Observations suggest an abundance of water and paucity of methane in the majority of observed exoplanetary atmospheres. We isolate the effect of atmospheric processes to investigate possible causes. Previously, we studied the effect of effective temperature, surface gravity, metallicity, carbon-to-oxygen ratio, and stellar type assuming cloud-free thermochemical equilibrium and disequilibrium chemistry. However, under these assumptions, methane remains a persisting spectral feature in the transmission spectra of exoplanets over a certain parameter space, the Methane Valley. In this work we investigate the role of clouds on this domain and we find that clouds change the spectral appearance of methane in two direct ways: 1) by heating-up the photosphere of colder planets, and 2) by obscuring molecular features. The presence of clouds also affects methane features indirectly: 1) cloud heating results in more evaporation of condensates and hence releases additional oxygen, causing water dominated spectra of colder carbon-poor exoplanets, and 2) HCN/CO production results in a suppression of depleted methane features by these molecules. The presence of HCN/CO and a lack of methane could be an indication of cloud formation on hot exoplanets. Cloud heating can also deplete ammonia. Therefore, a simultaneous depletion of methane and ammonia is not unique to photochemical processes. We propose that the best targets for methane detection are likely to be massive but smaller planets with a temperature around 1450 K orbiting colder stars. We also construct Spitzer synthetic color-maps and find that clouds can explain some of the high contrast observations by IRAC's channel 1 and 2.

中文翻译：

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云对受辐照系外行星透射光谱中甲烷和水的消耗的作用

观测表明，在大多数观测到的系外行星大气中，水含量丰富，甲烷含量极低。我们隔离大气过程的影响以调查可能的原因。之前，我们研究了有效温度、表面重力、金属丰度、碳氧比和恒星类型的影响，假设无云热化学平衡和不平衡化学。然而，在这些假设下，甲烷仍然是系外行星在特定参数空间甲烷谷的透射光谱中的持久光谱特征。在这项工作中，我们研究了云在这个领域的作用，我们发现云以两种直接方式改变了甲烷的光谱外观：1）通过加热较冷行星的光球层，以及 2）通过模糊分子特征。云的存在也间接影响甲烷特征：1) 云加热导致更多的冷凝物蒸发，从而释放出额外的氧气，导致较冷的贫碳系外行星的水主导光谱，以及 2) HCN/CO 的产生抑制了枯竭的这些分子具有甲烷特征。HCN/CO 的存在和甲烷的缺乏可能表明热系外行星上有云形成。云加热也会消耗氨。因此，甲烷和氨的同时消耗并不是光化学过程所独有的。我们提出，甲烷探测的最佳目标可能是温度在 1450 K 左右、围绕较冷恒星运行的大质量但较小的行星。