File:Erupting Volcano on Jupiter's Moon Io.jpg

English: In this close-up Hubble Space Telescope picture of Jupiter's moon Io (bottom right) the mound rising from Io's surface is actually an eruption from Pillan, a volcano that had previously been dormant.

Measurements at two ultraviolet wavelengths indicate that the ejecta consist of sulfur dioxide "snow," making the plume appear green in this false-color image. Astronomers increased the color contrast and added false colors to the image to make the faint plume visible.

Pillan's plume is very hot, and its ejecta move extremely fast. Based on information from the Galileo spacecraft, Pillan's outburst was at least 2,240 degrees Fahrenheit (1,227 degrees Celsius). The volcano was spewing material at speeds of 1,800 miles per hour (2,880 kilometers per hour). The hot sulfur-dioxide gas expelled from the volcano cools rapidly as it expands into space, freezing into snow.

Io is well known for its active volcanoes, many of which blast huge plumes of volcanic debris into space. Io has hundreds of active volcanoes, but only a few, typically eight or nine, have visible plumes at any given time.

For more information, visit: hubblesite.org/news_release/news/1999-13

Credit: John Spencer (Lowell Observatory) and NASA