Someday, Mars’ moon Phobos will slip past a certain point in its degrading orbit and get ripped apart by tidal forces, forming a ring. This illustration depicts Phobos midway through that process, overlooking the Red Planet. Credit: Ron Miller for Astronomy

Phobos, it seems, is not long for this universe — at least on the large cosmic timescale.

Astronomers have long known that Phobos, the larger and nearer of the two martian moons, is slowly spiraling inward to eventual destruction. The end result won’t be pretty: Phobos will slip closer and closer toward Mars, then strike a gravitational line where the planet’s tidal forces will be strong enough to rip it apart. The rubble pile-like moon will break into smaller boulders, rocks, and dust, and will spread out in orbit around Mars.

Mars will join the gas giants in having a spectacular feature: a ring system.

It could be 25 million years from now. It could be up to 75 million years.…