The two lumpy moons, Phobos and Deimos, were discovered in 1877 and are named after the sons of Ares from Greek mythology (Mars is named after the Roman god of war, who is Ares in Greek). Phobos means fear or panic and Deimos means fear.
For a long time it was thought that they were actually asteroids captured by the planet’s gravity. They both seem to be similar in composition to blackish carbonaceous chondrite asteroids. But scientists realized that the moons are almost on the same plane as the Martian equator. This suggests that they formed around the same time that Mars did so about 4.5 billion years ago.
While both are irregular, Deimos is slightly rarer than Phobos – its orbit is tilted two degrees.
“The fact that Deimos’s orbit is not exactly in the plane with the equator of Mars was not considered important, and no one cared to try to explain it,” said Matija Ćuk, lead author of the study and a research scientist at the SETI Institute. “But once we had a great new idea and looked at it with new eyes, Deimos’ orbital tilt revealed his great secret.”
A cyclical destination for the moons of Mars?
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 break it into pieces that form a ring around the planet. This is estimated to happen in 50 million years.
What if Mars ever owned other moons that had the same fate? Over time, the rings would form new, smaller moons in an apparently endless cycle.
Something pushed Deimos off the planet and made him bow, like interacting with another larger moon.
Phobos, slightly larger than Deimos, is only 3,700 miles above the Martian surface; No known moon in our solar system is closer to its planet. 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 one orbit.
But what if Phobos wasn’t always, well, Phobos? What if it was once much bigger and went through a few cycles where it went from a moon to a ring, and back?
Researchers believe Phobos once had a grandfather 3 billion years ago that they called proto-Phobos, probably 20 times more massive than the small moon 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.
A prominent ring around Mars would have pushed the proto-phobes out, away from the planet. This would also have affected Proto-Phobos’ orbital resonance with Deimos and would also have alienated it from Mars.
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. This may reveal more about the moon’s past.