Geophysical Observations of Phobos Transits by InSight

Abstract

Since landing on Mars, the NASA InSight lander has witnessed eight Phobos and one Deimos transits. All transits could be observed by a drop in the solar array current and the surface temperature, but more surprisingly, for several ones, a clear signature was recorded with the seismic sensors and the magnetometer. We present a preliminary interpretation of the seismometer data as temperature-induced local deformation of the ground, supported by terrestrial analog experiments and finite-element modeling. The magnetic signature is most likely induced by changing currents from the solar arrays. While the observations are not fully understood yet, the recording of transit-related phenomena with high sampling rate will allow more precise measurements of the transit times, thus providing additional constraints for the orbital parameters of Phobos. The response of the seismometer can potentially also be used to constrain the thermoelastic properties of the shallow regolith at the landing site. Plain Language Summary The geophysical lander, InSight, has been operating on the surface of Mars since November 2018. Since then, the Martian moons Phobos and Deimos have been partially blocking the Sun, as seen from the InSight landing site, multiple times. Multiple InSight instruments have been measuring the effect of those transits; this surprisingly includes the seismometer and the magnetometer. We conclude that temperature-induced deformation and tilt are responsible for the seismic measurements. The change observed in the magnetometer measurements are most likely the result of a drop in the solar array currents. We do not observe atmospheric modulations with InSight's weather station during the transit. These observations help constrain orbital parameters of the Martian moons, and the seismometer signal might allow investigating thermoelastic properties of the shallow Martian material.

Near Surface Properties of Martian Regolith Derived From InSight HP3‐RAD Temperature Observations During Phobos Transits

“…All rights reserved. The bad fit of these transits occurring late in the afternoon is likely a result of scattering and refraction in the atmosphere, as discussed in Stähler et al (2020). This appears to reduce the effect of the eclipses although at solar elevations greater or equal to that of sol 499 (34 • ) the deviation is within the uncertainty from the position of Phobos.…”

“…instruments (Stähler et al, 2020) including the SEIS Very Broad Band seismometer and the infrared radiometer (RAD) of the HP 3 instrument (Spohn et al, 2018;Mueller et al, 2020). RAD is mounted under the lander deck and has two unobstructed 20 • fields of view of two spots in 1.5 and 3.5 m distance NNW from the lander center.…”

“…The THEMIS instrument on the Mars Odyssey orbiter observed the Phobos shadow but could not resolve the temperature response (Piqueux & Christensen, 2012). At the InSight landing site, the effect of several Phobos transits was observed with different geophysical instruments (Stähler et al, 2020) including the SEIS Very Broad Band seismometer and the infrared radiometer (RAD) of the 3 HP instrument (Mueller et al, 2020;Spohn et al, 2018). RAD is mounted under the lander deck and has two unobstructed 20 fields of view of two spots in 1.5 and 3.5 m distance North-North-West from the lander center.…”

Soil Thermophysical Properties Near the InSight Lander Derived From 50 Sols of Radiometer Measurements

“…While excellent fits are obtained within error bars for homogeneous soil configurations (Figure 5), material layering could still be present: First, the lower albedo zone around the lander formed at the time of touchdown indicates that an optically thick layer (but thermally thin at the diurnal cycle time scale) of fine dust was originally present and has been at least partially removed . Surface cooling during Phobos transits Mueller et al, 2021;Stahler et al, 2020) is also consistent with an up to a 4 mm thick top layer characterized by a relatively low conductivity and thermal inertia, for example, ∼75 J m −2 K −1 s −1/2 on a higher thermal inertia soil with ∼200 J m −2 K −1 s −1/2 Mueller et al, 2021). Finally, images of the HP 3 mole support structure feet after the first set of hammering on sol 92 is most consistent with a ∼1 cm (or more) thick layer of loose material resting on stronger, more cohesive material estimated to reach at least 10 cm in thickness (Figure 4) Hudson et al, 2020).…”

Space Weather Observations With InSight

“…In addition to the crustal magnetic field, the IFG has also measured external time-varying fields at the surface for the first time. Some of these signals are periodic, such as diurnal variations or waves with periods of 100-1,000 s, so-called ultra-low-frequency waves (Johnson et al, 2020;Mittelholz et al, 2020), but others are of a transient nature (Johnson et al, 2020;Stähler et al, 2020). External fields are of interest because they can elucidate the interaction of the Interplanetary Magnetic Field (IMF) with the ionosphere and the resulting surface signals.…”