Mars has liquid insides and strange insides, suggests InSight Ars Technica

Image of a lander on a reddish dry planet, showing two circular solar arrays and an array of instruments.
Zoom in / Artist’s impression of what InSight would look like after landing.

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

Whether Mars’ core is solid or liquid has long been debated. While it’s not possible to observe the Martian core directly, InSight has tried. His seismometer, SEIS, was the first instrument to find possible evidence of a liquid core. Meanwhile, its Rotation and Interior Structure Experiment (RISE) instrument had measured tiny changes in the rotation of the planets as it orbited, wobbles in its axis caused by the push and pull of the sun’s gravity.

Our analysis of InSights 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 in the mantle, write the researchers behind the instrument in a recently published study in Natures.

Slow to CLIMB

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

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 has proven challenging. It took a while for the signals to emerge from the noise of the planets’ movements. Mars is also full of dust storms, and the storms that occurred before and after InSight landed changed the planet’s rotational speed for a while. Its axis of rotation also undergoes slight changes due to the gravitational forces exerted by its moons, Phobos and Deimos.

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

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

You have nutations in your rotation

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

Nutation analysis based on radiometric measurements is the only technique capable of providing direct estimates of [the] properties of the Martian core, the researchers in the study also said.

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

Scientists say it’s possible Mars’ lower mantle could also be molten, which would affect the size and shape of the core. A molten mantle would allow for subsurface mass anomalies, regions where the material is more or less dense than the surrounding material. It turns out that one of these anomalies appears to be much deeper beneath the surface than the other. The anomalies could partly explain the slight flattening of both Mars’ surface and core as it rotates on its axis.

In the future, Le Maistre hopes to analyze more RISE data in the same dataset that unveiled the anomalies and liquid core. There is still massive amounts of data from InSight just waiting to tell us more about Mars. RISE is not only about depth, but also about atmosphere and rotation, she said in a press release. [It can] provide a model of orientation and rotation that can serve as a reference for the scientific community.

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

Elizabeth Rayne is a writing creature. Her work has appeared in SYFY WIRE,, Live Science, Grunge, Den of Geek and Forbidden Futures. When she’s not writing, she shapeshifts, draws, or cosplays as a character no one has ever heard of. Follow her on Twitter @quothravenrayne.

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