The two inner rocky planets of our solar system have no moons. The Earth has an unusually large, the product of a massive collision. And the two moons of Mars, Phobos and Deimos, are…well, foreign, which look like asteroids but don’t behave like them. Now a new paper suggests the moons’ peculiarity could be explained by a cycle of ring formation and destruction that began more than four billion years ago.
The idea solves a lot of problems, it can create an entirely new problem and, best of all, it has some testable consequences.
Phobos and Deimos don’t look particularly strange on the surface. In fact, their surfaces look a lot like a fairly commonplace class of asteroids. So it has been suggested that the two moons are simply asteroids that enter Mars’ gravitational field and become trapped there. Phobos appears to be transient; in about 70 million years, it is expected to fall close enough to Mars to be torn apart by the planet’s gravity.
But asteroids have no reason to approach Mars from a particular direction, and they could even end up in the opposite direction of Mars’ rotation. But that doesn’t describe Phobos and Deimos at all. The two have neat, circular orbits around Mars’ equator and travel in the same direction as the Red Planet’s rotation, all of which is more consistent with the fact that they formed where they are.
To understand this situation, people have suggested that they formed like Earth’s moon from a giant impact. This would also explain the differences between the northern and southern hemispheres of Mars, with the north being a low basin and the south extensive highlands. An impact could also explain why the current moons also have a very different composition than Mars itself. But it doesn’t explain why Phobos is on the brink of destruction or why both moons are so small – estimates are that the collision was about 1020 kilograms of debris in orbit.
Two Purdue astronomers, Andrew Hesselbrock and David Minton, decided to follow what happens after the collision to see if you could get to the current moons. In their model, the debris initially forms a ring near what’s called the Roche limit, the distance where all bodies would be torn apart by the planet’s gravity. From there, the debris begins to spread. The majority scatter back to Mars and eventually fall to the planet’s surface. About 80 percent of the ring’s original mass returns to the planet.
But some of the rest will scatter beyond the Roche limit, after which it may begin to condense into moons. The model suggests that several moons would eventually form, the largest of which is farthest from Mars. Over time, however, gravitational interactions would gradually pull the moons back toward Mars. This happens because the moons orbit Earth faster than Mars rotates, so their tidal interactions constantly slow them down. The large outer moon would swallow the inner one on its way back, eventually reaching 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 is deposited on Mars, creating fewer and smaller moons. In fact, given the mass of a satellite, the authors can calculate the size of the ring that started the cycle. And if you have the mass of that ring, you can calculate the size of the moon that spawned it, working backwards.
An ongoing cycle
The authors find there may have been six separate ring-moon cycles. The former had a lot of mass and moved quickly, but the later ones gradually slowed down, to the point where the existing cycle had been going on for about 2.5 billion years. As noted above, Phobos has about 70 million years left before it is destroyed by Mars’ gravity and creates a new ring, starting the cycle again.
All in all, the idea seems to neatly tie up a lot of loose ends related to the moons of Mars. “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, the process is anything but. After all, massive amounts of material would periodically rain into the Martian sky. In fact, the original ring would drop enough material to place every square meter of Mars under 370 meters of debris. If the debris rained preferably near the equator, the depth would be more than 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 more than 1,000 kilometers near Mars’ equator. Is there really enough of this loosely packed material to explain all the debris that must have rained down?
According to Asphaug, who called the debris “the model’s main testable hypothetical prediction,” we may already have some data relevant to this! One that can be tested because there’s already a rover in the middle: “Indeed, if it’s true, then the Curiosity rover is crawling over proto-Phobos ejecta, which the giant impact models don’t even think are Martian material.”
Natural Geosciences2017. DOI: 10.1038/NGEO2916 (About DOIs).