Voyager As
Voyager 1 approached the Jupiter system in March 1979, it acquired numerous images of the planet and its four largest satellites, including Io. One of the most distinctive features of these distant images of Io was a large, elliptical, footprint-shaped ring on the satellite's trailing hemisphere (the side facing away from the direction of motion in a synchronously rotating satellite like Io). During the encounter itself on March 5, 1979,
Voyager 1 acquired high-resolution images of the footprint-shaped region. At the center of
bow tie-shaped dark region in the middle of the ring was a depression partially filled with dark material, by in size. This depression, later found to be the source of the Pele volcano, is at the northern base of a rifted mountain later named Danube Planum. With the other dramatic evidence for volcanic activity on the surface of Io from this encounter, researchers hypothesized that Pele was likely a
caldera. At first, she suspected the cloud to be a moon behind Io, but no suitably sized body would have been in that location. The feature was determined to be a volcanic plume tall and wide, generated by active volcanism at Pele. Based on the size of the plume observed at Pele, the ring of reddish (or dark as it appeared to Voyager's cameras, which were insensitive to red-wavelengths) material was determined to be a deposit of plume material. When
Voyager 2 flew through the Jupiter system in July 1979, its imaging campaign was modified to observe Io's plumes in action and to look for surface changes. Pele's plume, designated Plume 1 at the time as it was the first of Io's volcanic plumes to be discovered, was not seen by
Voyager 2 four months later. Surface monitoring observations revealed changes with the red ring surrounding Pele. While it was heart- or hoofprint-shaped during the
Voyager 1 encounter, it was now more elliptical with the notch in the southern part of the plume deposit now filled in, possibly due to changes in the distribution of plume sources within the Pele
patera. Thermal emission from Pele was detected in nearly every occasion Io's trailing hemisphere was imaged while the moon was in the shadow of Jupiter. Subtle changes in the shape and intensity of the large red-ring plume deposit surrounding Pele were observed in daylight images of the volcano, with the most notable change seen in September 1997 when dark
pyroclastic material from an eruption of
Pillan Patera covered up a portion of Pele's plume deposit. During ''Galileo's''
encounters with Io between October 1999 and October 2001, the spacecraft observed Pele on three occasions using its camera and infrared spectrometers while the volcano was on Io's night-side. The cameras revealed a curved line of bright spots along the margin of the Pele patera (a term used for volcanic depressions on Io, akin to calderas). Within the east–west dark band along the southeastern portion of the patera, a large amount of thermal emission was observed, with temperatures and distribution consistent with a large, basaltic lava lake. ==Physical characteristics==