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Europa Image from Galileo

Europa is a puzzle. The sixth largest moon in our Solar System, Europa confounds and intrigues scientists. Few bodies in the Solar System have attracted as much scientific attention as this moon of Jupiter because of its possible subsurface ocean of water. The more we learn about this icy moon, the more questions we have. Because the nature of science is to ask questions, we cannot resist the mystery of Europa and its potential for possessing an ocean.

Early Imaging
The United States sent two spacecraft, Pioneer 10 and 11, to Jupiter in the early 1970s. No one knew if a spacecraft could survive a flyby of Jupiter, but the Pioneers did survive, and they sent back valuable information for the next space mission. However, the Pioneer photographs of Jupiter's largest moons were fuzzy and dim. The twin Voyager spacecraft flew by Jupiter and its moons in 1979, giving us our first close-up view of Europa. Voyager pictures show pale-yellow icy plains with red and brown mottled regions. Long cracks run for thousands of kilometers over the surface. On Earth, these cracks would indicate such features as tall mountains and deep canyons. But none of these features are higher than a few kilometers on Europa, making it one of the smoothest objects in our Solar System.

Images from Galileo
If we look at the surface more closely, as we have with the instruments on the Galileo spacecraft, we see some fascinating features. Europa looks like broken glass that is repaired by an icy glue oozing up from below. Low ridges, straight and curved, crisscross the surface. Flows and fractures, pits and frozen "puddles" - all hint at a unique geologic history. Large circular features could be the sites of impacts or the result of upwelling of material from beneath the surface. Making sense of this chaotic landscape is a challenge to planetary scientists.

Europa's Puzzling Surface
Despite the chaos of its surface, Europa is probably the kind of puzzle that science can solve. Some of our questions are: "How old is the surface? How were the cracks and other features made? What is under the ice?" To answer them, we collect data and make careful observations, applying what we know about geology, physics, and chemistry. Geologists figure out the age of a surface by counting the impact craters formed where comets, meteorites, and other debris hit the surface.