[meteorite-list] How Life Might Have Formed in Martian Impact Craters

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:06:16 2004
Message-ID: <200211191628.IAA13776_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/crater_life_021119.html

How Life Might Have Formed in Martian Impact Craters
By David Tenenbaum
Astrobiology Magazine
19 November 2002

Mars may be smaller than Earth, but it's still huge to a roving spacecraft
that can cover only 100 meters a day. For that reason, Mars mission planners
must go to great lengths to find landing sites that might still carry
evidence that life once existed on Mars.

A key zone of speculation exists just beneath Mars' cold, dry, dusty and
inhospitable surface - where two prerequisites for life, water and heat, may
be found. Such heat may come from volcanism, and indeed Olympus Mons is the
largest volcano in the solar system.

Asteroid impacts (most likely in the first half-billion years of the solar
system but conceivably even today) are a second possibility. When a big
piece of rock crashes into Mars at about 5 kilometers per second, could that
liberate enough heat to melt underground ice, drive the circulation of
liquid water, and perhaps allow the formation or survival of life?

Julie Rathbun, who now teaches astronomy and physics at the University of
Redlands (Redlands, California), and Steven Squyres, a planetary scientist
at Cornell University, decided to answer the question by modeling
hydrothermal circulation - the flow of liquid water through geologic
structures.

"We were looking to see if a hydrothermal system would set up, and if so,
what kind of temperature would be established, and for what period," says
Rathbun.

The model indicated that lakes might have lingered for thousands of years
after an impact, conceivably long enough for life to form. The lakes were
much warmer than the planet as a whole. And they may have been deep enough
to connect to aquifers - underground water bodies -- where microbial life
may already have been living.

Like much of astrobiology, the study was a bit of a shot in the dark,
Rathbun says. "A lot of efforts were incredibly theoretical, and ours was
certainly one of those. But there hadn't been any strict physical modeling
of what water would do in the temperatures available in an impact crater,
just qualitative work."

In the journal Icarus (June 2002), Rathbun and Squyres described two
theoretical impact craters on Mars. In both cases, a lake formed from melted
ice in the Martian permafrost and was soon covered by ice.

The smaller crater was 7 kilometers (4.3 miles) across, and the lake
probably froze rather quickly. (Under current Martian conditions, any water
at the surface will rapidly boil and freeze, then eventually sublimate into
the atmosphere.) The larger crater, with more astrobiological interest, was
180 kilometers (112 miles) in diameter. Water in parts of that lake ranged
from 50 degrees C (122 F) to 100 degrees C (212 F). Depending on assumptions
used for geologic conditions, the lake may have persisted for 15,000 years.

Whether life could begin quickly enough to form in that transient lake,
Rathbun admits, is "an open question." Although she says biologists have not
provided "any hard and fast numbers" for how rapidly life could start, "a
lot of biologists believe life would have emerged quickly" in the right
circumstances.

Virginia Gulick, an astrobiologist with the SETI (Search for
Extraterrestrial Intelligence) Institute, agrees that hydrothermal systems
may be hospitable to life. "From what we know about life, life requires
water, an energy source, and time. Hydrothermal systems can provide such an
environment."

However, she notes that hydrothermal systems can also be powered by rising
magma, volcanism and tectonic shifting. "It's not clear whether craters
would be a better place to look, especially considering that hydrothermal
systems powered by the intrusion of magma may last far longer - millions or
hundreds of millions of years."

In addition, she says the very warmth that makes craters such alluring
targets may backfire. "Large impact events have a tendency to sterilize the
surrounding environment, leaving the area initially devoid of life. However,
with time life may migrate to such areas through warm water being circulated
through the extensive fracture systems generated by the impact."

While the search for past life on Mars may seem a long shot, Gulick thinks
recent biological discoveries indicate otherwise. "We know on Earth that
microbial life inhabits environments formerly thought to be inhospitable,
such as in the deep subsurface, in the extremely cold and dry Antarctic
soils, rocks and ice-covered lakes, in deep ocean basins at mid-oceanic rift
hydrothermal systems, and also in high altitude (20,000 foot) icy volcano
lakes. Given that life is found in these extreme environments on Earth, it
isn't such a far stretch to think that similar microbial life may have
existed deep in the subsurface of Mars."

Similar speculation also indicates the type of life that may have lived in
Martian hydrothermal systems. Rathbun and Squyres expect to see evidence of
organisms akin to those found in deep-sea vents and geysers on Earth. These
members of the kingdom Archaea live in anaerobic (oxygen-free),
high-temperature conditions; some metabolize rocks for energy and can live
without sunlight.

Rathbun agrees that the accuracy of estimates of conditions on Mars is only
as good as the assumptions of Martian conditions on which they rest. All
bets are off, for example, if any water is absent from the near-surface
environment of Mars.

Perhaps the most important limitation of any Mars modeling effort is the
reliance on estimates rather than data for key parameters. "Normally on
Earth you have far more information about what you're trying to model," says
Horton Newsom, a solar-system geologist from the University of New Mexico
who has been speculating about hydrothermal systems on Mars for 20 years.
"It's a much more difficult job to try to model on Mars, where you have no
geological constraints."

The lifetime of a lake, Newsom notes, "depends on the amount of heat, and
permeability. But in geological material, permeability can vary over many
orders of magnitude, and this has a major influence" on when the
hydrothermal system will freeze up.

Nonetheless, even transient hydrothermal systems might be a smart place to
look, Newsom says. "In a 150-kilometer crater, you could have hot rock and
hot water around for thousands of years. Even if life did not originate
there, the lake will draw in groundwater, so you have essentially a giant
Petri dish that can culture and grow microorganisms that may have grown
elsewhere."

Thus he, like Rathbun and Squyres, agree that large impact craters are
promising sites for evidence of life. Indeed, Newsom says, two impact
craters (Gusev and an unnamed, buried, 150-km crater rim) are top candidates
for the Mars Exploration Rover, scheduled for launch in 2003.

What's next

Short of actual exploration, perhaps the best way to verify the results of
computer modeling, Rathbun says, would be to investigate historic craters on
Earth, and check whether their conditions match those that the computer
model predicts.

Editor's Note: This story is presented in cooperation with Astrobiology
Magazine, a web-based publication sponsored by the NASA astrobiology
program.
Received on Tue 19 Nov 2002 11:28:26 AM PST