EUROPA one of the satellites of Jupiter, has long been thought to be a dormant icy body(1), unlike its volcanically active neighbour, Io. Europa lies deep within Jupiter's magnetosphere, however, and is continuously bombarded by energetic ions, which modify the surface ices(2) and are probably responsible for creating Europa's tightly bound oxygen atmosphere(3,4). Here we report the discovery of an atmosphere of atomic sodium that extends to at least 25 times Europa's radius. We suggest that this sodium is originally released by Io's volcanoes, after which it is ionized in the magnetosphere and implanted into Europa's surface ice; subsequent sputtering of the ice by magnetospheric ions releases the sodium to form the extended atmosphere. Although sodium is a minor constituent of Europa's atmosphere, it traces the distribution of the major atmospheric components which are not themselves directly observable. The sodium and oxygen could represent the extremes of the distribution of the atmospheric components, with only the heaviest molecules (such as the oxygen) being tightly bound; alternatively the sodium might be in the form of an extended corona, analogous to Io's atmosphere. [References: 11]
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