The Mars Diurnal CO2 Cycle as Observed in the Tharsis Region.

Abstract:

The Mars atmosphere is composed of 95% CO₂, 25% of which is cycled through the seasonal CO₂ ice caps annually. During the polar night, surface temperatures drop to ~145 K and CO₂ ice can begin to directly condense. Also, atmospheric temperatures can drop even lower, allowing for the formation of CO₂ snow. During the spring, the CO₂ ice sublimes thus completing the annual CO₂ cycle. While significant study has been conducted to characterize and model the annual CO₂ cycle, little has been done to investigate the possibility of a diurnal CO₂ cycle.

There are two general locations where the diurnal exchange of surface CO₂ ice with the atmosphere can occur: (1) the higher elevations of the Tharsis region and (2) the edges of the seasonal polar caps. This presentation will focus on the Tharsis region. The low surface thermal inertia, due to a thick layer of dust, and the low atmospheric pressure of the Tharsis region allow for diurnal temperature ranges from at or near the CO₂ frost temperature (~140 K at 2 mbar) during pre-dawn hours to temperatures above the triple point of water during mid-day.

The Mars Global Surveyor Thermal Emission Spectrometer showed evidence of CO₂ ice deposition near the summit of Olympus Mons at 2 am local time. Spectra from the coldest data observed are generally flat through the 25 μm region, suggesting slab or coarse grained CO₂ ice. The 15 μm region of the spectra shows a warm atmosphere, suggesting that the formation of CO₂ snow is unlikely. The Mars Odyssey Thermal Emission Imaging System showed consistently that much of the surface is at or near the frost point of CO₂, suggesting the possible widespread presence of CO₂ ice deposits during the pre-dawn hours.

Simple energy balance considerations suggest that a nightly CO₂ deposit that completely sublimes away shortly after dawn, could be as thick as 1.6 kg/m³ (~1 mm if deposit is slab ice with no porosity). Further constraints on the details of the Tharsis diurnal CO₂ cycle will be presented.