Remote characterization of physical surface characteristics of Mars using diurnal variations in apparent thermal inertia

Abstract

Analysis of the Martian surface today can provide insight into the processes that may have affected it over its history. Information about the physical surface characterization of a region can help determine the degree of sorting it has experienced and/or its geologic maturity. Sub-resolved, horizontal “checkerboard” mixtures of materials with different thermal inertias can lead to differences in the apparent thermal inertia values inferred from night and day radiance observations. Standard methods for deriving thermal inertia from orbit via the THermal EMission Imaging System (THEMIS) can give values for the same location that vary by as much as 20% between images. Such methods assume that each pixel contains material of a single, uniform thermal inertia. Here, it is proposed that if a mixture of low and high thermal inertias is present within a pixel, the apparent thermal inertia inferred will be biased toward the thermal inertia of whichever component of the surface is warmer at the time of the measurement. This effect will result in a variation in apparent thermal inertia values inferred from measurements taken at different times of day and night. Therefore, a correlation is hypothesized between the magnitude of diurnal variations in apparent thermal inertia values and the degree of non-uniformity of thermal inertias present within a given pixel location.This work shows that the magnitude of such diurnal variation in apparent thermal inertias is sufficient to detect geologically useful differences in surfaces mixtures both theoretically and through in-situ analysis. Mapping the difference in apparent thermal inertias from day and night THEMIS observations may prove to be a new way of distinguishing surfaces that have relatively uniform thermal inertias from those that have horizontally-mixed thermal inertias. Note that this study makes no attempt to characterize the effects of vertical (subsurface) inhomogeneity on apparent thermal inertia, which could also be important for some locations.