Nick Hoffman is by far the most qualified to answer your question - we are fortunate that he frequents this habitable zone. I have posted some of Dr. Hoffman's research concering gullies along the rim of the basin that may begin to answer your question:

Dr. Hoffman and colleagues from the US Geological Survey (USGS) presented their most recent evidence in the latest Geophysical Research Letters.

Their research focused on gullies formed by areas of collapsed terrain along the rim of the 3.5 billion year old Hellas basin, an impact crater 2,000 kilometers wide and 9 kilometers deep.

Their data suggests that the gullies were scoured by debris helped along by flowing liquid and gaseous CO2 rather than hot springs or exploding groundwater as originally proposed.

Hoffman believes the Hellas Basin Gullies formed when volcanic magma mixed with trapped and compressed frozen carbon dioxide. This expanded violently, causing the ground to collapse and flow down the slope, gouging out a gully as it went. The now liquid and gaseous carbon dioxide acted as a lubricant for the debris to continue flowing.

A personal idea - wind/dust storms. Could they in effect "clean out" the basin? The dust storm that enveloped most of the planets surface last year started in the Southern Hemisphere near Hellas Basin.

See ya, VINCE