Abstract
Gullies were not only investigated for scientific endeavor, but also were prime targets for the search for presence of water and life on Mars, as habitats might have or are existing. Gullies on Mars were first identified in the milestone paper by Malin and Edgett in 2000. Since then more than 174 papers were published on Martian gullies. A typical gully consists of an upper theatre-shaped alcove that tapers downslope to converge on a channel that extends further downslope to terminate in a triangular apron of deposited material. A number of processes were proposed for gully formation. These include liquid-induced processes, such as overland flow, headward sapping, debris flow, and other wet mass movements. The formation of liquid is attributed to groundwater sapping, to supply from deep subsurface aquifers by cryovolcanic processes, to melting of snow or melting of ground ice from surficial to shallow depths. Liquid-free mass movements, such as dry granular flows and dry ice outbreak, are also invoked as formation processes. Supporting and opposing morphologic evidence is shortly discussed. Tens of thousands of individual gullies were identified on Mars, which are concentrated on mid- to high-latitudes in both hemispheres. Gullies might have been active from 3 Ma ago to present. Future research may learn lessons from terrestrial gully research. On Earth, linear gullies may gradually develop oversteepened sidewalls, which in turn initiate deep-seated mass movements. Such systems are named gully complexes. Gullying can also be induced by sliding. Such landforms were termed slide complexes. These process sequences may occur also on Mars. In future investigations, identification of such complexes on Mars requires a focus on phases of incision and infilling to elucidate gully evolution.
