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Gazing off toward the horizon is thought-provoking no matter what body's horizon it is. Rhea's horizon is slightly irregular and battered by craters, so thoughts inevitably turn towards the forces that shape these icy worlds. The surface of Rhea (949 miles or 1527 kilometers across) has been sculpted largely by impact cratering, each crater a reminder of a collision sometime in the moon's history. On more geologically active worlds like Earth, the craters would be erased by erosion, volcanoes or tectonics. But on quieter worlds like Rhea, the craters remain until they are disrupted or covered…
