The two theories for how Mars got its moons, and how Japan's MMX probe aims to test them

Nobody knows yet how Mars got its two small moons, Phobos and Deimos. The team behind Japan's Martian Moons eXploration (MMX) mission sums up the debate as two competing ideas. In the "giant impact" hypothesis, a massive collision with Mars threw up debris that came together as the moons. In the "capture" hypothesis, the pair began as asteroids elsewhere in the solar system and were caught by Mars's gravity. MMX, which the Japan Aerospace Exploration Agency (JAXA) launched on October 20, 2026, aims to settle the question for Phobos by studying it up close and bringing more than 10 grams of its surface back to Earth, a return targeted for 2031.

Two small, dark moons

The American astronomer Asaph Hall discovered both moons in 1877, Deimos on August 11 and Phobos on August 17, and named them after the sons of Ares, the Greek counterpart of Mars: Phobos means fear and Deimos dread. Phobos measures 27 by 22 by 18 kilometers (17 by 14 by 11 miles) and Deimos 15 by 12 by 11 kilometers. Phobos goes around Mars three times a day and Deimos once every 30 hours.

Phobos is not going to last. It is moving closer to Mars by about 1.8 meters every hundred years, and at that rate it will crash into Mars or be pulled apart into a ring within 50 million years.

The case for capture, and the doubts

The moons' make-up is the main argument for capture. Both have low densities and dark surfaces, and the way they reflect light resembles carbonaceous asteroids, the MMX team writes. Both appear to be made of C-type material, similar to blackish carbonaceous chondrite asteroids, and Deimos's surface resembles asteroids in the outer asteroid belt. MMX's principal investigator, Kuramoto Kiyoshi, says the moons "closely resemble carbonaceous asteroids," the kind of body that could carry the ingredients for an atmosphere and water.

The resemblance does not settle it: capture remains possible, but some researchers have put forward evidence against it. Under the giant impact idea, the moons would instead have formed from the debris of a large body's collision with Mars. On the pages we read, the MMX team does not say which idea it favors.

Why the answer matters

The question reaches beyond Mars. By finding where the material in Phobos came from and when it reached the Martian system, the team says, MMX seeks evidence about how the solar system formed and how material moved within it. One of its stated goals is to investigate how planets formed in the transition region between the inner and outer solar system, where Mars sits, including how key materials such as water and organic matter were transported to the rocky planets.

Phobos may also hold a record of Mars itself. It orbits close to the planet and is thought to have collected a lot of material thrown off the Martian surface by meteorite impacts, over 100 times as much as Deimos by the team's estimate. Such grains could preserve clues about how Mars's surface changed, including signs of water from the time of oceans and lakes about 4 billion years ago, organic matter, or even possible signs of past life, the mission says.

How MMX will test the ideas

MMX lifted off on JAXA's H3 rocket from the Tanegashima Space Center at 04:41:03 Japan time on October 20, and a day later JAXA said it had completed its initial critical operations and was in good health. The mission's schedule has it spend about a year reaching the Martian system in 2027, then enter a quasi-satellite orbit around Phobos and observe it from 2027 to 2030. It is to collect its sample in two soft landings and observe Deimos in the final phase, leave Mars in 2030 and release its return capsule for atmospheric entry in 2031, with recovery planned in Australia.

The spacecraft carries 13 instruments. In the first half of the mission it is to map Phobos's shape and terrain and choose a landing site. A telescopic camera, TENGOO, can image the surface at a resolution of about 40 centimeters, and a gamma-ray and neutron spectrometer, MEGANE, will measure which chemical elements make up Phobos, information the team says is key to determining its origin. A rover named IDEFIX, developed by France's space agency CNES and the German Aerospace Center (DLR), is to land before the main spacecraft to test the ground. Two samplers will gather the material: a corer that digs to about 2 centimeters, and a pneumatic sampler that lofts surface grains with nitrogen gas, contributed by NASA and built by Honeybee Robotics.

The team chose Phobos over Mars and Deimos for practical and scientific reasons. Mars's strong gravity makes returning samples from its surface a major technical challenge, while a small body lets JAXA build on the sample collection methods of its Hayabusa and Hayabusa2 asteroid missions. Phobos has also been observed more than Deimos, which helps with picking a landing site, and its surface shows distinct "red" and "blue" regions that suggest more varied material. MMX describes itself as the world's first sample return mission to the Martian system; its sample is due back in 2031.