土卫六的甲烷湖泊：液体气体构成的奇异世界
When the Cassini spacecraft peered through Titan's thick orange haze, it revealed a landscape surprisingly similar to Earth's yet utterly alien. Radar images showed vast dark patches near the moon's poles, which scientists soon confirmed as lakes and seas. But these were not filled with water; instead, they consist of liquid methane and ethane. Titan, Saturn's largest moon, is the only other place in our solar system known to have stable liquids on its surface.
The methane on Titan acts much like water on Earth. It evaporates from the lakes, forms clouds, and then falls as rain, carving river channels and shaping the icy terrain. Because Titan's surface temperature hovers around minus 179 degrees Celsius, methane exists comfortably as a liquid, while water ice is rock-hard. This complete hydrological cycle—but with methane instead of water—makes the moon a natural laboratory for understanding planetary processes.
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