Scientists have made the most precise measurements ever of Mars’ rotation, for the first time detecting how the planet wobbles due to the “sloshing” of its molten metal core.

The findings, detailed in a Nature paper, rely on data from NASA’s InSight Mars lander, which operated for four years before running out of power during its extended mission in December 2022. To track the planet’s spin rate, the study’s authors relied on one of InSight’s instruments: a radio transponder and antennas collectively called the Rotation and Interior Structure Experiment, or RISE. They found the planet’s rotation is accelerating by about 4 milliarcseconds per year – corresponding to a shortening of the length of the Martian day by a fraction of a millisecond per year. It’s a subtle acceleration, and scientists aren’t entirely sure of the cause. But they have a few ideas, including ice accumulating on the polar caps or post-glacial rebound, where landmasses rise after being buried by ice. The shift in a planet’s mass can cause it to accelerate a bit like an ice skater spinning with their arms stretched out, then pulling their arms in.

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NASA InSight Study Finds Mars Is Spinning Faster