A very high resolution image taken by Galileo from a distance of 540 miles (top) shows icy plates that probably broke apart and moved laterally when the surface was slush or water. The cliffs are a few hundred feet high, and the blocks of debris below them are the size of house. A few are as big as the Rhode Island state capitol, the large white building in the upper left of the bottom image, which shows Providence at the same scale.
At a campus press conference jointly held by Brown and NASA on March 2, Head unveiled dramatic photos taken a few months before by the Galileo spacecraft - photos so detailed that scientists are able to pick out objects on Europa's surface the size of a large truck. According to Head, the images show patterns of surface debris that "strengthen evidence for the idea that there is a subsurface of warm, slushy material." The existence of such an underground ocean would indicate that, despite a surface temperature of 260 degrees below zero, the moon has sufficient heat, water, and organic material for life to develop.
Describing the evidence for a liquid ocean at the press conference were graduate students Geoffrey Collins and Louise Prockter and postdoctoral researcher Robert Pappalardo, all of whom, like Head, are members of the Galileo imaging team. Collins described a shallow crater on the surface of Europa named Pwyll, whose shape suggests "that flowing ice or slush filled it in pretty quickly," much like honey flowing up into a bowl. Pappalardo pointed out large blocks in one of the Galileo photos that were configured like icebergs floating in a "rough, jumbled matrix" that more closely resembles slush than water that has frozen solid. The images, he said, "suggest that the surface was warm and slushy at one time." Finally, Prockter, who has studied mid-ocean ridges on Earth, described patterns of striations on the surface and ice sheets that appear to have moved apart, like tectonic plates floating on an ocean.
Definitive answers about the existence of a subsurface ocean could come early next century, after another spacecraft is launched toward Europa in late 2003. The orbiter, which will reach Europa about five years later, will not only prove or disprove the existence of an ocean, but also should be able to measure its depth.
Head and his team are betting that the ocean's existence will be confirmed. "We are now 80 to 90 percent sure," Pappalardo concludes. Then again, he won't be 100 percent certain, he says, "until I can go swim in it."