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 SummaryThe 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

“…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 () the deviation is within the uncertainty from the position of Phobos.…”

“…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 instrument (Mueller et al., 2020; Spohn et al., 2018). RAD is mounted under the lander deck and has two unobstructed fields of view of two spots in 1.5 and 3.5 m distance North‐North‐West from the lander center.…”

Seismology in the Solar System

“…This may mean that seismic events need to be identified autonomously by the lander itself. Given the phenomenology of non-seismic events, even in terrestrial planets (Ceylan et al 2021;Dahmen, Zenhäusern, et al 2021;Stähler, Widmer-Schnidrig, et al 2020), let alone on icy ocean worlds, this process is non-trivial and will require significant theoretical work over the next years. Given the surprises we saw with both moonquakes and marsquakes, it is unlikely that a fully autonomous processing can find all kinds of events without prior knowledge of typical examples.…”

Investigation of magnetic field signals during vortex-induced pressure drops at InSight

“…A few pressure drop events have been found to exhibit magnetic signals; however the origin of these was unclear (Charalambous et al, 2021;. A passing vortex, if a DD, could be observed in IFG data via one or more of the following possible mechanisms: (1) Triboelectrically charged lofted dust, as suggested for terrestrial DD magnetic emission (Farrell et al, 2006;Kurgansky, Baez, & Ovalle, 2007), (2) Changing solar array currents as a result of entrained dust of a passing DD obscuring the solar arrays (as observed at InSight during transits of Phobos (Stähler et al, 2020), (3) Ground tilting resulting from the pressure lows at the center of convective vortices (Lognonné et al, 2020 that changes the orientation of the IFG in the ambient crustal magnetic field, and (4) similarly, wind-induced tilt of the lander as explored previously for the seismometer on the lander deck (Murdoch et al, 2017, Panning et al, 2020. The goal is therefore to establish, if, when, and why pressure drops at the InSight lander have associated magnetic signals and in particular to evaluate whether any such signals can be attributed to DDs.…”