detected by Cassini for the first time in 2004. A fraction of the oxygen gets ionized, creating a small population of O2+ ions in the magnetosphere. The influence of Saturn's magnetosphere on its moons is more subtle than the influence of Jupiter on its moons. In the latter case, the magnetosphere contains a significant number of sulfur ions, which, when implanted in surfaces, produce characteristic spectral signatures. In the case of Saturn, the radiation levels are much lower and the plasma is composed mainly of water products, which, when implanted, are indistinguishable from the ice already present. Exploration As of 2014 the magnetosphere of Saturn has been directly explored by four spacecraft. The first mission to study the magnetosphere was Pioneer 11 in September 1979. Pioneer 11 discovered the magnetic field and made some measurements of the plasma parameters. In November 1980 and August 1981, Voyager 1–2 probes investigated the magnetosphere using an improved set of instruments. From the fly-by trajectories they measured the planetary magnetic field, plasma composition and density, high energy particle energy and spatial distribution, plasma waves and radio emissions. Cassini spacecraft was launched in 1997, and arrived in 2004, making the first measurements in more than two decades. The spacecraft continued to provide information about the magnetic field and plasma parameters of the saturnian magnetosphere until its intentional destruction on September 15, 2017. In the 1990s, the Ulysses spacecraft conducted extensive measurements of the Saturnian kilometric radiation (SKR), which is unobservable from Earth due to the absorption in the ionosphere. The SKR is powerful enough to be detected from a spacecraft at the distance of several astronomical units from the planet. Ulysses discovered that the period of the SKR varies by as much as 1%, and therefore is not directly related to the rotation period of the interior of Saturn. Notes References Bibliography Further reading Saturn Sends Mixed Signals External links NASA site about the emissions Saturn Saturn