Saturn is, of course, famous for its exquisite ring system, but other planets have rings as well - Jupiter, Uranus and Neptune all have them, they just aren't nearly as prominent. Now it turns out that Mars may also have once had rings, and could have them again in the future.
None of the other rocky planets in our Solar System have rings, and there's no evidence they ever did, so it would be surprising if Mars ever did, but certainly not impossible.
The new theory has been proposed by David Minton and Andrew Hesselbrock, NASA-funded scientists at Purdue University in Lafayette, Indiana, and the new findings have been published in the journal Nature Geoscience. Their new model suggests that there is rocky debris left over from an asteroid impacting Mars around 4.3 billion years ago, which over time alternates between forming a planetary ring and clumping together to form a moon. The Borealis Basin on Mars, also known as the North Polar Basin, may have been created by that impact. The massive basin covers about 40 percent of Mars' northern hemisphere. Such an impact would have sent an enormous amount of material into space.
"That large impact would have blasted enough material off the surface of Mars to form a ring," Hesselbrock said.
Hesselbrock and Minton suggest that the ring formed first, but then as material continued to spread out, it gradually began to clump together to form a moon. Eventually, the moon was pulled in close enough to the planet by gravity that it was torn apart, leaving only the two smaller moons we see today, Phobos and Deimos. This process may have been repeated three to seven times over billions of years. Each time a new moon formed, it would be five times smaller than the previous moon which broke apart. Debris raining down on the planet may also explain some of the sedimentary deposits found near Mars' equator.
"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," Minton said. "And now it's possible to study that material."
It is thought that Phobos will also meet this fate, and form a set of rings about 70 million years from now.
Ring material falling to the surface may also explain the weird equatorial ridge on Saturn's moon Iapetus and tiny moonlets Pan and Atlas.
There are other theories that Phobos was created from the impact that formed the basin, but Minton thinks that is unlikely. He noted Phobos would had to have formed far out from Mars and then cross through the resonance of Deimos, the smaller of the two moons. But since Deimos' orbit is within one degree of Mars' equator, that would suggest Phobos has had little or no effect on Deimos.
An asteroid with rings
"Not much has happened to Deimos' orbit since it formed," Minton said. "Phobos passing through these resonances would have changed that."
"This research highlights even more ways that major impacts can affect a planetary body," said Richard Zurek of NASA's Jet Propulsion Laboratory, Pasadena, California. Zurek is the project scientist for NASA's Mars Reconnaissance Orbiter, whose gravity mapping provided evidence for the hypothesis that the northern lowlands were formed by a massive impact.
If the researchers are correct, then Mars probably once had a ring system of its own, although it would have been nowhere near as massive as Saturn's. Perhaps more like Jupiter's, Uranus' or Neptune's very thin, almost invisible rings. It would also suggests that ring systems may be quite common among exoplanets as well, since rings have even been found around asteroids/centaurs in our own Solar System, such as 10199 Chariklo. Chariklo is only 248 kilometers (154 miles) across.
Saturn may be the jewel of the Solar System when it comes to rings, but now we are learning that ring systems may be more common, in various forms, than first thought - not only in our Solar System, but perhaps beyond as well.