Erosion in Martian Gullies
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Erosion in Martian Gullies

October 19, 2009
Since their discovery early during the Mars Global Surveyor's Mars Orbiter Camera investigation, as first reported in June 2000, Martian gullies have presented a puzzle for the Mars science community: what fluid was responsible for the erosion that created the channels, and where did it come from? The gullies seem to be quite young in a geologic sense (millions of years or less), yet modern and geologically-recent Mars is an extremely dry place, where water ice sublimates directly to gas when the temperature is warm enough.

Since June 2000, many hypotheses have been discussed at scientific meetings, in the scientific journals and elsewhere. The original June 2000 hypothesis held that the fluid was liquid water (either pure, salty, acidic, etc.) that came to the surface where slopes intersected conduits of groundwater. Such slopes include crater walls, valley walls, hills, massifs and crater central peaks. Later investigators explored the possibility that rather than liquid groundwater, the source was ground ice, which, under some climate conditions, melted to produce liquid runoff. Still others noted that thick mantles covered a fraction of the gully-bearing slopes, suggesting that the mantles were ancient, dust-covered snow or ice packs that might melt at the base to make liquid water runoff. Water was not the only fluid considered by various colleagues; carbon dioxide can be fluid at some pressures and temperatures. Fluid carbon dioxide was also proposed as a candidate fluidizing agent. Even dry mass movement, or land sliding, of unconsolidated granular material can exhibit some fluid-like behavior. Such mass movements were considered as an explanation for the gullies

The presence of channels primarily formed by erosion but also displaying features representing along-channel deposition, such as levees and meanders, and terminal depositional aprons consisting of dozens to hundreds of individual flow lobes, contributed to the general acceptance of the hypothesis that gullies involved the action of liquid water.

Throughout the Mars Global Surveyor mission, the Mars Orbiter Camera team continued to image gullies at every opportunity, looking for new gullies, taking higher resolution images of previously identified gullies, and monitoring the gullies for changes that might occur. Among the results of this extensive survey are numerous examples of gullies that have geological relations to other things in their vicinity. This provides support for the hypothesis that the fluid responsible for the gullies came from beneath the ground, either as groundwater or melting of ice in the Martian subsurface. Three of the best examples are presented here.

Figure A: The first picture shows a pair of gully channels that emerge, fully-born at nearly their full width, from beneath small overhangs on the north wall of Dao Vallis. These overhangs are probably created by the presence of a hard-rock layer. Liquid, probably water, percolated through permeable layers just beneath these harder, more resistant rock layers. The arrow points to the place where one of the two neighboring channels emerges. This is a sub-frame of an image acquired on Jan. 10, 2006, located near 34.2 degrees south latitude, 268.1 degrees west longitude. The 150-meter scale bar is about 164 yards wide.

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