Pluto’s atmosphere is cold and hazy. Recent observations have shown it to be much colder than predicted theoretically, suggesting an unknown cooling mechanism. Atmospheric gas molecules, particularly water vapour, have been proposed as a coolant; however, because Pluto’s thermal structure is expected to be in radiative–conductive equilibrium, the required water vapour would need to be supersaturated by many orders of magnitude under thermodynamic equilibrium conditions. Here we report that atmospheric hazes, rather than gases, can explain Pluto’s temperature profile. We find that haze particles have substantially larger solar heating and thermal cooling rates than gas molecules, dominating the atmospheric radiative balance from the ground to an altitude of 700 kilometres, above which heat conduction maintains an isothermal atmosphere. We conclude that Pluto’s atmosphere is unique among Solar System planetary atmospheres, as its radiative energy equilibrium is controlled primarily by haze particles instead of gas molecules. We predict that Pluto is therefore several orders of magnitude brighter at mid-infrared wavelengths than previously thought—a brightness that could be detected by future telescopes.

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阴霾加热冥王星的大气但解释了它的寒冷温度

冥王星的气氛寒冷而朦胧。最近的观察表明它比理论上预测的要冷得多，这表明存在未知的冷却机制。大气中的气体分子，尤其是水蒸气，已被提议作为冷却剂；然而，由于冥王星的热结构预计处于辐射-传导平衡，在热力学平衡条件下，所需的水蒸气需要过饱和许多数量级。在这里，我们报告说，大气雾霾而不是气体可以解释冥王星的温度分布。我们发现，雾霾颗粒比气体分子具有更大的太阳加热和热冷却速率，主导着从地面到 700 公里高度的大气辐射平衡，高于此高度的热传导保持等温大气。我们得出结论，冥王星的大气在太阳系行星大气中是独一无二的，因为它的辐射能量平衡主要由雾霾粒子而不是气体分子控制。因此，我们预测冥王星在中红外波长处比以前认为的要亮几个数量级——这种亮度可以被未来的望远镜探测到。