Frosty white, freezing cold, bleak and barren--that's how Jupiter's moon Europa appears when viewed through Earthly telescopes. But zoom in closer and the question arises: Is this frigid world home to living creatures?
Scientists were abuzz with that exciting possibility last week when NASA released new, close-up pictures of Europa taken by the Galileo spacecraft. Galileo, which has been fixing its sensors on Jupiter and its moons for more than a year, came within 363 miles of Europa--an astronomical eyelash away.
Pictures Of A Water World
In the new photos, Europa appears to be wrapped in a vast ocean. Scientists believe that this sea is the largest body of water in the solar system. The surface is frozen. That's no surprise, since Europa's surface temperature is estimated to hover around -360° F. But the sheet of ice appears to be thin in places, deeply cracked, and dotted with what look like icebergs. The icebergs lean at crazy angles, as though they have drifted on liquid water. The
scene resembles Earth's polar seas during springtime thaws.
"These are really mind-blowing pictures," says Richard Terrile of NASA's Jet Propulsion Laboratory in Pasadena, California, where the new images were released. According to Terrile, if Europa's ice crust could be melted, the volume of water would be greater than all Earth's oceans combined!
Europa's tilted icebergs and its cracked and rutted surface suggest to scientists that liquid water is, or was, moving beneath the moon's frozen crust. And where there's water, there is the possibility of life.
More hints of life: brownish stains on the ice. These could be organic chemicals, the building blocks of living cells. "If we have organic chemicals mixed into a bath of relatively warm water," says Terrile, "that's a recipe for life."
Heat Within The Ice
How could liquid water flow in a frozen world? What source of heat might exist on a moon that is half a billion miles from the sun? Astronomers suspect that Europa is heated from within by the constant push and pull of gravity--the gravity of Jupiter and its other moons.
On Earth, the force of our moon's gravity is so great that it causes the ebb and flow of ocean tides. Europa, which is slightly smaller than Earth's moon, is subject to similar forces. The tidal pulsing inside a moon produces enormous friction, and that friction probably generates heat.
Tidal heating on Europa could keep the waters thawed below the icy surface. Water that is even a degree above freezing could be home to living creatures. On Earth, deep-sea creatures like tube worms and bacteria "thrive in environments like this," says Galileo scientist Paul Geissler.
Last week, as pictures of Europa's sea were released, oceanographer John Delaney of the University of Washington threw scientific caution to the wind. "I'm sure there's life there," he declared.
Other Moons, Other Worlds
Astronomers have been aware of Jupiter's biggest moons since 1610, when they were
discovered by Italian scientist Galileo Galilei, for whom the Jupiter probe is named. But only in the past 20 years have they taken a serious look at them, along with the moons of other planets.
What they have seen, thanks to space probes like Galileo, has blown apart old ideas. Take, for instance, the notion that only Earth has volcanoes. In 1981, images of Io (Eye-oh), another moon of Jupiter's, showed it to be erupting in 10 places.
Volcanoes may also exist on Enceladus (En-sell-uh-duss), a moon of Saturn's. As Enceladus races around the giant planet, its eruptions leave a trail of sparkly ice crystals--forming Saturn's ghostly fifth ring.
Astronomers are intrigued as well by Saturn's Titan, a moon larger than the planet Mercury. Titan has a dense atmosphere that might be a fine breeding ground for living microbes, except its temperature is a chilly -290° F.
More Explorations Ahead
With luck, Galileo will continue to help scientists learn about some of the 61 moons of our solar system. The trusty space probe is working so well that NASA hopes to keep it active in the Jupiter system through 1999. Meanwhile, in November, the space agency will launch the new Cassini-Huygens spacecraft on a seven-year journey to Saturn. In its bag of tricks is a probe designed to puncture Titan's cloud cover, parachute to the surface and relay data from the giant moon to Earth.
Scientists like Terrile can't wait. The moons of our solar neighborhood form a wild cosmic family, he says. Some of them "may be up to something truly fantastic."