These false-color images of Io and Jupiter were taken with the NASA Infrared Telescope Facility at Mauna Kea, Hawaii, as part of a campaign to support closeup Io observations by NASA's Galileo spacecraft. These and other Earth-based observations show that Io's most powerful volcano, Loki, began one of its periodic major eruptions about a month before Galileo's October Io flyby, and that the eruption was continuing during the Galileo flyby. These infrared images (taken at a wavelength of 3.8 microns) show Sunlight reflected from the edge of Jupiter's disk on the left-hand side, and the heat from several glowing volcanoes on Io on the right. Io is in Jupiter's shadow, so no Sunlight falls on it -- the volcanoes are all we see.

On August 9, 1999 (left), several volcanoes glowed faintly with roughly equal brightness. However, on October 10, 1999, roughly 20 hours before Galileo flew past, a single volcano, Loki, dominated the image. Loki brightened by a factor of ten in the period between these images. Other observations from the NASA Infrared Telescope and from Wyoming Infrared Telescope near Laramie operated by the University of Wyoming show that most of this brightening occurred during September.

Earth-based observations since the 1980s have shown that these periodic bright eruptions are typical behavior for Loki. They occur about once per year and last several months. Galileo has given us our first chance to see one of these eruptions up close.

The Jet Propulsion Laboratory, Pasadena, CA manages the Galileo mission for NASA's Office of Space Science, Washington, DC. JPL is a division of the California Institute of Technology, Pasadena, CA.

This image and other images and data received from Galileo are posted on the Galileo mission home page at http://solarsystem.nasa.gov/galileo/. Background information and educational context for the images can be found at http://galileo.jpl.nasa.gov/gallery/io.cfm.