Mars’ moons could be remnants of earlier rings, destined to form a new one

Our Solar System's two innermost rocky planets have no moons. Earth has an unusually large one, the product of a massive collision. And Mars' two moons, Phobos and Deimos, are... well, weird, looking like asteroids but not behaving like them. Now, a new paper suggests that the moons' oddity could be explained by a cycle of ring building and destruction that started more than four billion years ago.

The idea solves a bunch of problems, it may create a brand new one, and, best of all, it has some testable consequences.

Phobos and Deimos don't look especially odd on the surface. In fact, their surfaces look a lot like a pretty mundane class of asteroids. So it has been suggested that the two moons are simply asteroids that wandered into Mars' gravitational field and got caught. Phobos appears to be transient; in about 70 million years, it's expected to drop close enough to Mars to be torn apart by the planet's gravity.

But asteroids have no reason to approach Mars from any particular direction, and they could even have ended up orbiting in the opposite direction from Mars' rotation. But that does not describe Phobos and Deimos at all. The two have neat, circular orbits at Mars' equator and travel along in the same direction as the Red Planet's rotation, all of which is more consistent with them having formed where they are.

Giant impacts

To make sense of this situation, people have suggested that they formed much as the Earth's Moon did, through a giant impact. This would also explain differences between Mars' northern and southern hemispheres, with the north being a low basin and the south having extensive highlands. An impact could also explain why the current moons also have a composition quite different from Mars itself. But it doesn't explain why Phobos is on the verge of destruction or why both moons are so small—estimates are that the collision would put about 10²⁰ kilograms of debris into orbit.

Two astronomers from Purdue, Andrew Hesselbrock and David Minton, decided to follow up on what happens after the collision to see whether you could get to the present moons. In their model, the debris initially forms a ring near what's called the Roche limit, the distance where any bodies would be torn apart by the planet's gravity. From there, the debris begins to spread. The majority spreads back toward Mars and ends up falling to the planet's surface. Roughly 80 percent of the ring's initial mass ends up back on the planet.

But some of the remainder will spread out beyond the Roche limit, at which point it can start condensing into moons. The model suggests that several moons would eventually form, with the largest being the farthest from Mars. Over time, however, gravitational interactions would gradually pull the moons back toward Mars. This occurs because the moons orbit faster than Mars rotates, so their tidal interactions constantly slow them down. The large outer moon would swallow up the inner ones on its way back in and eventually reach the Roche limit again. At this point, it would be torn apart and form a new ring.

From there, the cycle can be repeated. Each cycle reduces the amount of material in the ring, as more gets deposited back on Mars, producing fewer and smaller moons. In fact, given a satellite's mass, the authors can calculate the size of the ring that started the cycle. And if you have that ring's mass, you can figure out the size of the moon that produced it, working backward.

An ongoing cycle

The authors find that there may have been six separate ring-moon cycles. The first few involved a lot of mass and moved quickly, but the later ones gradually slowed down, to the point where the existing cycle may have been running for about 2.5 billion years. As noted above, Phobos has about 70 million years left before it's destroyed by Mars' gravity and creates a new ring, starting the cycle over again.

In all, the idea seems to neatly tie up a lot of loose ends regarding Mars' moons. "For what it's worth, I believe the model is right," Arizona State's Erik Asphaug told Ars, "because it solves too many things."

While the ideas may be neat and tidy, however, the process is anything but. After all, vast amounts of material would periodically rain down into the skies of Mars. The original ring, in fact, would drop enough material to place every square meter of Mars under 370 meters of debris. If the debris preferentially rained down near the equator, the depth would be over two kilometers. Subsequent cycles would add hundreds of meters more.

The authors of the paper point out that there are many areas on Mars that appear to be filled with loosely packed material, including the Medusae Fossae, which stretch for over 1,000 kilometers near the Martian equator. Is there really enough of this loosely packed material to account for all the debris that must have rained down?

We may already have some data relevant to this, according to Asphaug, who called the debris "the most important testable hypothetical prediction of the model!" One that can be tested because we already have a rover sitting in the middle of it: "Indeed if it is true, then the Curiosity rover is crawling all over proto-Phobos ejecta, which according to the giant impact models, isn't even Mars material."

Nature Geoscience, 2017. DOI: 10.1038/NGEO2916  (About DOIs).