A way out of this difficulty was provided by a theoretical investigation carried out by Stanton Peale of the University of California at Santa Barbara and Pat Cassen and Ray Reynolds at the NASA Ames Research Laboratory. Working in the months before the first Voyager flyby, they calculated that the tidal effects of Jupiter on Io could generate large-scale heating of the satellite. Io is about the same distance from Jupiter as the Moon from Earth, but the much greater mass of Jupiter raises enormous tides in its satellite. These tides distort its shape, but no other effect would be present if Io remained at a constant distance from Jupiter. What Peale, Cassen, and Reynolds realized was that the distance of Io from Jupiter varies as the result of small gravitational perturbations from the other Galilean satellites. Therefore the tidal distortions also vary, in effect squeezing and unsqueezing Io each orbit. Such flexing pumps energy into the interior of Io in the form of heat; theorists calculated that the heat supplied could be as high as 10¹³ watts. They predicted, in a paper published just three days before the Voyager flyby of Io, that “widespread and recurrent surface volcanism might occur,” and that “consequences of a largely molten interior may be evident in pictures of Io’s surface returned by Voyager.” [Illustration: Voyager 2 obtained beautiful views of the volcanic eruptions during its ten-hour Io volcano watch on July 9. On the edge of the crescent image are the volcanoes Amirani (P₅) and below it Maui (P₆), each sending up fountains about 100 kilometers above the surface. The blue color is probably the result of sunlight scattered by tiny particles of sulfur dioxide snow condensing in the erupting plume. [P-21780]] VOLCANIC ERUPTIONS ON IO Plume Name Location Height During Activity During Number (latitude/longitude) Voyager 1 Voyager 2 Flyby Flyby (kilometers) 1 Pele -20°/255° 280 ceased 2 Loki +20°/300° 100 increased 3 Prometheus -5°/155° 70 increased 4 Volund +20°/175° 95 no data 5 Amirani +25°/120° 80 similar 6 Maui +20°/120° 80 similar 7 Marduk -25°/210° 120 similar 8 Masubi -40°/ 50° 70 similar