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 similar to 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 similar to 100 mu m at the farthest area observed on the images. We find that the deposit of grains coarser than 500 mu m is controlled mainly by gravity. Grains finer than 100 mu m are present in the regolith, but they are not quantifiable with this method because they are blown away by the wind.