Goddard Mars Model (GMM) 3 is a global map of the gravity field of Mars. It was created by studying the flight paths of three Mars-orbiting spacecraft — Mars Global Surveyor (MGS
), Mars Odyssey (ODY
), and Mars Reconnaissance Orbiter (MRO
). If Mars were a perfectly smooth sphere of uniform density, the gravity experienced by the spacecraft would be exactly the same everywhere. But like other rocky bodies in the solar system, including the Earth, Mars has both a bumpy surface and a lumpy interior. As the spacecraft fly in their orbits, they experience slight variations in gravity caused by both of these irregularities, variations which show up as small changes in the velocity and altitude of the three spacecraft.