If you look at the photographs of Deimos and Phobos you will notice one point in common – their colour. Both the Martian moons look dark in colour, darker than Mars. In fact, they are said to be two of the darkest bodies in our solar system. On closer inspection, scientists discovered that they are made up of a peculiar type of carbon-rich material and ice – a composition they share with the space rocks drifting in the main asteroid belt (that lies between the orbits of Mars and Jupiter) of our solar system. This gave rise to the idea that Deimos and Phobos may be two runaway asteroids that wandered too close to Mars! After all, asteroids do break out of the main asteroid belt occasionally due to collisions or Jupiter’s gravitational effects. And during their journey afterwards, if they happen to pass by a planet like Mars, they could easily get pulled into its orbit!
But in such a case, the orbits of these asteroids would be elliptical at first. It would take them many, many years to settle into the kind of near-circular orbits that the Martian moons now have. Also, the thin atmosphere of Mars would perhaps not be enough to slow down the asteroids before they could fall into orbit. All these points make the asteroid-capture theory unlikely, but not impossible!
Picture Credit : Google
