Daily temperature variations on Mars

Abstract

The Viking infrared thermal mapper (IRTM) has measured the infrared brightness temperature of Mars for more than a full Martian year. The daily variation of the 20‐μm brightness temperature can be predicted to within ±5 K for most of the Martian surface during any season if the radiometric albedo and the thermal inertia of the model are allowed to increase with increasing atmospheric dust opacity. However, for approximately 32% of the Martian surface area, no values of thermal inertia or albedo can fit the thermal observations. These temperature anomalies isolate one large region on Mars which connects the low thermal inertia regions of Arabia, Tharsis, and Elysium. The anomalies do not correlate with elevation, geologic units, morphology, or atmospheric dust content. All regions with Lambert albedo less than 0.18 can be well fit with the standard thermal model, but all areas with albedo greater than 0.28 are anomalous. There is a strong inverse correlation between the magnitude of the anomaly and the thermal inertia. This correlation indicates that some surface property is responsible for the anomaly. In the anomalous region the observed temperatures are warmer in the morning, cooler late in the afternoon, and decrease more slowly during the night than the Viking model temperatures. Of all the physical processes likely to occur on Mars, but not included in the Viking thermal model, only a layered soil can explain the observations. A low thermal inertia layer covering a high thermal inertia material will heat up quickly in the morning, and cool down quickly in the afternoon. At night the high thermal inertia material will moderate the temperature. A possible interpretation for the layering deduced from the IRTM observations is a layer of aeolian deposited dust about one thermal skin depth thick (1 to 4 cm), covering a duricrust. Because dust accumulates in some areas after every global dust storm, the duricrust must be actively forming. A one thermal skin depth dust layer covering a duricrust is compatible with Viking lander observations, IRTM spectral observations, and Viking orbiter imaging.