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 . 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".