A new study challenges the belief that the tiny, irregular moons of Mars were passing asteroids caught by the Red Planet’s gravitational field. The new theory is that they may be fragments that broke off of Mars during impact with an asteroid.
The researchers ran a computer simulation adapted from a simulation that was used to describe how Earth’s moon was formed, after having been slammed an object the size of Mars slamming into proto-Earth. They simulated an object the size of Pluto hitting Mars. This threw one-thousandth of the planet’s mass in orbit. A couple of the broken fragments, over time, may have arranged themselves into misshapen disks. The outer part of the disks would have had to have been spinning fast enough to keep gravity from pulling them back down to the planet. Instead, two large fragments formed into two moons with two circular orbits. Thus, Phobos and Deimos were created.
The idea is that Phobos and Deimos are the only two survivors of a once much larger population of satellites.
This idea is reinforced by Mars itself. Mars shows signs of impact. There is a dramatic difference of surface height between its northern and southern hemispheres. The planet also has a very fast rotation on its axis. And, if these two moons were captured asteroids, it is unlikely that, accidentally, they would both have regular, circular orbits.
It makes sense to think about a big impact for Mars.
Some collisions are a good thing.