Earth has one large, round Moon. Mars has two small, potato-shaped ones, and both are deeply weird. Phobos, the larger, is only about 22 kilometres across, orbits Mars faster than Mars rotates, and is slowly falling toward the planet. Deimos, the smaller, is about 12 kilometres across and drifts in a distant, quiet orbit. Together they raise a question that still has no settled answer: where did they come from?

Phobos orbits Mars in just over seven and a half hours, which means it rises and sets twice per Martian day. From the surface of Mars it would appear to move backwards across the sky compared with most moons, because it laps the planet's rotation. It is so close to Mars that it orbits below the height where a geostationary satellite would sit around Earth, skimming just a few thousand kilometres above the surface.

Deimos is the opposite: small, far out, and calm, taking about thirty hours to circle Mars. Both moons are dark, cratered, and irregular, more like asteroids than like the spherical moons we picture. Their surfaces are covered in a fine, dark dust, and Phobos carries a set of strange parallel grooves that scientists have debated for decades.