Dissecting the polar dichotomy of the noncondensable gas enhancement on Mars using the NASA Ames Mars General Circulation Model

摘要

The atmospheric processes underlying the observed spatial and temporal enhancement of noncondensing gases in Mars' atmosphere are investigated. The Gamma Ray Spectrometer (GRS) on board Mars Odyssey has obtained measurements indicating that the absolute and relative column abundance of noncondensing gases (primarily argon and nitrogen) maximizes at high latitudes in both hemispheres during winter as CO2 gas condenses and forms the seasonal polar ice cap. This condensing CO2 “leaves behind” noncondensing gases whose local absolute and relative column abundances can increase at a rate controlled by mixing with less-enhanced air from lower latitudes. Understanding the processes responsible for the magnitude and seasonal variations of these enhancement values is an aid in understanding atmospheric transport processes. The NASA Ames Mars General Circulation Model is employed to help understand the atmospheric thermodynamical mechanisms that give rise to the observed temporal and magnitude variations in the polar enhancement values. The model produces a threefold noncondensable gas enhancement in the south polar region and an approximate 1.4-fold increase in noncondensables in the north polar region. These model results are temporally consistent with observed values, but the observed enhancement magnitudes exceed those modeled by up to a factor of two. The difference in strength and the season of formation between transient eddies in the southern and northern hemispheres may play a large role in determining the different character of the two polar enhancements. Model simulations also illuminate the effect that topography, orbital eccentricity, and atmospheric dust opacity have on producing the north versus south polar enhancement dichotomy.