Craft eyes new evidence of a slushy Europa.

The promise of a watery world lurking beneath the surface of a faraway moon just got more real. The sharpest images ever taken of Jupiter's icy satellite Europa offer fresh evidence that it possesses either a vast liquid ocean or a partially frozen sea as slushy as a snow cone.

Unveiled this week, the pictures were recorded by the Galileo spacecraft on Dec. 16, 1997, when it came within 900 kilometers of the moon--its closest pass. These images show details as small as 6 meters across. Several features are best explained by water or slush a few kilometers below the surface, scientists asserted at a briefing at Brown University in Providence, R.I.

Water remains frozen on Europa's chilly surface, but the gravitational tug of Jupiter and other moons causes flexing that may generate enough heat to melt subsurface water (SN: 8/9/97, p. 90).

One area examined by Galileo contains the crater Pwyll. Several properties of this 26-km-wide crater, including its girth and the presence of debris ejected during its excavation, suggest that Pwyll formed as recently as 10 million years ago. A crater this young, if it forms in rock or solid ice, typically retains its original size and depth. Yet stereo pictures constructed from Galileo images reveal that Pwyll's bottom is at the same level as its surroundings and that the crater's central peak rises far above the rim. A crater of similar size on Earth's moon would have a hole as deep as the Grand Canyon, says James Head of Brown.

He argues that Pwyll's shallowness stems from the presence of fluid beneath the surface. This material, displaced by the impact that created Pwyll, quickly flowed back, lifting the entire structure, Head and his colleagues propose.

Galileo also took a close look at a jumbled collection of fractured chunks of ice, each the size of several city blocks. Previous images indicated that the chunks are either gliding on warm ice or floating like icebergs in an ocean. The new images reveal that the material between the chunks has a rough, ropy texture. Such a texture is most likely formed by slush that froze in place when it rose to the moon's frigid surface, says Robert T. Pappalardo of Brown.

In addition, Galileo homed in on a region containing wedges of dark material thought to be new crust. The images show more clearly than ever that the wedges are scarred by linear ridges and parallel grooves. Such markings resemble those seen in new crust on Earth's seafloor, where lava has pushed up, cracking and separating the old surface. Instead of lava, the molten material on Europa could be water or slushy ice, says Louise Prockter of Brown.

The new pictures provide "a compelling case for liquid in recent times" beneath parts of the surface, says William B. McKinnon of Washington University in St. Louis. More images, though not quite as detailed, are expected as Galileo flies past Europa another six times over the next 22 months.

McKinnon notes, however, that the "concrete proof that people want" awaits a new mission, set for launch in 2003. That craft, orbiting Europa rather than repeatedly flying past it, will generate radar maps to deduce more directly the presence of an ocean.