Mars' puzzling moons, Phobos and Deimos, may be formed from debris of asteroid

Computer simulations suggest that Mars' puzzling moons, Phobos and Deimos, may have been formed from debris created when a large asteroid wandered dangerously close to the Red Planet. 

This new model says Phobos and Deimos resulted from the wreckage of a larger asteroid that wandered too close to Mars and crossed its Roche limit. It's good to explore a new option for the making of Phobos and Deimos — the only moons in our solar system that orbit a rocky planet besides Earth's, Jacob Kegerreis of NASA's Ames Research Center said in a statement.

Mars' moons are not easy to explain. Both are small — Phobos is 16 miles (26km) across at its widest point, Deimos is just 10 miles (16km) — and lumpy, which makes them look like captured asteroids. However, objects captured in orbit tend to have elongated, inclined and sometimes retrograde trajectories around their new parent planet — Neptune's moon Triton, or Saturn's moon Phoebe are good examples. However, Phobos and Deimos have neatly circular orbits aligned with Mars' equatorial plane, which seems more likely if they had formed in orbit around Mars.

Another hypothesis is that Phobos and Deimos formed very much like Earth's moon did — that an impact on the surface of Mars threw debris into orbit that eventually coalesced into the two moons. However, Phobos and Deimos have very different heights above Mars' surface — about 6,000km (3,700 miles) and 23,000km (14,577 miles) respectively — which models simulating an impact on the surface have difficulty explaining.

So Phobos and Deimos have been left needing a third option, and now that may have arrived thanks to this new model developed by Kegerreis and his team. Using the supercomputers at Durham University's Advanced Computing Systems, Kegerreis and company performed hundreds of simulations of such an event, varying the asteroid's diameter, rotation, velocity and distance from Mars during its closest approach. While some of the debris is lost to space, they found that enough fragments from the original asteroid survived in orbit in many of the simulations,  where they continually collided and ground themselves down into smaller particles that settled into a disk around Mars, from which Phobos and Deimos were built.

This new model both satisfies the presence of Phobos and Deimos in circular, equatorial orbits around Mars, and also how Deimos could have formed so relatively far from the planet.

"Our idea allows for a more efficient distribution of moon-making material to the outer regions of the disk," said Jack Lissauer of NASA Ames. "That means a much smaller 'parent' asteroid could still deliver enough material to send the moons' building blocks to the right place."

Of course, it is all relative. Earth's moon orbits at an average distance of 384,400km (238,855 miles), but the scale of the impact that ultimately sparked its formation was much grander than the asteroid that formed Phobos and Deimos.

Kegerreis and Lissauer acknowledge that their idea remains just a hypothesis for the moment. However, it will soon be put to the test. In 2026 the Japanese Aerospace Exploration Agency (JAXA) are launching the Martian Moons eXploration (MMX) spacecraft, which is a sample-return mission that will bring pieces of Phobos back to Earth for study in laboratories. 

"This new model makes different predictions about the moons' properties that can be tested against the standard ideas for this key event in Mars' history," said Kegerreis. The simulations could also be adapted to look at other interactions between planets and smaller bodies such as asteroids and comets throughout the solar system's history, perhaps exploring how Saturn's rings may have formed, or other puzzling moons. 

On board MMX will be a NASA instrument called MEGANE, the Mars-moon Exploration with GAmma rays and Neutron experiment. MEGANE will determine the identity of chemical elements on Phobos and help select sampling sites. 

Hopefully, the MMX mission will tell us about the composition of Phobos (and presumably, by proxy, Deimos), which will provide a big clue as to their origin. If they contain traces of rocks from Mars, then that would suggest they formed from impact ejecta, but if their composition is more like an asteroid, then it could support Kegerreis' model.