These images were acquired to test hypotheses about water on Mars.

From the first HiRISE image at this location (PSP_008312_1415) it appeared that there was a gully and bright deposit, indicating recent activity, on an unusually shallow slope, less than 20 degrees.

Most gullies are on steeper slopes, steep enough that dry granular flows could explain the movement and deposition of materials. If the slopes are too shallow, then some extra mechanism of fluidization is required, such as the action of volatiles like water or carbon dioxide.

We acquired a new stereo pair (with better illumination that the first image) and produced a Digital Terrain Model (DTM). The DTM shows that the slopes near the alcove where movement was probably initiated is actually between 20 and 30 degrees. Since acquiring these images we have also learned that most gully activity occurs in the winter and in places where there is carbon dioxide frost on the ground. This frost may coat grains to reduce friction, or it may sublimate to create a flow of gas that fluidizes the particles. Thus many of the gullies no longer appear to require the presence of water on Mars today.

Although there is abundant evidence for water on Mars in the past, the transient existence of water (in a liquid state) on the surface in the present climate remains only a hypothetical possibility. MRO will continue to search for evidence of water on Mars today.

Written by: Alfred McEwen