NASA’s Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight) provided the first direct measurement of the martian crust’s thickness by monitoring marsquakes’ seismic waves. The crusts of terrestrial planets preserve information on how these bodies formed and were changed by later events such as magmatism or impacts. According to the currently preferred model, the crust at the InSight landing site extends to 39 ± 8 kilometers deep.

Mark A. Wieczorek of Université Côte d’Azur, along with collaborators, combined the local constraints from InSight with global gravity and topography data to infer the global character of the crust. Generally, anomalies in the observed planetary gravity field are due to relief at a planet’s surface (i.e., its topography), the crust-mantle interface, and the mantle-core interface. By assuming the internal density structure, the relief at the crust-mantle interface can be calculated (and thus also the crustal thickness) to satisfy both the observed global gravity field and the local crustal thickness measured by InSight.

By considering several reasonable density profiles, Wieczorek and collaborators found that the average crustal thickness of Mars is between 30 and 72 kilometers, with a maximum permissible density of 2850 to 3100 kilograms per cubic meter. This density is lower than that of the basaltic materials observed on the surface and consistent with more felsic materials, which are relatively enriched in silica and could have been derived from basalts by magmatic fractionation. Based on this result, the researchers suggest that the martian crust that formed during the initial differentiation of the planet was covered by subsequent volcanism. The wide range of estimated average crustal thickness is partly due to the uncertainty in InSight’s measurement of crustal thickness. Placing additional seismometers — especially in the southern highlands — to measure the local crustal thickness at multiple points on Mars would help constrain the global estimate. READ MORE