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

[Gary K. Foote]

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