Thermal studies of Martian channels and valleys using Termoskan data

Abstract

The Termoskan instrument on board the Phobos '88 spacecraft acquired the highest spatial resolution thermal infrared emission data ever obtained for Mars. Included in the thermal images are 2 km/pixel, midday observations of several major channel and valley systems including significant portions of Shalbatana, Ravi, Al‐Qahira, and Ma'adim Valles, the channel connecting Valles Marineris with Hydraotes Chaos, and channel material in Eos Chasma. Termoskan also observed small portions of the southern beginnings of Simud, Tiu, and Ares Valles and some channel material in Gangis Chasma. Simultaneous broadband visible reflectance data were obtained for all but Ma'adim Vallis. We find that most of the channels and valleys have higher thermal inertias than their surroundings, consistent with previous thermal studies. We show for the first time that the thermal inertia boundaries closely match flat channel floor boundaries. Also, buttes within channels have inertias similar to the plains surrounding the channels, suggesting the buttes are remnants of a contiguous plains surface. Lower bounds on typical channel thermal inertias range from 8.4 to 12.5 (10−3 cal cm−2 s−1/2 K−1) (352 to 523 in SI units of J m−2 s−1/2 K−1). Lower bounds on inertia differences with the surrounding heavily cratered plains range from 1.1 to 3.5 (46 to 147 SI). Atmospheric and geometric effects are not sufficient to cause the observed channel inertia enhancements. We favor nonaeolian explanations of the overall channel inertia enhancements based primarily upon the channel floors' thermal homogeneity and the strong correlation of thermal boundaries with floor boundaries. However, localized, dark regions within some channels are likely aeolian in nature as reported previously. Most channels with increased inertias have fretted morphologies such as flat floors with steep walls. Eastern Ravi and southern Ares Valles, the only major channel sections observed that have obvious catastrophic flood bedforms, do not have enhanced inertias. Therefore, we favor fretting processes over catastrophic flooding for explaining the inertia enhancements. We postulate that the inertia enhancements were caused either by the original fretting process or by a process involving the bonding of fines due to an increased availability of water, either initially or secondarily.