D: Hey Yaël, can you picture Mars with a ring around it?
Y: I guess I can. But I don't know why I would.
D: That's what scientists think Mars is going to look like in tens of millions of years. They're predicting that one of Mars' moons, Phobos, is going to break apart, and its particles are going to make a ring around Mars.
Y: Phobos is the moon that orbits really close to Mars, right?
D: Right. And it's orbiting a few centimeters closer every year. That's making the tidal forces caused by the gravitational pull of the planet on the moon get stronger, and all that stretching is slowly deforming Phobos. The moon isn't very solid -- it's considered a "rubble pile" -- so scientists think the tidal forces will eventually get strong enough to tear the moon apart. The whole process will probably take less than forty days once it starts. Of course, the universe has a long time to wait for all that to happen -- it won't be for another twenty to forty million years.
Y: Do we know how long Mars gets to keep its new ring?
D: It depends on how far Phobos is from Mars when it finally breaks apart. If the moon breaks up when it's closer to Mars, the new ring would persist for about a million years before it spread to Mars' atmosphere and started losing material. But if the moon breaks apart farther away from Mars, the ring could persist for a hundred million years.
Y: One million and one hundred million. I wonder if there'll be anyone on Earth in twenty or forty million years from now for it to make a difference.