Thermal and albedo mapping of the polar regions of Mars using Viking thermal mapper observations: 1. North polar region

Abstract

We present the first maps of the apparent thermal inertia and albedo of the north 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 50‐day period in 1978 during the Martian early northern summer season. The maps cover the region from 60°N to the north pole at a spatial resolution of 1/2° of latitude. 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 wide range of assumptions concerning aerosol optical properties and aerosol optical depths. The results of these calculations show that the effects of the Martian atmosphere on remote determinations of surface thermal inertia are more significant than have been indicated in previous studies. The maps of apparent thermal inertia and albedo show a great deal of spatial structure that is well correlated with surface features. The north residual polar cap has a very high apparent thermal inertia, and is interpreted to contain dense, coarse‐grained, or solid water ice that extends from within 2 mm of the surface to depths of at least l m below the surface. Detached, bright water ice deposits surrounding the residual cap also have very high apparent thermal inertias, and are interpreted to have similar properties. Polar layered deposits surrounding the north residual cap also have high apparent thermal inertias, and are also interpreted to contain near‐surface water ice. Mariner 9 images of the north residual cap obtained in 1972 show much less bright water frost coverage than Viking images obtained three Mars years later in 1978, and it is suggested that layered deposits may be actively forming in these areas over interannual timescales. Dark transverse dune deposits adjacent to the north residual cap have relatively low apparent thermal inertias, as do arcuate scarp regions within the polar layered deposits that appear to be major sources of polar dune material. The apparent thermal inertias of these north polar dune deposits are significantly lower than those of intracrater dune deposits at lower latitudes. The north polar dunes are interpreted to be composed of dark unconsolidated material that is being eroded from the layered deposits and transported away from the pole by saltation. The region poleward of 60°N contains no large low thermal inertia regions, which is interpreted as evidence that atmospheric dust is not accumulating in the north polar region under present climatic conditions.