A Mars probe from the Japan Aerospace Exploration Agency (JAXA) may answer some longstanding questions about the Red Planet and its moons. The mission—dubbed MMX for “Martian Moons eXploration”—aims to collect at least 10 grams of samples from Phobos, one of two moons orbiting the planet, and if successful, these will be the first rocks from a Martian moon ever brought to Earth.

They won’t arrive quickly, however. The MMX probe will take approximately one year to reach Mars. It will then collect samples during three years of operations while orbiting the red planet before taking another year to return to Earth. If all goes well, scientists expect to receive the moon rocks in 2031.

One of the most interesting questions the probe can answer is how Mars’ moons, Phobos and Deimos, formed. There are two main possibilities. One is that the moons condensed from material ejected during a massive collision with Mars. The other hypothesis is that the moons were asteroids from the outer solar system that were captured by Mars’ gravity.

The moons are dark in color, and their light-reflecting properties closely resemble those of asteroids rich in water and carbon. Their orbits, however—which are nearly circular along Mars’ equatorial plane in the same direction as the planet’s rotation—are better explained by the giant impact hypothesis. Comparing samples from Mars’ moons to those previously collected from the Martian surface could help settle this debate.