Winter Frost at Viking Lander 2 Site

摘要

A key question in the study of Mars is water exchange between atmosphere and surface on daily, seasonal, and astronomical timescales. We believe that small-scale processes are a key for enhanced understanding of the global water behavior of Mars. The principal data for this study of small scale properties of the Martian surface were collected by the second Viking lander (VL 2) and by both Viking orbiters. The annual deposition and retreat of the frost layer were observed in situ by VL 2. The frost is inferred to be H_2O frost but with some properties suggesting a much thicker layer than would be expected from the simple mass balance calculation. Our original contribution is in considering the effect of cold trapping (frost redeposition) which has been previously neglected and which enables us to reconcile all the observations with environmental conditions. In addition, we believe that this study points to a more general phenomenon of cold trapping in the Martian environment. Our study of the VL 2 observations suggests that H_2O frost occurs in two forms: (1) thin, almost continuous, early frost and (2) much thicker, patchy, later frost. Both frost forms contain essentially the same total water content, but they cover different fractions of the surface. The transition between the two frost forms occurs by recondensation at local cold traps when solar insolation sublimates the first frost but the atmosphere is still too cold to transport the resultant water vapor away. These cold traps are created by shadowing from the small-scale surface roughness, rocks, troughs, etc. This hypothesis hinges on the disparity between local and long-range transport of water vapor by the atmosphere. The local transport is driven by abundant insolation energy available at the time of transition. This results in a large fraction of surface frost being redistributed rapidly into locally thermodynamically preferable locations, cold traps. Long-range transport is constrained by the atmospheric carrying capacity. At the time of transition, the atmosphere is still cold, not far from its winter minimum, and is almost saturated by residual water vapor (5–8 precipitable micrometers). Therefore it cannot carry much additional water vapor to lower latitudes. This disparity delays the global transport of water vapor by the atmosphere.