A radioactive sea of magma hides beneath the surface of Mars
In 2021, it seemed like Mars had a surprisingly big heart. Scientists used InSight, a robotic lander, to study the planet’s interior. The spacecraft had listened to enough Martian quakes to form a picture of the layered nature of the Martian underworld.
The crust and mantle were not particularly strange. However, the core was too large and not very dense for such a small planet.
For some researchers, that core measurement was incorrect.
“We missed something,” he says Amir Khan, a geophysicist at ETH Zurich in Switzerland who studied InSight’s data. “But what?”
It turns out that the core of Mars is small after all, Dr. Khan and other researchers discovered.
In two studies Published Wednesday in the journal Nature, researchers reevaluated InSight’s seismic record. Both teams independently concluded that the core of Mars is more similar to the heavy metal heart of our own world than previously suspected. The initial estimate of the larger size was the result of an undetected ocean of molten rock 90 to 200 kilometers deep, making the underlying core appear larger than it actually is.
But the deep sea of magma, hidden beneath Mars’ solid mantle and kept molten by radioactive elements, is exotic. “It doesn’t exist on Earth,” said Dr. Khan, and its presence may require a rethink of the red planet’s chaotic evolution.
Scientists have been studying the Earth’s geological layers for more than a century using the illuminating power of earthquake-induced seismic waves. InSight, which landed on Mars in November 2018, was sent to investigate whether the innards of the rusty world were similar.
But studying Mars with a single seismometer proved difficult. InSight’s instruments detected only a few modest vibrations that mainly came from a convulsive region close to the spacecraft, and only a small slice of the Martian pie was seismically imaged. For a time, Marsquakes also appeared to bounce back, but not plunge through the planet’s inner sanctum, revealing very little information about the core.
Researchers found that the core of Mars had a radius of about 1,100 miles (1,800 kilometers), indicating that the core is not very dense. Terrestrial planet cores should be iron-rich, but Mars’ puffy core was finally confirmed completely liquid – seemed 27 percent lighter than one made of pure liquid iron. The implication was that Mars’ core was strangely enriched with lighter elements such as sulfur, carbon, oxygen and hydrogen – nebulous matter that the young Sun should have blown away before Mars formed.
Puzzled, scientists hoped that a stronger seismic yaw would provide clarity. And on September 18, 2021, the firmament delivered the following: A meteor barreled into the hemisphere opposite InSight, screaming seismic waves that shot through the core and bounced off the edges.
“That was the turning point,” he said Henry Samuela geophysicist at the Université Paris Cité and author of one of the new studies.
Based on a model of the thermal and chemical evolution of Mars, Dr. Samuel and his colleagues proposed the existence of one the core of the magma ocean in 2021. But “we had no seismological evidence,” he said. With that meteor impact, his team confirmed the existence of this super-hot radioactive soup.
The team of Dr. Khan has also used the impact to re-examine InSight’s seismic data, combining it with computer simulations that explore how iron-rich alloys behave at the molecular level – and independently found Mars’ hidden magma ocean.
Its existence means that the radius of the liquid core is closer to 1,600 kilometers – a denser, iron-rich sphere with fewer lighter elements, which is easier to explain.
The discovery is “very cool” and the shared conclusions of the studies are compelling, the researchers said Paula Koelemeijer, a seismologist at the University of Oxford, was not involved in the study. “But they could pose a new problem.”
Before Mars collapsed 3.8 billion years ago, it had a magnetic field shielding its atmosphere. Scientists thought that the magnetic field was generated by a cooling, and therefore vigorously swirling, liquid iron core. But a radioactive, magmatic blanket wrapping it would have kept the core too warm.
So a new origin story for Mars’ magnetic bubble is needed. Dr. Samuel offered one suggestion: Perhaps Mars long ago possessed moons more massive than its current Lilliputian pair, the kind whose strong gravity could create magnetism-making movements in its core. But for now, he said, that’s just a hypothesis.
After four years, InSight died in 2022. But the discovery of this magma ocean likely won’t be the mission’s last surprise. “This is just the beginning,” said Dr. Samuel.