The two lumpy moons, Phobos and Deimos, were both discovered in 1877 and named after the sons of Ares from Greek mythology (Mars is named after the Roman god of war, which is Ares in Greek).

Being so close, Phobos completes three orbits around Mars in one Earth day, while Deimos, which is more distant, takes about 30 Earth hours to complete an orbit.

What if it was once much larger and went through a few cycles where it went from a moon to a ring, and back again?

The researchers believe that Phobos once had a grandparent 3 billion years ago that they called proto-Phobos — likely 20 times more massive as the small moon is now.

This would have also affected proto-Phobos' orbital resonance with Deimos and pushed it further from Mars as well.

The Japanese space agency, JAXA, plans to send a spacecraft to Phobos in 2024 to collect samples from the surface and return them to Earth.