Near Surface Properties of Martian Regolith Derived From InSight HP 3 ‐RAD Temperature Observations During Phobos Transits

N. Mueller (1) , S. Piqueux (2) , M. Lemmon (3) , J. Maki (2) , R. D Lorenz (4) , M. Grott (1) , T. Spohn (1, 5) , S. E Smrekar (2) , J. Knollenberg (1) , T. L Hudson (2) , C. Krause (1) , E. Millour (6) , F. Forget (2) , M. Golombek (2) , A. Hagermann (7) , N. Attree (8) , M. Siegler (9) , W. B Banerdt (2)

1 DLR Institute of Planetary Research
2 JPL - Jet Propulsion Laboratory
3 SSI - Space Science Institute [Boulder]
4 APL - Johns Hopkins University Applied Physics Laboratory [Laurel, MD]
5 ISSI - International Space Science Institute [Bern]
6 LMD - Laboratoire de Météorologie Dynamique (UMR 8539)
7 LUT - Luleå University of Technology = Luleå Tekniska Universitet
8 University of Stirling
9 Huffington Department of Earth Sciences [SMU Dallas]

Abstract We use the Martian surface temperature response to Phobos transits observed next to the InSight lander in Elysium Planitia to constrain the thermal properties of the uppermost layer of regolith. Modeled transit lightcurves validated by solar panel current measurements are used to modify the boundary conditions of a 1D heat conduction model. We test several model parameter sets, varying the thickness and thermal conductivity of the top layer to explore the range of parameters that match the observed temperature response within its uncertainty both during the eclipse as well as the full diurnal cycle. The measurements indicate a thermal inertia (TI) of urn:x-wiley:00948276:media:grl62794:grl62794-math-0002 in the uppermost layer of 0.2–4 mm, significantly smaller than the TI of urn:x-wiley:00948276:media:grl62794:grl62794-math-0003 derived from the diurnal temperature curve. This could be explained by larger particles, higher density, or some or slightly higher amount of cementation in the lower layers. Plain Language Summary The Mars moon Phobos passed in front of the Sun from the perspective of the InSight lander on several occasions. The Mars surface temperatures measured by the lander became slightly colder during these transits due to the lower amount of sunlight the surface received at this time. The transits only last 20–35 s and therefore only the very top layer, about 0.3–0.8 mm, of the ground has time to cool significantly. The top layer cools and heats up faster than we expected based on the temperature changes of the day-night cycle, which affects about 4 cm of the ground. Based on this observation we conclude that the material in the top mm of the ground is different from that below. A possible explanation would be an increase of density with depth, a larger fraction of smaller particles such as dust at the top, or a layer where particles are slightly cemented together beginning at 0.2–4 mm below the surface.

Mots clés: phobos transit; thermal inertia; thermal conductivity

Domaines: Sciences de la Terre

N. Mueller, S. Piqueux, M. Lemmon, J. Maki, R. D Lorenz, et al.. Near Surface Properties of Martian Regolith Derived From InSight HP 3 ‐RAD Temperature Observations During Phobos Transits. Geophysical Research Letters, 2021, 48 (15), ⟨10.1029/2021GL093542⟩. ⟨hal-03354056⟩