WHAT CAN BE DEDUCED ABOUT THE CAPPING UNIT OF MARTIAN ‘INVERTED CHANNELS’ FROM THEIR THERMOPHYSICAL PROPERTIES?

In denuded landscapes, inverted fluvial landforms are preferentially preserved due to their greater resistance to erosion from clast armoring, lava infilling or cementation. These induration agents have differing material properties that should influence the thermal response of the surface. On Mars, the bulk thermal conductivity is the parameter that dominates the thermal inertia. Because there is an order of magnitude difference in thermal conductivity values for unconsolidated clasts (whether dust, sand or gravel-sized) relative to rocks, the corresponding thermal inertia for these materials could differentiate induration mechanisms for martian fluvial sinuous ridges (FSR). We test the hypothesis that thermophysical properties of the FSR are due to their induration agent. This hypothesis predicts lower thermal inertia associated with gravel-armored surfaces, and higher thermal inertia corresponding to well-cemented channel deposits or lava capped FSRs.

Using thermal inertia derived from 2001 Mars Odyssey Thermal Emission Imaging System (THEMIS) data, we assessed the relative thermal inertia difference of FSR compared to the surrounding terrain. The thermophysical contrast for twenty large ‘inverted channels’ on Mars are often similar to well-indurated plains units. Where friable units are present, the FSR exhibit an elevated thermophysical contrast. We find few instances of depressed thermophysical contrast, as would be predicted for gravel-armored surfaces. These cases are explained by mantling overburden or aeolian ripples sourced from weathering of FSR materials.

The thermal signature, together with regional geology and morphology attributes, leads us to disfavor clast armoring as an induration agent for most martian FSRs. The available evidence is consistent with cementation as the most likely induration agent for FSRs, although lava-infilling is a plausible explanation especially for instances of nearby volcanic edifices. FSR cementation, probably by near-surface, solute-rich groundwater, extends aqueous processes beyond the period of widespread fluvial activity. Future work on the temporal response of the surfaces of inverted channels has the potential to further test the inferred induration mechanism and potentially constrain the grain size distribution.