NASA Mars MRO HiRISE Image: Formation of a new gully channel on a crater-wall slope

This pair of before (left) and after (right) images from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter (MRO) documents the formation of a substantial new channel on a Martian slope between Nov. 5, 2010, and May 25, 2013. 

Credit: NASA /JPL-Caltech /Univ. of Arizona 

A comparison of images taken by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter (MRO) in November 2010 and May 2013 reveal the formation of a new gully channel on a crater-wall slope in the southern highlands of Mars.

Gully or ravine landforms are common on Mars, particularly in the southern highlands.

This pair of images shows that material flowing down from an alcove at the head of a gully broke out of an older route and eroded a new channel.

The dates of the images are more than a full Martian year apart, so the observations did not pin down the Martian season of the activity at this site.

Before-and-after HiRISE pairs of similar activity at other sites demonstrate that this type of activity generally occurs in winter, at temperatures so cold that carbon dioxide, rather than water, is likely to play the key role.

The location is on the inner wall of a crater at 37.45 degrees south latitude, 222.95 degrees east longitude, in the Terra Sirenum region.

Gully or ravine landforms are commonly found in the mid-latitudes on Mars, particularly in the southern highlands.

These features typically have an alcove at the upper end, feeding into a channel and an apron of debris that has been carried from above.

Researchers using HiRISE have discovered many examples of gully activity likely driven by seasonal carbon-dioxide frost (dry ice).

The changes visible by comparing the 2010 and 2013 observations at this site formed when material flowing down from the alcove broke out of an older route, eroded a new channel and formed a deposit on the apron.

Although this pair of observations does not pin down the season of the event, locations HiRISE has imaged more often demonstrate that this sort of event generally occurs in winter, when liquid water is very unlikely.

Despite their resemblance to water-formed ravines on Earth, carbon dioxide may play a key role in the formation of many Martian gullies.

NASA Mars HiRISE Image: Gully Monitoring on Crater Slopes in Terra Sirenum

These crater gullies lie on the northern wall of an unnamed 9-kilometer diameter southern hemisphere crater in Terra Sirenum. The image was acquired during early winter in the southern hemisphere, so the crater wall is in shadow.

These gullies were first imaged by HiRISE in 2006. Since that time the possible role of seasonal frost in gully formation along with the association of polygonal terrain with these and other gullies has garnered considerable interest.

As a result, these gullies have become one of several locations being monitored by HiRISE throughout multiple Mars years. Over a dozen images of these gullies have been acquired to date throughout different Mars seasons.

In this image, frost (likely water-ice) is once again forming on these southern hemisphere mid-latitude crater slopes.  

The subimage shows gullies on the shadowed polar-facing slope. The large dynamic range of the HiRISE camera allows one to see into the shadows dimly lit by sunlight scattered by the surface and the atmosphere.

These gullies are thinly veiled with frost and range in width from several meters to tens of meters and in length from a couple kilometers or so. Dark regions within the gullies are warmer areas where frost likely evaporated or melted exposing the darker underlying surface.