The latest discoveries show the oppositethat such geological activity on Mars is recent and ongoing.
"[These studies] provide an overwhelming case for new thinking about recent geological activity on Mars," Victor Baker, a geologist at the University of Arizona in Tucson, writes in an accompanying article in Nature.
In a third paper, scientists describe how recurring glacial activity has left its mark at what are now tropical and mid-latitude regions on Mars.
Ernst Hauber of the Institute of Planetary Research at the German Aerospace Center in Berlin says the surface of Mars today is not stable, so the glaciers must have formed in a different climate and are protected now by a dust cover, which may be several meters thick.
"That would mean the climate has changed within the last few million years, which is nothing for planetary time scales," said Hauber, a co-author on all three studies.
Scientists believe the Martian poles contain ice. And for some time researchers predicted they would find water close to the surface of the equator too.
The frozen body of water fills a low-lying region in the southern Elysium region, five degrees north of the equator. It is 800 by 900 kilometers (about 500 by 560 miles) wide, and averages 45 meters (147 feet) deep. On the surface it is covered by traces of pack ice.
The theory is that water erupted less than five million years ago from the Cerberus Fossae, deep cracks on the Martian surface. These cracks opened up and tapped a huge reservoir of liquid water deep beneath the surface.
Pack ice formed on top of that water. It then broke up, before freezing rigid. A crust of dust and volcanic ash, perhaps just a few centimeters thick, has prevented sublimation, the process by which ice erodes over time into water vapor.
Scientists say the same eruptions of vast amounts of water have occurred throughout geological time elsewhere on Mars.
"The fact that they are still occurring today means that we have had huge pockets of liquid water beneath the surface of Mars for thousands of millions of yearsplenty of time for life to develop," Murray said.
If life can develop in these subsurface oceans, as many scientists believe, this frozen sea may be the ideal place to look not just for fossils of past life, but for the actual frozen organisms themselves, Murray said.
"We now need to land there and drill into these deposits, which hopefully will happen in 2011, when both the European Space Agency and NASA have plans for landers," he said.
"This mission has certainly utterly changed my ideas about Mars and the possibility of finding life there now, let alone the remains of past life."
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