They were long thought to actually be asteroids captured by the planet's gravity. Both appear to be similar in composition to blackish carbonaceous chondrite asteroids. But scientists realized that the moons are nearly in the same plane as the Martian equator. This suggests that they formed around the same time Mars did around 4.5 billion years ago.
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The researchers looked at Phobos, which loses height as it interacts with Martian gravity over time. Eventually, its orbit will be too low and Mars will essentially rip it into pieces that form a ring around the planet. It's estimated that this will happen within 50 million years.
What if Mars once possessed other moons that met the same fate? Over time, the rings would form new, smaller moons in a seemingly endless cycle.
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Something pushed Deimos out away from the planet and caused it to tilt — like interacting with another, larger moon.
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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. Over the course of a few moon-ring cycles, it became Phobos — which would explain why some researchers believe it only formed about 200 million years ago.
