The Two of Mars is a reference to the two moons of Mars, Phobos and Deimos. These moons were discovered in 1877 by Asaph Hall, an American astronomer. Phobos is the larger of the two moons and orbits closer to Mars than Deimos. Both moons are believed to be asteroids that were captured by Mars' gravitational pull. They are both heavily cratered and have a low density, indicating that they are composed of mostly rock and ice.