Hundreds of enigmatic small troughs are seen to carve into the slopes of these dark sand dunes lying within Russell Crater on Mars. These features were previously identified as gullies in images from the Mars Orbiter Camera on Mars Global Surveyor, but the higher resolution HiRISE image brings out many new details and mysteries. The troughs extend from near the top of the dunes to their bases, indicating that some fluid material carved into the sand. The troughs commonly begin as smaller tributaries joined together, suggesting several sources of fluid. Distinct dark spots are located near where the troughs seem to originate. Several troughs appear to begin at alcoves. Several of these troughs have sinuous middle reaches whereas others are straighter. Further down slope, some trough edges appear elevated above the surrounding terrain, particularly in the lower reaches. The troughs seem to terminate abruptly, with no deposition of material, unlike at the bases of some other gullies on Mars that are not on dunes. One hypothesis for the origin of these troughs, which has been previously been proposed by the MOC team, is that CO2 (or maybe H2O) frost is deposited on the dunes in shadows or at night. Some frost may also be incorporated into the internal parts of the dunes due to natural avalanching. When the frost is eventually heated by sunlight, rapid sublimation triggers an avalanche of fluidized displaced sand, forming a gully. HiRISE will continue to target small trough features such as these and may return to search for any changes over time.
Image credit: NASA/JPL/University of Arizona
+ High resolution image