The gravitational tides that keepwater in a liquid state under the frozen surface of Jupiter's moon Europa mightalso foster the evolution of life by forcing it to adapt to the wild changes,says University of Arizona professor Richard Greenberg.

"Necessity driveschange," Greenberg reminds us.

And change is what Europa is allabout. At about the same size as Earth's Moon, Europa is the smallest of fourso-called Galilean moons. As Europa orbits the giant planet, its innards arepulled this way and that by gravity. Ocean tides fluctuate by more than 1,600feet (500 meters). The very shape of the moon is stretched along its equator.

Heat generated by all this tidalmotion keeps the subsurface liquid, scientists say.

In recent years, researchers havecreated a growing body of evidence suggesting that Europa's under-ice oceanmight harbor life, or at least that the conditions are ripe. No one knows ifanything is there, however. And most of the speculation has centered aroundsimplistic life, the implication being that whatever microbes might be therewould be barely scraping by.

But Greenberg thinks the tides,in combination with other factors, would not just support life but alsoencourage it to evolve to thrive in various niches. He's not suggestingdolphins or anything, but he does figure that Europan life may surprise us.

Greenberg, a professor ofplanetary sciences and member of the imaging team for NASA's Galileospacecraft, which is nearing the end of its mission to study Jupiter and itsmoons, discussed his views in the February issue of American Scientistmagazine.

Over the past four years or so,images of Europa returned by the Galileo spacecraft have revealed a surfaceriddled with cracks, ridges and other jumbled features, not unlike the shoresof a polar region in spring, where heaving icebergs protrude from the surface.

The ridges are believed to bebuilt over thousands of years by water seeping up the edges of cracks andrefreezing to form higher and higher edges. Other chaotic-looking areas may beevidence that the ocean melts through periodically.

"Everything on and under thesurface is driven by the tides," Greenberg said. "The ocean isinteracting with the surface."

In addition to tidal heat,undersea volcanoes might generate melt throughs, he said.

If the ocean is sometimesexposed, light and chemicals could get in. And all the action would mixsubstances needed to support life. But exposed areas would eventually refreeze.

"That would require anorganism to adapt in some way," he said.

Life on Europa -- if it exists atall -- could resemble that of simple sea-dwelling organisms of Earth, Greenbergsaid, possibly utilizing photosynthesis for energy.

"Plenty of Earth's organismslive at 32 degrees Fahrenheit or below," he said.

Microbes in the Antarctic areknown to hibernate for up to a million years in the ice, he points out. Suchorganisms on Europa might thaw out when the next warm tide flowed through.

"There is a possible biospherethat extends from way below the surface to just above the crust,"Greenberg speculates.

Most researchers agree that proofof life on Europa will only come with a mission to the moon. No firm plansexist for such a project.

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