Modeling the atmospheric limb emission of CO 2 at 4.3 μm in the terrestrial planets

Abstract

The MIPAS instrument on board Envisat, in Earth orbit, the PFS and OMEGA instruments on Mars Express, and VIRTIS on board Venus Express are currently providing a dataset of limb measurements of the CO 2 atmospheric fluorescence emission at 4.3 ‐ μ m from the upper atmosphere of the three planets. These measurements represent an excellent dataset to perform comparative studies between the terrestrial planets’ upper atmospheres, and also to test our theoretical understanding of these emissions. In order to exploit these datasets, we apply a set of non-local thermodynamic equilibrium (non-LTE) models developed at the IAA/CSIC, in Granada, Spain, to a selection of data. In general, the models can explain the main spectral features of the measurements, and also the altitude and solar zenith angle variations. However, the simulations for Mars and Venus give an incorrect ratio of the emissions at two wavelengths, 4.4 and 4.32 μ m . In order to explain this deficiency, a revision of the most uncertain non-LTE energy transfer parameters has been performed. The quenching rate of ν 3 quanta of high-energy CO 2 states by CO 2 itself could reduce the model-data discrepancy if increased by a factor 2–4, still within its current uncertainty range. This factor, however, is subject to the uncertainty in the thermal structure. A number of simulations with the non-LTE models were also used to study and compare the role of radiative transfer in this spectral region in the three terrestrial planets. Sensitivity studies of density and temperature are also presented, and they permit an analysis of how the differences between the planets and between the three instruments affect their sounding capabilities.