An Ocean Discovered:
Europa Surrenders Her Secrets

Sixteen months into the Jovian system tour, public interest has shifted increasingly towards Europa--the newest superstar on the celestial stage. Prospects for subsurface water, and with it, life, have never looked brighter.

When the Galileo Orbiter swooped within 587 km (363 mi.) of Europa's icy crust, on the E6 pass of February 20, its Solid-State Imaging (SSI) camera captured an array of breathtaking, mind-blowing images. The panel of eight investigators (moderated by David Seidel) that reviewed the E6 science on April 9 at JPL's von Kármán Auditorium was certainly the most diverse Galileo group, and perhaps the most excited, to face the international press.

Michael Carr of the U.S. Geological Survey was not new to the crowds at von Kármán, but his previous appearance had been to discuss hidden water on Mars. Now, in his first appearance on a Galileo panel, he spoke of hidden water on Europa. Seated below a large, full-color image of the Europan surface, he maintained Galileo had discovered the "smoking gun" that indicates the signature of a subsurface ocean in the place where nature marked the spot with a large "X." The image above, centered over the trailing hemisphere, showed the bright crater Pwyll to the south, and, near the northern edge, two dark-red triple bands, one crossing the other at a right angle--the "X." And just south of the intersection, an irregular dark-red patch, maybe 50-km wide. This was the patch from which Robert Sullivan, of Arizona State University, had introduced (only minutes before) the celebrated image of the Europan icebergs, floating in their now-frozen sea (see first image). Carr was visibly impressed with the way these enormous, 3-to-6-km-wide blocks, scarred with ridges, were tipped and rotated. This motion, he explained, could not be accounted for by wind or slope, but could be caused only by the traction of currents in a liquid medium.

Max Coon of the Northwest Research Association displayed a picture of pack ice in the open water of the Earth's own Arctic Ocean for comparison. Such floes, he explained, frozen in a winter sea, would resemble Europa's bergs even more closely. Open water on Europa would boil and freeze at the same time; the rapid freezing would seal in further loss; the water vapor released into space would settle as snow and help color the whitest, brightest surface in the solar system.

Larger areas of the surface show a bewildering complexity of features. Here, the ice-bergs appear to be frozen in an area surrounded by sound, unbroken, grooved crust. Yet portions of this crust also show smaller areas within which the surface seems to have melted, then refrozen as a choppy, rubbly patch that obliterates the older pattern of ridges and grooves.

Even stranger, some areas look as smooth and flat as skating ponds. Sullivan's introduction included this image from the E4 pass that showed both a sharp-edged, jumbled patch and a smooth, flooded area. These two features are separated by only 5 km, yet the older, eastern one shows what may have been the sudden collapse of a 4-km-wide section of crust that refroze as a mass of broken, floating chunks; the younger, western one shows what may have been the gradual sinking of an equally wide area and a gentle flooding from below.

How Old a Surface?

Close up, neither of these features shows even the slightest resemblance to impact craters, but from a distance they appear dark against the whiter, ridged crust. As "freckles," they were considered impact craters and included in preliminary counts. Clark Chapman, of the Southwest Research Institute, noted that this confusion resulted in crater counts about 100 times greater than now observed! This means a surface proportionately much younger, maybe even less than a million years. This is a geological eye blink, and Chapman credited only the Earth and Io with more active crusts.

Michael Carr, echoing some of the passion in the surface-dating debate, pointed out that the cratering rate in the Jovian neighborhood is much less well known than the rate around the Earth and the surface could be much older. Given such uncertainty, Clark Chapman thought it might even be younger, while Torrence Johnson, Project Scientist from JPL, felt that even much older than a million years would still be geologically young.

The Case for an Ocean--and Life!

The evidence for a deep ocean, then as Richard Terrile from JPL, suggested, is "strong." If, he added, it underlies the whole surface, then the Europan ocean would contain more water than exists in all the oceans of the Earth! As best we can tell, life on Earth appeared within 700 million years of its formation, 4.6 billion years ago. From its beginning, the Europan environment with abundant water, rich in dissolved minerals and organics, would be suitable for the origin of life. As old as the Earth, this moon has had time for life to evolve.

John Delaney, an oceanographer from the University of Washington, the first of that discipline at a Galileo press conference, shared some of his passion for exploring the Europan ocean and discovering life. He noted that about the time the Voyagers were downlinking the first pictures of Europa's flat, icy crust, deep-diving oceanographers were discovering the first volcanic-vent communities on the Eastern Pacific sea floor.

Delaney could confidently report that today, wherever sea-floor volcanism is found, so will be rich and diverse life. This suggests a new biological paradigm--a new way to view life: wherever you find volcanic activity and liquid water, even within rock, you will find life.

The panel speculated on possible follow-on missions to Europa--orbiters to map the sub-surface water, "cryobot" penetrators to reach the hidden sea, and "hydrobot" submersibles to plumb its sunless depths.

As Project Galileo continues to study Europa's mysteries, we discover even more. Europa has proved to be a far more interesting place than ever imagined. And we've only begun.

--Larry Palkovic

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