We already knew that the surface of Europa is covered with water ice. That this water would be ice was no surprise: the surface temperature is in the neighborhood of -225 degrees (and that's without windchill). But the ice has a story to tell. Eugene Shoemaker of the U.S. Geological Survey says the surface is very young. He bases that assertion on the rarity of craters on Europa. He has catalogued all the known comets that have been captured by Jupiter's gravitational field. Some of those eventually crash into the giant planet (SMITHSONIAN, June 1994) while others hit the Galilean satellites. Knowing what's out there, Shoemaker has calculated the current cratering rate on Europa. The surface should be liberally sprinkled with craters, but it is not. Therefore the surface must be renewing itself. Some parts of its surface show so few craters that Shoemaker estimates they are less than ten million years old.
There's more: the surface is laced with cracks and fissures, some of which look as though they have been pushed apart after forming, as if something -- presumably water, possibly in the form of slush -- were pushing up from below. This sort of activity would account for the obliteration of old craters. So it just may be that there is enough heat moving up and out of the solid moon to melt the ice from below, or in other words, an ocean of liquid water is hiding under that ice. Under that water, submarine volcanoes may be spewing out the minerals, including sulfides, from which life could arise.
If we wanted to look for life elsewhere in the Solar System (and if we had the money), our best bet might be to send an oceanographer the 400 million miles to Europa where she could drill through the ice and, just maybe, shout "Eureka!" Naturally we would not expect to find any forms of life "higher" than bacteria because we know all the reasons why that is improbable. But there are those white worms and crabs around thermal vents here at home. The undisciplined mind envisions a new world of intelligent crustaceans. Think of the movie possibilities.
By John P. Wiley, Jr.