Current Status of Atmospheric and Surface Retrievals in the Mars Polar

Introduction: The Mars polar regions and the global atmospheric circulation are intimately coupled through the CO2 condensation/sublimation cycle driven by the polar energy balance [1-4]. The caps and the atmosphere also interact on the regional scale [5]. Signatures of these interactions may be expected in the surface and atmospheric properties retrieved from remote sensing observations, but polar retrievals to-date have been somewhat limited in scope. For example, the opacity product in the Planetary Data System (PDS) retrieved from Thermal Emission Spectrometer (TES) radiances is essentially non-existent when the surface temperature drops below about 220 K. This is principally due to the generally small thermal contrast between the atmosphere and the surface, particularly in situations when the surface has near-black-body emissivities [6]. This limitation has been addressed through a modification of the TES opacity retrievals [7], but this modification does not attempt a simultaneous retrieval of atmospheric temperatures, relying instead on the PDS profiles [8]. The latter have been obtained without specifically accounting for either dust or the polar surface emissivities (which are often very different from the non-polar emissivities) and they exhibit little vertical structure (see below). Surface emissivities are also not reported in the retrievals performed from the Planetary Fourier Spectrometer (PFS) data [9]. Polar atmospheric retrievals from the Mars Climate Sounder (MCS) limb radiances are emerging [10], but with a scarcity of concurrent surface observations and without a scattering parameterization. Simultaneous Atmospheric/Surface Retrievals: In a pilot study [11], we performed simultaneous atmospheric and surface retrievals on a limited number of TES spectra in the northern polar ring data around 87N during fall and winter. The retrieved quantities were atmospheric temperatures, spectral surface emissivities, and optical depths of atmospheric water ice and dust. For the atmospheric particulates, we ignored scattering, instead using their spectral absorption coefficients available from the PDS [6] and retrieving their optical depth. The impact of neglecting scattering is, in general, not large. For the small dust and ice optical depths we retrieve, the differences between nonscattering radiances and radiances computed using the scattering version of our radiative transfer code [12] are, on average, within the instrument noise level. Figure 1 shows the atmospheric temperature profiles retrieved for locations characterized by near-unity emissivities (“low-BD25” [13], panel a) and the socalled “cold spots” (“high-BD25”) where emissivities are significantly lower than unity (panel b). Since cold spots are usually attributed to the occurrence of snowfall [5, 13-18], it is encouraging to see that the associated temperature profiles do fall below the CO2 condensation line (plotted in green in Figure 1) more often than in the “low-BD25” locations (where the CO2 frost is likely to form directly on the ground). The supersaturated region in Figure 1 is confined to the lowest 20 km, consistent with the altitude range of previous detections of CO2 clouds [19, 20].