The moons of Mars weren't discovered until after many of the moons around other planets had already been found. In the late 1800s astronomer Asaph Hall finally found the two moons of the Red Planet that he named Phobos and Deimos.
A century later, images from spacecraft revealed that moons didn't look much like moons, but instead they looked more like asteroids. They were dark, heavily marked with craters and shaped like potatoes. This suggested that Mars had stolen them from the asteroid belt.
Now, planetary scientists have conducted the first computer simulations that actually support an alternate idea that like Earth's moon, the two satellites formed after a large object smashed into the planet kicking up debris.
This hypothesis has been around for decades but continues to be hotly contested. "It was embarrassing how badly people came after me," says geologist Robert Craddock of the Smithsonian Institution's National Air and Space Museum in Washington, D.C., who presented the idea at a 1994 conference. "The general tone was this couldn't possibly work." He submitted his work to several journals that all rejected it. Years later, Craddock returned to his work after another scientists became interested in his work and some of his critics either retired or died.
This once dismissed work has now inspired not one, but two teams of planetary scientists to model what might have happened if an object 10 times the size of Ceres, the largest asteroid in the asteroid belt, hit Mars.
Planetary scientists Robin Canup and Julien Salmon of the Southwest Research Institute in Boulder, Colorado, presented their simulations at a conference late last year. And planetary scientists Robert Citron of the University of California, Berkeley, and Hidenori Genda and Shigeru Ida of the Tokyo Institute of Technology published their own calculations in the 15 May issue of Icarus.
Although the two teams were unaware of each other, they both reached the same conclusion. A large impact can indeed form two moons. Canup also suspects that the impact created not two, but three moons with the third moon being much larger with an estimated diameter of 300 kilometers.
Canup believes that this moon probably lasted a few hundred million years before being pulled in by the gravity of the Red Planet and crashing into its surface.
"I think they're absolutely on the right track," says William McKinnon, a planetary scientist at Washington University in St. Louis. "It makes it much more believable to have a simulation that actually works." McKinnon says that if the moons are really captured asteroids, they should have elliptical orbits far from the planet, but they actually have circular paths close to the planet, as expected if the satellites formed by impact.
Not all scientists agree, however. "I think by far the interpretation most consistent with observable characteristics of the moons aside from their orbits-[their] density, spectral properties, [dark color]-would be that they're captured asteroids," said Scott Murchie, a planetary scientist at Johns Hopkins University in Baltimore, Maryland, who has been studying the moons of Mars for more than 25 years.
All the scientists do agree on one thing, however. To solve the controversy, a spacecraft must be sent to the moons to learn the truth. A collision would vaporize any water ice or hydrogen on the surface. If they were to find ice and hydrogen, then there is a good chance they are stolen asteroids, if not, it was most likely a collision.
