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.