordinary claims require extraordinary
evidence" --Carl Sagan
• Proponents remain steadfast, despite criticism...
– Carbonate minerals: precipitated from a
once thicker, warmer, atmosphere?
– Magnetite grains: similar in shape to
magnetite formed bacterially
– Complex organic molecules:
specifically PAH molecules
– Segmented, "bacterial" shapes
Landmark paper published by McKay et al.
273, p. 924
(1996)
Science,
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Life on Europa?
The real message of ALH84001
• Whether or not ancient fossil microbes actually
exist in this Mars meteorite may be secondary
• ALH84001 and data from telescopes and space
missions appear to show that:
–liquid water existed in the Martian subsurface
–complex organic molecules were there too
–energy was provided by volcanoes, impacts, geothermal
• The ingredients for life all appear to have existed
at one time on Mars. Do they still exist today??
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Europa may have a subsurface liquid water ocean
The ocean may be warmed by tidal energy
Organic molecules delivered by comets over time?
Could there be life down there?
Finding out will not be easy
– First, we must prove that there's an ocean
– Then, we must figure out how to access it
• And there are ethical issues to face as well,
especially if we find evidence for life there
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Life on Titan?
Other possible places for clues...
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Complex organic chemistry in the clouds
Molecules should sink and accumulate on surface
There are lakes of liquid ethane (C2H6)
What happens to the organics on the surface?
– Simple accumulation?
– Geologic "recycling"?
– "Evolution"?
• At T=90K, chemistry likely to be sluggish...
• Huygens probe revealed channels cut by liquid methane
• Images from Cassini show a surface shaped largely by Earth-like
processes of tectonics, erosion, winds, and perhaps volcanism
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• "Hospitable" planetary atmosphere levels
– High up on Venus?
– Near the 1 bar level on Jupiter, Saturn?
• Subsurfaces of small bodies
– Comets
– Asteroids
– Planetary satellites
• What surprises await?
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• Orbiters: Imaging,
spectroscopy,
geochemical mapping,
• Searching for evidence of life in our solar
system and beyond has become one of
NASAs most important goals
• It's not just words, it's actions...
– Mars Odyssey, Mars
Express
– Mars Reconnaissance
Orbiter
Next 10 years of Mars exploration
Cassini/Huygens Titan investigation
Europa Orbiter?
Continued study of extreme life on Earth
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Mars is the Main Focus...
NASA's Astrobiology Focus
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Exobiology has a prominent
role in Mars exploration
Mars Rovers: Water Evidence!
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2008 and Beyond...
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Mars Science Laboratory: Planned Launch: 2009
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Roving long-range, long-duration science
laboratory
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Pave the way for a future sample return mission
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Beyond that…
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Human Exploration
• Substantial increase in cost and complexity
• But substantial gain in scientific
capabilities...
• Maybe Mid 2020s?
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A Moessbauer spectrum that indicates the
presence of jarosite. Jarosite is an iron
sulfate hydrate, a mineral that requires
water for its formation.
Summary
Phoenix Mars Scout: Landing: May 25, 2008
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A high-latitude lander to dig trenches up to half a
meter (1.6 feet) into the layers of water ice at the
northern pole of Mars
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Goal: to search for evidence of
ancient Mars environments that
may have been suitable for life
Based on findings from Opportunity,
scientists stated: “Liquid water was
once intermittently present at the
martian surface at Meridiani, and at
times it saturated the subsurface.
Because liquid water is a key
prerequisite for life, we infer
conditions at Meridiani may have been
habitable for some period of time in
martian history.”
152 microscopic images of
this rock allowed scientists to
see features that look like the
distinctive signature of ripples
formed in flowing water.
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We must not cease from exploration. And the end of
all our exploring will be to arrive where we began
and to know the place for the first time."
-T.S. Eliot
• Life exists in extreme environments on Earth
– Wide range of temperature, pressure, acidity, salinity, ...
– Simplest organisms (prokaryotes) occupy the extremes
– But even more complex life (eukaryotes) are diverse
• Discovery of life in extreme environments on Earth
provides insight on possible niches for life
elsewhere in the solar system (and beyond)
– Mars: A prime candidate and a major NASA focus
– Europa: Subsurface ocean?
– Titan: Complex organic chemistry?
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Next (Last!) Lecture...
• Life in the Universe
– Extrapolating from our solar system experience...
• The Search for Extraterrestrial Intelligence (SETI)
– Is anyone else out there?
– How can we find out?
– What would it mean?
• Reading: Chapters 24.424.4-24.5
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