Volumes of data collected by the Mars Global Surveyor spacecraft have led some scientists to conclude that liquid water may have existed on the surface of Mars relatively recently in its history.

In a paper published in a recent issue of the journal Nature, University of Arizona hydrologist Victor Baker concludes that, rather than being cold and dry for most of the last 3.9 billion years, liquid water and ice has shaped the surface of Mars within the last 10 million years.

Mars as seen by Hubble. Credit: NASA and the Hubble Heritage Team (STScI/AURA) Acknowledgment: J. Bell (Cornell U.), P. James (U. Toledo), M. Wolff (Space Science Institute), A. Lubenow (STScI), J. Neubert (MIT/Cornell)

Baker points to several lines of evidence in support of his claim. For example, MGS images of the planet's northern plains and southern highlands shows regions relatively free of craters, implying they were altered in the recent past. These regions also have a polygonal terrain that is similar to what is seen in permafrost regions on Earth.

Elsewhere on the planet, Baker and other scientists have seen evidence of massive floods, gullies, and even lakebeds that appeared to harbor liquid water for up to ten thousand years, a possibility only if the Martian climate was much warmer and wetter than at the present.

MGA images have also shown evidence of glacial activity in the recent past, in the form of moraines -- piles of rock carried and deposited by glaciers on Earth -- and crevasses that are similar to those on Earth carved by moving sheets of ice. There is no visual evidence of glaciers themselves on Mars today, though.

The glacier evidence is particularly interesting to Baker. "If there are glaciers, then Mars is very different than many people have described it," he said. "Glaciers aren't ice cubes, sitting there to waste away. In order to have glaciers, you have to have precipitation. Water has to move through the atmosphere, and it precipitates and makes the ice grow."

"The presence of glaciers means that Mars once was a lot warmer, and that there was much more water on the Martian surface," Baker added. "The presence of glaciers suggests that there must have been standing bodies of water as well."

Other evidence suggests that liquid water may exist on Mars today. High-resolution MGS images released last June showed evidence of gullies that appear to have been formed virtually in the present day. Some planetary scientists have argued that those gullies formed when groundwater deposits reached the surface in the sides of crater walls, although other have argued that carbon dioxide ice deposits could form similar features.

Baker thinks that Mars may go through cycles of extended cold, dry periods punctuated by warmer, wetter spells. While the cold periods can last hundreds of millions of years, the warmer periods may last just a few tens of thousands of years. Moreover, Baker believes that there may be cold interludes within those brief warmer periods, and that Mars today may be in such a brief cold period.

"We don't know the answer to that yet -- that's very speculative," he said. "But if it's true, it would have major implications for sending people to Mars, because it may mean that water is more available than otherwise thought."

The warm periods are triggered, Baker believes, by a period of massive volcanic activity. The heat from that activity melts ice trapped below the surface, possibly enough to form a temporary ocean in the planet's northern region. A greenhouse effect created by carbon dioxide released into the atmosphere through the volcanic activity warms the atmosphere and allows water to remain in liquid form at the surface.

However, the warm, wet atmosphere eventually generates precipitation, which washes carbon dioxide out of the atmosphere. This cools the atmosphere again, freezing the water and eventually returning the planet to the cold, dry conditions that existed before the volcanic activity began.

While such conditions may seem harsh, Baker notes that it may still be hospitable to life. "This is the type of environment in which the extremophile progenitors of Earth's biosphere probably evolved," he said. "Indeed, early Mars provided an arguably better habitat for the inception and incubation of early life than did early Earth."