Mars-GRAM 2010: Improving the Precision of Mars-GRAM

It has been discovered during the Mars Science Laboratory (MSL) site selection process that the Mars Global Reference Atmospheric Model (Mars-GRAM) when used for sensitivity stu-dies for Thermal Emission Spectrometer (TES) Ma-pYear=0 and large optical depth values, such as tau=3, is less than realistic. Mars-GRAM’s perturba-tion modeling capability is commonly used, in a Monte-Carlo mode, to perform high fidelity engi-neering end-to-end simulations for entry, descent, and landing (EDL) [1]. Mars-GRAM 2005 has been validated [2] against Radio Science data, and both nadir and limb data from TES [3]. Traditional Mars-GRAM options for representing the mean atmosphere along entry corridors include: (1) TES mapping year 0, with user-controlled dust optical depth and Mars-GRAM data interpolated from NASA Ames Mars General Circulation Model (MGCM) [4] results driven by selected values of globally-uniform dust optical depth, or (2) TES mapping years 1 and 2, with Mars-GRAM data com-ing from MGCM results driven by observed TES dust optical depth. From the surface to 80 km alti-tude, Mars-GRAM is based on NASA Ames MGCM. Above 80 km, Mars-GRAM is based on the University of Michigan Mars Thermospheric General Circulation Model (MTGCM) [5]. MGCM results that were used for Mars-GRAM with MapYear=0 were from a MGCM run with a fixed value of tau=3 for the entire year at all loca-tions. This choice of data has led to discrepancies that have become apparent during recent sensitivity studies for MapYear=0 and large optical depths. Unrealistic energy absorption by time-invariant at-mospheric dust leads to an unrealistic thermal energy balance on the polar caps. The outcome is an inac-curate cycle of condensation/sublimation of the polar caps and, as a consequence, an inaccurate cycle of total atmospheric mass and global-average surface pressure. Under an assumption of unchanged tem-perature profile and hydrostatic equilibrium, a given percentage change in surface pressure would pro-duce a corresponding percentage change in density at all altitudes. Consequently, the final result of a change in surface pressure is an imprecise atmos-pheric density at all altitudes.