Mars is spinning faster and it’s gradually cutting short the length of each Martian day, scientists have found.
The red planet is accelerating at about four milliarcseconds per year, bringing the days to a close a fraction of a millisecond earlier.
As those measurements would suggest, the changes are subtle – but the scientists who observed them are not sure what the cause is.
Potential causes could be ice accumulating on the planet’s polar caps or post-glacial rebound, which is when land masses rise after being buried by ice.
NASA says such shifts 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.
Scientists detected the shift in spin speed using data from NASA’s InSight Mars lander, which operated on the surface for four years until running out of power last December.
Sebastien Le Maistre, lead author of a paper documenting the findings in the Nature journal, said the variations were “just a few tens of centimetres over the course of a Martian year”.
“It takes a very long time and a lot of data to accumulate before we can even see these variations,” he added.
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The paper examined data from InSight’s first 900 days on Mars.
The lander’s advanced radio technology, coupled with NASA’s Deep Space Network (an international array of giant radio antennas back on Earth), worked together to provide scientists with observations.
They would beam a radio signal to the lander using the antennas, which would then be reflected back to Earth.
When scientists received the reflected signal, they would look for small changes in frequency to determine how fast Mars rotates.
Mr Le Maistre said the “historic experiment” was just the tip of the iceberg so far as the data InSight had collected.
There is still “a lot to reveal” and further information will be mined for years to come.