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Fig. 6.16. Europa wedge region. A Galileo spacecraft image showing crustal separation, with the dark bands being areas where the icy crust has completely pulled apart. Dark material has welled up from below and filled the void created by this separation. (Credit: NASA/JPL/Caltech)
Fig. 6.17. This image, taken by the Galileo spacecraft, reveals the ice-rich crust of Europa. Crustal plates up to 13 km (8 miles) across have broken apart and "rafted" into new positions. (Credit: NASA/JPL/Arizona State University)

during springtime thaws." According to Paul Geissler the motion of the ice rafts could not be explained by convection in ice; only a fluid medium could account for such rotation and tilting. According to Michael Carr, the rafts had been clearly caught up in a strong current in a fluid medium, which was almost certainly water. Because they had been floating, they were true icebergs and most of their bulk would have been below the water level, just like on Earth (Fig. 6.18). Thus, we see strong evidence for some kind of liquid ocean. Carr believes the icebergs provide the proof that Europa had to have had liquid water exposed at the surface at some time in its past [372] (Fig. 6.19).

Fig. 6.18. This mosaic of images taken by the Galileo spacecraft reveals ridges, plains, and mountains on Europa. There are hundreds of ridges that cut across each other, indicating m ultiple episodes of ridge formation either by volcanic or tectonic activity within the ice. There are also numerous isolated mountains or "massifs". Irregular shaped areas where the ice appears to be lower than the surrounding plains may be related to the "chaos" areas of iceberg-like features. (Credit: NASA/JPL/Arizona State University)

Fig. 6.18. This mosaic of images taken by the Galileo spacecraft reveals ridges, plains, and mountains on Europa. There are hundreds of ridges that cut across each other, indicating m ultiple episodes of ridge formation either by volcanic or tectonic activity within the ice. There are also numerous isolated mountains or "massifs". Irregular shaped areas where the ice appears to be lower than the surrounding plains may be related to the "chaos" areas of iceberg-like features. (Credit: NASA/JPL/Arizona State University)

Fig. 6.19. A very high resolution Galileo spacecraft image of the Conamara Chaos region on Europa. Icy plates have been broken apart and moved around laterally. There are corrugated plateaus ending in icy cliffs over 100 m high. (Credit: NASA/JPL/Caltech)
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