A navigation engineer spotted the first volcano erupting beyond Earth
Shortly after Voyager 1 swept past Jupiter's moon Io in March 1979, navigation engineer Linda Morabito noticed an odd plume rising from its edge in one image. Other pictures soon revealed nine of them. Io was erupting, the first active volcanism ever seen on another world, and a team had predicted it in print just before the flyby.
Io is slightly bigger than our Moon and is now known to have more than 400 active volcanoes, making it the most geologically restless object in the Solar System. The heat comes from tides. Io's orbit is locked in rhythm with its neighbours Europa and Ganymede, so Jupiter's gravity keeps squeezing and relaxing its interior, generating friction. Stan Peale, Patrick Cassen and R. T. Reynolds calculated this tidal heating in the paper published shortly before Voyager arrived.
The results are spectacular. Plumes of sulfur and sulfur dioxide shoot up to 500 kilometres high, lava flows stretch for hundreds of kilometres and sulfur compounds paint the ground yellow, red, white, black and green. More than 100 mountains rise from the crust, some taller than Everest. Unlike icy outer moons, Io is silicate rock around an iron core, has the highest density of any moon and holds less water than any other known body in the Solar System. Its ejected material feeds a ring of plasma around Jupiter that makes its surroundings intensely radioactive.
Galileo first glimpsed Io on 7 January 1610 through a 20-power telescope, though he could not split it from Europa until the next night. Simon Marius claimed to have seen the moons first, and while Galileo got the credit, Marius supplied the names, following a suggestion from Johannes Kepler to honour lovers of Zeus; they only caught on in the mid-20th century. In the 17th century, Io's predictable eclipses helped with finding longitude and gave the first measurement of the speed of light.
Radiation has always been a hazard for visitors. Pioneer 10's planned close-up pictures in 1973 were lost to it, and only Pioneer 11 returned a decent image. When Voyager 2 passed four months after its twin, seven of the nine plumes were still erupting; only the volcano Pele had gone quiet. Many of Io's features are named after fire and thunder gods and places in Dante's Inferno.
Source: Io (moon)
