The spatial distribution of rocks on mars

Abstract

The spatial distribution of rocks exposed at the surface of Mars has been mapped using Viking Infrared Thermal Mapper (IRTM) observations. Overall, there are no regions on the surface of Mars at a scale of 1° × 1° in latitude and longitude that are rock free. The modal value of rock abundance is 6% areal coverage, with no areas having more than 30–35% rock cover. The model developed to determine rock abundance relates the thermal emission in each of the four surface-sensing IRTM bands to temperature contrasts within the field of view, non-unit thermal emissivity due to absorption bands in the surface materials, and the absorption and scattering of the outgoing energy by atmospheric dust and water ice. Each of these effects produce characteristic spectral and diurnal signatures, allowing them to be separated. The temperature contrasts provide a means to determine both the abundance of exposed rocks and the fine-component thermal intertia. Rock abundance alone does not produce the observed variation in bulk thermal inertia of the surface. Low-inertia (fine), bright surfaces have fewer rocks exposed than do high-inertia (coarse), dark surfaces. Rock abundance does not correlate well with the RMS slope nor reflectivity determined from delay-Doppler radar measurements. There is, however, a possible correlation between RMS slope determined from continuous-wave radar observations. Dual-polarization radar measurements, which provide the best radar measure of small-scale roughness, indicate that the Tharsis volcanic region is very rough, whereas thermal measurements indicate very few rocks and a covering of dust. Together these observations suggest an approximately 1-m-thick mantle of fines covering a very rough subsurface. This fine material may be an aeolian deposit of dust storm fallout. Valles Marineris, together with several major channels and chaotic terrains, have high rock abundances, as does the Acidalia Planitia basin. This observed distribution may reflect the initial deposition of coarse material associated with channel formation. Syrtis Major is unique in having a high inertia but a very low rock abundance, together with low radar roughness. These observations are consistent with mantling of rocks by sand. The observed distribution of rocks and fines indicates that aeolian processes, both erosional and depositional, play a dominant role in shaping the present Martian surface.