Gas-phase contamination modeling for space systems typically looks at the free molecular flow regime, Knudsen number » 1, wherein transport is characterized by collisionless motion of contaminant molecules and deposition proportional to grey- or black-body view factors. Such an approach, however, was not applicable to the contamination transport environment [to be] encountered by the NASA Mars Science Laboratory (MSL) during surface operations on the Red Planet. For MSL, we required an understanding of contaminant transport under the Mars-ambient conditions of an approximately 8 Torr CO2 atmosphere in order to provide traceability between hardware outgassing limits and the allowable vapor-phase contaminant concentrations in the vicinity of atmospheric sampling sensors and deposition to prospective solid sample sites on the Martian surface.
In setting outgassing requirements for the MSL surface system, an engineering upper-bound estimate--rather than a precise result based on an all-inclusive simulation of the dynamic flow field--of the local contamination density was needed. Here we describe a 3-D, low-speed computational fluid dynamics approach, including molecular diffusion, to determine mixing ratios of contaminants at the atmospheric sample inlets and solid sample inlet deposition rates. Turbulence enhances the effective diffusion, leading to the dilution of the volatile contaminants, which reduces contamination concentration at a distance far from the source in comparison to inviscid or laminar flow fields: Therefore, the approach employed here results in a conservative upper bound compared to one in which turbulence is explicitly addressed. Because contaminant transport in this environment (Peclet number in the range of 50-1000) is advection dominated, spatial contamination concentration is a strongly-peaked function of the wind direction. Results of sample calculations for expected Mar wind speeds (u = 1-20 m/s) and several wind directions are presented.