Simulating the density and thermal structure of the middle atmosphere (∼80–130 km) of Mars using the MGCM–MTGCM: A comparison with MEX/SPICAM observations

Abstract

The objective of this work is to advance the understanding of the structure and dynamics of the middle altitude region of the martian atmosphere. While numerous advancements have been made in the level of scientific understanding of Mars’s upper and lower atmospheres over the past several decades, insight into the mechanisms of the middle atmosphere has come at a significantly slower pace due to the small number of datasets available for this region. Over the past decade this has begun to change, with renewed interest by NASA and ESA to send spacecraft to Mars. The result of these recent efforts is a growing database for Mars’s middle atmosphere, enabling long-awaited and necessary studies characterizing the middle altitude region. Various numerical models of the martian atmosphere can now be validated and constrained using this database. We utilize the Mars Express/Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (MEX/SPICAM) density and temperature datasets to characterize the middle atmosphere as well as validate and constrain the coupled multi-dimensional Mars General Circulation Model–Mars Thermosphere General Circulation Model (MGCM–MTGCM) at middle altitudes in order to explore the underlying physics controlling the structure and dynamics at these levels. The results of this study stress the importance of proper dust prescription within the MGCM–MTGCM for accurately reproducing the density and thermal structure of the middle and upper atmosphere regions on Mars. Simulations conducted with horizontal dust opacities that are consistent with the SPICAM observation period (i.e. Mars Odyssey/THEMIS opacities) yield modeled densities and temperatures that are closer to the observed values than simulations conducted with “typical” dust conditions (i.e. Mars Global Surveyor/TES opacities). We show that the MGCM–MTGCM closely reproduces the observed densities during low-dust and high-dust scenarios but displays difficulty during the pre-dust-season “ramp-up” period ( L s ∼ 120–200°) during MY27. In addition, we show that the MGCM–MTGCM accurately reproduces the temperature profiles below the mesopause, but, the mesopause altitude is too low and its temperature warmer (5–10 K) than observations. This may be related to nightside dynamical heating processes that require further refinement. In addition, CO 2 15-μm cooling rates may be too small, which would be consistent with underestimated atomic O abundances.