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| ne of the most remarkable features of the
 solar system is the variety of planetary
 atmospheres. Earth and Venus are of
 comparable size and mass, yet the surface of Venus
 bakes at 460 degrees Celsius under an ocean
 of carbon dioxide that bears down with the
 weight of a kilometer of water. Callisto and Titan—planet-size
 moons of Jupiter and Saturn,
 respectively—are nearly the same size, yet Titan
 has a nitrogen-rich atmosphere thicker than
 our own, whereas Callisto is essentially airless.
 What causes such extremes? If we knew, it would
 help explain why Earth teems with life while its
 planetary siblings appear to be dead. Knowing
 how atmospheres evolve is also essential to determining
 which planets beyond our solar system
 might be habitable.
 A planet can acquire a gaseous cloak in
 感觉这个更像
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