Astronomy Picture of the Day

Explanation: Is Io the solar system's Fissure King? Well, probably not ... but it is the most active volcanic moon. Active volcanoes on Jupiter's moon Io were a surprise discovery of the Voyager missions of the late 1970s. The extent of Io's volcanic activity today is being investigated close-up by the Galileo spacecraft currently exploring the Jovian system. The two frames above show a roughly 300 mile square area around the Io volcano called Marduk. The left-hand view of Marduk was made by Voyager in 1979, the right-hand view by Galileo earlier this year. A comparison reveals that dramatic changes have occured, including the creation of a dark, linear feature running diagonally through the Galileo image that is probably a huge volcanic fissure.