Their results, which will be published in "Icarus", provide strong evidence that multiple wet and/or icy climate cycles have shaped the topography of the planet's large craters. "Icarus" is the journal of the American Astronomical Society's Division of Planetary Sciences.

"Studying crater degradation in potentially ice-rich environments is vital to understanding the geology of craters and their surroundings, as well as for determining whether the ice comes from the atmosphere or from below the ground," said Daniel Berman, a PSI associate research scientist and lead author of the paper.

Berman, along with PSI Senior Scientist David Crown and PSI Research Scientist Leslie Bleamaster III, surveyed the geologic features in two sets of mid-latitude craters.

Each set included about 100 craters, with the first set in the Arabia Terra region of the northern hemisphere and the second set in an area east of Hellas basin in the southern hemisphere.

The researchers selected craters that are greater than 20 km (about 12.5 miles) in diameter that have been completely or nearly completely photographed by cameras on various spacecraft, including the Mars Odyssey THEMIS VIS camera, the Mars Global Surveyor Mars Orbiter Camera, and the Viking Orbiter cameras.

They looked specifically for the following erosional or depositional features, the number and sizes of those features, and how the features are oriented (i.e., whether they face the equator or the planet's pole in their hemisphere):

+ Lobate flows - Lobe-shaped flow features that have pitted surfaces and raised ridges on their lateral margins are observed on the walls of some craters. These lobes resemble rock glaciers on Earth.

+ Channels - Narrow channels often breach crater walls and extend outside the craters, as well as across crater floors, These channels may have been formed by flowing water.

+ Crater-wall valleys - Trough-like crater-wall valleys, wider than the above-mentioned channels, typically start at the top of the crater rim and terminate where the wall meets the floor. These valleys are sometimes filled with rough-textured deposits, which may be glacial.

+ Gullies and alcoves - Gullies are typically composed of three parts: alcoves at the head of a channel, channels, and debris fans, and are thought to have been formed by flowing water.

+ Arcuate ridges - These are small, arc-shaped ridges that enclose depressions at the base of crater walls, often below gullies. Berman interprets these to be glacial moraines, remnants of glacial deposits that have since evaporated.

+ Debris aprons - These aprons are pitted and lineated deposits on crater floors. They are similar to debris-covered glaciers or ice-rich landslides seen on Earth.

All of these features suggest a landscape shaped by liquid water and/or ice, Berman said. He found that lobate flows, gullies, and arcuate ridges on the crater walls between latitudes of 30 to 45 degrees face the pole in their hemisphere, whereas equator-facing orientations are more common than pole-facing ones at latitudes between 45 and 60 degrees.

In the southern study area, narrow channels generally had pole-facing orientations, whereas wider valleys generally have equator-facing orientations.

The features' pole-facing or equator-facing orientations could result from uneven heating of the crater walls. Ice on walls that get more sunlight would melt faster, causing more water to flow and form the gullies and other features.

Unlike Earth, whose axis only oscillates through an arc of about four degrees over millions of years, Mars appears to have an axis that tilts between vertical and as much as 60 degrees, according to recent studies.

Such tilting could enhance ice deposition, Berman said. When one pole begins leaning toward the sun, ice evaporates and then falls as snow at the other pole, which is getting little sunlight. Such tilting could have caused ice sheets to form in areas that are now ice-free, he added.

Further evidence for flowing ice is found on the crater floors, Berman observed. He found that the floors of small craters slope away from the walls that exhibit erosional/depositional features toward the more pristine ones.

These slopes have inclines of about 0.5 to 3 degrees. This suggests that ice-rich materials flowed from one crater wall to the other. Tilting floors are less evident in larger craters, although some have gradually sloping floors where debris apron material is evident.

The PSI team's crater study has led them to make the following conclusions:

+ The orientation of erosional/depositional features (whether they face the equator or the pole) suggests a direct relationship to total solar heating along the crater walls.

+ Differences in the shape and size of various erosional/depositional features can be explained by differences in crater-wall slopes, local topography and orientation.

+ The geologic features and suites of features found in the craters may have been created by multiple cycles of ice-sheet formation in response to changes in the tilt of Mars' rotation axis.

This work was funded by a grant to PSI from NASA's Mars Data Analysis Program (PI David Crown).

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