The Japan Aerospace Exploration Agency (JAXA) has announced a mission to visit the two moons of Mars and return a rock sample to Earth. It’s a plan to uncover both the mystery of the moons’ creation and, perhaps, how life began in our Solar System.
The Solar System’s planets take their names from ancient Greek and Roman mythology. Mars is the god of war, while the red planet’s two moons are named for the deity’s twin sons: Deimos (meaning panic) and Phobos (fear).
Unlike our own Moon, Phobos and Deimos are tiny. Phobos has an average diameter of 22.2km, while Deimos measures an even smaller 13km. Neither moon is on a stable orbit, with Deimos slowly moving away from Mars while Phobos will hit the Martian surface in around 20 million years.
The small size of the two satellites makes their gravity too weak to pull the moons in spheres. Instead, the pair have the irregular, lumpy structure of asteroids. This has led to a major question about their formation: were these moons formed from Mars or are they actually captured asteroids?
Our own Moon is thought to have formed when a Mars-sized object hit the early Earth. Material from the collision was flung into the Earth’s orbit to coalesce into our Moon.
A similar event could have produced Phobos and Deimos. The terrestrial planets were subjected to a rain of impacts during the final throes of Solar System formation.
Mars shows possible evidence of one such major impact, as the planet’s northern hemisphere is sunk an average of 5.5km lower than the southern terrain. Debris from this or other impacts could have given birth to the moons.
Alternatively, Phobos and Deimos could be asteroids that were scattered inwards from the asteroid belt by the looming gravitational influence of Jupiter. Snagged by Mars’s gravity, the planet could have stolen its two moons. This mechanism is how Neptune acquired its moon, Triton, which is thought to have once been a Kuiper belt object, like Pluto.
There are compelling arguments for both the #TeamImpact and #TeamCapture scenario.
The orbits of the two moons are circular and in the plane of Mars’s own rotation. While the chance of this happening during a capture event are extremely low, observations of the moons suggest they may have a composition similar to that of other asteroids.
Definite determination of the moons’ composition would act as a fingerprint to distinguish the two models. A collision event…