Life-Swapping Scenarios for Earth and Mars

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.

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.

•	Life-Swapping Scenarios for Earth and Mars Space.com, Byline: Leonard David

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