Models of climate cycles recorded in Martian polar layered deposits

Layered strata observed in orbital images of polar sedimentary terrains on Mars have been hypothesized to contain information about recent variations in the Martian climate. Celestial mechanicians have determined that Mars has experienced large-amplitude quasi-periodic variations in obliquity and eccentricity: planetary meteorologists have inferred that the resulting changing patterns of solar heating must have had dramatic effects on Martian climate. Accordingly, it might be possible to link the known recent history of orbit/axial variations with the physical characteristics of the most recently deposited strate determined from orbital imagery. Our paper explores that possibility quantitatively for various models of the formation of the polar layered deposits and the layering developed within them. We modeled the formation of polar layered deposits during the past 10 my for two general classes of model. Uniform deposition rate (UDR) models assume that the major constituent of the layered deposits is deposited at a uniform rate and that layering is established by climate-induced variations in the concentration of a minor constituent. Climate-modulated deposition rate (CMDR) models assume that the deposition rate of the major constituent is controlled by climate and that layer boundaries correspond to intervals of nondeposition. In both models, abrupt changes in deposition regime occur when a function of the orbit/axial parameters crosses a threshold value. Detailed physical mechanisms for the control of sedimentation by orbit/axial variations were developed for two specific models of the CMDR type. In one model, the deposits are assumed to be primarily composed of dust; in the other, they are assumed to be predominantly water ice. We calculated the times at which abrupt changes in deposition regime occur and determined the relative thicknesses of successive layers which can be compared against layer thicknesses made from orbital imagery. In all these models, obliquity variations exert the dominant influence on the sequential variations in layer thickness, although eccentricity and precession index (longitude of perihelion) may take recognizable contributions as well. The layer patterns for the north and south polar deposits are identical for the dust model and only slightly different for the ice model, but this is at least partly a consequence of simplifying assumptions. The sequential variation in layer thickness is also very sensitive to those model para