Analogue simulation of the temperature response of Mars surfaces to Phobos transits, and vice-versa

The objective of this series of experiments was to support the data interpretation of short period changes in insolation of planetary bodies such as solar eclipses or transits. The temperature response of Martian regolith to the transit of Phobos has been observed by InSight and interpreted in terms of layering in the near surface [Mueller et al. 2021]. Current and future mission may observe similar transits if a suitable thermal infrared instrument is available. An opportunity to observe a similar event with the roles of Mars and Phobos switched may be the rover on the Martian Moons eXplorer (MMX) carrying the Mini-RAD radiometer [Ulamec et al. 2023]. The original mission plan did not foresee a landing on the Mars facing side, but in case the plan is revised due to the changing launch date there might be a chance. There are however open questions about the interpretation of such observations using 1D models of heat conduction [e.g. Mueller et al. 2021]. These models assume that the subsurface is a continuum, while in reality the material consists of particles that are not necessarily small compared to the characteristic depth scale (skin depth) of the material responding to the changes in insolation. The Planetary Ices Laboratory has the necessary equiment to recreate such events on Earth in form of a thermal vacuum chamber and a solar simulator (Fig. 1). The chamber walls can be cooled with liquid nitrogen and a window with mirror on the chamber ceiling allows illumination of an approximately 15 cm diameter spot by the solar simulator. For our test set-up we have placed a tray of about 38x18x2cm filled with regolith analogue material in the center of the chamber (Fig. 2) i.e. encompassing the illuminated spot. The radiometer is set up within the chamber on a rotary stage that allows to move the Field of View of the instrument along the long axis of the tray. The FoV is small enough to fall within the illuminated spot and the rotary stage allows adjustments within the running setup. A small TIR camera also observed the tray. This instrument has lower temperature resolution but provides data on temperature inhomogeneity of the sample. The general approach of simulating the eclipses was to first evacuate the chamber, either refill with 5 mbar CO2 or to leave at