New and recent gully activity on Mars as seen by HiRISE

These deposits have latitudinal and azimuthal dependences similar to the overall preferences of southern-hemisphere gullies, suggesting that most gullies can undergo such events. Definite changes are seen at ten sites, including two previously reported.

These include visible modification of gully channels and aprons. Those formation intervals constrained to better than one Mars year tend to include winter and exclude summer, suggesting seasonal activity. This seasonal activity is consistent with proposed models for gully formation driven by CO2 frost, although at least some of the new deposits are probably due to dry granular flow with no volatile involvement.