Thermal and albedo mapping of the polar regions of Mars using Viking thermal mapper observations: 2. South polar region

Abstract

We present the first maps of the apparent thermal inertia and albedo of the south polar region of Mars. The observations used to create these maps were acquired by the infrared thermal mapper (IRTM) instruments on the two Viking Orbiters over a 30‐day period in 1977 during the Martian late southern summer season. The maps cover the region from 60°S to the south pole at a spatial resolution of 1° of latitude, thus completing the initial thermal mapping of the entire planet. The analysis and interpretation of these maps is aided by the results of a one‐dimensional radiative convective model, which is used to calculate diurnal variations in surface and atmospheric temperatures, and brightness temperatures at the top of the atmosphere for a range of assumptions concerning dust optical properties and dust optical depths. The maps show that apparent thermal inertias of bare ground regions decrease systematically from 60°S to the south pole. In unfrosted regions close to the south pole, apparent thermal inertias are among the lowest observed anywhere on the planet. On the south residual cap, apparent thermal inertias are very high due to the presence of CO2 frost. In most other regions of Mars, best fit apparent albedos based on thermal emission measurements are generally in good agreement with actual surface albedos based on broadband solar reflectance measurements. However, in the frost‐free region poleward of 75°S, best fit apparent albedos are significantly higher than measured Lambert albedos, implying that measured brightness temperatures in this area are up to 15 K colder than would be expected for surfaces with the same measured albedos during this season. The one‐dimensional atmospheric model calculations also predict anomalously cold brightness temperatures close to the pole during late summer, and after considering a number of alternatives, it is concluded that the net surface cooling due to atmospheric dust is the best explanation for this phenomenon. The observed systematic decrease in apparent thermal inertia from 60°S to the south pole during this season is also consistent with the predictions of atmospheric model calculations. The region of lowest apparent thermal inertia close to the pole, which includes the south polar layered deposits, is interpreted to be mantled by a continuous layer of aeolian material that must be at least a few millimeters thick. The low thermal inertias mapped in the south polar region imply an absence of surface water ice deposits, which is consistent with Viking Mars atmospheric water detector (MAWD) measurements which show low atmospheric water vapor abundances throughout the summer season. However, these observations do not necessarily imply a complete absence of water. Thermal model calculations that can reproduce observed diurnal temperature variations in the south polar region show that annual maximum subsurface temperatures at depths ranging from 4 to 20 cm are cold enough to permit the stability of ground ice deposits, even if they are in excellent diffusive contact with the atmosphere. Therefore, the presence of near‐surface ground ice in the south polar region is not inconsistent with the Viking IRTM and MAWD observations.