Does Mars sometimes have rings?

It’s possible that Mars’ closer moon – Phobos – might alternate between becoming a planetary ring and clumping together to form a moon.

Artist’s concept of Mars with a ring. Made using Celestia, by the Celestia Development Team.

The closer of Mars’ two moons – Phobos – might sometimes exist in the form of rings. After spending some time as rings, it might then reform as a moon. It’s possible this cycle has repeated some three and seven times over billions of years, according to the output of a new computer model developed by scientists at Purdue University in Lafayette, Indiana. NASA funded the study and reported on the findings – which were published in the peer-reviewed journal Nature Geoscience – on March 20, 2017.

The idea of Phobos someday shattering and forming a ring for Mars isn’t new. It’s long been known that Phobos is getting closer to Mars and that it might someday break apart. According to the new model by David Minton and Andrew Hesselbrock, Phobos will break apart upon reaching the Roche limit – the minimum distance a moon can orbit without being gravitationally shredded – in roughly 70 million years. At that time, it’ll become rings for Mars.

Depending on where the Roche limit is, Minton and Hesselbrock believe this cycle might have repeated between three and seven times over billions of years of Mars history. Each time a moon broke apart and reformed from the resulting ring, its successor moon would be five times smaller than the last, according to their model. The model also suggests that debris from the moon-shattering events would have rained down on Mars, possibly explaining enigmatic sedimentary deposits found near Mars’ equator. Minton said:

You could have had kilometer-thick piles of moon sediment raining down on Mars in the early parts of the planet’s history, and there are enigmatic sedimentary deposits on Mars with no explanation as to how they got there.

And now it’s possible to study that material.

Where did the material to form the rings – or the moons – come from? It may be debris that was pushed into space from an asteroid or other body slamming into Mars around 4.3 billion years ago.

In fact, Mars’ large North Polar Basin or Borealis Basin — which covers about 40 percent of the planet in its northern hemisphere — is believed to have been created by that impact, sending debris into space. Hesselbrock said:

That large impact would have blasted enough material off the surface of Mars to form a ring.

Other theories suggest that the impact with Mars that created the North Polar Basin led to the formation of Phobos 4.3 billion years ago, but Minton said it’s unlikely the moon could have lasted all that time.

Read more from NASA: Does Mars Have Rings? Not Right Now, But Maybe One Day

Bottom line: A new computer model suggests that Mars’ moon Phobos might alternate between being a moon and being rings around Mars. This cycle might have repeated between three and seven times over the last 4 or so billion years.

Deborah Byrd