Formation of the Moon. Moons could come and go over the billions of years of the Earth’s history. For example, Mars has two Moons, but not for long. Phobos, the larger moon, is spiraling inward and expected to crash into the planet within the next 10 million years. And so, in the future, Mars will only have a single Moon, Deimos. It’s also possible that the Earth might capture a Moon in the future.