Sputtering, still the dominant source of plasma at Dione?



It is generally assumed that the heavy-ion plasma in the inner Saturnian magnetosphere is formed by the energetic plasma ions impacting and ejecting material from the icy satellites (Figure 1), a self-sustained process known as ion sputtering [Lanzerotti et al., 1983; Eviatar, 1984; Richardson et al., 1986].

Plasma-ion sputtering of a surface is a well-established laboratory technique for surface analysis and producing a vapor from a surface with a low vapor pressure; therefore, its potential relevance in Saturn's magnetosphere is intriguing. If the source of plasma in Saturn's inner magnetosphere is the surface of the icy satellites, then measurements of the composition of the freshly produced plasma can be used to determine the satellite surface composition [Johnson and Sittler, 1990]. This exciting prospect may be realized during the proposed encounter of Cassini with Saturn's satellite Dione in ∼2005 because of the high mass resolution of the plasma instruments carried on the spacecraft.