A new study of gas in meteorites suggests Mars was bitterly cold for pretty much all of the past 4 billion years, putting the freeze on hopes that the red planet had any extended wet periods during which life could have flourished.
Several rocks that were once near the surface of Mars, and have in the past few million years been kicked up by impacts that sent them to Earth, have been freezing cold for most of the past four billion years, the study concludes.
While the findings don't rule out the possibility of life on Mars, they indicate that biology's best shot would have come in the first 500 million years of the red planet's 4.5-billion-year existence.
"Our research doesn't mean that there weren't pockets of isolated water in geothermal springs for long periods of time, but suggests instead that there haven't been large areas of freestanding water for four billion years," said Caltech graduate student David Shuster.
Shuster and MIT assistant professor Benjamin Weiss present their results in the July 22 issue of the journal Science.
Ancient Mars
Many scientists have tried to open ancient chapters in the book of Mars geology by modeling the past based on large channels carved into the dusty surface. In some scenarios -- very popular a few years ago -- the computers said Mars was warmer and wetter during much of its early time.
But recent evidence of past water, provided by the Mars rovers, has not revealed the sorts of huge and deep oceans that some might have hoped for. Instead, water might have existed in shallow lakes that did not necessarily last too long, providing only lukewarm support for the warmer and wetter theory.
Life as we know it requires liquid water, so much of the money spent to explore Mars is geared toward searching for signs of liquid water, past or present.
Yet scientists have failed to conclude whether the channels on Mars, some deeper and wider than any on Earth, were carved mostly by water or whether other substances -- such as carbon dioxide -- might have been involved. It is also not clear if the riverbeds were created gradually or, as many scientists have come to believe in recent years, by catastrophic floods of water and mud that came in short, hellish bursts.
"Our results seem to imply that surface features indicating the presence and flow of liquid water formed over relatively short time periods," Shuster said.
There has been water on Mars, scientists agree, and much of it may still lurk beneath the surface, recent evidence from orbiting craft has shown. And if life ever did get going on Mars, it could still exist in subsurface aquifers, biologists say.
But the new study and other recent work is building a strong case to suggest that Mars has always been pretty inhospitable compared to Earth.
How the study was done
Shuster and Weiss studied previously published data on the amount of argon gas in seven meteorites that are known to have arrived from Mars after millions of years in space.
Argon decays at a known rate that varies with temperature. The amount of argon in a rock can be used to infer the maximum temperature the rock has experienced. Only a tiny amount of argon that would have existed in the rocks initially has leaked away.
"Any way we look at it, these rocks have been very cold for a very long time," Shuster said.
One of the rocks in the study was the infamous ALH84001, which contains etchings that some scientists have interpreted as being created by microscopic life forms. Shuster said the rock could not have been above freezing for more than a million years during the past 3.5 billion years.
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