The Galileo spacecraft continues to return tantalizing images of Jupiter's frozen moon, Europa. In previous passes by Europa, Galileo sent back snapshots of ice volcanoes, charted long, straight ridges that could be evidence of tidal churning and identified the presence of a tenuous oxygen atmosphere. All of these data have led to speculation that beneath its icy surface, Europa could harbor a hidden ocean of liquid water--and, just possibly, life.
These latest images, released by NASA's Jet Propulsion Laboratory on April 9, 1997, offer the closest views yet of Europa's fractured terrain. They were captured on February 20, when Galileo swung within 586 kilometers (363 miles) of the Jovian moon.
Among the intriguing details in these new views are relatively smooth, crater-free patches and features that appear to be chunky ice rafts. To some researchers, these formations suggest that Europa's crust of frozen ice is younger and thinner than previously believed, improving the odds that a warm ocean of liquid water lies below.
The most compelling image (top) shows rafts of ice resembling those seen on Earth's polar seas during springtime thaws. The crustal plates are up to 13 kilometers (8 miles) across and seem to have broken apart and "floated" into new positions. "The size and geometry of these features lead us to believe that there was a thin icy layer covering water or slushy ice and that some motion caused these crustal plates to break up," says Ronald Greeley, an Arizona State University geologist and member of the Galileo imaging team.
Other images hint at recent geologic activity on Europa. A mosaic of two Galileo images reveals a double ridge that stretches about 2.6 kilometers (1.6 miles) wide and stands some 300 meters (330 yards) high. This complex landscape implies that the icy crust of Europa has been modified by intense faulting and disruption, driven by energy from the moon's interior.
One especially spectacular closeup shows smooth areas that obliterate older terrain. The view, which covers an area of 13 kilometers by 18 kilometers (8 miles by 11 miles) and has a resolution of 26 meters (28 yards), includes a flat, circular feature that JPL scientists speculate formed when a liquid--water?--erupted onto the surface and flooded older ridges and grooves. Such activity again testifies that there was once significant energy churning Europa's surface.
As impressive as the new images are, they leave many key questions unanswered. First and foremost, they do not reveal precisely when these geologic disruptions occurred. If they are recent, Europa may still have a vast ocean of liquid water locked away beneath a thin veneer of ice; the presence of warm water would open at least the possibility of life. But if the events took place in the distant geologic past, Europa could now be frozen solid.
Galileo imaging team member Clark Chapman believes the smoother regions displaying few craters indicate Europa's surface is quite young. Chapman, a planetary scientist at Southwest Research Institute in Boulder, Colo., bases his age estimate on current knowledge about the rates at which craters form. "We're probably seeing areas a few million years old or less, which is about as young as we can measure on any planetary surface besides Earth," Chapman says. "Europa's extraordinary surface geology indicates an extreme youthfulness--a very alive world in a state of flux."
On the other hand, Michael Carr, a geologist with the U.S. Geological Survey, puts Europa's surface age at closer to one billion years old. "There are just too many unknowns," he asserts. "Europa's relatively smooth regions are most likely caused by a different cratering rate for Jupiter and Earth."
More answers may be forthcoming; intrepid Galileo is not done with Europa yet. The spacecraft will visit Europa for the last time in its primary mission on November 6, 1997. And NASA has recently extended Galileo's mission by another two years. During the additional time, it is scheduled to make eight more flybys of Europa. The extended mission will also include encounters with two of Jupiter's other giant moons, Callisto and Io.
If luck is with them, scientists might even see the definitive proof of a Europan ocean: a geyser of liquid water shooting onto the surface, spotted in the act. "We want to look for evidence of current activity on Europa, possibly some erupting geysers," Greeley says.