These days, our neighbor Mars has just two moons: Phobos and Deimos. But according to a study published Monday in the journal Nature Geoscience, these little satellites may actually be the planet’s last holdouts. They may have outlived a whole host of Martian moons.
According to the study, the two potato-shaped modern moons probably aren’t just misshapen asteroids caught in Mars’s gravitational pull — a theory some scientists have suggested, though the consistent and regular orbit of the two moons seems to contradict this origin story. Instead, the new study reports, a huge impact event on Mars could have created them.
The reason this idea isn’t already widely accepted — after all, we’re pretty sure our own moon formed after a big impact — is that it poses a few problems: For starters, scientists would expect the moons formed by this kind of impact to be larger. Phobos and Deimos are just 14 and 8 miles wide, respectively.
But the researchers led by Pascal Rosenblatt of the Royal Observatory of Belgium think they’ve found a solution: According to their simulations, the kind of large inner satellite one would expect to result from this impact could have coexisted with — or perhaps even created — the moons we know today.
And as time went on, their simulations show, that moon could easily have been lost.
Previous research has suggested that a collision massive enough for moon-making formed the so-called Borealis basin, which covers about two-fifths of the planet’s surface. It’s thought that an object about 1,250 miles wide slammed into the red planet and spewed debris into space.
But the amount of material ejected would have totaled 100 times the mass of Phobos and Deimos combined. That’s where the big mystery moon — and likely a whole gang of little lunar cousins — come into play.
The debris, the researchers say, would have orbited Mars to form a disk and could have created an inner moon at least 125 miles in diameter. Once this moon existed, its gravitational clout could have helped other moons clump together from the remaining debris, including ones as puny and lumpy as Phobos and Deimos.
So, where’s the moon?
You can look to Phobos for clues: Because of how slowly Mars spins relative to the speed of its moon, it’s dragging it slowly inward. (This is the opposite of the dynamic between Earth and our own moon, as our relatively quick spin is sending our satellite farther and farther afield.)
Deimos has a stable orbit for now, but Phobos is expected to break up sometime in the relatively near future — as soon as 20 million years from now — giving Mars a Saturn-like ring in lieu of its second potato-moon. According to the new study, the original, larger moon met this fate about 5 million years after its formation, as did any of the other moons it may have helped create.
“It would have broken into very small parts that could in fact burn through the atmosphere or create very, very small impact craters,” Rosenblatt told the Guardian.
“Unlike other ideas invoking planets or moons that have long since disappeared, this hypothesis is more testable than it sounds,” Erik Asphaug of Arizona State University, who wasn’t involved in the study, wrote in a commentary for Nature. “The destruction of an object hundreds of kilometers in diameter, a few million years after the formation of Borealis, would have left a profound geologic record.”
Scientists could go looking for impact craters that match those that would have been left behind when the missing moons crashed into the Martian surface. They could also test Phobos and Deimos to see if the moons are made of the same stuff as Mars, which would further support a collision origin.
A Japanese mission called the Martian Moons Explorer would collect and return the kinds of samples from Phobos that could make this possible. But it’s not slated for launch until 2022 at the earliest.