Phobos and Deimos may be a chip off the old Martian block. The Red Planet’s tiny, misshapen moons are often thought to be captured asteroids, but an alternative theory suggests they are shrapnel left over from an ancient impact on Mars. Now we have a model showing how this could have happened.

The argument against the moons being asteroids rests on the fact that both have a roughly circular orbit around Mars. If they were asteroids snared by Mars’s gravity, they would probably have much less regular orbits. One circular orbit could happen by chance, but two seems unlikely, says Julien Salmon of the Southwest Research Institute in Boulder, Colorado.

In previous simulations of a moon-forming impact on Mars, the material that is thrown into an orbiting disc eventually comes back down, meaning nothing is left to form Phobos and Deimos. Now Salmon and his colleague Robin Canup have adapted models used to study the formation of Earth’s moon, which is also thought to be the result of a large collision. They have found that an impacting object with around 3 per cent of the mass of Mars could create the right kind of disc.

The results show that such an object – with roughly the mass of Pluto – would throw around a thousandth of Mars’s mass into orbit, and the edge of the disc would reach beyond the 24,000-kilometre orbit of Deimos, the outer moon.

Over time, the material nearer to Mars would coalesce into large bodies, but the planet’s gravity would eventually drag them back down. But the outer part of the disk would spin fast enough to keep it out of gravity’s clutches, and the material would form into the Phobos and Deimos we see today.

“The idea is that Phobos and Deimos are the only two survivors of a once much larger population of satellites,” says Salmon, who presented the work at the Lunar and Planetary Science Conference in The Woodlands, Texas, last month.

Such a large object hitting Mars in its past could also explain some other features we see today, like the planet’s relatively fast rotation and the large differences in average surface height between its northern and southern hemisphere, says Salmon. “It makes sense to think about a big impact for Mars.”

Source: New Scientist