Although there is evidence that Mars was warmer and wetter in the distant past, today it is a frozen desert. Temperatures average minus 50 C (minus 58 F) for the planet as a whole. The poles reach a summertime high of minus 70 C (minus 94 F).
|Close-up of a Mars meteorite, showing what some argue appears to be fossilized evidence of ancient microbial life.
Image Credit: NASA
Nothing is known to be able to live under such deep-freeze conditions. So why does Bruce Jakosky, Director of the Center for Astrobiology at the University of Colorado at Boulder, think Mars's polar regions, where it is coldest, are good places to look for evidence of life?
Because, although the Martian poles are frigid today, in the recent past, some 10 to 20 million years ago - to a geologist like Jakosky, that's recent - they were much warmer. Warm enough, perhaps, to support life.
"Most discussions of [Martian] habitability have centered either on the ancient surface of Mars, where we think the climate might have been different, or the deep subsurface, where the temperatures might be warm enough due to geothermal heating to give liquid water," Jakosky says.
"We wanted to explore the near subsurface, the top couple of meters of the regolith [the mixture of rock and dirt that lies at the Martian surface]. We know that there's ice there, based on the Odyssey results, and it's accessible. You can actually drill down within the top meter and access ice."
Jakosky presented his findings last week at the NASA Astrobiology Institute General Meeting in Tempe, AZ.
To examine the possibility of life in the polar ground ice, Jakosky posed three related questions.
"First," he wondered, "what is lowest temperature at which liquid water can exist in thin films sufficient to support an organism?" The answer: about minus 20 degrees C (minus 4 F).
|Recent research has found that cold-adapted bacteria, or psychrophiles, live in the Siberian permafrost (shown above).
"Down to minus 20 degrees you still have liquid water that is present in thin films that are big enough to physically hold an organism," says Jakosky.
But just barely. The liquid-water films Jakosky is talking about exist at the boundaries between grains of ice, or of ice and dirt. They are roughly 10 microns thick, or about one-tenth the width of a human hair.
His second question: "What is the lowest temperature at which terrestrial organisms can grow or metabolize?" And, coincidentally, the answer is the same as the answer to the first question: minus 20 C.
Recent research has found that cold-adapted bacteria, or psychrophiles, live in the Siberian permafrost. They grow and divide at temperatures as low as minus 10 degrees Celsius (14 F). At temperatures as low as minus 20 C, they take in needed materials from their environment and appear to be capable of repairing damage.
That leaves the third question: How warm does ground ice get at the Martian poles during periods of high obliquity?
Obliquity is the angle at which a planet is tilted on its axis. The higher the obliquity, the greater the tilt.
Today, both Mars and Earth are tilted at approximatelly the same angle: Earth at 23.5 degrees, Mars at 25.2. But unlike Earth, Mars's obliquity changes over time. Over the course of tens of millions of years, Mars's obliquity is believed to fluctuate between standing almost straight up and leaning over as much as 60 degrees.
When its tilt is high, the poles get the lion's share of the planet's sunlight. And the poles just happen to be where most of Mars's water is found.
Today, of course, that water is frozen solid. But during periods of high obliquity, Jakosky says, the polar regions get enough sunlight to raise the temperature significantly. Up to minus 20 degrees C; perhaps even higher.
In other words, at periods of high obliquity, ground ice in the Martian polar and high-latitude regions should warm up enough for thin films of water to form. And those thin films of water should provide a suitable environment in which Martian microbes could live.
|Scientists hypothesize that liquid water burst out from underground, eroded the gullies, and pooled at the bottom of the Newton Crater (shown above) as it froze and evaporated. If so, life-sustaining ice and water might exist even today below the Martian surface.
Gene McDonald, of the Jet Propulsion Lab in Pasadena, CA, thinks Jakosky's scenario is "plausible." But, he adds, "it depends a lot on the length of time between the warming events." McDonald has studied organisms that live in the Siberian permafrost.
"We don't really know, in Siberian permafrost on Earth, how long an organism can stay dormant or nearly dormant and still survive," he says. "We've gone back to about 30,000 years or so and they seem to be holding their own.
"There are some hints that maybe it's as much as a couple of million years. There may still be some viable organisms that have been frozen essentially for a couple of million years. So if the time frame between warming events [on Mars] is on that scale, then it's quite possible."
At present, no mission to Mars's polar regions is planned. But Jakosky thinks one would be a good idea. Although there are many unknowns, the polar regions may offer the best hope of finding signs of life on Mars.
Most scientists believe that at lower latitudes any signs of viable life - if it exists - are likely to lie deep below the surface. Finding it will require sending complex and expensive drilling equipment to Mars. But at high latitudes, if Jakosky is right, it may be as easy as scratching the surface.
Note: In a related story, "Evidence of Snow on Mars - And Perhaps an Abode for Life?" posted last week, Phil Christensen of Arizona State University proposed a similar explanation for the origin of Mars's mysterious gullies.