Where did Mars’ moons come from?

The origin of Mars’ two moons is a mystery. We once thought these moons were asteroids captured by Mars’ gravity. Their small sizes – Phobos is only 27 kilometres

(17 miles) along its longest axis, while Earth’s Moon has an average diameter of 3,500 kilometres (2,200 miles) – irregular shapes and non-Martian compositions suggested an asteroidal origin. However, a capture scenario tends to produce moons with highly eccentric orbits. Phobos and Deimos are in nearly circular orbits.

An alternative explanation is that the two moons formed around Mars following a giant-impact scenario. Similar to our understanding of the formation of Earth’s Moon, a giant impact could have led to the formation of a disc of debris around Mars. Material in this debris gradually coalesced to form the two moons we see today.

However, many questions still remain, such as why the material on these two moons looks distinctly unlike that of Mars. In an attempt to end this debate, the Japanese Space Agency (JAXA) developed the Martian Moons eXploration (MMX) mission, which will launch in 2024. It will travel to one of these moons, collect a sample and return it to Earth. Laboratory analysis of this sample may finally resolve these questions and likely introduce new clues and questions for how our Solar System formed.