Looking across the various moons in our solar system, it seems fair to say the Martian moon Phobos is a bit of a black sheep. It probably wasn’t part of its planet’s formation, isn’t a contender for supporting life, nor does it hold much gravitational sway over it’s closest neighbor. What was once theorized to have been a captured asteroid is looking more and more like a jumbled aggregation of rocks. New simulations from Purdue University suggest that much of Phobos’ history wasn’t spent as a moon, but was instead set of planetary rings circling Mars that just ended up clumped together into what is now a pretty fragile natural satellite.
Created through collisions and clumping
The latest hypothesis about Phobos’ origins is that a large object struck Mars around 4.3 billion years ago, ejecting smaller debris into orbit. That debris, like most orbiting objects, eventually flattened out into a set of rings, similar to the iconic rings around Saturn (…and Uranus … and Neptune.) Eventually, the pull of Mars’ gravity would have helped the rings clump together to form the tiny moon, Phobos.
We know that Phobos is slowly spiraling towards Mars, as well as beginning to be pulled apart by the tug of Mar’s gravity. There’s been some speculation about if Phobos will eventually fall and impact Mars as a single object, or if it would be slowly shredded by tidal forces first. This new model favors the latter option, and suggests that Phobos will hit the Roche limit, or threshold before Mar’s gravity pulls it apart, it around 70 million years. That material has to of course go somewhere, and most of Phobos would likely be returned to being a set of rings for the Red Planet.
Repeatedly becoming rings
Taking the model into the future, researchers also suggest that Phobos may repeat this rings-to-moon cycle three to seven times in the next few billion years. Each cycle would cost Phobos some of its mass though, as some of the loose rock would end up falling on Mars in the process. As a result, each generation of Phobos descendants would be around five times smaller than their predecessors. Even if that sounds like a rather dramatic cycle of events, it apparently still makes more sense than something as small and close to Mars as Phobos somehow sitting happily in a relatively tight orbit for the last 4.3 billion years. And in the end, it’s really something to brag about, since not every moon can also claim to have been, and eventually become, orbital rings.
Source: Does Mars have rings? Not right now, but maybe one day, Scienmag