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

Measurements from the InSight lander radiometer acquired shortly after landing are used to characterize 41 the thermophysical properties of the martian soil in Homestead hollow. This dataset is unique as it stems 42 from a high measurement cadence fixed platform studying a simple well-characterized surface, and it 43 benefits from the environmental characterization provided by other instruments. We focus on observations 44 acquired before the arrival of a regional dust storm (near Sol 50), on the furthest observed patch of soil 45 (i.e., ~3.5 m away from the edge of the lander deck) where temperatures are least impacted by the 46 radiative/reflective presence of the lander and where the soil has been least disrupted during landing. 47 Diurnal temperature cycles are fit within error bars using a homogenous soil configuration with a thermal 48 inertia of 183 ± 25 J m-2 K-1 s-1/2 and an albedo of 0.16, corresponding to very fine to fine sand with the 49 vast majority of particles smaller than 140 μm. A pre-landing assessment leveraging orbital thermal 50 infrared data is consistent with these results, but our analysis of the full diurnal temperature cycle acquired 51 from the ground further indicates that near surface layers with different thermophysical properties must be 52 thin (i.e., typically within the top few mm) and deep layering with different thermophysical properties must 53 be at least below ~4 cm. The low thermal inertia value indicates limited soil cementation within the upper 54 one or two skin depths (i.e., ~4-8 cm and more), with cement volumes <<1%, which is challenging to 55 reconcile with visible images of overhangs in pits.