Phobos and Deimos, the small irregular moons of Mars, have fascinated astronomers for a long time, and it was previously believed they were passing asteroids captured by the Red Planet long ago. However, a new model challenges this idea and suggests that the satellites might actually be fragments of Mars itself.

Researchers Julien Salmon and Robin Canup fom the Southwest Research Institute in Boulder, Colorado ran a computer simulation where an object roughly the size of Pluto hit Mars, throwing one-thousandth of the Red Planet’s mass into orbit. This model was presented at last month’s Lunar and Planetary Science Conference in Texas.

That material arranged itself into a disk, extending far beyond the current orbit of Deimos that is located about 24,000 kilometers (15,000 miles) from the surface. Salmon and Canup believe that the material in the disk coalesced into larger objects, most of which have fallen back to Mars over time. The outer regions of the disk, however, would have escaped such a fate.

“The idea is that Phobos and Deimos are the only two survivors of a once much larger population of satellites,” said Salmon, reported New Scientist.

This scenario could explain several mysterious features of both the Red Planet and its moons. Captured asteroids usually have peculiar orbits around planets, such as those often seen in the minor satellites of Jupiter and Saturn. Phobos and Deimos, on the other hand, orbit Mars in roughly circular orbits. It appears extremely unlikely that both objects ended up in such regular orbits by chance.

An impact scenario can also be used to explain other apparent peculiarities we have observed. Mars has a relatively fast rotation on its axis, and there’s a dramatic surface height difference between the northern and southern hemisphere. “It makes sense to think about a big impact for Mars,” added Salmon.

Before this work, other scientists have attempted without success to use simulations to test an impact scenario for the formation of Phobos and Deimos. Salmon and Canup’s model was adapted from a computer simulation that was used to describe how our own Moon formed 4.4 billion years ago after an object the size of Mars slammed into the proto-Earth. 

[H/T: New Scientist]