Science Puzzle
Why Are Planets Round but Asteroids Lumpy?
Every planet is a ball, and so is our Moon. But most asteroids, and small moons like Phobos, which goes round Mars, are lumpy, like potatoes covered in craters.
Why are big worlds round when small ones aren't?
The Answer
A big enough world's own gravity is stronger than the rock or ice it's made of, so it pulls itself into a ball. A small world's gravity is too weak to reshape it, so it keeps whatever shape it was made in.
Every bit of a world pulls on every other bit. In a small asteroid that pull is feeble, and solid rock is easily strong enough to hold up a lump or a spike. In a big world the pull is enormous. Over millions of years, rock deep inside under all that weight slowly flows, like extremely thick toffee. High parts sag and low parts fill in until every point on the surface is as close to the centre as it can get. The only shape where that's true is a sphere.
The change happens at a few hundred kilometres across. Ice is weaker than rock, so icy worlds go round sooner: Saturn's moon Mimas, about 400 kilometres across, is round. The asteroid Vesta is bigger, at about 525 kilometres, but it's made of rock and is still slightly lumpy. Ceres, at about 940 kilometres, is round, which is one reason it counts as a dwarf planet.
The same tug of war limits mountains. On Earth they can't grow much beyond 10 kilometres high before the rock beneath them starts to give way. Mars has about a third of Earth's gravity, and its biggest volcano, Olympus Mons, stands about two and a half times as tall as Everest.
The principle: Hydrostatic equilibrium. Once a world is more than a few hundred kilometres across, its own gravity overpowers the strength of its rock or ice and pulls it into a sphere.