Mars south polar spring and summer behavior observed by TES: seasonal cap evolution controlled by frost grain size

Thermal Emission Spectrometer (TES) observations of the recession phase of Mars' south polar cap are used to quantitatively map this recession in both thermal and visual appearance. Geographically nonuniform behavior interior to the cap is characterized by defining several small regions which exemplify the range of behavior. For most of the cap, while temperatures remain near the CO₂ frost point, albedos slowly increase with the seasonal rise of the Sun, then drop rapidly as frost patches disappear over a period of ∼20 days. A “Cryptic” region remains dark and mottled throughout its cold period. TES observations are compared with first-order theoretical spectra of solid CO₂ frost with admixtures of dust and H₂O. The TES spectra indicate that the Cryptic region has much larger grained solid CO₂ than the rest of the cap and that the solid CO₂ here may be in the form of a slab. The Mountains of Mitchel remain cold and bright well after other areas at comparable latitude, apparently as a result of unusually small size of the CO₂ frost grains; we found little evidence for a significant presence of H₂O. Although CO₂ grain size may be the major difference between these regions, incorporated dust is also required to match the observations; a self-cleaning process carries away the smaller dust grains. Comparisons with Viking observations indicate little difference in the seasonal cycle 12 Martian years later. The observed radiation balance indicates CO₂ sublimation budgets of up to 1250 kg m⁻². Regional atmospheric dust is common; localized dust clouds are seen near the edge of the cap prior to the onset of a regional dust storm and interior to the cap during the storm.