Possible stream channels in the equatorial region of Mars are associated with chaotic terrain boundaries. Scalloped edges and slump features at the edge of chaotic terrain in the Chryse region of Mars indicate that the relatively smooth “upland” areas have collapsed to form chaotic terrain that has been further modified in part by both eolian and fluvial processes.

Circular depressions near chaotic terrain could result either from impact or collapse (or both). Eolian infilling may have modified impact craters to the extent that they do not resemble terrestrial or lunar counterparts. However, the dominance of fractures originating within circular depressions may indicate an internal source of heat. Collapse of the surface because of local heating, either internal or external, would provide a mechanism for both the subsidence responsible for chaotic terrain and the release of subsurface water leading to channel formation.

Analyses of Mariner 9 imagery and ERTS photographs show that drainage basins on Mars and earth have similar characteristics, but both sets of photographs have fewer small tributaries than are actually present in terrestrial drainage basins. On earth, small tributaries are not observed because of the resolution of ERTS cameras. On Mars, both resolution and eolian infilling may account for this characteristic. There is evidence of the effects of both surface and subsurface water on Mars, but erosional characteristics of Martian channels show a closer relation to formation by subsurface water than to formation by rainfall.

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