[meteorite-list] Life-Swapping Scenarios for Earth and Mars

From: Gary K. Foote <gary_at_meteoritecentral.com>
Date: Sat, 27 Jan 2007 18:25:49 -0500
Message-ID: <45BB992D.5677.257A752_at_localhost>

http://www.space.com/scienceastronomy/mars_life_041213.html

By Leonard David
Senior Space Writer
posted: 13 December, 2004
7:00 a.m. ET

Evidence is mounting that the time-weathered red planet was once a warm and water-rich
world. And a Mars awash with water gives rise to that globe possibly being fit for
habitation in its past ? and perhaps a distant dwelling for life today.

As sensor-laden orbiters circle the planet, NASA?s twin Mars rovers -- Spirit and
Opportunity -- have been tooling about and carrying out exhaustive ground studies for
nearly a year.

The Opportunity robot at Meridiani Planum, for instance, has found telltale signs that
water came and went repeatedly within that stretch of Martian real estate. While that
intermittent water at Meridiani Planum is thought to be highly acidic and salty, its
ability to sustain life for some period of time cannot be ruled out.

What scientists now see is a Mars different in its first billion years of geologic
history than once thought ? and conceivably an extraterrestrial address for home-grown
life.

Rainfall: From years to decades

Mars is one complex and perplexing world.

That was strikingly evident at the Second Conference on Early Mars: Geologic, Hydrologic,
and Climate Evolution and the Implications for Life, held Oct. 11-15 in Jackson Hole,
Wyoming. Nearly 140 terrestrial and planetary scientists took part in that seminal
meeting hosted by the Lunar and Planetary Institute (LPI), NASA, and NASA?s Mars Program
Office.

"One of the most significant new findings reported at the meeting was that it appears
Mars underwent many of its most important changes much earlier in its history than
previously thought," said Steve Clifford, an LPI planetary scientist. That includes core
formation, the development of the crustal dichotomy, a rapid decline in geothermal heat
flow, and the loss of a planetary magnetic field.

"Surprisingly, all of these events appear to have occurred within the planet?s first 50
million to 100 million years of existence," Clifford explained. A related discovery is
the potential role played by large impacts during this same period, he said, a
topographic record of which is preserved in the ancient cratered highlands and has now
also been detected beneath the planet?s northern plains.

Clifford said simulations indicate that the very largest of these impacts may have blown
away a significant fraction of the early Martian atmosphere. Impacts that produced
craters greater than some 60 miles (100 kilometers) in diameter might have affected the
climate on a regional and global scale, creating transient environmental conditions
capable of sustaining continuous rainfall lasting from years to decades, he said.

Water-rich world

"There now appears to be overwhelming evidence that early Mars was water-rich ? and may
have possessed standing bodies of water and ice that ranged from large seas to a
primordial ocean, perhaps covering a third of the planet," Clifford said.

Supporting evidence ranges from orbital observations of extensive layered terrains
within, and possible paleoshorelines surrounding, the northern plains to on-the-spot
investigations of the mineralogy and sedimentary record recently discovered by the
Opportunity rover in Meridiani Planum.

"The implications of these findings are just beginning to be absorbed by the Mars
community, yet they have already substantially revised our understanding of the planet?s
early evolution. They are sure to be a continued focus of attention as the intensity and
scope of Mars exploration increases over the next decade," Clifford observed.

Now mix in recent findings about the origin and range of life here on our own planet.

"Life is incredible and the envelope for what we know about where life can live -- data
from planet Earth -- is ever expanding and is far beyond what we might have
hypothesized," suggested Lynn Rothschild, a scientist in the Ecosystem Science and
Technology Branch of NASA?s Ames Research Center, Moffett Field, California.

"There is a difference in perspective between planetary folks and biologists regarding
where life might thrive. Organisms don't look for a global average. As a microbe, just
give me 100 microliters of liquid water and I am happy. In any case, I certainly don't
need an ocean! So think microenvironment," Rothschild advised.


Water and energy for microorganisms

Given the wealth of Mars Exploration Rover (MER) data, the likelihood that life could
have existed on Mars -- or still does -- is viewed as more probable according to Carrine
Blank, Assistant Professor of Molecular Geobiology in the Department of Earth & Planetary
Sciences at Washington University, St. Louis, Missouri.

The MER results indicate that there were both large bodies of liquid water on Mars and
there were fluids carrying redox (oxidizing and reducing) gradients through the near
surface which resulted in precipitation of the blueberries, Blank told SPACE.com. "Life
not only requires liquid water, but it also needs a source of metabolic energy," she
added, "and redox gradients are great sources of energy for microorganisms.

Blank said in her mind the really big question is just how long was this liquid water and
energy present on the surface of Mars. Be it brief or extended, so goes drawing the life
line in the sands of Mars.

"If it was for just a brief time in the geologic history of Mars, then perhaps the
potential for life is low," Blank said. "If, on the other hand, it was for an extended
period of time, then the potential for life at the surface becomes much higher."

What is needed now, Blank noted, is more information about how widespread sedimentary
deposits are on Mars, and then identify age constraints on the presence of liquid water
at the surface.

Planet swapping microbes

The idea that the seeds of life hobnob between far-flung celestial localities is known as
panspermia.

Could Mars be a domain for both microbes flung off Earth due to asteroid and comet
impacts, as well as a planet where a "second genesis" might have also occurred?
Furthermore, if this was the case, could external life and made-on-Mars biology co-exist?

"Absolutely," advised Blank, adding yet another scenario: That life originated on Mars
and was transferred to the Earth, and then went extinct on Mars.

"At present, there is no geologic evidence that the origin of life occurred on the Earth.
So one hypothesis is that the origin could have occurred elsewhere, like Mars, and then
transferred to the Earth," Blank suggested. Alternatively, life could have originated on
the Earth -- but left no evidence since we don't have any rocks for the first billion
years of Earth history -- and then transferred to Mars, she said.

"If life was transferred between the planets, then Martian life, past and present, should
have similar characteristics to early Earth life," Blank said. "On the other hand, if
there was a second genesis, then life on Mars should be very different than life on
Earth, and may in fact be quite difficult to detect or even recognize as
life
particularly if it has gone extinct!"

Deepest branches on the tree of life

Meanwhile back on Earth, Blank said that more research is needed to understand whether
interplanetary transfer of life could have been possible. In particular, additional work
on hyperthermophiles -- microbes that live at very high temperatures and that form the
deepest branches on the tree of life -- is required, as they were the early inhabitants
of the Earth and therefore were the ones most likely to have been transferred around the
solar system by impacts, she said.

"We know very little about the origin of life on the Earth
how it happened, what kind of
environment it might have happened in, and how long it look to go from the origin to the
last common ancestor of life as we know it - a very complex organism very much like
modern life," Blank said.

Casting her eye back on Mars, Blank also said an unknown is whether conditions on early
Mars were similar to what they were like on the early Earth when the origin of life
likely happened.

"If they were similar, then perhaps a ?second genesis? could have been possible on Mars.
Even if conditions were different on Mars, there could still have been a second genesis
only with a very different result than what happened on the Earth," Blank stated. "If
these different life forms were spread throughout the solar system, then they might have
co-existed if they could learn to depend upon each other. If, on the other hand, they
were in direct competition for resources, then you might expect that one would ?win? and
survive, and the other go extinct," she advised.

War of the worlds?

Jack Farmer, an astrobiologist at Arizona State University in Tempe, also contends that
the chance for life having existed on Mars is definitely in the cards. He is a Mars
Exploration Rover science team member.

"We now have what I consider to be definitive evidence for standing bodies of water on
Mars and this has opened up a serious and focused discussion of habitable environments on
Mars early in the planet's history. This discovery marks a first step in implementing a
strategy for Mars exopaleontology," Farmer told SPACE.com.

Farmer said the idea that Mars could have played host to Earth-launched microbes, as well
as being a planet where a second genesis might have also taken place "are both contenders
for an origin of Martian life and deserve serious consideration."

"I also think the idea of a ?War of the Worlds? on Mars between life forms that
originated there and those that arrived from Earth is a serious possibility," Farmer
said. And that prospect, he continued, raises some key questions: Who would win? Is there
the possibility for a competitive co-existence between life forms that originated on a
different basis?

"The good news is [that] these alternative hypotheses appear to be testable in the
context of future missions. But this discussion also points, again, to the importance of
planetary protection and the potential for back-contamination arising from a Martian
sample return," Farmer concluded.

Gary Foote
Received on Sat 27 Jan 2007 06:25:49 PM PST