The Martian moon Phobos is thought to be a pile of rubble that’s nearly a third empty space inside. It circles its planet twice a Martian day, and in a few million years it will disintegrate into rings.
Phobos is one of the stranger moons in our solar system. There’s a lot of competition for that title, because because our solar system has some pretty strange moons: ice volcanoes, polar vortices, underground oceans, methane rain, dunes hundreds of kilometres long… and that’s just Titan.
Anyway, Phobos is thought to be a celestial rubble pile. Essentially, some big chunk of solid rock (say an asteroid or moon) gets smashed up, and all the various bits of rock fly apart. But the weight of those bits of rock pull each other back into a semi-solid lump. You’re left with a loose aggregation of rubble that has just enough gravity to stick together, albeit with a bunch of holes and gaps inside it. Most asteroids and perhaps some comets are thought to be rubble piles. But rubble pile moons are pretty rare.
Phobos, like most rubble piles, is full of holes. We cannot see them directly because the surface of the moon has a layer of dirt / dust (regolith) about a hundred metres thick, but if you measure the density of the moon it is estimated to be between a quarter and a third empty space inside. Back in the 1950s a Soviet scientist thought it might be a hollow sphere built by Martians, but alas this moon is completely natural.
This moon orbits really close to Mars. 6,000 kilometres above the surface of the planet, on average. Our moon, by comparison, is on average more than 60 times further away from Earth (384,400km). Because it is so close it orbits really quickly: over one Martian day Phobos will circle the planet twice. And the distance between the moon and planet is shrinking by about 1.8 centimetres a year as Phobos is pulled downwards.
In a few million years (30 to 50) Phobos will hit the Roche limit. Essentially, this is the point at which orbiting material is torn apart by the tidal forces of the planet beneath it. Above the Roche limit, you have a moon. Below the Roche limit, you have rings. If you’re old enough to remember the disintegration of the comet Shoemaker-Levy 9 back in the 1990s, you’ll understand that it broke up because it passed within Jupiter’s Roche limit.
Phobos also has a 90-metre-high monolith sticking out of it. It’s probably a natural feature, but I still guess we should attempt no landing there.