A seismometer maps the anatomy of Mars

Unlike the eight orbiters that currently monitor the planet and the six rovers that have explored its chemistry and geology, InSight is the only spacecraft to directly probe the planet’s interior. Orbital measurements of Mars’ moment of inertia and gravity field have provided indirect clues about internal anatomy: its central metallic core, slimy mantle, and brittle crust. An international collaboration of 65 seismologists and planetologists from 12 countries has now published three papers that describe the first direct observations of these distinct layers. To date, the instrument has captured more than 1000 seismic events. Of the few hundred earthquakes in the sample, the vast majority were small and none exceeded a moment magnitude of 4. This low level of seismicity was not unexpected. Unlike Earth, whose well-defined tectonic plates intersect at the boundaries that wrap around the planet like the seam of a baseball, Mars has a single thick plate. The planet’s crust is thin, between 15 km and 47 km, and porous. And just below is the Mars lithosphere, a thick plate that includes the crust and reaches 400-600 km in the mantle. It is twice as deep as the Earth’s lithosphere.

The collaboration used seismic wave reflections from the core-mantle boundary to determine the size of the metallic core of Mars. They measured a radius of 1,830 km, about 100 km longer than previous estimates. This large size implies a relatively low core density, with a higher than expected concentration of light elements, such as sulfur, carbon, silicon and hydrogen, which are sequestered inside. Enrichment lowers the melting temperature of the nucleus, perhaps to a point that keeps the nucleus as a completely molten liquid. If this is the case – and the absence of shear waves passing through the nucleus suggests it – the absence of a solid inner core is probably one of the reasons why the geodynamo of Mars went extinct ago. billions of years and left the planet without a global magnetic field.

The large size of the core also influences the convection of heat from the mantle. The mantle of Mars is mineralogically similar to Earth’s upper mantle, but it never reaches the high pressures necessary to produce a stable phase transition from ringwoodite – a high pressure phase of olivine – to bridgmanite, the most abundant mineral on Earth. It is believed that the absence of this mineral on Mars allows its core to cool quickly. (A. Khan et al., Science 373, 434, 2021; B. Knapmeyer-Endrun et al., Science 373, 438, 2021; SC Stähler et al., Science 373, 443, 2021.)

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