SEISMOLOGIST Daoyuan Sun and his colleagues at the University of Science and Technology of China found an anomaly when reexamining data from NASA's InSight lander.

Seismic waves that should have passed cleanly through a molten Martian core were arriving 50 to 200 seconds late. This delay, the team reported in a new study in Nature, strongly suggests the waves were slowed by a dense, solid inner core at the planet's center. After digging back through the data, the group also identified a faint wave that appeared to have reflected directly off the inner core's surface. "The inner core is real, and we are confident in our results," Sun says.

The finding, however, immediately clashes with other recent analyses and raises fundamental questions about Mars's history. Two years ago, studies suggested a layer of molten mantle rock rests just above the core, heating it like a thermal blanket. That intense heat would make it extremely difficult for a solid core to cool down and crystallize, says Henri Samuel, a planetary dynamicist at the Paris Institute of Earth Physics, who finds the new result puzzling. The claim also complicates the story of Mars's dead magnetic field; on Earth, the crystallization of the inner core is a primary engine driving the magnetic dynamo, leaving scientists to wonder why the same process would not be happening on Mars.

MARS IS EARTH-LIKE NASA's InSight Mars lander acquired this image of the area in front of the lander using its lander-mounted, Instrument Context Camera (ICC). (Photo from NASA/JPL-Caltech)

This startling discovery about the planet's core is just one of two radical new portraits of Mars to emerge from the InSight data, years after the lander's mission officially ended. Taken together, the findings suggest Mars is a planet of contradictions, with an interior that is simultaneously more and less like Earth's than researchers expected. But both conclusions are drawn from the faint vibrations recorded by a single, wind-buffeted seismic station, leading to significant scientific debate. "A lot of people were very comfortable with the nonexistence of the inner core," says Simon Stähler, a seismologist at ETH Zürich.

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The second result, published in Science, probes the Martian mantle -- the vast layer of rock between the core and crust. Researchers led by Constantinos Charalambous of Imperial College London found that high-frequency seismic waves were progressively delayed as they traveled deeper. They concluded the waves were hitting "seismic obstacles," which they believe are the crystallized, hardened remnants of ancient magma oceans from when asteroids pummeled an infant Mars.

This suggests that unlike Earth's hot, churning mantle, the Martian mantle is cool, sluggish, and has not mixed away its ancient history. This might explain the weak volcanism on Mars and confirms its status as a planetary "time capsule."

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While both studies are compelling, Stähler notes that the InSight data are fundamentally noisy and that the claims are not yet settled science. He feels the assertion of a solid inner core is about as certain as those that propose one at the Moon, where the evidence remains weak. While he believes the team has ruled out obvious errors, the conclusion is far from an open-and-shut case.

"I wouldn't bet my house on it," he says. "But I would bet $200. -- reports from Space.com