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题目材料：
The large amount of methane in the atmosphere of Saturn's giant moon Titan puzzles astronomers. Because methane is converted into heavier hydrocarbons by the Sun's ultraviolet light, all the methane in Titan's atmosphere should, at the present conversion rate, have disappeared in a few million years, long ago in Titan's 4.5 billion years of existence. Something must be continuously feeding methane into Titan's atmosphere. One possibility is that the methane comes from volcanic eruptions, but unless the supply rate were somehow precisely calibrated at a moderate level, either the methane would have run out by now or excess methane would have started to accumulate as a liquid ocean on the Surface. However, data indicate that the atmosphere just above the surface is not humid, as would be expected above a pure methane ocean. A possible explanation was suggested by Lunine: when sunlight breaks down methane, the main product is ethane. Lunine noted that at Titan's surface temperature, ethane is also a liquid. A liquid mixture of methane and ethane would produce low Humidity. Such an ocean would slowly become richer in ethane while keeping the atmosphere supplied with methane.
以上解析由 考满分老师提供。
