Geological observations of Martian arroyos

Martian channels are interpreted as dry riverbeds (arroyos). The serious problem of fluid sources and arroyo ages is addressed. Arroyos originating in collapsed chaotic terrain may have been fed by melting permafrost, but major arroyos with tributaries and the network of fine equatorial channels imply a more widespread source of fluid. Some channels probably had episodic activity. Time intervals between episodes are uncertain, but marginal cratering evidence suggests that the episodes may have been more than 107 years apart. Cratering evidence also indicates that some fluvial activity occurred at least some 108 years ago, although additional activity may have occurred earlier and more recently. Some arroyos are now sites of variable albedo markings, probably owing to deposition and removal of dark windblown deposits. The observations are difficult to reconcile with short-term periodicities, such as Ward's (1973) 1-m.y. obliquity variations, as the major cause of Martian fluvial episodes. Longer-term climate variations are favored.