Moons of Mars binary asteroid that separated under tidal forces. Phobos could be a second-generation Solar System object that coalesced in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars. Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large planetesimal. The high porosity of the interior of Phobos (based on the density of 1.88 g/cm, voids are estimated to comprise 25 to 35 percent of Phobos' volume) is inconsistent with an asteroidal origin. Observations of Phobos in the thermal
Moons of Saturn in the Solar System (after Jupiter's Ganymede), with a nitrogen-rich Earth-like atmosphere and a landscape featuring dry river networks and hydrocarbon lakes found nowhere else in the solar system; and Enceladus since its chemical composition is similar to that of comets. In particular, Enceladus emits jets of gas and dust, which could indicate the presence of liquid water under its south pole region, and may have a global ocean below its surface. Twenty-four of Saturn's moons are "regular satellites"; they have prograde orbits not greatly inclined to Saturn's equatorial plane. They include the seven major satellites, four small moons that