Nature and origin of layered deposits of the Martian polar region.

Layered deposits in the polar regions of Mars may be formed from fine dust ultimately derived from the canyons and other eroded terrains of the equatorial regions. An analysis of dust deposition in the area of annual frost cover predicts the formation of vast featureless domed plateaus underlain by layered deposits of dust. Television observations suggest that these plateaus once existed, but they have now been dissected and stripped from most of the area of annual frost cover. A possible explanation is that a major secular change has occurred in the erosional environment of the Martian polar regions. Estimates of depositional rates of dust under current atmospheric conditions place the time span represented by the accumulation of layered deposits at about 500 m. y. Because of erosion the present surface appears very young and lacks any impact craters. The present rate of water ice accumulation in the area of perennial frost is comparable to that of dust, and thus it is suggested that large quantities of water ice may be trapped with dust beneath the perennial frost caps. The formation of terraced erosional surfaces corresponding to individual layers or groups of layers indicates variations in response to erosion. One possible explanation is that reworking of the surface has occurred during intervals of nondeposition. Perennial frost appears to inhibit erosion of the layered deposits. One speculative possibility is that the secular change in erosional conditions corresponds to a reduction in the area of the perennial polar cap. Materials eroded from the layered deposits appear to have been redeposited in the mid-latitudes of Mars.