Seismic data reveal Mars’s molten mantle

Planets, unlike rocks, cannot simply be cracked in half for scientists to study what’s inside. On Earth, researchers use seismic waves to gather information about the structure and properties of the crust, mantle, and core. NASA’s InSight mission was designed to do the same on Mars. By studying the waves from marsquakes (see Physics Today, October 2021, page 17 ), researchers would learn about the elastic and density structure of the crust, mantle, and core. Using the Seismic Experiment for Interior Structure (SEIS) instrument, InSight collected measurements from 2019 to 2022. And although the mission has ended, the data still have a story to tell.

Most marsquakes are small, just as most earthquakes are (see Physics Today, 22 November 2021 ). During the majority of the InSight mission, SEIS was recording seismic waves from small-magnitude quakes and surface impacts that were located on the same hemisphere as InSight. From those data, researchers determined the depth of a solid–liquid boundary and interpreted it as the boundary between the solid mantle and the liquid core. The differences in the arrival time of different waves implied a core size of 1830 km.

The researchers’ early results indicated that the liquid core was less dense than expected given its measured volume and the expectation that it be made up of mostly iron and nickel, like the cores of other terrestrial planets. And for that density to be realized, the fraction of light elements that would need to be present in the core didn’t seem possible given the formation history of Mars.

Then, in September of 2021, a fortuitous meteorite struck the other side of the planet. For the first time, SEIS was able to detect seismic waves that had traveled almost straight through the planet, specifically along the bottom of the solid mantle. Independently, research groups from the Institute of Geochemistry and Petrology at ETH Zürich (led by Amir Khan) and an international team (led by Henri Samuel at Paris Cité University) analyzed the travel times of the trans-planetary waves. The refracted waves gave them a more detailed understanding of the different density regions of Mars’s interior. They then re-examined the old InSight data in light of the new data and concluded that there must be an additional layer in Mars’s mantle, just above the core.

The previously undiscovered layer had been tricking the researchers into believing that the core was larger than it really was. Instead, a layer of molten silicate—much lighter than the liquid metal core—resolves the discrepancy between the density and elemental makeup of the previous measurements. A new model of Mars’s interior means redoing analysis of InSight seismic data. Researchers say they hope those findings will help them reach a better understanding of Mars’s magnetic field evolution and crust formation. (H. Samuel et al., Nature 622, 712, 2023 ; A. Khan et al., Nature 622, 718, 2023 .)