In the quest to find evidence of life beyond Earth, Europa's star is rising.
Scientists trying to unlock the secrets of this frozen Water World have unveiled the most detailed images yet of its surface. The photos reveal an ice-scape pinched, pushed, and punctured by heat-driven processes, apparently similar to those that shape the Earth's crust.
The fresh portraits of Jupiter's second-closest moon, for example, show the first ice flows seen on any Jovian satellite. Hundreds of meters thick, these flows indicate that "water is mobile enough to emerge through the surface and there is sufficient heat to pump it out," says Ronald Greeley, a geologist from Arizona State University and a member of the Galileo imaging team.
He adds that organic molecules are likely to be present as well, because they permeated the cloud of dust and gas that spawned the solar system. Evidence for these molecules, which could come as Galileo returns data on Europa's spectra, would give the moon the three conditions needed for life to form: liquid water, heat, and organic molecules.
Some of the larger features that intrigue planetary geologists appeared in images Galileo returned last August. But those were coarse compared with the latest images. Coming within 676 km (437 miles) of the surface, Dec. 19, this pass picked up objects the size of the Superdome.
As a result, Europa's surprises "make the 'X Files' look like a predictable soap opera," says James Head, a planetary scientist at Brown University in Providence, R.I. Areas between large ridges seen in August are themselves crisscrossed with smaller ridges that make some parts of the moon "look like a ball of twine," he says. Galileo spotted a trough similar to a slot-like volcanic vent found in Idaho's Snake River plain.
Light-colored regions of ice separated by darkened bands of parallel ridges resemble areas where Earth's sea floor is spreading. The images point to a surface broken into "plates" floating on a liquid substrate. "I showed these pictures to my daughter and said, 'If you ever see ice like this, don't skate on it,'" Dr. Head quips.
Dr. Greeley cautions that for now, the most that can be said is that the evidence shows past geological activity. What's missing, researchers agree, is a "smoking gun" - an image of a geyser erupting or a comparison of Galileo and Voyager images showing significant changes in the shapes or positions of the ice plates.
Yet the prospect that Europa's icy crust may mask an ocean underneath is prompting scientists to lay the next steps in exploring what some say may be the solar systems leading candidate for hosting extraterrestrial life. And it is becoming an interdisciplinary effort. Last November more than 100 biologists, oceanographers, and planetary scientists met at the San Juan Institute in San Juan Capistrano, Calif., to lay the ground work for future Europa exploration.
Based on observation and theory, "most everyone now thinks that Europa has a subsurface ocean," says Bruce Betts, a research scientist at the institute. "There is enough tidal heating to melt the ice" and probably enough heat from radioactive decay in the moon's rocky core to generate hydrothermal vents similar to those found beneath Earth's oceans.
These vents provide the minerals and heat that sustain primitive organisms. Researchers hope to look for chemical evidence of an ocean with the Europa Ice Clipper. Proposed for launch in the fall of 2000, the craft would arrive in 2008 and drop a 10 kilogram ball onto one of the zones between plates where material is thought to be welling up from beneath the ice. The craft would pass through the plume of ice shards, analyze their composition and perhaps return samples to earth.