超级地球的大气特征：如何区分富氢和贫氢大气
The Atmospheric Signatures of Super-Earths: How to Distinguish Between Hydrogen-Rich and Hydrogen-Poor Atmospheres
摘要：
太阳系外的超级地球（1-10 M $_{\earth}$）很可能存在广泛的大气层。一些超级地球可能能够保留大量富含氢的大气层。其他行星可能永远不会积累氢或经历轻质元素的大量逃逸，导致大气层更像我们太阳系中的类地行星。我们研究了观察者如何通过模拟超地球发射光谱和透射光谱来区分富氢和贫氢大气，并且我们发现仅通过观察透射光谱就可以区分富氢和贫氢大气。一个类似地球的大气层，主要由重元素和分子组成，由于它的大气尺度高度很小（因为尺度高度与分子量成反比），因此传输信号非常弱。另一方面，一个大的富氢大气揭示了一个相对较大的传输信号。超级地球的发射光谱还可以控制大气成分和温度结构。具有大量氢大气层的超级地球将由于水而显示出强烈的光谱特征，而那些已经失去大部分氢（并且没有液态海洋）的超级地球将以CO $_2$特征和缺乏H $_2$ O为标志。我们将我们的研究具体应用于围绕M星运行的低质量行星Gl 581 c（$M sin i$ = 5 M $_{\earth}$），尽管我们的结论与一般的超级地球有关。区分富氢大气层的能力对于解释岩石超级地球和海王星类行星之间过渡的行星质量和半径观测至关重要。
英文摘要：
Extrasolar super-Earths (1-10 M$_{\earth}$) are likely to exist with a wide range of atmospheres. Some super-Earths may be able to retain massive hydrogen-rich atmospheres. Others might never accumulate hydrogen or experience significant escape of lightweight elements, resulting in atmospheres more like those of the terrestrial planets in our Solar System. We examine how an observer could differentiate between hydrogen-rich and hydrogen-poor atmospheres by modeling super-Earth emission and transmission spectra, and we find that discrimination is possible by observing the transmission spectrum alone. An Earth-like atmosphere, composed of mostly heavy elements and molecules, will have a very weak transmission signal due to its small atmospheric scale height (since the scale height is inversely proportional to molecular weight). On the other hand, a large hydrogen-rich atmosphere reveals a relatively large transmission signal. The super Earth emission spectrum can additionally contrain the atmospheric composition and temperature structure. Super-Earths with massive hydrogen atmospheres will reveal strong spectral features due to water, whereas those that have lost most of their hydrogen (and have no liquid ocean) will be marked by CO$_2$ features and a lack of H$_2$O. We apply our study specifically to the low-mass planet orbiting an M star, Gl 581c ($M sin i$ = 5 M$_{\earth}$), although our conclusions are relevant for super-Earths in general. The ability to distinguish hydrogen-rich atmospheres might be essential for interpreting mass and radius observations of planets in the transition between rocky super-Earths and Neptune-like planets.
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