Japan is launching a mission to Mars’s largest moon Phobos

We may be on the verge of solving the mystery of the Martian moons. On 20 October, the Martian Moons eXploration (MMX) spacecraft launches towards the larger of the two moons, Phobos, with the goal of bringing back a sample that may finally explain how Phobos and its sibling, Deimos, formed. MMX will launch from Tanegashima Space Center in Japan. If all goes to plan, it should arrive at Mars in August 2027, entering an orbit that keeps it alongside Phobos so it can build a detailed map of the surface. Along the way, it will also observe the surface of Deimos. Once the map is complete enough, the spacecraft’s operators at the Japan Aerospace Exploration Agency (JAXA) will use it to pick out a spot on Phobos for MMX to land. During one or two brief landings, it will take on two tasks: to drop off a small rover called IDEFIX and to pick up samples. The rover is the first anyone has ever placed on such a small cosmic body, so part of its purpose aside from scientific observations is to confirm that a wheeled rover even works on an object with such low gravity. Advertisement The samples, though, are the most important part of the mission. MMX has two different sampling devices: one that shoots a puff of gas onto the surface, blowing dust and pebbles back up into a sampling container, and another that blasts a hollow cylinder into the ground to take a sample a several centimetres deep. After the samples are taken, they will be stowed away inside the spacecraft, which will then head back towards Earth. It is expected to arrive home in 2031. Once these samples get back to Earth and are analysed by scientists on the ground, they should finally solve the mystery of how Phobos and Deimos formed. For more than a century, researchers have debated whether the two are captured asteroids or formed from an impact on Mars, similar to how Earth’s moon formed. On first look, their shapes and surfaces resemble asteroids, but their circular orbits around Mars’s equator are more consistent with the impact hypothesis. Figuring out what they are made of should settle it once and for all. This will also help illuminate what the solar system was like billions of years ago when these strange moons formed. “Having the answer will help illuminate that early era when Earth and the other inner planets may have been acquiring much of their water and organic compounds,” said Tom Statler, part of the NASA team that is collaborating on the MMX mission, in a statement. If we are very lucky, the samples from Phobos may even contain bonus bits of dust and debris from Mars itself. Impacts on the planet could have launched material into space, which would have collected on the moon’s surface and been preserved there for billions of years. That means that these samples could illuminate not just the history of Phobos and Deimos, but also of Mars.