Mars' 2 weird moons began as 1 large moon, study suggests

An impact may have split the moon in two.
By Kellen Beck on February 24, 2021

An artist's interpretation of the event where a single moon orbiting Mars was struck and split into the two moons Phobos and Deimos. Credit: Mark Garlick

Mars may have had a single moon before something smashed into it, tearing it asunder into the two moons we see today.

In a study published Monday in Nature, scientists explained how they used the orbital patterns of Phobos and Deimos along with seismic data from NASA's InSight mission to create a simulation of where their paths began, like a digital time machine. When they looked far enough back, it indicated the orbits once intersected, giving rise to the new smashed-moon theory.

While many moons in our solar system are spherical bodies like our own moon, Phobos and Deimos are not, explained Amirhossein Bagheri, lead author on the study and a doctoral researcher at the Institute of Geophyics, ETH Zürich.

“The Martian moons are quite irregular,” he said in a phone interview. “They are really potato-like things. They don't look like our moon.”
What makes Phobos and Deimos stand out

As moons go, Phobos and Deimos are quite small. The larger, oblong Phobos is roughly 157 times smaller than Earth’s moon with an equatorial circumference of around 43 miles. It and its little brother are much less dense than our Moon, meaning their interiors are more porous and possibly fractured, a fact gleaned from Phobos's large Stickney Crater.


Stickney Crater is visible on the lower right side of this image of Phobos, the larger of Mars's two moons. Credit: Nasa / Jpl-Caltech / University Of Arizona

“After the Viking mission in the '70s, there was almost a consensus that Phobos and Deimos were captured asteroids,” Bagheri said, meaning Phobos and Deimos were asteroids traveling through our solar system that were trapped in Mars’s orbit.

The problem is the moons' orbits don’t line up with that theory.

Bagheri said an asteroid caught by Mars should have an elongated orbit.

“We expect that the orbit would be really eccentric, but that's not the case for the Martian moons," Bagheri said. "They're quite circular.”

Not only are the orbits near-circular, they also occur around the equator of Mars. That's what you would expect to see from moons that were formed alongside or from a rotating planet — called “in situ” (Latin for “on site”). Asteroids, meanwhile, can begin orbiting a planet at a wide range of angles and would likely not be orbiting as close to the equator as Phobos and Deimos are.

Earth’s moon is generally believed to have formed after Earth was hit by something roughly the size of Mars 4.5 billion years ago. The impact kicked up a bunch of molten debris, which swirled around the rotating Earth and eventually formed our nice, round moon with its circular, equatorial orbit.
Stepping through the orbital time machine

Phobos and Deimos are not locked at a certain distance from Mars. Phobos, the closer moon, is slowly moving closer toward Mars at a rate of roughly six feet per year. Deimos on the other hand is moving away from Mars several inches each year.

Taking that in consideration along with other, complex factors, the team was able to create the simulation, explained fellow study author Michael Efroimsky, research scientist at the U.S. Naval Observatory.

“It turns out that at some point, the orbits of Phobos and Deimos intersected or were extremely close, which gives us an opportunity to think they are probably remnants of a larger body,” he said in a phone interview.

He said if the impact was intense, the parent body could have just disintegrated. If it wasn’t very intense, it’s possible it could have split into two large pieces, one of which fell a little closer to Mars and the other spun out a little further.

Some key pieces to putting together the puzzle of the simulation were the interiors of Mars and the moons.

We don’t know much about Phobos and Deimos because we don’t have spacecraft on them. But we have new information about Mars from NASA’s InSight, which measures seismic activity. By looking at marsquakes, scientists can piece together a map of the planet's deep interior. This information paired with tidal calculations help create a more accurate simulation.
Solidifying the theory

To make this moon-smashing theory more solid, more evidence is needed to more accurately define Phobos and Deimos, two bodies that haven’t seen any visitors on their surfaces.

“Of course, the most reliable way to test our theory would be to have some geological probes,” Efroimsky said. “That would be absolutely wonderful.”

Luckily, a mission called Martian Moons Exploration from the Japanese Aerospace Exploration Agency will investigate both moons. The plan is to launch in the mid 2020s. It will also return a sample of Phobos back to Earth, if everything goes well.

“We have been in touch with them,” Bagheri said.
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If the mission’s instruments are accurate enough and the tidal forces are large enough (which Bagheri thinks they are), they could better understand the interiors of the moons.

“Basically, that can solve the $10 question,” he said.