The origin of Mars's two moons is a longstanding mystery: just how did the Red Planet get its pair of satellites?
Martian moons Phobos and Deimos may have been formed by a passing asteroid being ripped apart by Mars's gravitational pull, according to a study.
Mars moon origins
A team of scientists led by Jacob Kegerreis at NASA’s Ames Research Center used a computer simulation to investigate whether an asteroid passing close to Mars may have been the source of material that formed the planet's moons.
Simulations show fragments of the asteroid thrown into orbit around Mars.
Half the asteroid fragments would have escaped Mars's gravitational pull, but some would have stayed in orbit.
Then, the asteroid debris in orbit around Mars would have collided more and more over time, being ground down in the process and spreading the debris wider.
Credit: NASA
Eventually, this finer debris could have formed a disk of material surrounding the planet.
And after even more time, this would have begun to clump together, accumulating more material and eventually forming Mars's two moons, Phobos and Deimos.
Scientists are able to use advanced computer simulations to test out theories about the cosmos, and in this case the team used a computing code known as SWIFT to investigate the scenario.
They were able to explore hundreds of different simulations, changing the asteroid’s size, spin, speed and distance at its closest approach to the planet.
The team say that, in many of the scenarios they explored, there would have been enough asteroid fragments that survived and collided in orbit to serve as raw material to form the moons.
"It’s exciting 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," says Kegerreis.
"Furthermore, this new model makes different predictions about the moons’ properties that can be tested against the standard ideas for this key event in Mars’s history."
Mars moon formation scenarios
The study provides further fuel for one of two formation theories, regarding the origins of Mars's moons.
One idea suggests that passing asteroids were captured whole by Mars’s gravity.
Indeed, the two moons do look similar to asteroids.
The other says a huge object smashed into Mars and ejected material into orbit around the planet, creating a disk of material.
This is similar to the theory as to how Earth's moon formed.
Scientists say the impact theory better accounts for how the moons orbit Mars today: in near-circular orbits that are roughly aligned with Mars’s equator.
But, Phobos and Deimos's orbits are probably too far away to have been caused by ejected material, which would be more likely to stay close to the disk.
"Our idea allows for a more efficient distribution of moon-making material to the outer regions of the disk," says Jack Lissauer, a research scientist at Ames and co-author on the paper.
"That means a much smaller ‘parent’ asteroid could still deliver enough material to send the moons’ building blocks to the right place."
Perhaps the mystery surrounding the formation of Mars's moons will be solved by the upcoming Martian Moons eXploration (MMX) sample return mission led by JAXA (Japan Aerospace Exploration Agency).
Read the full paper at www.sciencedirect.com/science/article/abs/pii/S001910352400397X
