Plasma Observations at Io with the Galileo Spacecraft

Science  18 Oct 1996:
Vol. 274, Issue 5286, pp. 394-395
DOI: 10.1126/science.274.5286.394


Plasma measurements made during the flyby of Io on 7 December 1995 with the Galileo spacecraft plasma analyzers reveal that the spacecraft unexpectedly passed directly through the ionosphere of Io. The ionosphere is identified by a dense plasma that is at rest with respect to Io. This plasma is cool relative to those encountered outside the ionosphere. The composition of the ionospheric plasmas includes O++, O+ and S++, S+, and SO2+ ions. The plasma conditions at Io appear to account for the decrease in the magnetic field, without the need to assume that Io has a magnetized interior.