Non-condensable gas in a Mars General Circulation Model

We model the variation of non-condensable trace gases that results from the seasonal cycle of CO2 on Mars. A simple condensation scheme has been incorporated into MarsWRF, a 3-dimensional numerical model for the atmospheres of Mars. Non-condensable trace gas (mostly N2 and Ar) mass mixing ratios are affected by the phase change of CO2 and by transport. The distribution of Ar abundance has been observed by the Gamma Ray Spectrometer on the Mars 2001 Odyssey spacecraft. We are able to qualitatively reproduce the Ar observations, including the seasonal evolving latitudinal distribution. However, the modeled magnitudes of maximum enrichment are lower than observed. Smoothing Ar enrichment in the vertical reduces susceptibility to transport by near-surface, off-cap circulation, therefore gives further enhancement of non-condensable tracer in the winter pole. We suggest that a missing process in the model may account for the underestimation. An extra buoyancy term in the dynamics should result from the vertical gradient in mean molecular mass as Ar mass mixing ratio increases.