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题目材料：
Using radar instruments to analyze the exact nature of geological features on Saturn's moon Titan is challenging, because radar images are unlike ordinary photographs. With radar, the brightness of the reflected signal correlates with the roughness of the surface. Thus, the smooth surfaces of Titan's methane-filled lakes look dark in radar images, while bright areas usually suggest rough hilly terrain. However, the data don't always follow this pattern. Many detected river valleys and lakes appear bright compared to the surrounding terrain, suggesting a generally dry riverbed or lake bed filled with gravel or cobble. As for the dark, meandering river valleys, one theory proposes that they are deeply incised bedrock channels whose steep walls shadow the signal, making the radar images appear dark.
以上解析由 考满分老师提供。
