On 26 November 2018, the InSight lander—whose acronym stands for the Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport—touched down on Mars’s Elysium Planitia. Within two months of landing on the flat, volcanic plain, the lander’s robotic arm placed the mission’s seismometer on Mars’s surface, as shown here, where it started listening for vibrational signals.

Unlike the eight orbiters currently surveying 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’s moment of inertia and gravity field have provided indirect clues about the internal anatomy- its central metallic core, viscous mantle, and brittle crust. An international collaboration of 65 seismologists and planetary scientists from 12 countries has now published three papers that describe the first direct observations of those distinct layers.

To date, the instrument has picked up more than 1000 seismic events. Of the several hundred marsquakes among the sample, the vast majority were small, and none exceeded a moment magnitude of 4. That low level of seismicity wasn’t unexpected. Unlike Earth, whose sharply defined tectonic plates intersect at boundaries that wind around the planet like the seam of a baseball, Mars has a single, thick plate. The planet’s crust is on the thin side, between 15 km and 47 km, and porous. And just underneath it is Mars’s lithosphere, a thick plate that includes the crust and reaches 400–600 km into the mantle. That’s twice as deep as Earth’s lithosphere.

The collaboration used reflections of seismic waves from the core–mantle boundary to determine the size of Mars’s metal core. They measured a radius of 1830 km, about 100 km larger than previous estimates. That large size implies a relatively low core density, with a greater-than-expected concentration of light elements, such as sulfur, carbon, silicon, and hydrogen, that are sequestered inside. The enrichment lowers the core’s melting temperature, possibly to a point that sustains the core as completely molten liquid. If that’s the case—and the lack of shear waves passing through the core suggests it is—the absence of a solid inner core is likely one of the reasons Mars’s geodynamo turned off billions of years ago and left the planet without a global magnetic field.

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