The Science of The Callisto Incident
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A doublet ridge is one of the many names given to the surface features on Europa. A picture of many of them is shown below: it's basically a pair of parallel ridges, with a trough in-between, probably formed as faults when ice plates move with respect to each other. Arthur C. Clarke has even suggested that these new pictures look like a freeway system, so it's natural to use them as roads in the story.
Original Caption Released with Image:
This high resolution image of the icy crust of Europa, one of Jupiter's moons, reveals a surface criss-crossed by multiple sets of ridges and fractures. The area covered by this image is approximately 9 miles (15 kilometers) by 7 miles (12 kilometers), located near 15 North, 273 West. North is to the top, and the sun is illuminating the terrain from the right. The large ridge in the lower right corner of the image is approximately 1.5 miles (2.5 kilometers) across, and is one of the youngest features in this image, as it cuts across many of the other features. Note that one ridge has been sheared by a right-lateral fault. < P>This image was taken by the Galileo spacecraft on February 20, 1997 from a distance of 1240 miles (2000 kilometers).
Here's a 3-D view. Check out the picture here.