Bodies Without Atmospheres in the Outer Solar System

Abstract

After the invention of the telescope in the 17th century, it was discovered that the Earth was not the only planet to have a satellite. In 1610, Galileo observed the largest satellites of Jupiter and Huygens discovered Titan, the largest of Saturn’s satellites in 1655. Gradually the number of satellites in the realm of the outer planets grew to about a dozen. Not until the 19th century, however, were the martian satellites, Phobos and Deimos, observed for the first time. It is only very recently, with the Pioneer and Voyager deep-space missions, that numerous new satellites have been discovered, and their main physical properties have been determined. What is striking is that this family of bodies orbiting the outer planets should show such an extraordinary diversity in their surface conditions, degree of internal activity, internal structure and evolution. Nevertheless it has been possible to establish some general laws regarding orbital parameters and macroscopic properties. For example, almost all the satellites have synchronous rotation, with equal periods of orbital revolution and sidereal rotation: they present the same face to the planet throughout their orbits. This implies that tidal forces have stabilized the rotation in an equilibrium state.