Whether found at the equator or the poles, water ice also might provide habitats for underground Martian life.
In the new study of thousands of images from the Mars Global Surveyor, Brown University geologist John F. Mustard and his colleagues spotted hundreds of smooth regions that all appear to have been created at roughly the same time and are roughly the same depth. From these smooth plains, pits or valleys then somehow formed.
The resulting terrain is described by Mustard as hummocky and is distinct from wind-blown features that have been previously studied.
What's important is not so much the newly mapped features but how they formed.
Ice age relics
Mustard figures the process began roughly 100,000 years ago, when Mars is thought to have been colder. First, he explains, a layer of dust and water ice accumulated on much of the surface, mixing to form a sort of icy cement that was 3 to 33 feet (1 to 10 meters) thick. This icy cement hardened into a relatively stable soil and smoothed out the landscape, much like mud might fill in the ruts in a rough driveway after a heavy rain.
Later, some of the ice in the soil mixture became unstable and evaporated directly into the atmosphere, skipping the liquid stage in a process called sublimation. This left the soil unstable and it broke apart, leaving behind the hummocky valleys, along with some remaining smooth areas.
Mustard and his colleagues expect that not all the water ice sublimated. Instead, a reservoir remains, spread around the globe at between 30 and 60 degrees latitude, trapped in the pores of Martian soil.
"This is the most direct evidence of the last ice age on Mars," Mustard told SPACE.com, adding that "the amount of water represented by the ice is substantial."
If melted, the researchers figure there would be enough water to cover all of Mars in a layer of water 4 to 12 inches (10-30 centimeters) deep.
The hummocky terrain has been noticed before but never systematically mapped as in the new study. In a paper in the July 26 issue of the journal Nature, the researchers note that the smooth areas are only rarely marred by impact craters, an indication that they have not been in place for very long.
Support for previous water findings
In June of 2000, photographs of apparent gullies, washes and channels
by researchers Michael Malin and Kenneth Edgett as evidence for relatively recent surges of underground water to the surface. Mustard said some of the features mapped in his study are associated with the channels studied by Malin and Edgett, and might represent the water that carved those channels. He also explained how the hummocky terrain, in general, provides clues to the most recent cold spell on Mars.
Long-term changes in Mars' orbit, and thus its distance from the Sun, are thought have caused an ice age 100,000 years ago. Cooler conditions would have allowed water ice to exist at or near the surface, possibly even at the equator.
"We have suspected that Mars should have ice ages, but we have never quite known what they would look like, and we have never had direct evidence for them," Mustard said. "On Earth we see glaciated terrains (moraines, eskers, drumlins, etc.) that mark the presence of former ice sheets. On Mars I believe the terrain we have mapped is the equivalent marker of the ice-affected terrains."
Mustard said some of the features his team has found appear to represent icy soil flowing down slopes. "These in fact may be the glaciers of the last Martian ice age," he said.
advertisement
