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Recent explorations discover evidence of subsurface water on one of Jupiter's ice-ridden satellites.
by Ronald Greeley, Department of Geology, Arizona State University, Tempe
On a cold, clear night in 1610 in northern Italy, Galileo Galilei made the remarkable discovery of moons swirling around Jupiter. I wonder what he would think today about a spacecraft named in his honor, returning the first close-up pictures of those marvelous objects? Of particular interest is the second large satellite outward from Jupiter, the moon now known as Europa. It is likely that Galileo would be as delighted and as mystified as today's planetary scientists in seeing this bizarre, icy world.
Europa is one of the brightest objects in the Solar System, and the way it reflects sunlight shows that water ice is a dominant part of the surface. Europa is almost the same diameter as Earth's Moon, but has a slightly lower overall density. These characteristics suggest that Europa is mostly a rocky object with an ice-rich outer layer. But how thick is the ice? Is some of the ice melted below the surface? Could there be a subsurface global ocean of liquid water? These are questions that are currently being asked by the planetary science community.
Tantalizing clues to Europa's history and characteristics came in the 1970s and early 1980s from data returned from the flybys of NASA's Pioneer and Voyager spacecraft. Pictures showed a bright, splotchy-orange surface crisscrossed with long thin lines, giving Europa the appearance of a string-wrapped ball. Very few impact craters were found on its surface, suggesting geologically recent activity. Before the Voyager flybys, geophysicists modeled the tidal heating induced in a neighboring moon, Io, in Europa's elliptical orbit around Jupiter, and predicted active volcanism. In one of the great moments in science, within two weeks of this prediction, the Voyager team discovered active volcanoes on Io. Later, similar calculations were made for Europa, but because this moon is farther from Jupiter, the tidal heating is less than on Io, and it was not clear that sufficient energy was present to drive active volcanoes. The apparent lack of superposed impact craters, however, suggested that some process had "resurfaced" Europa, and volcanism seemed the logical candidate. In the case of Europa, the "volcanism" would involve water-rich "magmas," and geysers might be a more proper term. If geysers were erupting, then many planetologists speculated that there could be enough heat to maintain a water ocean beneath Europa's icy crust.
The Galileo spacecraft began operations in the Jupiter system in December 1995. By the summer of 1996, the first data were returned from Europa, paving the way for subsequent close flybys that yielded pictures with resolutions as high as 16 meters per pixel and other data obtained from a near infrared mapping spectrometer, photo-polarimeter radiometer, magnetometer, etc. Of particular interest were the results from the spacecraft tracking. By monitoring how the path of the spacecraft was perturbed during close flybys, insight was gained into the nature of the interior of Europa.
John Anderson of the Jet Propulsion Laboratory and his colleagues have analyzed these data and conclude that Europa's interior could be subdivided into a metallic core some 1250 kilometers in diameter, surrounded by a rocky mantle and an outer shell of water composition. Although they note that, alternatively, the interior could be a homogeneous mixture of rock and metal, the presence of a slight magnetic field suggests that a core and mantle configuration is more likely. This does not mean that liquid water is present, but it does suggest sufficient heating of the interior in the past to lead to differentiation.
The best evidence for liquid water or "warm" ice beneath the ice crust comes from high-resolution pictures of the surface (see photos on this page and 13). These pictures, obtained by Mike Belton and the Galileo imaging team, show a surface that has been disrupted, leaving blocks of ice as small as a few kilometers across that have been rafted into new positions. Many of the blocks fit back-to-back like pieces of a jigsaw puzzle, but with some space left over. This suggests that some surface materials have been consumedCperhaps meltedCor that Europa experienced global expansion. In either case, the style and amount of disruption suggest that the crustal blocks were either "floating" in water or suspended in warm, mobile ice or slush within a few kilometers of the surface.
While the pictures strongly suggest liquid water or warm ice in some places, they do not indicate that these conditions exist today. How can we tell when these features formed? Estimates of planetary surface ages can be derived from the number of superposed impact cratersCthe more craters, the older the surface. By knowing the rates of impact, in principle, a surface's age can be estimated by counting the total number of craters. The key, then, is knowing the rate and unfortunately, only crude estimates are available for the rate of impacts in the Jupiter environment today.
Another approach is to extrapolate crater-count data from Earth's Moon to Jupiter. In this approach, the number of impact events on the Moon through time is determined from crater counts made for different-aged surfaces, sampled by the Apollo astronauts and dated by radiogenic methods. The results are then extrapolated from the Moon to the Jupiter system. But adjusting the data to a part of the Solar System far removed from the vicinity of Earth's Moon introduces large uncertainties in the results. In fact, using these two methods, ages for Europa are as young as 3 million years or as old as 3 billion years, based on the same crater countsCnot very satisfying!
What, then, are the prospects for determining if a subsurface Europan ocean exists today? This report is very much a "work in progress." So far in the Galileo mission, we have had only three close flybys of Europa. In addition, authorization has been approved for the Galileo Europa Mission (GEM), which will allow continued operation through 1999. As the name, GEM, implies, the focus will be on Europa with at least eight close flybys, increasing the amount of data nearly tenfold, enabling near-global imaging at a resolution of about 1 kilometer per pixel and coverage of selected areas in color, stereo, and super high resolution (better than 10 meters per pixel). Part of the strategy is to search for signs of current activity, such as erupting plumes of the type observed on Io or changes that might have occurred on the surface during the four years of Galileo operations. Although the likelihood of such occurrences is small, their discovery would provide definitive evidence for sufficient heat to maintain liquid water.
The increased imaging anticipated by GEM, both in areal coverage and resolution, will enable a much better understanding of the relative ages of different surfaces on Europa. If expanses of smooth terrain are found that completely lack impact craters, then they would clearly represent very recent activity. On the other hand, if no such areas are found, then the ages of the surfaces and the times of surface activity must be ancient.
Galileo was designed more than 20 years ago, before Voyager provided the first clues to Europa's surface, and the instruments on board Galileo are not really appropriate for searching for subsurface water. That is why NASA is currently investigating how future missions might be designed for the next step in the exploration of Europa. One instrument that could be used in this way is a microwave sounder, or radar system. Placed on an orbiting spacecraft around Europa, it could send signals through the ice to a depth of a kilometer or more to search for liquid water. In addition, precise mapping of the global shape of Europa would allow for determining how distorted the moon might be in response to tidal forces. If subsurface liquid water is found, either globally or locally, plans for future exploration call for landed spacecraft with the ability to drill or melt through the ice to sample and explore the potential water world.
In many respects, Europa is unique in the Solar System. During the next two years of Galileo operations, we can expect a steady stream of new data. Undoubtedly, many surprises will be revealed.
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