Mars appears to be a frozen expanse of red dust, gaping craters, and rocky terrain on the outside—but what lies beneath its wind-blasted surface? NASA’s InSight lander might have discovered this before it took its proverbial last breaths in a dust storm.

Whether the core of Mars is solid or liquid has been long debated. While there is no way to observe the Martian core directly, InSight tried. Its seismometer, SEIS, was the first instrument to find possible evidence of a liquid core. In the meantime, its RISE (Rotation and Interior Structure Experiment) instrument had been measuring minuscule changes in the planet’s rotation as it orbited, “wobbles” in its axis caused by the push and pull of the Sun’s gravity.

“Our analysis of InSight’s radio tracking data argues against the existence of a solid inner core and reveals the shape of the core, indicating that there are internal mass anomalies deep within the mantle,” write the researchers behind the instrument in a study recently published in Nature.

Slow to RISE

RISE works by transmitting radio signals to Earth. By tracking changes in these signals, researchers can detect extremely small changes in its location relative to our receivers. These changes are caused by wobbles in Mars’ rotation called nutations. The distance and direction in which the axis moved because of these nutations can be used to infer information about Mars’ internal composition.

The Red Planet was previously suspected to have a liquid core based on measurements of seismic waves. But detecting these changes based on radio signals proved challenging. It took a while for signals to emerge from the noise of the planet’s motions. Mars is also swirling with dust storms, and storms that occurred before and after InSight landed changed the planet’s rotational speed for a while. Its rotation axis also experiences slight changes due to the gravitational forces exerted by its moons, Phobos and Deimos.

For the RISE experiment to work, researchers needed to know precisely where InSight landed on Mars. Landers have planned landing sites, but those aren’t exact—not even the scientists following them can tell precisely where they are until they interpret the first data that the lander transmits to Earth.

The first RISE data was processed by radio scientist Sebastien Le Maistre of the Royal Observatory of Belgium, and a positional estimate was uploaded to the Mars Reconnaissance Orbiter (MRO), which took a picture of the location. The image showed that InSight had been located with amazing accuracy.

You have nutations in your rotation

After RISE knew where exactly on Mars its lander was, how did the nutations it detected hint at a liquid core? Nutations can be prograde (the axis moving counterclockwise relative to its surroundings) or retrograde (the opposite of that). Le Maistre and his team already knew that if Mars really had a liquid core under a solid mantle, it would have to mean the axis wobbled in retrograde and also moved slightly more than it would if the core was solid. When they tested this against the InSight data, it was a match.

“Nutation analysis based on radiometric measurements is the only technique that can provide direct estimates of [the] properties of the Martian Core,” the researchers also said in the study.

Further analysis determined that the Martian core is most likely made out of an alloy of liquid iron and sulfur, and that it’s constantly going through convection, with hotter fluid rising and cooler fluid sinking. Unlike Earth’s core, it is also thought to be completely liquid. The outer core of Earth is an alloy of liquid iron and nickel, while the inner core is solid and mostly made of iron.

The scientists say it is possible that Mars’ lower mantle may also be molten, which would affect the size and shape of the core. A molten mantle would enable subsurface mass anomalies, regions in which material is more or less dense than the surrounding material. It turned out that one of these anomalies appears to be located much deeper below the surface than the other. Anomalies could partly explain the slight flattening of both the surface and core of Mars as it rotates on its axis.

In the future, Le Maistre hopes to analyze more RISE data in the same data set that gave away the anomalies and liquid core. There are still enormous amounts of data from InSight just waiting to tell us more about Mars. “RISE is not only about the deep inside but also about the atmosphere and the rotation,” he said in a press release. “[It can] provide an orientation and rotation model that can serve as a reference for the scientific community.”

Nature, 2023.  DOI: 10.1038/s41586-023-06150-0

Elizabeth Rayne is a creature who writes. Her work has appeared on SYFY WIRE, Space.com, Live Science, Grunge, Den of Geek, and Forbidden Futures. When not writing, she is either shapeshifting, drawing, or cosplaying as a character nobody has ever heard of. Follow her on Twitter @quothravenrayne.