If a potato-sized meteorite from Mars has sparked fresh speculation about organic life elsewhere in the solar system, a moon orbiting Jupiter may well add fuel to the fire.
Fresh images of the icy surface of Jupiter's satellite Europa, taken by the Galileo spacecraft in July, give hints that the Jovian moon could host an environment capable of supporting primitive forms of life.
One of the key requirements for such organisms is water. Europa long has been known to have a surface encrusted in a layer of ice up to 60 miles thick. But the shots from Galileo detailed Tuesday at a briefing at NASA's Jet Propulsion Laboratory in Pasadena, Calif., show surface features that resemble ice flows in the polar seas on Earth, geysers, and crustal ridges where new material may be welling up from beneath the satellite's surface.
Whole plates and sheets seem to have rotated intact, suggesting that they are riding on a layer of slush or water, says Ronald Greeley, one of the Galileo project's imaging scientists and a geologist at Arizona State University. This implies the existence of a heat source driving Europa's frost-bound equivalent of plate tectonics. The heat comes from the impact of tidal forces on the moon, with Jupiter's gravity tugging from one direction and the gravity from two other large moons, Ganymede and Callisto, tugging from another.
When researchers consider likely targets in space as candidates for biological research, they look for water, the presence of organic materials, and sufficient heat to support life. "The things we're seeing in the pictures, these mobile zones, are places that could be environmentally favorable for life," Dr. Greeley says.
The images of Europa, taken as Galileo passed within 96,000 miles of the moon on July 27, cover an area of the surface roughly equal in size to the United States west of the Mississippi River. They have allowed researchers to pinpoint details "not even suspected" from pictures returned in 1979 from the Voyager.
Among these are formations that look like geysers and impact craters from meteors. In addition, thin stripes crisscrossing the moon's surface, which appeared faintly in Voyager images, become what Greeley calls "the great interstate highway system of Europa" when seen by Galileo. These stripes, with three distinct bands, are five to six miles wide and continue for thousands of miles across Europa's surface. Researchers interpret these features as the ridges where fresh crust formed.
Less clear, and important to the issue of whether Europa could harbor life, is how active the Jovian moon is today. In December, Galileo will take another pass by Europa, this time at a distance of only about 600 miles. If the moon is still tectonically active, researchers should see changes in surface features since last month's images were taken.
Besides images of Europa and another moon, Io, Galilee's recent photo session provided other significant Kodak moments.
Dramatic images of the region of Jupiter's Great Red Spot, a hurricane more than 300 years old, confirmed a notion about how energy is transferred in Jupiter's atmosphere. When scientists first learned that circulation around the outer edge of the spot topped 250 miles an hour, "this was a surprise, because in the outer solar system you have less [solar] energy to drive winds," says Andrew Ingersoll, another member of the Galileo science team. This led researchers to ask: What supplies and dissipates the energy? Drawing on analogies with Earth, they surmised that thunderstorms were responsible.
"The news is," he adds, "we think we've found them." The evidence is contained in images designed to help determine relative cloud heights. As on Earth, thunderheads on Jupiter stick up like an over-exercised thumb. Dr. Ingersoll estimates that these rapidly evolving storms are roughly equal in size to the largest Earth has to offer - some 30 to 50 miles wide with clouds that reach 50 miles high.