"Gravity is a force of attraction that exists between objects. Gravity is related to the mass and density of an object or planet. The greater a planet’s mass, the greater its gravity. A certain amount of gravity is needed to hold on to the kind of atmosphere we have on Earth. With less gravity, a planet cannot hold on to the atmosphere that we need to survive." As I was saying the moon would have to be recreated. http://quest.nasa.gov/projects/astrobiology/astroventure/challenge/Articles/gravityandatmosphere.pdf