At a glance, Phobos looks like a space potato. It’s not as neatly rotund as our moon, nor is it as striking as Saturn’s smoggy Titan or Jupiter’s icy patterned Europa. If anything, it looks like a rogue asteroid that Mars ensnared and tamed into a pet (or a Mark Watney Martian experiment gone awry).
Though they’re irregularly shaped and share some asteroid DNA, astronomers don’t actually think they’re asteroids.
Mars actually has two domesticated pets: There’s Phobos, and then there’s the petite Deimos, which orbits farther away. Other than the fact that they orbit around Mars, not much about them is what we consider to be moonlike. Most notably, they’re lumpy, and they zip around Mars quickly. Phobos clocks orbits Mars three times every day, and Deimos clocks in at about once a day.
When comparing them to other moons in our solar system, they’re also among the smallest. Phobos measures about 14 miles (22 kilometers) across, and Deimos is only 8 miles (13 km) long. (Titan, on the other hand, is about the size of Mercury.)
For a long time, humans didn’t even know Mars had moons. Though Johannes Kepler had a hunch, it was American astronomer Asaph Hall who discovered the duo in 1877. The moons had been traveling so close to Mars, they had been hidden in the planet’s glare. And because their surfaces are the same substance as asteroids in the outer asteroid belt — specifically, carbonaceous chondrites — scientists believed they were trapped asteroids.
The key to their origin, according to Space.com, is in their orbits. If Mars had actually harnessed them with an assist from Jupiter’s gravity, their orbits wouldn’t be as clean. Phobos and Deimos have circular paths around the planet, whereas a captured object moves much more unevenly.
It’s also possible that the moons formed from Mars’ debris during its birth — or that a young Mars suffered an impact that broke it into pieces. Though much is still unknown about their beginnings, if the moons were indeed caught, the capture mechanism remains unknown and the scenarios improbable. But one thing is certain: one way or another, Mars is home to potatoes.