Which moon of Mars will crash or shred into a ring within 50 million years?

Phobos, which orbits closer to its planet than any moon in the solar system - about 6,000 km above the surface - and is spiralling in by a couple of centimetres a year. Tidal forces are already opening long parallel grooves across it, which are thought to be early structural failure.

It orbits faster than Mars rotates, which produces something odd: from the surface it rises in the west and sets in the east, and does so twice a day. That same fast orbit is why it is doomed - the tidal interaction that pushes our Moon away pulls Phobos in.

What it does at the end depends on how solid it is, and it does not appear to be very. Its density suggests a rubble pile perhaps a quarter empty space, in which case it will be pulled apart before impact and Mars will have a ring for a few million years before the debris rains down. Both Martian moons are also unexplained: they look like captured asteroids, but their near-circular equatorial orbits are hard to reconcile with capture, so the current preference is for an ancient impact throwing up debris that re-formed.