Atmospheric and Surface Retrievals in the Mars Polar Regions from the Thermal Emission Spectrometer Measurements

Introduction: The Thermal Emission Spectrometer (TES) aboard the Mars Global Surveyor (MGS) spacecraft has generated an unprecedented wealth of information about Mars. Although TES is primarily a surface-oriented instrument [1, 2], analyses of TES spectra have also yielded abundant information about the martian atmosphere, including its thermal structure, dust opacity, column abundance of water vapor, and optical properties of airborne dust and water ice particles [3-7]. The information about the spatial and temporal variability in these parameters has in turn enabled a wide range of scientific studies. In particular, TES retrievals have led to a description of the amplitudes, dominant wave periods, and seasonal evolution of planetary waves [8-12], provided insights into dust storm generation mechanisms [13], and, together with the Viking data, have served as a reference to studies of the Mars water cycle [14]. The focus of the TES retrieval and analysis work performed to date has been on the non-polar regions. For example, the opacity product in the Planetary Data System (PDS) 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]. Furthermore, the polar temperature profiles in the PDS have been obtained without specifically accounting for the polar surface emissivities (which are often very different from the non-polar emissivities) and they exhibit little vertical structure (see below). To some extent, the lack of vertical structure in the TES temperature profiles retrieved to date stems from the coarse vertical resolution of the instrument (10 km or more in the nadir mode). However, the TES temperature profiles in mid-latitudes do exhibit vertical structure and are generally consistent with the MGS radio science (RS) retrievals [15]. This points to other factors playing a role. Indeed, using an independent retrieval algorithm [16], we have performed simultaneous retrievals of atmospheric temperatures and surface emissivity in the polar regions covered by seasonal CO2 frost and established that, even when the coarse vertical resolution of TES is taken into account, the retrieved temperature profiles exhibit inversions around 1 mbar that are present in the newest GCM polar simulations, but largely absent in the PDS profiles. These results are discussed in more detail below. Results: In our TES retrieval work to date [16], we have focused on the polar regions covered with the seasonal CO2 frost, where the presence of both nonporous (slab-like) CO2 ice and more porous, fluffy frost were predicted [17], a prediction confirmed by subsequent analyses [18, 19] (for completeness, we note that dust coating can also mimic the slab-ice behavior [20]). A fluffy texture can be distinguished from a slab layer by the shape of the 25-μm band of solid CO2 [21] seen in the TES spectra, with large band depth BD25 indicative of high porosity. BD25 is defined as the fractional drop in the measured band radiance relative to the expected blackbody radiance at the brightness temperature of adjacent continua [18]. Sample TES spectra of the southern seasonal cap with small and large BD25 are shown in Figure 1.