InSight (Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport) is not your typical Mars mission. Whereas others, such as the recently landed Perseverance, were sent to scientifically rich destinations that may have once supported life, InSight’s landing zone in Elysium Planitia was decidedly mundane, described by some as a “parking lot.” Flat and smooth—nearly featureless save for scattered rocks and impact craters—the site was the perfect place for the stationary lander to study the Martian interior. The Seismic Experiment for Interior Structure (SEIS) instrument, provided by France’s space agency and place gently on the surface by InSight’s robotic arm in December 2018, was encased in a domed shield, allowing it to detect waves moving through Mars without interference from wind or dust storms. storms. SEIS “can see motions on the order of atomic-sized vibrations,” says Andrew Lazarewicz of the Massachusetts Institute of Technology, who took part in a 1976 attempt to detect seismic waves with a seismometer on NASA’s Viking 2 lander.
In a series of papers published today in the journal Science, researchers describe how they used this instrument to trace seismic waves caused by dozens of detected marsquakes through the Martian interior. These events were possibly caused by meteorites hitting the planet’s surface or even by the stirrings of magma (some were localized to nearby Cerberus Fossae, a geologic formation displaying signs of recent volcanic activity). At less than magnitude 4 on the moment magnitude scale, all of these quakes were so small that they would be barely noticeable on Earth. But SEIS registered them clearly, allowing researchers to track their reverberations through the interior of Mars, all the way down to its core, revealing what was going on inside.
Simon Stähler of the Institute of Geophysics at the Swiss Federal Institute of Technology Zurich and his colleagues measured the waves’ reflections off the core to calculate its size and bulk composition. They found that it is likely 1,830 kilometers in radius, several hundred kilometers larger than predicted. And the strength of the reflected waves suggested they were bouncing off a core mostly composed of molten iron and nickel. The size of the core was a “surprise,” Stähler says. “People were assuming it must be on the order of 1,500 or 1,600 kilometers,” based on the fact that, kilogram for kilogram, Mars is a bit less dense than Earth, and the core would be expected to be mostly iron and nickel, which is heavier than rock. Instead the results show that the ratio of Mars’s core radius to its planetary radius is similar to that of Earth—which counterintuitively means the relatively low-density Martian core must be enriched with other elements, such as sulfur and oxygen, that are comparatively less abundant in our planet’s core. Why Mars’s core would have a different composition than ours is unclear. “If you assume that Mars was made from the same building blocks as Earth, then it is not so easy to explain,” Stähler says.
Moving outward, Amir Khan of the Institute of Geophysics and his colleagues used the seismic waves to probe Mars’s mantle, the region between the planet’s core and surface crust. Although Earth has an insulating liquid lower mantle layer that sits above its core, there is no such feature on our neighboring world. “That lower mantle does not exist on Mars,” Khan says. Instead, abo
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