How a missing moon could explain Mars's mysterious little satellites

The Martian moons, Phobos and Deimos, have long intrigued scientists. The two small, potato-shaped space rocks that whirl around Mars aren't anything like our own moon. They're much smaller, oddly shaped, and orbit the Red Planet farther away than scientists would expect to see in a system with just two little moons. So how did they get there?

Some scientists argue Mars's moons are asteroids, stolen from the Asteroid Belt eons ago. Others say they formed from the debris shot into space after a large celestial body slammed into the planet billions of years ago, in a scenario similar to how our own moon formed. But neither theory fully explained their unique orbits. 

To sort out the complexities, an international team of researchers created a computer simulation of the impact theory. Their model suggests another, larger moon facilitated the formation of Phobos and Deimos. Their findings are reported in a paper published Monday in the journal Nature Geoscience.

When scientists first began puzzling over the two moons, most assumed that the lumpy satellites, just 14 and 8 miles across, must have been asteroids captured by Mars's gravitational field. 

But "you can't, in any reasonable way, make the moons of Mars by capturing them as asteroids," says David Stevenson, a planetary scientist at the California Institute of Technology who was not part of the new study, in an interview with The Christian Science Monitor. "The dynamic challenges in capturing asteroids and putting them in orbits like Phobos and Deimos are so huge that people, for the most part, decided long ago that that was not a good idea."

So then the question was, can an impact scenario explain it? Yes, says the lead author of the new paper, Pascal Rosenblatt of the Royal Observatory of Belgium. But it was probably a complex process, he acknowledges.

The story goes something like this: 

Billions of years ago, a celestial body about one third the size of Mars slammed into the Red Planet. Debris from the impact shot out into space but didn't escape the Mars system. A ring of dust and debris, similar to the one around Saturn, formed around the planet. 

Over time, the close-in debris spiraled back into Mars, while the outer rings combined to form the planet's satellites. 

That story sounds simple enough, but there's a catch. The current system doesn't match what scientists would expect based on calculations of their formation. 

One such challenge of this story is the "synchronous radius," which is the "dead zone" within which satellites will fall back into their host planet. In the Mars system, that distance is about 18 Mars radii.

Mars rotates slowly enough that satellites or debris orbiting within the dead zone have to move faster than than the planet spins. The problem is that the satellites outpace the tidal bulge they pull up, which creates enough torque to steal momentum from the satellite, ultimately causing it to spiral down into Mars. 

Deimos orbits Mars outside the synchronous limit, so that little moon is relatively stable. But the closer and larger moon, Phobos, is getting sucked in, and is on track to be torn apart by the pull of Mars in 20 to 40 million years.

But for both Phobos and Deimos to form outside the dead zone, there would have had to be more going on, according to Dr. Rosenblatt's models. There wouldn't have been enough collisions among particles that far out to form these little moons without another force in action.

So Rosenblatt and his colleagues tried introducing other satellites into their models. And they found that if at least one moon tens of times larger than Phobos and Deimos also formed in the accretion disk, but closer to Mars, it could have helped concentrate the debris to facilitate the formation of Mars's current satellites. Then, because it was within the destruction zone, that larger moon would have been destroyed long before astronomers on Earth ever had a chance to spot it.

"The proposed scenario can explain why Mars has two small satellites instead of one large moon," Rosenblatt and his colleagues write in the paper.

"There could have once been many moons around Mars, the most massive sculpting the system and the smallest being the last to come down," explains Erik Asphaug, a planetary scientist at Arizona State University, in a Nature News and Views article about the new paper. 

And, he writes, as Phobos is already within the synchronous radius, "Phobos could be the straggler in a series of crashing moonlets, readying for its final approach."

It is possible to test Rosenblatt's model, as the destruction of the larger inner moon would have left debris on Mars's surface. Studying the composition of Phobos and Deimos could also help confirm an impact model, as debris from both the impactor and Mars would have been kicked out to space, where it accreted into moons.

"This is not a new story," Dr. Stevenson says of the impact model. But this paper is "a more complete description of the kind of story that we think was needed."

He cautions, "It's just one piece of the jigsaw puzzle." 

Understanding the Mars system could help scientists better understand satellite formation more generally, he says.

"We are trying to understand if there is some universal mechanisms to form moons around planets," Rosenblatt tells the Monitor. This could help researchers better understand the history of our own solar system and exoplanetary systems, as astronomers continue to probe farther into the universe.