Research Press Release

Mars' suicidal moon Phobos may put a ring on it

Nature Geoscience

November 24, 2015

One of the two moons that orbit Mars may break apart in 20-40 million years and gift the red planet its own ring, according to a study published online in Nature Geoscience. The study predicts the ring will persist for millions of years and eventually have a mass density similar to those of Saturn.

Phobos, the larger of Mars’s two moons, is gradually spiralling inwards towards the planet, unlike our own Moon, which is gradually spiralling away from the Earth. Eventually, Phobos will either break apart due to increasing tidal stresses induced by the gravitational pull of Mars or crash into Mars, fates that are expected for any inwardly migrating moon.

Benjamin Black and Tushar Mittal use observational data and a geotechnical model to calculate the strength of Phobos, and find that much of the moon is composed of weak materials. They predict that these will break apart once sufficient tidal stresses are reached in 20-40 million years. The particles are then expected to disperse to form a ring around Mars that the authors estimate will persist for 1-100 million years. They note that any large piece of Phobos that is strong enough to remain intact during the tidal breakup will eventually collide with Mars and form a crater.

Although only the outer Solar System gas giants currently have rings, the study suggests there might be a ring in Mars’s future and offers a glimpse into how other inwardly migrating moons in our Solar System may have self-destructed long ago.


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