Mars May One Day Have A Ring System Like Saturn

Phobos, the larger of Mars' moons, imaged from a distance of 6,800 kilometers (4,200 miles). The Stickney impact crater dominates one hemisphere of the moon. NASA/JPL/University of Arizona

Phobos is getting closer and closer to Mars, and it’s liking the fall so much that it might just put a ring on it. Astronomers think that in 20 to 40 million years, Phobos will break apart, giving the Red Planet a ring comparable with the outer planets of the Solar System. 

According to the research, published this week in Nature GeosciencePhobos is being pulled apart by Mars’ gravity. The closer it gets to the Red Planet, the more intense these forces will be. If the moon breaks apart before it enters the Martian atmosphere, it would create a long-lived ring system, capable of remaining stable for millions of years.

Phobos is the larger and closer of the two natural satellites of Mars. The object and its companion, Deimos, are most likely captured asteroids, stolen from the main asteroid belt. It has a mass of more than 10 trillion metric tons, which is comparable to Uranus' brightest ring, but less than one-thousandth of the mass of the Saturn ring system.

The research details a model for the demise of the satellite. There are uncertainties though, both on the timescale and actual formation of the Martian ring, which depend on the still mysterious composition of Phobos. The moon, or at least large chunks of it, could still survive Mars' relentless gravity and rain down on the planet in the next 50 million years.

The forces acting on Phobos from Mars are called tidal forces, and they are very weak. If Phobos is a solid object, there won’t be any tidal disruption before it passes the point of no return and enters the Martian atmosphere. Not every asteroid is a solid object, though; some are a big aggregation of material kept together by their very weak gravity. These objects are considered “rubble-piles”, and there are some indications, based on surface stretchmarks that Phobos might be one of them. If it is, it would create a ring around Mars.

“A future mission to Phobos, such as the proposed PADME, PANDORA, and MERLIN Discovery-class missions, will provide better constraints on the interior structure and strength of Phobos, and consequently will allow us to test this conclusion,” said the authors, Benjamin Black and Tushar Mittal, in the paper. “We speculate that diminutive Phobos may be the last of many inwardly migrating prograde satellites in our Solar System.”

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