The small, misshapen moons of Mars, Phobos and Deimos, have puzzled and intrigued planetary scientists for a long time, though Phobos has captured the most attention. Scientists believe that in a few tens of million years the moon is destined to be pulled apart by Mars’ gravity, and when that happens a ring will form around the Red Planet.
A new piece of research presented at the 236th Meeting of the American Astronomical Society (AAS) held virtually on June 1-3, and accepted for publication in The Astrophysical Journal Letter, goes a step forward. The team believes that Mars had rings in the past, and this happened several times in a cycle of formation and destruction between rings and moons.
The evidence suggests this doesn’t come from Phobos but from the smaller and more distant Deimos. Unlike Phobos, Deimos’ orbit is tilted by 2 degrees with respect to the plane of Mars’ equator. Researchers at the SETI Institute and Purdue University tried to model this and came to the conclusion that the influence of something around Mars must have caused the shift, although with some caveats. The properties of Deimos make the most sense if a much larger moon than Phobos was being pushed outward.
This led to the suggestion of a ring system. A ring could push an object 20 times the mass of Phobos outwards, pushing Deimos into its current trajectory. But where did this moon go? It's likely the material in the ring fell in on Mars, as did the moon, eventually getting ripped apart and forming a new ring system.
The researchers believe it took two more ring-moon cycles to arrive at the current Phobos, and this may have happened just 200 million years ago.
“The fact that Deimos’s orbit is not exactly in plane with Mars’s equator was considered unimportant, and nobody cared to try to explain it,” lead author Matija Ćuk, a research scientist at the SETI Institute, said in a statement. “But once we had a big new idea and we looked at it with new eyes, Deimos’s orbital tilt revealed its big secret.”
This prediction will have the chance to be tested soon. The Japanese Space Agency, JAXA, is planning to send a probe to Phobos to collect samples before returning them to Earth. Those might tell us the age of the moon and if this curious idea is correct.