Mars Has a Surprise Layer of Molten Rock Inside

Alexandra Witze reports via Nature: A meteorite that slammed into Mars in September 2021 has rewritten what scientists know about the planet's interior. By analysing the seismic energy that vibrated through the planet after the impact, researchers have discovered a layer of molten rock that envelops Mars's liquid-metal core. The finding, reported today in two papers in Nature, means that the Martian core is smaller than previously thought. It also resolves some lingering questions about how the red planet formed and evolved over billions of years. The discovery comes from NASA's InSight mission, which landed a craft with a seismometer on Mars's surface. Between 2018 and 2022, that instrument detected hundreds of "marsquakes' shaking the planet. In July 2021, on the basis of the mission's observations of 11 quakes, researchers reported that the liquid core of Mars seemed to have a radius of around 1,830 kilometers3. That was bigger than many scientists were expecting. And it suggested that the core contained surprisingly high amounts of light chemical elements, such as sulfur, mixed with iron. But the September 2021 meteorite impact "unlocked everything," says Henri Samuel, a geophysicist at the Institute of Earth Physics of Paris and lead author of one of today's papers1. The meteorite struck the planet on the side opposite to where InSight was located. That's much more distant than the marsquakes that InSight had previously studied, and allowed the probe to detect seismic energy traveling all the way through the Martian core4. "We were so excited," says Jessica Irving, a seismologist at the University of Bristol, UK, and a co-author of Samuel's paper. For Samuel, it was an opportunity to test his idea that a molten layer of rock surrounds Mars's core5. The way the seismic energy traversed the planet showed that what scientists had thought was the boundary between the liquid core and the solid mantle, 1,830 kilometers from the planet's centre, was actually a different boundary between liquid and solid. It was the top of the newfound layer of molten rock meeting the mantle (see 'Rethinking the Martian core'). The actual core is buried beneath that molten-rock layer and has a radius of only 1,650 kilometers, Samuel says. The revised core size solves some puzzles. It means that the Martian core doesn't have to contain high amounts of light elements -- a better match to laboratory and theoretical estimates. A second liquid layer inside the planet also meshes better with other evidence, such as how Mars responds to being deformed by the gravitational tug of its moon Phobos. The second paper in Nature today2, from a team independent of Samuel's, agrees that Mars's core is enveloped by a layer of molten rock, but estimates that the core has a radius of 1,675 kilometers. The work analyzed seismic waves from the same distant meteorite impact, as well as simulations of the properties of mixtures of molten elements such as iron, nickel and sulfur at the high pressures and temperatures in the Martian core. Having molten rock right up against molten iron "appears to be unique," says lead author Amir Khan, a geophysicist at ETH Zurich. "You have this peculiarity of liquid-liquid layering, which is something that doesn't exist on the Earth." The molten-rock layer might be left over from a magma ocean that once covered Mars. As it cooled and solidified into rock, the magma would have left behind a deep layer of radioactive elements that still release heat and keep rock molten at the base of the mantle, Samuel says.

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