Soil Thermophysical Properties near the InSight Lander Derived from 50 Sols of 1 Radiometer Measurements

A subset of the measurements from the InSight lander radiometer acquired shortly after landing is used to characterize the thermophysical properties of the martian soil in Homestead hollow. This dataset is unique as it stems from a high measurement cadence fixed platform studying a simple well-characterized surface, and it benefits from the environmental characterization provided by other instruments. We focus on 8-12 μm observations acquired before the arrival of a regional dust storm (near Sol 50), on the furthest observed patch of soil (i.e., ~4.5 m away) where temperatures are least impacted by the radiative/reflective presence of the lander and where the soil has been least disrupted during landing. The diurnal temperature cycles are adequately fit within error bars using a homogenous regolith configuration with a thermal inertia of 183 ± 25 J m-2 K-1 s-1/2 and a thermometric albedo of 0.17, corresponding to fine sand with the vast majority of particles smaller than 140 µm. A pre-landing assessment leveraging orbital thermal infrared data is consistent with these results, but our analysis of the full diurnal temperature cycle acquired from the ground further indicates that any surficial layering must be thin (i.e., typically within the top few mm) and deep layering must be at least below ~4 cm. The low thermal inertia value indicates limited regolith cementation within the upper one or two skin depth (i.e., ~4-8 cm and more), with cement volumes <<1%, which is challenging to reconcile with visible imagery of two vertical pits.