By Sean Henahan, Access Excellence

PASADENA (Aug. 21, '96) Close on the heels of the discovery of bacterial fossils from Mars comes another compelling suggestion of life in the solar system, the discovery that water may have existed, or might even still exist on Jupiter's moon Europa.

CAPTION: Jupiter's moon Europa, as seen in this image taken June 27, 1996 by NASA's Galileo spacecraft, displays features in some areas resembling ice floes seen in Earth's polar seas.Click image for high resolution version.

NASA's Galileo spacecraft has sent back new images of Jupiter's moon Europa indicating signs of 'warm ice' or possibly liquid water beneath Europa's icy crust. Galileo also returned pictures of a huge volcano erupting on the moon Io and new information about Jupiter's Great Red Spot.

Galileo scientists are evaluating images that show places on Europa resembling ice floes in Earth's polar regions, along with suggestions of geyser-like eruptions and details of long dark bands centered with white stripes that stretch like interstate highways across Europa's face.

"This moon is a marvelous place," said Galileo imaging team scientist Dr. Ronald Greeley, a geologist at Arizona State University in Tempe, Ariz. "We're seeing evidence of a lot of geological activity on Europa. In some areas, the ice is broken up into large pieces that have shifted away from one another but that obviously fit together like a jigsaw puzzle. This shows the ice crust has been or still is lubricated from beneath by warm ice or maybe even liquid water."

The results bring scientists a step closer to discovering whether Europa has environmental "niches" warm and wet enough to meet the requirements to host life, Greeley said.

Europa is about the size of Earth's Moon and is covered largely with smooth white and brownish-tinted ice instead of large craters like so many other bodies in the solar system. Scientists believe its cracked cue-ball appearance is due to stressing caused by the contorting tidal effects of Jupiter's strong gravity. They speculate that the warmth generated by tidal heating may have been or may still be enough to soften or even liquefy some portion of Europa's icy covering.

Europa has long been considered by scientists and celebrated in science fiction as one of the handful of places in the solar system (along with Mars and Saturn's moon Titan) that could possess an environment where primitive life forms could possibly exist. Indeed, Europe was the destination of the fictional spacecraft in Arthur C. Clarke's "2010".

"A major goal of Galileo's studies of Europa is to search for signs of current or past activity to help answer the question: Is there a liquid zone on Europa?" said Greeley. "We are interested in identifying the time and places on Europa where liquid water might exist. We want to go back to some of these areas that suggest soft ice or liquid water under the ice and test some of the questions we're asking now."

The current images, taken from more than 155,000 kilometers (about 95,700 miles) distance, show just 1.6 kilometer-per-pixel resolution, or features about one mile across. Moon flybys later in the mission will bring Galileo to within 600 kilometers (370 miles) of Europa's surface. During those flybys, the best resolution from the camera will average about 22 to 30 meters per pixel (72 to 98 feet) and as fine as 11 meters (36 feet) per pixel, so that objects the size of buildings on Earth could be discerned. These close flybys will occur Dec. 19, 1996, Feb. 20, 1997 and Nov. 6, 1997.

Galileo's images are shedding new light on the nearly global, highway-like stripes that scientists call "triple bands" because of their dark-bright-dark appearance. The cracks are thought to reflect tidal stressing in Europa's icy crust. "The scale of fracture patterns -- extending a distance equivalent to the width of the western United States -- dwarf the San Andreas fault in length and width," said Greeley.

Planetary geologists have proposed several models that could be responsible for creating the banded roadway look of these features. One set of models calls for combinations of tectonic faulting and flooding caused by liquid water or warm ice mixed with darker silicates that well up through cracks and then freeze over.

Galileo scientists say the new data suggest another model where 'dirty geysers' erupt along a line, ejecting a mixture of ice and darker silicate debris along the surface. These events may be followed by a more gentle, continuous flow of cleaner water ice that paints the white stripe down the center of the feature.

One new Galileo image of the moon Io shows a huge new bluecolored volcanic plume extending about 100 kilometers (about 60 miles) into space. Scientists believe the blue color of the plume coming from the feature, called Ra Patera, is probably sulfur dioxide gas and "snow" that condenses from the gas as the plume expands and cools.

"This is very different from what we see with volcanic eruptions on Earth," said Galileo project scientist Dr. Torrence V. Johnson of JPL. "Terrestrial eruptions cannot throw materials to such high altitudes. We believe that on Io we are seeing geyser-like eruptions that are driven by sulfur dioxide or sulfur gas that erupts and freezes in Io's extremely tenuous atmosphere."