Saturn’s rings are, of course, a defining feature of the planet. But the other gas giants in the Solar System—Jupiter, Neptune and Uranus—also have faint, dark systems of rings around them. And it turns out that millions of years ago, another planet may have also had a ring: Mars.
New research published this week in the journal Nature Geosciences, suggests that one of Mars’ moons, Phobos, may be locked in a cycle where, over millions of years, it alternates between a ring of debris encircling the planet and a moon formed from that coalesced material.
Phobos is a small, pockmarked body that orbits about 3,700 miles above the surface of Mars—the closest orbit of any moon in the Solar System. But the gravity that keeps its celestial buddy nearby has also caused the tiny body stress, according to NASA. Phobos already has fractures on its surface and NASA estimates that it will be torn to shreds within 30 to 50 million years.
In the new study, researchers used computer modeling to examine Phobos' past and predict its future. The researchers suggest that an asteroid or other celestial body slammed into mars 4.3 billion years ago—an impact that created a massive basin on the planet's surface. This latest study, however, suggests that rather than creating the moons, the impact first sent debris shooting out into orbit around the planet. Eventually, that rocky debris ring coalesced into a large, lumpy moon.
Over time, Mars' gravity pulled that lumpy planetoid closer, bringing it within the so-called Roche Limit, or the distance at which a smaller body can exist as a self-contained unit under its own gravity. Any closer and the larger body's gravity rips the little moon apart.
When Mars' moon reached the Roche Limit in the past, it went from moon to ring. But again, over tens of millions of years, that debris clumped back together into a moon.
The simulation suggests that Phobos’ first iteration was likely a fairly large moon, reports Ryan F. Mandelbaum at Gizmodo. But over the last 4.3 billion years, it went through the ring-moon cycling three to seven times—each time losing a bit of mass to rocks that rain down on mars. The next time the moon crumbles, the model estimates it will lose another 80 percent of its mass. About 70 million years later, it will form another, much smaller Phobos version 8.0 (or so).
While the idea is compelling, it’s not the only proposal for the origin of Mars' moons. It does, however, offer something concrete for researchers to look for on the surface of mars: piles or layers of moon rocks from past moon explosions, according to a press release.
What about the other moon? As Mandelbaum explains, Deimos is outside the point where Mars' graity draws it in and could drift further and further away from the red planet, possibly escaping in the future.
The researchers plan to continue their work by looking deeper into the original ring around Mars or to try and investigate the potential sediment on the Martian surface.