Mars is spinning faster, its days getting shorter — one theory why

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By Dan Sears

There’s a mystery on Mars.

If you think time is whizzing by on planet Earth, try spending some time on the Red Planet.

Mars is spinning faster and gradually shortening each day slightly, scientists have found — and it has left them puzzled.

The findings, which were published in Nature, used data from NASA’s InSight Mars lander, which operated for four years before it ran out of power during a 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,” according to NASA’s Jet Propulsion Laboratory.

They found that Mars’ rotation is accelerating by about 4 milliarcseconds per year — making the length of a Martian day shorter by a fraction of a millisecond per year.

The authors theorized that the acceleration in Mars’ rotation could be ice accumulating on the polar caps — also known as post-glacial rebound — resulting in a rise of land mass from being buried by ice.

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They described the shift in mass as being similar to when an ice skater spins with their arms stretched out, then pulls them in, reported Sky News.

“What we’re looking for are variations that are just a few tens of centimeters over the course of a Martian year,” said lead author and RISE’s principal investigator Sebastien Le Maistre at the Royal Observatory of Belgium.

Mars close up.
The days are a bit shorter on Mars, scientists have found.

InSight lander on Mars.
Scientists used data from NASA’s InSight lander.

“It takes a very long time and a lot of data to accumulate before we can even see these variations,” he added.

Scientists examined data from InSight’s first 900 days on Mars that was collected by RISE, which tracked Mars’ spin rate.

The researchers would beam a radio signal to the lander using the Deep Space Network, and RISE would then reflect the signal back.

Once the signal was received, scientists looked for tiny changes in frequency caused by the Doppler shift, which is the change in observed wavelength or frequency.

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Measuring the data helped scientists to determine how fast the planet rotates.

Artist rendering of the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) robotic lander.
An artist’s rendering of the InSight lander.
Future Publishing via Getty Images

Le Maistre called RISE a “historic experiment.”

“We have spent a lot of time and energy preparing for the experiment and anticipating these discoveries,” he said. “But despite this, we were still surprised along the way — and it’s not over, since RISE still has a lot to reveal about Mars.”

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