Where did Mars’ moons come from?

Are Mars’ moons Phobos and Deimos captured asteroids or were they born in a collision, like our moon? New simulations suggest the latter – and the crash also spawned another larger moon that was torn apart and tumbled onto the planet’s surface a mere five million years later.

Planetary scientists from Belgium, France and Japan simulated collisions between the young Mars and a protoplanet about a third its size. Dust and rock that was blasted out clumped into one large moon close to Mars, while smaller moons, including Phobos and Deimos, formed further out.

The larger moon, they suggest, eventually crashed into Mars while Phobos and Deimos, being smaller and further away, survived in a more stable orbit. The work was published in Nature Geoscience.

Since Phobos and Deimos were discovered in 1877 by American astronomer Asaph Hall, scientists have wondered how they formed.

Unlike Earth’s moon, Mars’ moons are irregularly shaped, low-density clumps of rock, leading some to believe they’re asteroids netted into orbit by Mars’ gravitational pull. But they hurl around Mars in very circular orbits – unlike what you’d expect from a captured asteroid, which would have a long, elliptical orbit. 

The other main theory is they accreted from a disc of rubble and dust that formed after a large body slammed into Mars during the Late Heavy Bombardment in the solar system’s youth.

At the time, the solar system was awash with objects such as comets, asteroids and proto-planets. Earth’s moon, it’s thought, is the result of a collision. But why did Mars end up with two small moons – only 20 kilometres and 12 kilometres in diameter – and not a single large moon like ours?

Pascal Rosenblatt from the Royal Observatory of Belgium and colleagues ran simulations of a giant impact on Mars how the dust and rock kicked up by the crash behaved.

They found larger moons formed in the inner disc, where most of the matter accumulated. And while the sparse outer regions of the disc wouldn’t usually give rise to moons, given it comprised only around 1% of the disc’s matter, the presence of a large inner moon helped the outer disc clump into moons around the size of Phobos and Deimos.

At least one large inner moon, a few hundreds of kilometres in diameter with a mass of around 10 million billion tonnes, could exert enough of a gravitational tug on the outer debris, stirring it up and allowing it to start clumping. 

Of 288 simulation runs, almost a third yielded two smaller moons – the larger closer to the planet and orbiting faster that the planet’s rotation, and the smaller further out and not keeping up with the planet’s spin. This is the case for Phobos and Deimos today.

And 11 cases found the mass of the moons to lie within 5% and 30% of the mass of Phobos and Deimos respectively.

Being so close to Mars, the inner moon met its end quickly in cosmological terms, breaking apart thanks to Mars’ tidal tug and scattering across the Martian surface after only five million years.

Erik Asphaug from Arizona State University in the US writes in a News and Views article that the “ideas all fit together, but definitive proof of a giant impact formation might await the completion of Japan’s upcoming sample return mission to Phobos and Deimos, called Martian Moons Explorer”.

The low density, asteroid-like quality of the moons points to an accretion model, the researchers write, if they formed in the loosely packed outer disc of debris.

The simulations also explain why Mars will only have one moon in the future. Deimos’s orbit is stable, but Phobos is being slowly but inexorably drawn towards Mars at around two centimetres each year.

Once close enough, it will be ripped apart by tidal forces, forming a ring that will rain on the Red Planet, like its massive counterpart did 4.5 billion years ago.