Sun, 17 Dec 2000 press release from the Jet Propulsion Laboratory (JPL) NASA.

Evidence is building that yet another body in the solar system has liquid water.

It seems that magnetic readings taken by the Galileo spacecraft when it passed close to Ganymede in May 2000 can be best explained by a thick layer of salt water beneath its crust of ice. Unlike Europa and Callisto, two other moons of Jupiter thought to have hidden oceans, Ganymede has its own magnetic field, which complicated magnetic measurements. Dr Margaret Kivelson, a planetary scientist at the University of California, Los Angeles, and principal investigator for Galileo's magnetometer said Ganymede's field is "highly suggestive" of a salty ocean on Ganymede. "It would need to be something more electrically conductive than solid ice." She said that a melted layer several kilometers or miles thick, beginning within 200 kilometers (120 miles) of Ganymede's surface would fit the data if it were about as salty as Earth's oceans.

Dr. Thomas McCord, a geophysicist at the University of Hawaii, Honolulu, has been using Galileo's infrared spectrometer to study reflectance of Ganymede's surface materials. Ganymede is covered with lots of ice and frost, both in the older, dark terrains and younger, bright terrains, and portions of the moon appear to have types of salt minerals that would have been left behind by exposure of salty water near or onto the surface, he said.

"They are similar to the hydrated salt minerals we see on Europa, possibly the result of brine making its way to the surface by eruptions or through cracks," McCord said.  The infrared evidence does not indicate whether or not an ocean persists at Ganymede today, he said.

The images taken by Galileo as it passed within 809 kilometers (503 miles) of Ganymede on 20th May 20 show fractured surface features like those on Europa, another of Jupiter's moons, which is also believed to have a deep ocean beneath its ice. These faults may allow water or slushy ice to surface. You can see some of these images at http://www.jpl.nasa.gov/pictures/jovianmoons  They are the most detailed photos of Ganymede ever taken and an animated flyover of Ganymede's terrain. Some of the smooth, bright, new areas resemble parts of Europa.

Dr Robert Pappalardo, planetary scientist at Brown, examining the images taken by Galileo, commented that, "Bright broken swaths, disrupted dark plains and the astounding Arbela Sulcus suggest Ganymede may be more similar to Europa than previously believed," Arbela Sulcus is a relatively smooth, bright band interrupting a more cratered, older landscape. The new images show subtle striations along its length. "It is possible that Arbela Sulcus has formed by complete separation of Ganymede's icy crust, like bands on Europa, but unusual for Ganymede."

Dr. Dave Stevenson, planetary scientist at the California Institute of Technology, Pasadena points out that unlike Europa, Ganymede's naturally radioactive rock core should provide enough heat keep layer of liquid water between two layers of ice, about 150 to 200 kilometers (90 to 120 miles) below the surface. Europa derives its heat from interior flexing due to tidal effects of Jupiter's gravity. "I would have been surprised if Ganymede had not had an ocean, but the issue of whether it's there is different than the issue of whether you can expect to see it clearly in the data."

Galileo has been orbiting Jupiter since Dec. 7, 1995. It will fly past Ganymede again on Dec. 28, but will not come as close as it did in May. Additional information on Galileo is available at http://galileo.jpl.nasa.gov . The Galileo mission is managed for NASA's Office of Space Science, Washington, D.C. by JPL, a division of the California Institute of Technology.