The mystery of Neptune's rings
Aug 20, 1999
The rings surrounding Saturn are one of the best known features of the solar system. However, three other planets - Jupiter, Uranus and Neptune - also have thin narrow rings or arcs. Many astronomers have thought that the gravitational pull of local moons was responsible for the stability of these narrow rings. However, high-resolution observations by Bruno Sicardy of the Observatoire de Paris and colleagues at the University of Hawaii, Telespazio in Rome, and the Ecole Normale Supérieure, also in Paris, have shown that this "moon shepherding" effect cannot explain the stability of the ring arcs around Neptune (Nature 400 731).
Neptune's ring arcs were first observed by the Voyager spacecraft in 1984. In the absence of some external source of stability the rings would have been expected to decay within a matter of months. But the three rings - called Liberté, Egalité and Fraternité - are still visible, and it has been assumed that their stability was due to the influence of a small moon called Galatea. However, when Sicardy and colleagues observed the satellite with the Canada-France-Hawaii Telescope on Mauna Kea, they discovered that the arcs fall outside the gravitational resonance's associated with Galatea.
In a second paper, based on observations with the Hubble Space Telescope, Christopher Dumas of the Jet Propulsion Laboratory in California, and colleagues from Hawaii, the University of Arizona and the University of California at Los Angeles, reached the same conclusions (Nature 400 733). They suggest that instead of one moon, Galatea, there must be two moons keeping the arcs in position. However, no such moon has yet been discovered, and Sicardy and others have dismissed this explanation as 'ad hoc'.