Phobos and Deimos, Mars' lumpy, runty moons, were once pegged as captured asteroids. But the truth is shaping up to be far more interesting. These ruddy satellites could be the lone survivors of a giant impact that eviscerated half of Mars' surface billions of years ago. Image: Labex UnivEarths / Université Paris Diderot
That violent origin story is detailed in a new scientific paper, which used numerical models to show that a Deep Impact-style collision in Mars' past could have produced many moons, most of which are long gone. This hypothesis resolves several mysteries about Phobos and Deimos, and it can be tested by searching for geologic evidence on the Red Planet.
It's easy to see why Phobos and Deimos were first labelled asteroids — at 22km and 12km across respectively, the cratered, potato-shaped moons look an awful lot like rogue space rocks. But the asteroid hypothesis doesn't square with the moons' circular orbits and rotational rates, which cannot be produced by Mars' weak tidal pull.
Another possibility is that Phobos and Deimos formed in place from escaped chunks of Mars and another large object that collided long ago. This hypothesis is supported by Mars' vast Borealis basin, whose size and shape suggest it was punched out by an impactor thousands of kilometres wide.
Now, a series of computer simulations reconstruct a plausible sequence of events from that ancient cosmic smackdown to the moons of Mars today. Writing this week in Nature Geoscience, a team led by Pascal Rosenblatt of the Royal Observatory of Belgium show that within several hours of being struck, a vast debris disk formed around Mars. A large moon rapidly accreted from the inner part of the disk, where debris was most dense.
Next, the gravitational pull of the large moon concentrated material in the outer disk, allowing smaller moons like Phobos and Deimos to form. But the luckless inner moon — along, perhaps, with many other moonlets — was unstable. It broke up, raining its molten entrails back on the Red Planet's surface several million years later.
This hypothesis can explain several puzzling features of Phobos and Deimos, including their circular orbits, their unusual geologic composition and the fact that they both seem to be crumbly and porous, like granola bars that were crushed inside their packaging. And unlike many model simulations of our solar system, this one can actually be tested by hunting for the remains of an ancient, crashed moon on the surface of Mars.
"The destruction of an object hundreds of km in diameter, a few million years after the formation of Borealis, would have left a profound geologic record," astronomer Erik Asphaug wrote in a Nature News & Views article.
I, for one, am hoping the many moons of destruction hypothesis turns out to be correct. Not only does it add gory detail to the early history of our solar system, it makes Phobos' miserable fate a bit more fitting.