Large distinct Martian gully
Cool image time! The picture to the right, rotated, cropped, and reduced to post here, was taken on June 1, 2026 by the high resolution camera on Mars Reconnaisance Orbiter (MRO).
The object of focus is obviously that large gully, draining downhill about 5,000 feet to the northeast, over a distance of about three miles.
Gullies like this are found in many places in the Martian mid-latitudes. When first discovered scientists thought they might be related to the sublimation of underground ice. More recent research suggests they are formed by the seasonal dry ice frost cycle that in the high latitudes has carbon dioxide condense to fall as snow in autumn and then sublimate away in the spring.
This latter hypothesis however generally applies to a set of gullies on the interior rim of craters, all of which experience that cycle of freeze-thaw. In the case of this large gully the former hypothesis seems more likely, as it appears to be within a thick lobe of soft material that was a mass wasting event of glacial material off the crater wall (whereby a large section detaches and moves downward). Regardless, the near surface ice impregnated in the soil appears to be sublimating away, creating the gully.
The small rectangle on the overview map to the right marks the location, in the Nereidum mountains that surround Argyre Basin, one of three ancient gigantic impact basins on Mars. The location, at 48 degrees south latitude, is also within an area that orbital data has found a lot of evidence of glaciers.
The inset shows that this gully is in the southwest quadrant of a somewhat obscured 14-mile-wide crater more than a mile and a half deep. The inset also reinforces the supposition that this gully is within a mass wasting event that had a large part of the southwest rim break off and descend into the crater itself. My guess is that during that descent some of the ice impregnated inside that material was exposed to sunlight and sublimated away, forming the gully. As that sublimating ice fell downhill there was continually less of it, which is why the gully shrinks in width as it descends.
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