Like the gas giants in the outer region of the Solar System, Mars may have been circled by a ring of debris over three billion years ago.
Astronomers from the SETI Institute and Purdue University have spotted tantalizing signs that the red, rocky planet once supported a ring system. The clues lie in its two potato-shaped moons, Deimos and Phobos.
"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 Matija Ćuk, a research scientist at the SETI Institute and lead author of the research accepted at the Astrophysical Journal Letters. "But once we had a big new idea and we looked at it with new eyes, Deimos's orbital tilt revealed its big secret."
The paper is being presented at the virtual 236th Meeting of the American Astronomical Society this week.
Deimos, with a radius of just 6.2km, has a small orbital tilt of two degrees. Ćuk believes that its motion can be explained by a historic orbital resonance encounter that nudged it into a more distant and slightly wonkier orbit. Under this scenario, Mars is encircled by a ring, Deimos is being pushed into a further orbit and there's a third moon that lies beyond the ring.
The gravitational interactions between Deimos and the Martian ring system makes the satellite drift further away from its parent planet until it enters a 3:1 orbital ratio with the mystery moon. Deimos's orbit aligns with the mystery moon's three times as the more distant satellite completes one orbit.
Each time Deimos nears its neighbour, the third moon exerts a small gravitational kick that tilted Deimos's orbit. In order for this to happen, however, Mars had to have a ring of debris that was pushing Deimos out in the first place.
There's a second bit of evidence for Mars' ancient ring too. Phobos's orbit is being dragged downwards by the red planet's gravitational field. Over time, it'll drop so much that it will be ripped apart by tidal forces and all the leftover fragments will litter the Martian environment. No one is quite sure when that will happen. The best estimate is in about 40 million years or so, we're told.
Mars will capture the individual pieces and the debris will form a ring that orbits the planet. The researchers believe that the process is cyclical; larger chunks of rock will lead to a new, smaller moon that will in turn be pulled apart to create a new ring and so on and so forth.
"Since Phobos and Deimos have orbits close to the plane of Mars's equator, we think they must have formed from a flat ring and disk at some point," Ćuk told The Register. "Past work by my coauthor Dave Minton from Purdue proposed that Phobos is the latest product of the ring-moon cycle at Mars that happened several times and will happen again.
"Other people thought that Mars had a ring only at the very beginning – when the moons formed – and [will] never [have one] again. My result that a larger inner moon of Mars was moving outward requires that there was a ring and that Phobos had a larger precursor, and so I basically confirmed Dave's theory."
They hope to test their theory over the next few years. Japan's space agency, JAXA, plans to launch a spacecraft to Phobos in 2024 and bring samples back from the moon's surface. "I do theoretical calculations for a living, and they are good, but getting them tested against the real world now and then is even better," he concluded. ®