A Radioactive Sea of Magma Hides Under the Surface of Mars

In 2021, it seemed as if Mars had a surprisingly big heart. Scientists had been using InSight, a robotic lander to study the planet’s insides. The spacecraft had listened to enough marsquakes to develop a picture of the layer-cake nature of the Martian underworld.

The crust and mantle were not especially strange. The core, however, was too large, and not very dense, for such a small planet.

For some researchers, that core measurement didn’t ring true.

“We missed out on something,” said Amir Khan, a geophysicist at ETH Zurich in Switzerland who has studied InSight’s data. “But what?”

It turns out Mars’s core is small after all, Dr. Khan and other researchers have found.

In two studies published Wednesday in the journal Nature, researchers re-evaluated InSight’s seismic record. Both teams independently concluded that Mars’s core is more like our own world’s heavy metal heart than previously suspected. The initial higher-size estimate was a result of an undetected 90-to-125-mile-deep ocean of molten rock, which made the underlying core seem bigger than it is.

But the deep sea of magma, hidden below Mars’s solid mantle and kept molten by radioactive elements, is exotic. “It does not exist on Earth,” Dr. Khan said, and its presence may require a rethink of the red planet’s chaotic evolution.

Scientists have studied Earth’s geologic layers for more than a century using the illuminating power of quake-made seismic waves. InSight, which landed on Mars in November 2018, was sent to find if the rusty world’s viscera were similar.

But studying Mars with a single seismometer proved difficult. InSight’s instruments detected only a few modest temblors that came mostly from a convulsing region close to the spacecraft, and only a small slice of the Martian pie was seismically imaged. For some time, marsquakes also seemed to bounce off but not plunge through the planet’s innermost sanctum, revealing precious little information about the core.

Researchers worked out that Mars’s core had a radius of about 1,140 miles, suggesting it wasn’t very dense. Terrestrial planet cores should be iron-rich, but the puffy Martian core — eventually confirmed to be fully liquid — seemed 27 percent lighter than one made of pure liquid iron. The implication was that Mars’s core was strangely enriched in lighter elements like sulfur, carbon, oxygen and hydrogen — nebulous matter that the young sun should have blown away before Mars formed.

Puzzled, scientists hoped a stronger seismic yawp would provide clarity. And on Sept. 18, 2021, the firmament delivered: A meteor careened into the hemisphere opposite to InSight, screeching out seismic waves that blasted through the core and ricocheted around its edges.

“That was the turning point,” said Henri Samuel, a geophysicist at Université Paris Cité and an author of one of the new studies.

Based on a model of Mars’s thermal and chemical evolution, Dr. Samuel and his colleagues had proposed the existence of a core-straddling magma ocean in 2021. But “we had no seismological evidence,” he said. With that meteor impact, his team confirmed the existence of this superhot radioactive soup.

Dr. Khan’s team also leveraged the impact to re-examine InSight’s seismic data, combining it with computer simulations that explore how iron-rich alloys behave on a molecular level — and, in doing so, independently found Mars’s hidden magma ocean.

Its existence means the liquid core’s radius is closer to 1,000 miles — a denser, iron-rich orb with fewer lighter elements, which is easier to explain.

The discovery is “very cool,” and the studies’ shared conclusions are convincing, said Paula Koelemeijer, a seismologist at Royal Holloway, University of London, not involved in the research. “But they might open up a new problem.”

Before it collapsed 3.8 billion years ago, Mars had a magnetic field that shielded its atmosphere. Scientists thought the magnetic field was generated by a cooling, and thus vigorously churning, liquid iron core. But a radioactive, magmatic blanket swaddling it would have kept the core too toasty.

So a new origin story for Mars’s magnetic bubble is required. Dr. Samuel offered one suggestion: Perhaps long ago, Mars possessed moons more massive than its present-day Lilliputian pair, the sort whose strong gravity could stir up magnetism-making motions in the core. But for now, he said, that is just a hypothesis.

After four years, InSight died in 2022. But the discovery of this magma ocean probably won’t be the mission’s final surprise. “This is just the beginning,” Dr. Samuel said.