Biology on Europa.

New close-up images of Jupiter's moon Europa show a mesmerizing array of fine-scale ridges, cracks, and faults on the icy surface. Taken by the Galileo spacecraft and due to be released April 9, the pictures confirm previous findings that blocks of ice on Europa have moved and rotated, as if they had slipped across an underlying layer of warm ice or water.

This evidence suggests that Europa's surface was once-and may still be-thin and easily deformable, says Robert T. Pappalardo of Brown University in Providence, R.I. Beneath this crust, "there just about had to have been an ocean," he says.

Assuming that Europa still has a subsurface ocean, Jonathan I. Lunine and Ralph D. Lorenz of the University of Arizona in Tucson have calculated the effect of sunlight shining through cracks in the ice. These cracks fill so slowly, they find, that enough light can penetrate to the top of the water layer to produce prebiotic, complex organic compounds.

The team notes that Jupiter's gravity causes Europa to flex, generating heat within the moon and perhaps melting enough ice to form an ocean. The same heat may prevent ice from filling in surface cracks for several hundred years. Although photosynthesis can't occur if the ice layer exceeds 80 meters, organic molecules can form beneath even thicker layers of ice, where there is less light. Water or ice erupting through the cracks may fall back down as snow, which blocks more sunlight than ice does. Snow also acts as an insulator, however, and might keep the ice from thickening near cracks, Lunine notes.

On Earth, some organisms flourish at the seafloor, near the blasts of heat vented by underwater volcanoes. Although gravitational flexing on Europa may create similar subocean volcanoes, some researchers doubt that complex biological compounds could form in the presence of such high heat. Therefore, if organic compounds exist in the upper reaches of a Europan ocean, Lunine says, they would play a crucial role in fostering life on this distant moon.