THE MARS WATER CYCLE AT OTHER EPOCHS - RECENT HISTORY OF THE POLAR CAPS AND LAYERED TERRAIN

The Martian polar caps and layered terrain presumably evolve by the deposition and removal of small amounts of water and dust each year; the current attributes of the cap therefore represent the incremental transport during a single year as integrated over long periods of time. We have investigated the role of condensation and sublimation of water ice in this process by examining the seasonal water cycle during the past 107 years. In our model, axial obliquity, eccentricity, and Ls of perihelion vary according to dynamic models. At each epoch, we calculate the seasonal variations in temperature at the two poles, keeping track of the seasonal CO2 cap and the summertime sublimation of water ice into the atmosphere; net exchange of water between the two caps is calculated based on the difference in the summertime sublimation between the two caps (or on the sublimation from the other cap if one is covered with CO2 frost all year). Despite the simple nature of our model and the tremendous complexity of the martian climate system, our results suggest two significant conclusions: (1) Only a relatively small amount of the available water in the polar deposits actually cycles between the poles on orbital-evolution time scales, such that it is to some extent the same water molecules moving back and forth between the two caps. (2) The difference in elevation between the two caps results in different seasonal behavior, such that there is a net transport of water from south to north averaged over long time scales; this transport would cause the north cap to grow at the expense of the south cap until the product of water sublimation rate and cap area was equal for the caps, minimizing any subsequent net transport. These results can help explain (1) the apparent inconsistency between the timescales inferred for layer formation and the much older crater retention age of the cap and (2) the difference in sizes of the two residual caps, with the south cap being smaller than the north. © 1993 by Academic Press, Inc.