Pele is an active volcano on the surface of Jupiter's moon Io. It is located on Io's trailing hemisphere at A large, tall volcanic plume has been observed at Pele by various spacecraft starting with Voyager 1 in 1979, though it has not been persistent. The discovery of the Pele plume on March 8, 1979 confirmed the existence of active volcanism on Io. The plume is associated with a lava lake at the northern end of the mountain Danube Planum. Pele is also notable for a persistent, large red ring circling the volcano resulting from sulfurous fallout from the volcanic plume.
Observations
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.
On March 8, 1979, three days after passing Jupiter, Voyager 1 took images of Jupiter's moons to help mission controllers determine the spacecraft's exact location, a process called optical navigation. While processing images of Io to enhance the visibility of background stars, navigation engineer Linda Morabito found a tall cloud along the moon's limb. 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. Following this discovery, seven other plumes were located in earlier Voyager images of Io. Thermal emission from Pele detected by the Voyager 1 Infrared Interferometer Spectrometer (IRIS) detected a thermal hotspot at Pele, indicative of cooling lava, further indicating that volcanic activity at the surface was linked to the plumes observed by Voyager 1.
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.
Following the Voyager encounters, the International Astronomical Union officially named the volcano after the Hawaiian volcano goddess, Pele, in 1979.