New images show Jupiter's giant moon Ganymede has geologic hints of an early subsurface ocean and a chain of 13 craters that could have come from a broken-up comet.

But scientists still can't say whether life ever existed there."We don't know and that's why we're out there looking," said James Head, a Brown University planetary scientist. "You have heat, liquid water, organic material coming in from impacts from meteorites. The ingredients are there."

The close-ups of Ganymede's rich geology were released Wednesday by NASA. The unmanned Galileo spacecraft captured them during several flybys, some within just a few hundred miles of the surface, in June 1996 and June 1997.

Ganymede, the largest moon in the solar system, is 3,269 miles in diameter - bigger than Mercury and three-quarters the size of Mars. About half its surface is bright, clean ice and half dark, heavily cratered terrain made up of "dirty" ice and rocks. Scientists are looking for signs of liquid water.

The new images suggest that icy volcanoes spewed water instead of the rocky lava seen on Earth, Head, a member of the Galileo imaging team, said Tuesday from his office in Providence, R.I.

Because much of the moon is icy, scientists have wondered whether there is a warmer liquid layer beneath.

"The Galileo images show us quite dramatically that about half of the surface has been renewed by some process that put bright terrain out onto 50 percent of the surface," Head said.

From images snapped by the Voyager missions of the late 1970s, scientists suspected this was water, Head said. The water-spewing volcanoes suggested by the latest photos could provide the mechanism for how the ice was deposited.

At least 1 billion years into its history, Ganymede very likely had a global ocean, like the one that scientists suspect lies beneath the frozen expanses of another of Jupiter's moons, Europa.