Dione is was discovered in 1684 by the Genovan (coming from the Republic of Genova, part of modern Italy) astronomer Giovanni Domenico Cassini, the forth moon of Saturn to be discovered.
Dione orbits Saturn every 66 hours, at a distance of 377 000 km. It has a diameter of 1122 km, making it the fifteenth largest moon in the solar system. Its surface is made up of water ice, but planetary scientists calculate it is to massive to be icy throughout so it probably has an icy interior.
An image of Dione taken by the Cassini Space Probe.
In the 12 August edition of the journal Geophysical Research Letters contains a paper by a team lead by Sven Simon of the Institute of Geophysics and Meteorology at the University of Cologne, in which they announce the discovery of an atmosphere on Dione. This atmosphere was (unusually) not discovered by direct imaging but rather by its effect on the magnetic field of Saturn. As Dione moves through this field it creates a bow-wave ahead of itself, indicative of the presence of charged particles, particles which indicate the presence of an atmosphere, through in all probability a very thin one. This study gives no indication as to what the make-up of the atmosphere might be.
Dione is the third moon of Saturn where an atmosphere has been discovered.
Titan, Saturn's largest moon has long been known to have a thick, dense, atmosphere. This was first theorized by Josep Comas Solà, a Spanish astronomer at the Fabra Observatory in Barcelona in 1907, and confirmed in 1944 by Gerald Kuiper at the Yerkes Observatory. The atmosphere was studied directly by the Voyager Space Probes in the early 1980s and penetrated by the Huygens Probe, launched from the Cassini Spacecraft, in 2005. The atmosphere is 1.45 times as dense as that of Earth, and 1.19 times as massive. It is comprised mainly of nitrogen, with a small amount of methane; this methane content is about 1.6% of the total above a height of 32 km, increasing to 4.9% at 8 km, a concentration which remains constant to ground level.
Video taken during the Huygens Probe's decent into the atmosphere of Titan.
In 2010 NASA announced the discovery of a thin atmosphere on Rhea, the second largest moon of Saturn. This atmosphere is comprised of oxygen (60%) and carbon dioxide (40%). The oxygen content of the atmosphere is thought to be the result of the electrolysis of water ice on the surface of the moon, but the origin of the carbon dioxide is less clear; possibly the result of outgassing from somewhere inside the moon.
Video of the surface of Rhea from the Cassini Space Probe.
See also Running water on present day Mars? Pluto gains a fourth moon and Saturn on Sciency Thoughts YouTube.