Infrared radiative transfer in the dust‐free Martian atmosphere

Abstract

Gases in the Martian atmosphere, including CO 2 , H 2 O, CO, and O 3 combine to produce some absorption at most infrared wavelengths. Line‐by‐line and quasi‐random models are used to derive synthetic spectra of dust‐free Martian atmospheres. These spectra show where gases absorb most strongly and provide a baseline for comparison with the results from more complete models that include the effects of dust. Gas absorption and emission features at many infrared wavelengths provide a source of contamination that must be removed from remote sensing observations of the Martian surface. For example, the weak reflectance minimum observed at wavelengths near 2.35 μm, which has been interpreted as evidence for a variety of surface materials, is produced almost entirely by atmospheric CO and CO 2 absorption. Isotopic CO 2 bands near 7 and 8 μm and near‐infrared water vapor absorption bands partially overlap strong carbonate and hydrate features and frustrate systematic spectroscopic searches for these important candidate surface materials on Mars. In other spectral regions, gas absorption bands provide opportunities to study the structure and composition of the Martian atmosphere. Computed radiances within the strong CO 2 15‐μm band are incorporated into an atmospheric retrieval algorithm to derive the atmospheric temperature structure from Mariner 9 IRIS observations. Absorption and emission by gases also contributes to the energetics of the Martian atmosphere. Near‐infrared CO 2 bands absorb enough sunlight to produce globally‐averaged solar heating rates that vary from 1 K/Earth day at the surface, to 10 K/Earth day at pressures near 0.01 mbar. Other gases contribute 1–5% of the heating at some levels.