By showing that parts of equatorial and mid-latitudinal Mars have a variety of channels and channel-like forms, Mariner 9 photographs provide a basis for speculations concerning surface processes, crustal events, climatological environment, and evolutionary history.
Some large outflow channels display characteristics suggesting scour and plucking by torrential floods similar to the Spokane and Bonneville events of western United States, although such channels are probably not solely the product of flood action. Other channels with dendritic tributaries suggest runoff fed by seepage and headward growth and enlargement by sapping.
Some Martian channels and channel-like forms were probably created or initiated by endogenic processes such as faulting, subsidence, volcanism, fracturing, and crustal extension; others may be due to wind or lava erosion, but the features and relationships of many channels are best accounted for by fluvial action.
Reconciliation of fluvial erosion and the current hostile Martian environment may be possible if the channels are as old as 3 b.y. Such an age is suggested by recent re-evaluations of meteoroidal flux impacting Mars, Moon, and Earth and the chronology of lunar maria. A residual primitive atmosphere possibly congenial to running water on Mars may have permitted fluvial erosion 3 to 3.5 b.y. ago. Haphazard scattering of channels and the likelihood of seepage and sapping suggest that water was supplied to the Martian surface from the lithosphere, not the atmosphere.