Mars has two small moons, Phobos and Deimos, but their origins remain uncertain. Were they asteroids captured by Mars’ gravity, or did they form from debris following a giant impact on the planet? Japan’s Martian Moons Exploration (MMX) mission aims to solve this mystery by collecting samples of soil from Phobos, the largest of the two moons. Laboratory experiments in the UK are helping prepare scientists to interpret its findings.