Japan's MMX Space Mission Probes How Mars Got Its Moons

The general mechanism is similar to how scientists think Earth's natural satellite formed. But in the Martian impact theory, a single“giant” moon was created by the ejected material, along with a host of smaller satellites.

Over time, nearly every one broke up and fell to Mars' surface. Simulations by Pascal Rosenblatt, from the Laboratoire de Planétologie et Géodynamique (LPG) in Nantes, France, and his colleagues show that Phobos and Deimos would have been the sole moons to survive.

The other main idea is known as the captured asteroid theory, which goes back to the 1960s. This suggests that Phobos and Deimos were originally asteroids that wandered so close to Mars they were captured by the planet's gravity.

According to this idea, the moons may have originated from within or just beyond the asteroid belt that lies between Mars and Jupiter. The moons might even have been what's known as a binary – two asteroids that orbit each other.

The moons are characterised by their low density, which implies they have porous interiors.

The captured asteroid hypothesis, however, requires an extremely dense Martian atmosphere to explain how the asteroids were trapped by tidal forces. Mars today has a thin atmosphere mainly composed of carbon dioxide. Also, asteroids captured by planets tend to have highly eccentric orbits. The orbits of Phobos and Deimos around Mars are, by contrast, almost circular.

Scientists have also proposed other theories for the origins of these moons, but they command lower levels of support among the scientific community. One visualizes the origin of the moons as debris from the destruction of a single“proto-moon.”

Another lesser known idea proposes that Mars undergoes billion-year cycles in which rings of rocky material orbiting Mars coalesce over time into moons.

Once the moons are formed, they are eventually ripped apart by gravitational (tidal) forces to form new rings. Under this scenario, we just happen to live in an era in which the debris is in the form of moons and not of rings.

Amid this ongoing uncertainty surrounding the moons' origins, MMX aims to collect more than 10 grams of material from Phobos – marking the world's first sample return mission from that moon.