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Mars has started spinning faster – what does it all mean?

A ‘historic experiment’ using data from NASA’s InSight spacecraft (RIP) has revealed that days on the planet are getting shorter

NASA’s InSight spacecraft tragically passed away on the surface of Mars in December 2022, but the data it collected during its four years in service is still being analysed by scientists for new information about the red planet. The most recent discovery? Mars is spinning faster than it’s supposed to be, and it’s getting faster every year.

Yesterday (August 7), NASA’s Jet Propulsion Laboratory announced the surprising findings, which were uncovered by conducting the most precise measurements of the planet’s rotation to date. The exact results – which are outlined in more detail in a recently-published Nature study – show that Mars’s spin is speeding up by around four milliarcseconds per year. 

What is a milliarcsecond? Essentially, it’s a fraction of a tiny angle that’s used to conduct astronomical measurements. To put it into Earthbound terms, the amount that Mars is accelerating causes the Martian day to get shorter by a fraction of a millisecond every year. At first glance, this might sound slightly underwhelming. But, while the numbers involved are relatively small, the mere fact that Mars is speeding up – and, therefore, its days are getting shorter – could give researchers some clues about the planet’s mysterious past.

“It’s a historic experiment,” says the Royal Observatory of Belgium’s Sebastien Le Maistre, the study’s lead author, stressing that the small variations are significant nonetheless. “We have spent a lot of time and energy preparing for the experiment and anticipating these discoveries. 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.”

According to the researchers, they haven’t pinned down the exact cause of the accelerating spin yet, but one theory is that it could be caused by a shift in Mars’s mass due to the presence of polar ice. Despite mostly looking like a barren wasteland, Mars has water-based ice caps at both poles, and the growing or shrinking of these ice caps could affect the rate of its spin (a similar theory has been floated to explain why Earth started spinning faster). Picture spinning around on a desk chair, and pulling your arms in to spin faster – it’s a bit like that. Potentially, analysing the speed of Mars’ spin further could help uncover even more information about the materials on its surface in the distant past.

The InSight mission didn’t just shed light on the shortening of Martian days, though. Using RISE – AKA the Rotation and Interior Structure Experiment, a scientific instrument on InSight – researchers were also able to measure a wobble in the planet’s rotation, through which they could make new assumptions about the “sloshing” of its molten metal core.

“It’s really cool to be able to get this latest measurement – and so precisely,” says InSight’s principal investigator, Bruce Banerdt of NASA’s Jet Propulsion Laboratory in Southern California, in a statement. “I’ve been involved in efforts to get a geophysical station like InSight onto Mars for a long time, and results like this make all those decades of work worth it.”

It’s worth noting that such subtle variations in the rotation of Mars aren’t likely to have any effect on us, here on Earth. As space agencies prepare to establish colonies on the neighbouring planet, though, any new discoveries could have potentially groundbreaking implications.

Earth is actually also spinning faster – read about what all that means here.

Unfortunately, the Earth’s core has stopped spinning altogether! Read about what all that means here.

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