Mars’s moons, Phobos and Deimos, are believed to have formed from debris created when a large asteroid came dangerously close to the planet, according to computer simulations.
This new model suggests that the two moons originated from the remnants of a larger asteroid that crossed Mars’s Roche limit, the point where the planet’s gravitational tidal forces became strong enough to tear the asteroid apart.
Jacob Kegerreis from NASA Ames Research Center described the new theory as an exciting exploration of how Phobos and Deimos, the only moons in our solar system orbiting a rocky planet other than Earth, might have formed.
Although Phobos and Deimos are small—Phobos measures 16 miles (26 km) across at its widest point and Deimos is only 10 miles (16 km)—and irregularly shaped, they were once thought to be captured asteroids. Objects captured by a planet’s gravity typically follow elongated, inclined, or even retrograde orbits, as seen with Neptune’s moon Triton and Saturn’s moon Phoebe.
However, both Phobos and Deimos have nearly circular orbits aligned with Mars’s equatorial plane, suggesting that they may have formed directly in orbit around the planet.
