![]() ![]() ![]() Together, these three instruments produce mutually constraining datasets over a wide wavelength range, allowing detailed study of surface composition and the thermal behaviour of Io's volcanic processes. Galileo carries three instruments that have been used to examine Io: the Solid State Imaging experiment (SSI), the Near Infrared Mapping Spectrometer (NIMS) and the Photo-Polarimeter Radiometer (PPR), all described in the box “Galileo's instruments”. Because of the high radiation environment, this was the only close flyby of the nominal 11-orbit mission it would be another four years (October 1999) before a close flyby. Due to problems with the tape recorder on which data are stored prior to transmission to Earth, no imaging data of Io were collected during insertion into orbit around Jupiter in December 1995, when Galileo passed within 180 km of Io. That Galileo arrived at all as a functioning spacecraft was a tribute to the skill of the mission engineers and scientists who had to overcome many obstacles (including the failure of the main antenna, necessitating in-flight reprogramming of the spacecraft to use a secondary system, as well as unexpected radiation effects on the spacecraft and instruments) before data from Io finally arrived. Launched in 1989, the Galileo spacecraft arrived at Jupiter in December 1995 and opened a new era of monitoring volcanism on Io, providing regular observations at higher spatial, spectral and temporal resolution than ever before. In this paper the extraordinary discoveries pertaining to Io's volcanism made during the Galileo epoch are reviewed. Observing Io's volcanism, studying thermal output and eruption evolution, determining lava composition and measuring the resulting geomorphology were major objectives of the Galileo mission, building on the data collected by the Voyager spacecraft and continuing the monitoring of volcanic activity from ground-based telescopes (see, for example, Veeder 1994) augmented by Hubble Space Telescope observations (e.g. Since it began making observations of the Galilean satellites in June 1996, the spacecraft Galileo has produced extraordinary images of volcanism on Io, collected a wealth of data concerning thermal emission and composition, and greatly advanced what is known about this highly volcanic satellite. Io is the most volcanically active body in the solar system, as a result of tidal heating: Io is caught in a gravitational tug-of-war between Jupiter and Europa ( Peale 1979), and the intense heating manifests itself as widespread volcanism. ![]() The discovery of active volcanism on Io by the Voyager spacecraft ( Smith 1979) was the first extraterrestrial case of a process that constantly reshapes the surface of the Earth. ![]()
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