Unique in the Solar System

Asaph Hall was surprised Phobos revolves around Mars in 7.66 hours (Swift said 10 hours). Mars rotates on its axis in 24.6 hours. Hall said "...at first I thought there were two or three [inner] moons, since it seemed to me at that time very improbable that a satellite should revolve around its primary in less time than that in which the primary rotates..." and that Phobos "...made its revolution around the primary in less that one-third the time of the primary's rotation, a case unique in the Solar System".