The new hint involved Deimos, the smaller of the two Martian moons that raise a number of questions. It is rotating around the Red Planet at an insignificant tilt in accordance with Mars’ equator and could be the result of the gravitational stunts created by a planetary ring.
Ringed Celestial Systems
Ring systems are not that unusual, as a matter of fact. Some of these systems include Saturn, as well as about half of the Solar System with Uranus, Neptun, Jupiter, dwarf planet Haumea, and centaurs Chiron and Chariklo also featuring rings.
Back in 2017, a team of scientists theorized that Mars also had a ring. They carried out simulations of the larger Martian moon known as Phobos and discovered that it could have taken shape after an asteroid crashed into Mars, spreading debris into space, and forming a ring that assembled into an earlier form of Phobos.
Now the new study has considered Deimos as well and found that the models are in total agreement with the previous discoveries.
“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,” said astronomer Matija Ćuk of the SETI Institute. “But once we had a big new idea and we looked at it with new eyes, Deimos’s orbital tilt revealed its big secret.”
Deimos’ orbital tilt is not massive, and its orbit is rather normal, with a cycle taking about 30 hours, but when it comes to Phobos, there’s something weird about it. It is a lot closer to Mars, has an orbit of seven hours and 39 minutes, and it is getting closer and closer to the planet by 1.8 cm (0.70 inches) per year.
Employing numerical simulations, Ćuk and his fellow colleagues tried to figure out how such an outward-moving proto-Phobos would have impacted Deimos’ orbital tilt. They reached at proto-Phobos 20 times the moon’s current mass, which could have created an orbital resonance with Deimos at about 3.3 Mars radii distance that sent the latter’s orbit into a minor tilt.
This clearly designed the Deimos orbit we currently see, which then remained almost unchanged for billions of years.
This process must have occurred, Ćuk said, after the Late Heavy Bombardment of asteroids, which took place about 3.9 billion years ago, and probably would have shattered Deimos. Then, the moon would have rebuilt, but at zero inclination. However, the process couldn’t have happened too much after, due to the fact that the proto-Phobos-Deimos resonance needs low tilt at the start.
That presumable destruction and rebuilding at a low inclination of Deimos also means that the asteroid collision is probably not the cause of the disruption to the moon’s orbit. Also, an asteroid that traveled by would have messed with both inclination and eccentricity. However, since Deimos’ eccentricity is extremely low, that is not a probable scenario. When it comes to proto-Phobos, the moon would have been gravitationally squished by Mars again.
“Once the ring was gone, the moon also started falling because of Martian tides (just like Phobos),” Ćuk explained. “Once it was too close to Mars, tidal forces would pull it apart into a new ring, and the cycle would repeat, probably twice, to get to Phobos that we see.”
This means that current Phobos most likely took shape about 200 million years ago. The study has been presented at the 236th Meeting of the American Astronomical Society and published in The Astrophysical Journal Letters. It is currently available on arXiv.