Astronomers have long debated the origin of Mars’ tiny satellites. Some posit that Phobos and Deimos, which have diameters of 14 miles and 7.5 miles respectively, are actually intact asteroids that were captured by the gravity of Mars. But due to the fact that both moons orbit in nearly the same plane and trace almost circular paths, many astronomers lean toward the theory that Phobos and Deimos were produced inside a disk of debris that encircled Mars following a powerful collision with another object.Though prior researchers have attempted to model such a collision, the simulations typically fall short due to low resolution and oversimplified models. “Ours is the first self-consistent model to identify the type of impact needed to lead to the formation of Mars’ two small moons,” said lead author Robin Canup, an associate vice president in the SwRI Space Science and Engineering Division.“A key result of the new work is the size of the impactor; we find that a large impactor — similar in size to the largest asteroids Vesta [326 miles wide] and Ceres [587 miles wide] — is needed, rather than a giant impactor,” said Canup. “The model also predicts that the two moons are derived primarily from material originating in Mars, so their bulk compositions should be similar to that of Mars for most elements. However, heating of the ejecta and the low escape velocity from Mars suggests that water vapor would have been lost, implying that the moons will be dry if they formed by impact.”