Orbitally-Driven Change in the Martian Atmosphere

We consider the impact of orbital cycling (obliquity, eccentricity, perihelion precession) on the martian atmospheric water cycle through recent Amazonian history. Changes in obliquity from present-day values can have a significant effect on the abundance of water vapor in the martian atmosphere. In addition, the magnitude and distribution of surface water ice deposits can vary significantly, on timescales commensurate with the period of obliquity oscillation (100 ky). We have used results from both the GFDL Mars GCM as well as the Planetary WRF GCM to examine possible changes in the water cycle induced by these orbital motions. Preliminary results indicate that if a sublimation lag develops over extant polar deposits during an extended rise to high obliquity (105-106 yr), the amount of water vapor released into the atmosphere will be correspondingly and significantly reduced, maintained in balance with any ice deposits that developed in the lower latitudes prior to the ``shutting off'' of the polar ice deposits. Under these conditions, annual average water abundances at high obliquity may only be 20-80 prμm, one to two orders of magnitude less than previous estimates, and only a factor of a few higher than present-day values. Further, the ``wettest'' periods in recent Mars history may not be directly correlated with the highest mean obliquity but rather with brief periods of high obliquity when the mean obliquity was only slightly higher than present.