Sep. 23, 1999 TUCSON, Ariz. -- When Voyager flew by Jupiter's moon Europa in 1979, it photographed geological surface features unlike any others ever seen in the solar system. Near Europa's south pole, chains of scalloped lines joined arc-to-arc at the cusp ran for hundreds of miles across the frozen, fractured surface.
Until now, there have been no good ideas as to what formed these bizarre "cycloidal" features, or "flexi," as they were officially dubbed by the International Astronomical Union.
Now, planetary scientists at the University of Arizona in Tucson provide a model for how these features are created. It is perhaps the most convincing evidence yet for a global ocean. They report on it in today's issue of Science (Sept. 17).
Scientists know that Europa has a 100-mile-thick layer of water -- 20 times thicker than the Earth's oceans -- but the visible top layer is frozen. This new strong evidence for a liquid global ocean below the surface makes Europa a prime target in the search for life beyond Earth.
Gregory V. Hoppa, B. Randall Tufts, Richard Greenberg and Paul E. Geissler of the UA Lunar and Planetary Laboratory theorize that cycloidal cracks form in Europa's solid-ice surface with the daily rise and fall of tides in the subsurface ocean. They painstaking modeled and scrutinized images of Europa taken by the Galileo spacecraft between 1996 and 1999. The new images show that cycloidal cracks and ridges are widely distributed over all of the moon.
Hoppa has posted images and explanatory animation of cycloidal crack formation on the web site: http://pirlwww.lpl.arizona.edu/~hoppa/science.html
Europa is about the size of our moon. Tidal stresses on its ice-covered ocean ebb and flow as it orbits Jupiter, which is 300 times as massive as Earth. According to the UA researchers' model, Europa's ocean tides rise and fall a distance of 30 meters. By comparison, tides at most ocean beaches on Earth rise and fall 1 to 2 meters, or 4 to 6 feet.
"What causes the cycloid to form is that Europa is in a slightly eccentric orbit because of Io and Ganymede (other Jovian moons). Sometimes Europa is a little closer, other times a little farther from Jupiter. When Europa is closer to Jupiter, the tides are higher because Jupiter is pulling on it more. When Europa is farther, the tides fall because Jupiter's force falls. This causes Europa's ice shell to flex."
The UA model shows that when tidal stress reaches the tensile strength of ice, the ice begins to crack. It takes very little stress to form the initial crack -- something like the force it takes to break a saltine cracker -- because Europa's surface ice is weakened by countless linear fractures.
The crack propagates relatively slowly across the ever-changing stress field. It moves following a curving path until stress drops below the tensile strength of the ice, when it halts. A few hours later, when tidal stress again exceeds the tensile strength of ice, the crack begins a new curve in another direction.
"You could probably walk along with the advancing tip of a crack as it was forming --- if you could survive Europa's radiation environment," Hoppa said. "And while there's not enough air to carry sound, you would definitely feel vibrations as it formed."
One of their most striking conclusions is that each arc segment forms in 3.5 days -- the time it takes Europa to make one complete orbit around Jupiter. The cycloids faithfully record the 85-hour daily flexing of Europa's ice shell just as trees faithfully record each growing season in annual rings.
"We can look at a crack that has 4 or 5 cusps, each formed every 3.5 days, and know that the entire chain formed in about 2.5 weeks," Hoppa said.
Arc segments in the cycloid, each ranging from 75 km to 200 km long, form cracks stretching a thousand kilometers over the ice in a fraction of an instant in geological time. Eventually, cracks evolve into ridges, typically double ridges, according to the UA model.
The scientists also can determine which direction the cracks traveled as they formed based on the orientation of the arcs and the hemisphere in which they are found.
"What amazes me about this is just how long these features have been a mystery," Hoppa said. "We've been staring at pictures of them for 20 years, since Voyager. We didn't know what made them. And it seems what they've been telling us all along is that an ocean was there when these things formed."
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