How did Mars get its moons? Japan's MMX space mission aims to solve the puzzle

Mars, our neighboring planet, bears two moons - Phobos and Deimos. These moons have captivated scientists for centuries, leaving them puzzled about their origin. The Japanese-led Martian Moons Exploration (MMX) mission aims to unravel this mystery by gathering samples from Phobos and returning them to Earth.

Phobos, the larger of the two moons, has a diameter of 22.2km, while Deimos is smaller, with a diameter of 12.5km. These moons were discovered in 1877 by the American astronomer Asaph Hall. Two main theories dominate discussions on how Mars acquired its moons: the giant impact theory and the captured asteroid theory.

The giant impact theory suggests that a celestial object, a third of Mars' size, collided with the planet early in its history. This impact hurled molten rock into space, which eventually coalesced to form Phobos and Deimos. Simulations show that these moons would be the only survivors from the debris.

The captured asteroid theory proposes that Phobos and Deimos were originally asteroids that wandered too close to Mars and were captured by its gravity. This theory suggests that the moons originated from within or just beyond the asteroid belt between Mars and Jupiter. However, this idea requires an extremely dense Martian atmosphere to explain how the asteroids were trapped by tidal forces. Mars' thin atmosphere, mainly composed of carbon dioxide, contradicts this notion.

Scientists also propose other theories, such as the origin of these moons as debris from the destruction of a single proto-moon or Mars undergoing billion-year cycles where rings of rocky material coalesce into moons. Once the moons are formed, they are eventually torn apart by gravitational forces to form new rings, which may explain why we currently observe moons instead of rings.

The MMX mission, slated for launch on Oct. 19, 2023, aims to address this long-standing debate. The spacecraft will land on Phobos, collect a sample of soil, and return it to Earth. This sample will provide crucial insights into the moons' composition, helping scientists determine which theory best explains their origin. If the sample contains material from Mars itself, along with material from the celestial object that collided with the red planet, it would support the giant impact theory.

Conversely, if the composition of Phobos aligns with that of meteorites from known asteroid categories, it would favor the asteroid capture theory. The MMX mission's round trip to Mars will take five years, with the spacecraft arriving at the red planet in late 2027 and returning to Earth in 2031.