For four years, a lone robot sat on the dusty plains of Mars, listening. The InSight lander, a three-legged marvel of engineering, was sent to take the Red Planet’s pulse. There’s not much happening on Mars. Unlike the Earth, the Red Planet doesn’t have active plate tectonics, so its earthquakes (or rather, marsquakes) are much rarer and fainter.

But they do exist, and that’s important.

Such vibrations offer clues into the planet’s unseen interior. That’s how we know what the Earth’s structure is like. And now, that’s how we’re figuring out what the inside of Mars also looks like. Previous studies hinted at Mars having a liquid core. Now, this latest data suggests that while small, Mars’ inner core is actually solid.

Build-a-quake

When an earthquake (or a marsquake, or a moonquake) takes place, it sends out seismic waves in all directions. These waves propagate differently based on the medium they pass through. That’s how we know the Earth has a solid inner core, surrounded by an outer core and a mantle, even though the deepest hole we’ve ever dug barely scratches the plant’s interior.

Crucially, certain waves don’t pass through liquid environments, and researchers can use this to differentiate the nature of the core.

But detecting the core of another planet is a huge challenge. On Earth, seismologists have a global network of thousands of sensors that constantly record the powerful vibrations from earthquakes. These waves, as they travel through and bounce off different layers, allow us to build a detailed 3D picture of our planet’s interior.

Meanwhile, on Mars, we only have InSight.

For years, the planet’s deepest interior remained in a “shadow zone”. Seismic waves from most quakes simply didn’t have the right trajectory to dive deep enough to probe the innermost core and reach the lander. But the science team had an ace up its sleeve. They decided not to wait for one perfect, powerful quake. Instead, they treated a group of 23 quakes originating from a similar region as a “source array”. By digitally stacking the faint, coherent signals from these multiple events, they could amplify the signal and cancel out the noise, essentially creating a more powerful, virtual quake. This array analysis allowed them to finally hear the whispers from the planet’s heart.

Specific Waves

They were listening for two specific, almost mythical types of seismic waves. The first, known as PKIKP, is a wave that travels from a quake, dives through the mantle and liquid outer core, and then bounces directly off the boundary of a solid inner core before returning to the surface. Finding this echo is the smoking gun for a solid center. And find it they did. The team identified a clear, robust signal arriving about 604 seconds after the initial quake waves, a signal whose properties matched the predictions for a reflection from an inner core boundary perfectly.

The second key piece of evidence came from a wave called PKKP. This is a wave that performs an even more incredible journey: it travels all the way through the planet, reflects off the surface on the far side, and then dives back down through the core again before reaching the seismometer. The scientists noticed that these PKKP waves were arriving consistently early — up to 200 seconds sooner than predicted by models with a purely liquid core. There was only one logical explanation: for part of their journey, the waves must have traveled through a much faster medium. Since solid materials transmit seismic waves faster than molten ones, a solid inner core provided the most feasible explanation. This “fast lane” through the center of Mars was the definitive proof they needed.

The researchers calculate that the inner core extends to a radius of around 380 miles (613 km), plus or minus 10%. After that, Mars’ inner core is surrounded by a bigger liquid core that stretches as much as 1,100 miles (1,800 km) from the planet’s center.

The data also enables researchers to draw some conclusions about the properties of the inner core.

A Special Mix

A simple core of pure iron and nickel couldn’t account for the observed properties. The Martian core must be rich in “light elements” — things like sulfur, carbon, oxygen, and hydrogen that mixed with iron deep inside the planet during its formation.

The team ran models of different compositional mixes to see what could explain the data. A model where the core is significantly enriched with oxygen proved to be a very good fit. This is notable because it’s consistent with a leading theory of Martian evolution. Early in its history, the planet cooled rapidly, driving a powerful “thermal dynamo” that generated a global magnetic field. We see the ghost of this field today, locked away in the ancient crustal rocks of Mars’s southern hemisphere. But then, as the rate of cooling slowed, this thermal dynamo sputtered and died.

At that point, the “compositional dynamo” driven by inner core crystallization should have taken over. But on Mars, it appears it never did, or if it did, it was too weak or short-lived to sustain a global magnetic field. Perhaps the crystallization is happening too slowly, or the specific mix of light elements doesn’t create enough density contrast to churn the outer core effectively. Whatever the case, without a magnetosphere, Mars was vulnerable to solar wind and ultimately lost its atmosphere and became uninhabitable.

The study doesn’t answer all the questions about Mars’ core, but with InSight out of action, we’ll have to wait quite a bit for new information. There are no immediate plans for another seismometer mission on Mars after NASA’s InSight, which ended in 2022.

Journal Reference: Huixing Bi et al, Seismic detection of a 600-km solid inner core in Mars, Nature (2025). DOI: 10.1038/s41586-025-09361-9,