The brightest object in the solar system other than the Sun, Europa's frozen surface has fascinated scientists ever since the first Voyager spacecraft passed by Jupiter in 1979. At almost 300 degrees Fahrenheit below zero, Europa's surface temperature could deep-freeze an ocean over time, but many scientists now believe that heat from an inner dynamo and a tidal tug of war between Jupiter and Europa's neighboring moons could cover a hidden sea beneath the moon's icy crust that may be warm enough to support some primitive form of life.
Pictures of Europa taken by the Galileo spacecraft now in orbit around Jupiter reveal towering icebergs drifting in unseen currents, providing strong evidence of a hidden sea. How large could this hidden sea be? The most recent estimate of scientists studying the pictures taken by the Galileo spacecraft is that there is three times as much water under Europa's ice pack as there is in all of Earth's oceans, lakes and rivers.
"The evidence supports the hypothesis that liquid or at least partially liquid water (slush) existed at shallow depths below the surface of Europa in several places," said James Head, former member of the Apollo moon rock team, now professor of geological sciences at Brown University and a group leader of the Galileo research team. "The combination of interior heat, liquid water and infall of organic material from comets and meteorites means that Europa has the ingredients of life."
The photographic evidence includes a shallow impact crater, cliff-like surfaces that appear to be icebergs and huge gaps in the crust where a new ice crust appears to have formed between massive sheets of ancient ice. Debris and impact rays show that a large meteorite struck Europa as recently as 10 million years ago, excavating subsurface slush that quickly filled in the deep hole. A subsurface ocean warm enough to be slushy could also explain the broken blocks of ice crust the size of city blocks that show slush-like material between the chunks. These chunks appear to be sliding on soft glacier-like ice below the surface or floating like icebergs on the hidden sea.
So compelling are Galileo's pictures of Europa that the icy moon is one of the high priority targets for its own exploration mission. If liquid water exists on Europa, scientists say, it would not be unreasonable to speculate that life exists there, possibly forming near undersea volcanic vents the way they have formed in the deepest regions of the Pacific Ocean on Earth.
Thomas OToole is a freelance writer and former editor of Aerospace America magazine who covered space during much of his 21 years at the Washington Post.
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