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Liquid water may have flowed across Mars as recently as a few million years ago, according to two recent studies of the desert planet. One study found evidence of several cycles of thawing and freezing near the Martian equator, while the other found evidence of thin films of liquid water in the high northern plains.

Evidence of flowing water on Mars includes curved cliffs (lower left), patterns of mounds and grooves, and deposits of rocks and soil (upper right) in this Mars Reconnaissance Orbiter image. [NASA/JPL/University of Arizona]

There is little doubt that liquid water flowed across Mars several billion years ago, when the planet was young. Photographs by Mars-orbiting spacecraft show river beds and deltas, flood plains, and a possible ocean shoreline, and landers have discovered minerals that formed in watery environments.

Yet today, Mars is quite dry. Frozen water forms large polar ice caps, and water vapor forms fleecy clouds, yet there is no evidence of liquid water on the surface. Scientists are trying to understand how Mars dried out and what happened to all of that water.

In images from Mars Reconnaissance Orbiter, Matthew Balme of the Planetary Science Institute in Tucson discovered terrain near the Martian equator that resembles areas of permafrost on Earth. The pictures show evidence that the ice has melted and thawed several times over the last few million years, Balme reported, with the last thaw as recent as two million years ago.

And the team that operated the Phoenix lander, which studied the northern plains for about five months last year, reported that a thin film of liquid water could have coated the surface a few million years ago. The site also has chemistry that could have supported life, the team said.

The evidence of a more hospitable climate in the past suggests that life could have evolved on the planet and perhaps moved underground when long-term climate cycles rendered the surface too cold and dry. — Damond Benningfield