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
Solar System model Solar System model Solar System models, especially mechanical models, called "orreries", that illustrate the relative positions and motions of the planets and moons in the Solar System have been built for centuries. While they often showed relative sizes, these models were usually not built to scale. The enormous ratio of interplanetary distances to planetary diameters makes constructing a scale model of the Solar System a challenging task. As one example of the difficulty, the distance between the Earth and the Sun is almost 12,000 times the diameter of the Earth. If the smaller planets are to be easily visible to