In late 2018 NASA’s InSight—a lander possessing the agency’s only seismometer on Mars—began listening for vibrational signals. The seismometer shown in the photo picked up about 1300 distinct seismic events during its four years on the job. The majority were small, and none exceeded a moment magnitude of 5. But many were still large enough for researchers to work out the planetary structure.
Sound waves travel through the planet at different speeds, reflecting and refracting from the boundaries of its various layers. And those speeds vary with the layers’ stiffness, density, and temperature. By resolving the difference in the waves’ arrival times at a seismometer and their polarizations, the researchers could resolve where each seismic event took place—even using a single seismometer. Equipped with those locations and the details of when and where the sounds bounce around inside the planet, the scientists resolved the depth of the shallow crust and the boundaries where the mantle emerged from it. They even detected the fainter waves that reflect from the deeper core–mantle boundary. (See Physics Today, October 2021, page 17.)
The mission formally ended in December. But as Jessica Irving (University of Bristol in the UK), Vedran Lekić (University of Maryland), and an international collaboration of 32 other researchers continued to analyze the collected data, they realized that InSight had also picked up sound waves from two far more distant events—a meteorite impact and a marsquake—from the side of the planet opposite the seismometer. Seismic waves dissipate a lot of energy as they travel through the planet. And discerning the location of the very distant marsquake required teasing out its signals in the complex seismograms. Fortunately, with orbital imaging of the impact, the team members were able to pin down its location from space. By comparing the times required for the waves to traverse the core and reach InSight with others that traversed only the mantle, they estimated the core’s density and its resistance to compression.
Models that were based on those physical properties revealed a core slightly denser and a few tens of kilometers smaller than had been estimated just two years earlier. (Irving, Lekić, and their colleagues calculate a new, more accurate radius as 1780–1810 km.) Those results are consistent with a completely liquid iron-alloy core. And because the researchers’ measurements provided constraints on the velocities of seismic waves traversing the core, they were able to develop the first seismically based estimate of the core’s composition. It’s rich in sulfur, with smaller fractions of oxygen, carbon, and hydrogen. Both features distinguish Mars from Earth, whose interior is composed of a solid inner core and a liquid outer core. Mars’s core is therefore much more compressible than Earth’s. And that key difference may be one reason that the two planets have formed and evolved so differently. (J. C. E. Irving et al., Proc. Natl. Acad. Sci. USA 120, e2217090120, 2023.)
