As soon as gullies were discovered, researchers began to image many gullies over and over, looking for possible changes. By 2006, some changes were found. Later, with further analysis it was determined that the changes could have occurred by dry granular flows rather than being driven by flowing water. With continued observations many more changes were found in Gasa Crater and others. Channels widened by 0.5 to 1 m; meter sized boulders moved; and hundreds of cubic meters of material moved. It was calculated that gullies could be formed under present conditions with as little as 1 event in 50–500 years. So, although today there is little liquid water, present geological/climatic processes could still form gullies.Dundas, C., S. Diniega, and A. McEwen. 2014. LONG-TERM MONITORING OF MARTIAN GULLY ACTIVITY WITH HIRISE. 45th Lunar and Planetary Science Conference. 2204.pdf Large amounts of water or great changes in climate are not needed. However, some gullies in the past may have been aided by weather changes that involved larger amounts of water, perhaps from melted snow.Dundas, C., S. Diniega, C. Hansen, S. Byrne, A. McEwen. 2012. Seasonal activity and morphological changes in martian gullies. Icarus, 220. 124–143. With more repeated observations, more and more changes have been found; since the changes occur in the winter and spring, experts are tending to suspect that gullies were formed from carbon dioxide ice (dry ice). Recent studies describe using the High Resolution Imaging Science Experiment (HiRISE) camera on MRO to examine gullies at 356 sites, starting in 2006. Thirty-eight of the sites showed active gully formation. Before-and-after images demonstrated the timing of this activity coincided with seasonal carbon dioxide frost and temperatures that would not have allowed for liquid water. When dry ice frost changes to a gas, it may lubricate dry material to flow especially on steep slopes. In some years frost, perhaps as thick as 1 meter, triggers avalanches. This frost contains mostly dry ice, but also has tiny amounts of water ice.
