Images strengthen case for ocean on Jupiter moon

Newly released images from the Galileo spacecraft add to mounting evidence for a global ocean beneath the icy shell of Jupiter's moon Europa, researchers said here yesterday.

At a Tucson meeting of the Galileo imaging team, scientists unveiled a mosaic formed from 12 Europa pictures taken in November, when the aging Galileo spacecraft passed 58,000 miles from the satellite, which is about the size of Earth's moon.

The mosaic provides the sharpest views yet of the Jupiter-facing side of Europa. The pictures show dark, mottled regions where Europa's ice shell appears to have melted through to water or slush beneath, the researchers said.

This type of ``chaotic terrain'' has been seen elsewhere on Europa, including zones where iceberg-like rafts appear to have drifted on a sea before refreezing in new locations. The new Galileo images show that chaotic terrain is a global feature on Europa, said Arizona State University geologist Ronald Greeley, an imaging team member.

``This says that the zone of liquid or slush was global in extent,'' Greeley said during a news conference at the Doubletree Hotel.

NASA's $1.5 billion Galileo spacecraft was launched in 1989 from the space shuttle Atlantis. It entered orbit around Jupiter in December 1995, and its two-year main mission was followed by a two-year extension that focused on Europa.

The Europa extension ended in January, and the battered probe has embarked on a new one, called the Galileo Millennium Mission. The new mission extension will continue at least through December, when Galileo is scheduled to make simultaneous observations of the Jovian system with Cassini, a Saturn-bound NASA probe.

This week's Tucson gathering marks the 81st and final meeting of the Galileo imaging team, which began its work in 1979. The team is headed by Michael Belton of Kitt Peak National Observatory and includes several University of Arizona researchers.

Torrence Johnson, NASA's Galileo project scientist, ranked the accumulating evidence for a Europa ocean as Galileo's top discovery.

``Most of the lines of evidence have convinced the scientific community that there was an ocean at some time'' beneath Europa's ice, Johnson said at yesterday's news conference. Recently, measurements by Galileo's magnetometer provided strong evidence that liquid water is present today, he said.

If there is an ocean on Europa, it's conceivable that it harbors primitive life forms.

During its tour of the Jovian system, Galileo also observed active volcanoes on the moon Io. New mosaic images of Io volcanoes were displayed yesterday by UA researchers Elizabeth Turtle and Laszlo Keszthelyi.

The new Galileo pictures can be viewed at http://galileo.jpl.nasa.gov.