Volcanism On Io - Discovery

Discovery

Before the Voyager 1 encounter with Io on March 5, 1979, Io was thought to be a dead world much like the Earth's Moon. The discovery of a cloud of sodium surrounding Io led to theories that the satellite would be covered in evaporites.

Hints of discoveries to come arose from Earth-based infrared observations taken in the 1970s. An anomalously high thermal flux, compared to the other Galilean satellites, was discovered during measurements taken at an infrared wavelength of 10 μm while Io was in Jupiter's shadow. At the time, this heat flux was attributed to the surface having a much higher thermal inertia than Europa and Ganymede. These results were considerably different from measurements taken at wavelengths of 20 μm which suggested that Io had similar surface properties to the other Galilean satellites. It has since been determined that the greater flux at shorter wavelengths was due to the combined flux from Io's volcanoes and solar heating, while solar heating provides a much greater fraction of the flux at longer wavelengths. A sharp increase in Io's thermal emission at 5 μm was observed on February 20, 1978 by Witteborn, et al.. The group considered volcanic activity at the time, in which case the data was fit into a region on Io 8,000 square kilometres (3,100 sq mi) in size at 600 K (300 °C; 600 °F). However, the authors considered that hypothesis unlikely, and instead focused on emission from Io's interaction with Jupiter's magnetosphere.

Shortly before the Voyager 1 encounter, Stan Peale, Patrick Cassen, and R. T. Reynolds published a paper in the journal Science predicting a volcanically modified surface and a differentiated interior, with distinct rock types rather than a homogeneous blend. They based this prediction on models of Io's interior that took into account the massive amount of heat produced by the varying tidal pull of Jupiter on Io caused by the moon's slightly eccentric orbit. Their calculations suggested that the amount of heat generated for an Io with a homogeneous interior would be three times greater than the amount of heat generated by radioactive isotope decay alone. This effect would be even greater with a differentiated Io.

Voyager 1's first images of Io revealed a lack of impact craters, suggesting a very young surface. Craters are used by geologists to estimate the age of a planetary surface; the number of impact structures increase with the age of the planetary surface. Instead, Voyager 1 observed a multi-coloured surface, pockmarked with irregular-shaped depressions, which lacked the raised rims characteristic of impact craters. Voyager 1 also observed flow features formed by low-viscosity fluid and tall, isolated mountains that did not resemble terrestrial volcanoes. The surface observed suggested that, just as Peale and colleagues had theorized, Io was heavily modified by volcanism.

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 300-kilometre (190 mi) 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 plume generated by active volcanism at a dark depression later named Pele. Following this discovery, seven other plumes were located in earlier Voyager images of Io. 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.