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Scientists Uncover First-Ever Evidence of Tidal Squeezing On Europa
By Maia Weinstock
Staff Writer
posted: 02:04 pm ET
10 August 2000
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europa_squeeze_000809_MB_ Planetary scientists say they have found the first-ever evidence of folds in the icy surface of Europa, one of Jupiters moons, leading them closer to an intimate understanding of the moons interior dynamics. After analyzing a series of images taken by the Jupiter-orbiting Galileo spacecraft, Louise Prockter of Johns Hopkins Applied Physics Laboratory and Robert Pappalardo of Brown University found signs of surface folds, which are formed when the icy crust is squeezed like an accordion by gravitational forces. They say these folds all but complete scientists prediction of a flexing Europan crust, which is alternately stretched and squeezed as the moon orbits planet Jupiter. Europa, one of the most intriguing objects in the solar system, has always hidden its secrets well. When NASAs two Voyager space probes first whizzed by the moon in 1979, scientists could do little more than muse over tantalizing low-resolution images of an icy, cracked water world. But over the years, scientists have seen countless cracks in Europas icy crust. These cracks are produced as gravitational forces from Jupiter and other Jovian moons pull on the crust, causing it to split apart. Yet one problem scientists have been struggling with is how Europa can keep its shape when its crust is constantly being pulled apart. Is the moon gradually expanding in size? Could there exist so-called subduction zones like on Earth, where slabs of crust slip underneath other slabs of crust, thereby canceling out the moons stretching? Or could there be some other explanation?
Subsection of Europas crust, showing small-scale ridges and fractures, interpreted to be related to a process of crustal squeezing. Thanks to the Galileo spacecrafts high-resolution imaging capabilities, Prockter and Pappalardo say they have the answer. After analyzing a number of close-up images of Europas surface, the two uncovered evidence that Europas crustal extension (pulling apart) is being counterbalanced by an opposing tidal force -- a process called compression, or crustal "squeezing." "Its impossible to fully understand the geology of Europa or any icy satellite until we understand the nature of its extension and compression," said Pappalardo. "This is the first convincing evidence of folds on any icy satellite." ~ Detecting the squeeze Though planetary scientists had been trying to come up with a complete picture of Europas crustal movement for some time, it took diligent searching, and a little bit of luck, to find the folds that would turn out to be very important pieces of this planetary puzzle. One initial problem was that Pappalardo and his colleagues hadnt yet canceled out alternative explanations for Europas cracks, so they werent even sure that it was folds they were looking for. Whats more, said Pappalardo, even when they did search for folds, they "did not see any convincing examples." "Europa is such an alien landscape," added Prockter. "This makes trying to unravel its history even more exciting and challenging." Fortunately, the images that Prockter and Pappalardo were working with were taken near Europas "terminator," the moons day/night boundary, which highlighted surface features fairly well. This lighting allowed them detect a series of subtle hills about 16 miles (25 kilometers) in width, as well as small fractures and ridges within the larger-scale hills. According to Pappalardo, these smaller-scale features are the best evidence yet that the hills are folds created by compression of the moons crust. 
Crustal cracks due to extensional stress abound on Europas surface. The new discovery of folds implies the existence of compressional stress as well. Prockter and Pappalardo believe Europas folds come about as a result of a special kind of stress in which tidal forces cause the moons entire ice shell to slowly rotate above the moons more ductile inner layers. This rotation, said Pappalardo, causes enough stress to "form folds in warm areas of Europas icy shell." Later, these folds are relaxed, and may even allow some of the material near the surface to cycle back into deeper subsurface layers. Back to Europa While this discovery may be extremely significant in terms of unraveling Europas geological history, scientists concede that theyve only hit the proverbial tip of the iceberg with regard to Europas geology as a whole. Unfortunately, the Galileo spacecraft has already made its 12th and final close flyby of Europa, so researchers will have to rely on existing images and data for at least several years before any new data can be gathered. And while the Galileo mission has been a major success, going back to that area of the solar system for future studies poses some problems. "There are some very special challenges involved in studying the Galilean system," said Prockter. One of these, she said, is the presence of large amounts of intense radiation near Jupiter. Such radiation in high doses can spell disaster for instruments on nearby spacecraft. Still, the possible existence of liquid water on or somewhere inside the planet has many scientists pining for more information about this icy ball. "There are so many unanswered questions, particularly about whether or not there is an ocean and, if so, what its composition might be, how deep it is and whether or not it has the potential to harbor life," said Prockter. "I hope we get back to Europa before too long."Fortunately, the excitement over this icy moon, its odd geology, and its potential for harboring life has spread to NASA officials as well. The administration has already begun planning to launch a spacecraft in 2006, which will study Europa in-depth. Called the Europa Orbiter, this missions chief goals will be to measure the thickness and other properties of the moons surface ice, detect whether a subsurface ocean exists, and even survey the crust to see if it would be suitable for a follow-up lander mission.
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