Enlarge / Artist's view of what InSight looked like after landing. (credit: NASA/JPL-Caltech)

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.