Phobos Eclipse Observations with the HP3 Radiometer on Insight

Abstract

The Heat Flow and Physical Properties Package (HP³) includes an infrared Radiometer attached to the deck of the InSight lander. The main objective of this part of the instrument is to constrain the surface thermal boundary condition for the heat flow derivation by the instrumented tether deployed into the subsurface. The heat flow in the subsurface can be affected by seasonal and diurnal temperature variations, by radiation from the lander, its shadow, and the change in surface albedo caused by dust removal during landing and later deposition. The radiometer will determine daily average temperature as function of season and record diurnal temperature curves and shorter period temperature fluctuations, with a sampling rate of up to one sample every 2 s. On sol 96, 97 and 99 the radiometer observed the effects of Phobos eclipses, which corresponded to a ~30 s long reduction of solar flux between 3% and 13%, as observed by the camera and solar panel output. The diurnal temperature response provides an estimate of the thermal inertia, which is diagnostic of soil parameters such as grain size and cementation. We use output from the LMD1D model as boundary conditions to solve the heat conduction equation in the near surface and to calculate model curves of surface temperature. The model accounts for the Mars orbit and provides downwelling solar and infrared fluxes. Atmospheric dust opacity is derived from camera images of the sky. We vary the input thermal inertia to find the diurnal and eclipse responses that best match the data. Diurnal temperature changes on sol 97 indicate a thermal inertia of ~200 Jm-2K-1s-1/2, but the eclipse response indicates the presence of some low thermal inertia material (~120 Jm-2K-1s-1/2) at the surface. The temperature after the eclipse appears to return to normal values faster than predicted by either thermal inertia, which might indicate that only a fraction of the footprint is covered with low conductivity material. However, in this case with an even lower thermal inertia. The bulk thermal inertia is consistent with orbiter observations and indicates regolith particle sizes in the range of sand, and little cementation.