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. although processing images of io to enhance the visibility of background stars, navigation engineer linda morabito found a tall cloud along its 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 plume generated by active volcanism at a dark depression later named pele. following this discovery, eight other plumes were located in "voyager" images of io. these plumes were later named after mythological deities associated with fire, volcanoes, or mayhem: loki (two separate plumes), prometheus, volund, amirani, maui, marduk, and masubi. thermal emission from multiple sources, indicative of cooling lava, were also found. surface changes were observed when images acquired by "voyager 2" were compared to those taken four months previously by "voyager 1", including new plume deposits at aten patera and surt.