Almost a decade after NASA’s InSight mission put the first working seismometer on the Martian surface, researchers are still combing through its records of faint ground vibrations to reveal secrets of the planet’s deep interior.
In a recent analysis, scientists reported seismic evidence that Mars has a solid inner core, an unexpected finding that challenges earlier studies that suggested the planet’s core was entirely molten.
Like Earth—and onions and ogres—the interior of Mars has layers. These layers have different densities and can be solid or liquid. As seismic waves move through the layers, they are bent or reflected, especially at boundaries where density changes sharply. By analyzing how these waves propagate, scientists can trace their paths and infer the structure and properties of the materials they pass through.
Previous analyses of InSight data had already mapped the structure of the Martian crust and mantle and also revealed that the planet has a surprisingly large molten metallic core, spanning nearly half its radius. Such a large core, combined with measurements of the planet’s relatively low density, suggested that it must contain a lot of light elements such as sulfur, carbon, hydrogen, and oxygen. These light elements lower iron’s melting point, making it less likely to crystallize to form a solid inner core, which partly explains why the new finding caught InSight scientists off guard.
“None of us really believed that you would have a solid inner core,” said Amir Khan, a geophysicist at ETH Zurich who is part of the InSight science team but wasn’t involved in the new study.
A Long Way to the Core
Still, seismologist Daoyuan Sun of the University of Science and Technology of China in Hefei and his colleagues decided to look for signs of a solid core in the publicly available InSight data. Specifically, they reexamined data from a set of 23 marsquakes with seismic waves that passed through the planet’s core before returning to the surface.
To enhance the faint signals from the seismometer, the team combined—or stacked—recordings from these quakes. This revealed two types of compressional (P) waves that crossed the core. One set, known as P′P′ waves, traveled through the outer core to the farside of the planet, reflected off the surface there, and then passed back through the core to reach the seismometer. The other set, called PKKP waves, passed through the outer and inner core before being reflected back to the surface and encountering the core-mantle boundary on the way out.
“To me that’s the most exciting thing. That’s basically saying that you see this inner core structure. ”
Initially, the researchers could not find the PKKP waves at their expected arrival times. Instead, the waves were arriving 50–200 seconds earlier than predicted if the core was fully molten. The early arrivals suggested the waves had traveled through solid material, which transmits seismic P waves faster than liquids.
While looking for these early-arriving signals, the team also picked up a third set of seismic waves, called PKiKP. These are P waves that reflect back to the surface right at the boundary between the inner and outer core. This is the same type of seismic phase that seismologist Inge Lehmann used to reveal the existence of Earth’s solid inner core in 1936.
Finding these PKiKP waves in InSight data offered scientists a strong clue that Mars, too, may have a solid core.
“To me that’s the most exciting thing,” Sun said. “That’s basically saying that you see this inner core structure.”
By measuring the travel times of the seismic phases, Sun’s team estimated that Mars has a solid inner core with a radius of about 613 kilometers—roughly 18% percent of the radius of the planet itself. That ratio is very similar to that of Earth’s inner core, which is about 19% of Earth’s radius, and much larger than many researchers anticipated Mars could have. The new findings were published in Nature.
The team posited that their seismic observations could be explained by an outer core made up mostly of liquid or molten iron and nickel, as well as smaller amounts of sulfur and oxygen, and no more than 3.8% carbon, encasing a solid inner core enriched in more oxygen.
“It’s like Mars has lifted just the corner of its veil and allowed us to peek inside, but only a sneak peek—we could not get the full picture.”
These levels of light elements remain difficult for scientists to explain, Khan said. As light elements prefer to stay liquid, the existence of a solid inner core means that the outer core around it would have to be even richer in light elements than in previous models, which were already pushing the limits of what seemed plausible. On top of that, the building blocks from which scientists think Mars formed don’t contain enough of these elements to account for the abundance required by a solid core, Khan added.
The finding is also at odds with two studies published 2 years ago, one of them led by Khan, that proposed that a layer of molten rock sits at the bottom of the mantle, just above the core, insulating it like a thermal blanket. Such a layer would keep the core hotter, making it more difficult for it to crystallize and solidify.
“It’s like Mars has lifted just the corner of its veil and allowed us to peek inside, but only a sneak peek—we could not get the full picture,” Khan said. “We are not there yet.”
A Hibernating Dynamo
The new finding also renews questions about the absence of a global magnetic field on Mars. Earth’s magnetic field is sustained by the slow crystallization of the core, which drives magnetism-inducing convective motions in the liquid outer core. We know that Mars once had a magnetic field, but it died out billions of years ago.
If Mars does have a solid inner core, why is its magnetic dynamo inactive?
The likely reason is that core crystallization, and thus convection in the outer core, is too slow to power a global magnetic field on Mars, said Douglas Hemingway, a planetary scientist at the University of Texas at Austin and a coauthor of the new study. Mars’s early magnetic field was likely powered by primordial heat escaping from its core. As the planet cooled over billions of years, this convection weakened, and the magnetic field eventually disappeared.
Finding a solid core on Mars, however, opens up the intriguing possibility of a global magnetic field eventually reigniting, Hemingway said. The process of crystallization happens at the boundary of the outer core and the inner core, and if this surface grows larger over time, it could reach a point where there’s enough convective motion to kick-start the dynamo and revive the global magnetic field.
In earlier work, Hemingway predicted that if the Martian core is crystallizing from the center outward, the magnetic field could turn on sometime within the next billion years. “So, you know, if we wait a billion years and it doesn’t happen, then we were wrong,” he joked.
There may be no definitive confirmation of the existence of a solid core on Mars for a long time. The InSight mission ended in 2022, after dust piling up on the lander’s solar panels drained the device’s power supply, and new seismic data from Mars won’t be available for decades, most likely.
“Maybe when we send humans, we would be motivated to bring a few seismometers,” Hemingway said.
—Javier Barbuzano (@javibar.bsky.social), Science Writer
Citation: Barbuzano, J. (2025), Scientists may have finally detected a solid inner core on Mars,
Eos, 106, https://doi.org/10.1029/2025EO250367. Published on 1 October 2025.
Text © 2025. The authors.
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