Mars Will Have a Saturn-Like Ring After Its Biggest Moon Disintegrates

Slowly but surely the Martian moon Phobos is spiraling inward toward its guardian planet.

As it approaches Mars, one of two Phobos fates is foretold: Either the moon will smash into the planet's surface, or it will be ripped apart by ever-increasing tidal forces before it has a chance. Both outcomes have seemed plausible enough, but, as detailed in a paper published today in Nature Geoscience, observational data and a new geotechnical model indicate disintegration is the most likely result owing to the moon's weak geology.

If Phobos does in fact break apart as it approaches Mars, it will mean the formation of a ring similar to those surrounding Saturn. This will occur in 20 to 40 millions years, which, at planetary timescales, is roughly tomorrow or so.

While our own Moon is slowly spiraling away from Earth, Phobos will make no such escape. This is sort of just dumb luck—satellites with orbital periods less than the rotational periods of their central body (the planet) will migrate inwards, while those with larger orbital periods will migrate outwards. It's thought that inwardly migrating moons were more common in the early Solar System than they are now, and that Saturn's rings were formed as tidal forces stripped material away from that planet's own ill-fated moon.

"Phobos thus offers the last possible glimpse of the signatures and processes that applied to inwardly migrating moons and the interplay with ring formation early in our solar system's history," the new study explains.

A proposed timeline of Phobos's decay into a ring, and that ring's ultimate decay. Image: Nature

The researchers behind the report, planetary scientists Benjamin Black and Tushar Mitta, were able to come up with an estimate for the tidal forces likely to be exerted by Phobos on Mars as it approaches the planet's upper atmosphere by looking at how the gravitational field of Mars changes periodically and at how quickly the moon's orbit is decaying. For now, Phobos survives thanks to a balance between tidal forces and its own inward gravity, but whether this balance is able to persist in the future depends on the moon's geology.

Obviously, it's not all that easy to model the guts of a distant moon, but Phobos has a helpful feature in the form of the Stickney impact crater. At some point, a very large something smashed into the moon, leaving a scar occupying roughly one-sixth of its total circumference. Because Phobos didn't get obliterated in the collision, the scientists can conclude that it must have a fairly fragmented and porous interior (probably more so now after the hit). Otherwise, it wouldn't have been able to absorb the impact quite so efficiently.

So, we can say that Phobos is nowadays probably pretty fragile, and Phobos's most fragile regions are expected to break up first.

"We conclude that Phobos's inward migration will culminate with division of the satellite according to its internal distribution of strength and damage," Black and Mitta write. "The most damaged regions will disperse into a planetary ring."

The disintegration of Phobos into a planetary ring will probably happen pretty quickly, they note: 50 to 130 orbital periods or so. For future Mars dwellers, it will be quite a show.