Using Wind Dispersion Effects During the InSight Tether Burial Activities to Better Constrain the Regolith Grain Size Distribution.

In an attempt to improve the quality of the seismic signals provided by the seismometer of the InSight mission (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) on Mars, part of the tether linking the seismometer to the InSight lander was buried by some regolith using the scoop of the articulated robotic arm. The regolith in a source area was scraped into piles, scooped and dumped by the scoop from a height of ∼50 cm above the surface onto the tether. Part of the regolith was carried away by the wind and dispersed 1–2 m downwind, as evidenced by the comparison between images taken from the lander before and after the regolith pouring. Using both ballistic trajectory and wind dispersion effects as a sorter, the grain size range was determined through numerical fluid mechanics simulations. The trajectory of the poured grains is determined by the Martian atmospheric and gravimetry conditions, the initial conditions of scoop pouring and grain lithology. The spatial grain distribution on the ground shows a downwind decrease in grain size from the pouring point, with a size ranging from 1 mm near the dump point to ∼100 μm at the farthest area observed on the images. We find that the deposit of grains coarser than 500 μm is controlled mainly by gravity. Grains finer than 100 μm are present in the regolith, but they are not quantifiable with this method because they are blown away by the wind. Plain Language Summary: The soil on which the lander of the geophysical InSight mission on Mars has landed is partially known, mainly through visual observations by the lander's cameras and calculations based on their thermal properties (derived from both orbiter and lander measurements). Some soil scraped by the scoop of the lander's robotic arm was dumped on the tether connecting the InSight seismometer, so as to improve the quality of the seismic signals. During these dumpings, it was observed that some particles were blown away by Martian winds that were simultaneously recorded by the lander instruments. Given that they are subjected to the combined effect of gravity and wind, larger particles rapidly fall below the scoop, whereas smaller ones are blown away; the smaller the particle, the larger the distance. The trajectory of the poured particles was numerically calculated using fluid mechanics to account for wind effects, and the resulting downwind deposit was compared to that given by photos. It was shown that particles larger than 500 μm fell close to the pile formed below the scoop, whereas smaller ones (>100 μm) fell at distances of 2–4 m. Smaller particles (<100 μm) including dust (<5–10 μm) were blown further away. Key Points: The grain size distribution of the InSight Martian regolith is better constrained by using wind as a sorter during the burial of a tetherBased on image observations and numerical fluid mechanics simulations, the regolith mainly contains grains between 100 μm and 1 mmGrains smaller than 100 μm are not quantifiable because they have been blown away by the wind [ABSTRACT FROM AUTHOR]