Japan in planning to launch a mission to visit the two moons of Mars in 2024. The spacecraft will touchdown on the surface of Phobos, gathering a sample to bring back to Earth. But what is so important about a moon the size of a city?

Unlike the spherical shape of the Earth’s moon, the Martian moons resemble asteroids, with an asymmetric lumpy potato structure. This highlights one of the first mysteries about the pair: how did...