EUROPA

Ariz. researchers hope to learn if an ocean hides under moon's icy shell

SIGNS OF LIFE: Second of a three-day series

Arizona researchers are vying for the chance to help NASA answer one of the most intriguing questions in planetary science today: Is there an ocean beneath the icy shell of Jupiter's moon Europa?

In December, the space agency received five proposals for the $300 million Europa Orbiter mission, which is designed to settle the issue of a sub-surface ocean, first suggested by Voyager spacecraft pictures 21 years ago.

Arizona researchers play key roles in at least three of the five proposals for Europa Orbiter science instruments.

University of Arizona planetary scientist Jonathan I. Lunine heads one of the teams, and Arizona State planetary geologist Ronald Greeley leads another.

The space agency is expected to select the Europa Orbiter science instruments this month, and the target launch date is November 2003, said Jay Bergstralh, the mission's program scientist.

The possibility that Europa has a hidden global sea bigger than all of Earth's oceans combined is intriguing, but most tantalizing is the prospect that such an ocean could harbor primitive life.

Liquid water, carbon and an energy source are the three requirements of all known life forms, and Europa might have all three.

That realization has fired the imagination of science fiction writers such as Arthur C. Clarke.

Even the National Aeronautics and Space Administration has speculated that life may have sprung up near volcanic vents at the bottom of Europa's ocean.

But until the ocean issue is resolved by Europa Orbiter, talk of thriving ecosystems huddled around sea-floor volcanoes is just conjecture built on presumption.

``What's the evidence for life on Europa? There is none,'' Greeley said.

Greeley is a member of the Galileo imaging team, which is headed by Michael Belton of Kitt Peak National Observatory.

Researchers at Arizona State, the UA and Brown University did much of the initial analysis of Europa images sent back by the Galileo probe over the last few years. Some of the pictures were 100 times sharper than Voyager's.

The twin Voyagers saw a cream-colored ice world criss-crossed, like a big ball of twine, by long chains of curving cracks and ridges. The probes saw few large impact craters on Europa, which is about the size of Earth's moon.

The dearth of craters was a surprising finding, suggesting the ice had recently been resurfaced, just as a Zamboni machine smooths an ice rink's pits and blemishes between periods at a hockey game.

Some researchers suggested that the tidal flexing of Europa as it orbits giant Jupiter generates enough heat to melt sub-surface ice and form a hidden ocean.

They pointed to the neighboring moon Io, where the constant wrenching by Jove's gravity creates enough heat to drive the solar system's most active volcanism.

Years after Voyager, the Galileo probe measured Europa's gravitational field and found that the moon is mostly rock, with an outer shell of water about 60 miles thick.

Is the water layer solid ice all the way down to rock, or is it a mix of ice, slush and ocean? The gravitational data couldn't say.

Other Galileo instruments added evidence to the ocean hypothesis.

The near-infrared spectrometer found evidence for salts and, more recently, sulfuric acid that may have oozed up through cracks in the ice.

The magnetometer measured changes in Europa's magnetic field that researchers say are consistent with a salty liquid ocean.

UA researchers on the Galileo imaging team analyzed the tangled web of scalloped surface cracks first spotted by the Voyagers 20 years ago.

In an article published last fall in Science, they linked the crack pattern to the daily rise and fall of ocean tides beneath the ice.

But the UA computer model that tied the curved cracks to tides only works if Europa has a thin ice shell on top of a deep ocean.

If the moon was frozen solid or had a thick ice shell, it couldn't raise tides big enough to crack the ice, the researchers said.

Gregory V. Hoppa, lead author of the Science paper, said the ice may be less than a mile thick in some places, a few miles thick in others.

But other researchers, including Brown University's Robert Pappalardo, reject the UA ``thin shell'' model and suspect that the ice may be up to 20 miles thick.

``It sounds nice - and that's what people want to hear - but I don't believe it,'' said Pappalardo, a planetary geologist who studies the Galileo images.

NASA hopes to someday send ``cryobots'' to melt their way through Europa's ice, releasing ``hydrobots'' into the putative ocean to search for life.

Space agency engineers are exploring technologies for such missions now and plan to test them at Lake Vostok, an Antarctic lake covered by 2 1/2 miles of ice.

But a 20-mile-thick Europan ice shell would make tough sledding for the cryobot. And as the depth of the ice increases, the likelihood that the hydrobot would find diverse life plummets.

So the UA thin-shell model, if correct, would be welcome news at NASA.

UA planetary scientist Richard Greenberg sees additional evidence for the thin-shell model in the iceberg-like blocks of Europa's Conamara Chaos.

Detailed pictures of the region sent back by Galileo show ice rafts that appear to have broken away from the rest of the shell, drifting about on a slushy sea before refreezing in new positions.

When those pictures were relayed to Earth, Greenberg and other members of the Galileo imaging team began piecing together the giant jigsaw puzzle, trying to fit the jagged shards into their original locations.

When manipulating the images on his computer screen proved too tedious, Greenberg resorted to paper printouts, scissors, Post-It notes and glue.

Reconstructing the Conamara Chaos region helped convince him that ``the best, simplest and most obvious explanation is that there was melt-through'' from an ocean below the ice.

Greenberg suspects that portions of Europa's surface are resurfaced periodically by warm ocean water that melts through the thin ice sheet and bathes the surface.

He also suggests that water is ``gurgling up through the cracks'' on Europa's surface every 85 hours as tides flex the moon's global web of rifts and ridges.

``If you put bacteria there, on Europa, it would probably be a pretty comfortable place for it to live, right in the ice,'' he said.

``If we're right, and the ocean is really interacting with the surface on an ongoing basis, all you'd really have to do is go to the right place that's active now - the crack that's active now or a melt-through that's active now - just land there and scoop up the goodies.''

Greenberg presented his ideas at recent scientific meetings and discussed them in a paper entitled ``Habitability of Europa's Crust: The Role of Tidal-Tectonic Processes.''

Using the UA thin-shell model as a springboard, Stanford University researcher Christopher F. Chyba suggested in Nature in January that charged particles raining down on Europa from Jupiter could ``fuel a substantial Europan biosphere.''

In the darkness beneath Europa's ice, sunlight can't be tapped for energy. Sea-floor volcanoes could provide a life-sustaining energy source, but there's no evidence that they exist on Europa.

Even so, Chyba wrote, the fast-moving protons and electrons that bombard Europa's ice could fill the energy void.

The pelting by charged particles could trigger chemical reactions that transform ice and carbon dioxide into organic compounds that microbes can eat.

When Europa's thin ice shell melts through to the ocean at places like the Conamara Chaos, the sea would get a fresh supply of microbe food, Chyba theorized.

But some researchers aren't buying the life-friendly thin-ice model offered by UA researchers.

One critic is NASA oceanographer and microbiologist Kenneth H. Nealson, head of the astrobiology team at the Jet Propulsion Laboratory in Pasadena, Calif.

``It has nothing to do with reality,'' Nealson said of Greenberg's paper on the habitability of Europa's crust.

``Sure, it's a nice model. I read their paper. I've also read the one where there isn't any ocean, and I've also read the one where there's a slushy ocean. And I've also read the one where there's 30 kilometers of ice.

``In the absence of any data, one model is just as good as any other model, and it's just as likely to be true or untrue,'' Nealson said.

After listening to researchers opine on the possibility of diverse life beneath Europa's ice at a scientific meeting in late 1998, Nealson and California Institute of Technology graduate student Eric J. Gaidos responded in the journal Science.

If there is an ocean on Europa, a thick ice shell would essentially suffocate it, extinguishing most life forms, Gaidos and Nealson wrote.

``The whole point of our paper wasn't that there can't be life on Europa. It's that if you're looking for life, and it's really covered with kilometers of ice, don't look for oxygen-breathing, complex organisms. Look for simple things,'' Nealson said.

On Earth, organisms that can survive such conditions include methane-producing and sulfur-eating bacteria.

But Greenberg said the UA thin-shell model, with warm water sloshing onto the surface at cracks and melt-through zones, ``describes an environment in which life could form, survive, flourish and evolve.''

Instead of suffocating, the ocean essentially gets a chance to breathe freely.

Sounds great, but the evidence does not support a thin ice shell, Brown's Pappalardo said.

Brown scientists did their own jigsaw-puzzle reconstruction of the giant Conamara ice rafts, and they reached a different conclusion about their formation.

Brown researchers suspect that warm blobs of ice - not liquid water - rose like wax in a lava lamp, softening the surface ice to the consistency of slush.

The icebergs were then able to slide freely across the surface before the whole mess refroze.

Brown researchers still think the ice is up to 20 miles thick, Pappalardo said.

But no one knows for sure, and that's why NASA wants to launch Europa Orbiter.

At least two of the mission proposals submitted to the space agency late last year include a laser altimeter, said the UA's Lunine, who teamed up with Jet Propulsion Laboratory researchers on his proposal.

The altimeter would measure changes in Europa's shape as it orbits Jupiter and gets distorted by the changing gravitational tug.

If Europa is made of solid ice over a rocky core, its shape will only change about one yard throughout its orbit. But if it has a substantial ocean, tidal forces will cause distortions of up to 30 yards, Lunine said.

``The surest way, with the least complication, to determine whether Europa has a substantial ocean is to measure the change in its shape as it moves around Jupiter,'' he said.

Other instruments proposed for Europa Orbiter include a radar sounder that might be able to measure the thickness of the ice; high-resolution cameras to map the entire surface; a thermal sensor that would look for warm spots on the ice; and a spectrometer that would look for salts and other material that may have welled up from below.

``If there's an ocean on Europa, it's almost certain that there'll be a program of exploration analogous to the program of exploration for Mars. So we'll go back there many times,'' said Chyba, a former UA researcher who now works at Stanford and at the SETI Institute in Mountain View, Calif. SETI is the Search for Extraterrestrial Intelligence.

``If there isn't an ocean, then probably the focus of exploration will shift to some other place altogether.''

James S. Wood, The Arizona Daily Star: University of Arizona planetary scientist Jonathan I. Lunine heads one of the research teams

NASA/University of Arizona photo: Red areas are cracks on Europa, Jupiter's icy moon; blue areas are its plains.

Graphic by Warren Huskey, The Arizona Daily Star: Europa: A hidden ocean?