We present a retrieval method based on Bayesian analysis to infer the atmospheric compositions and surface or cloud-top pressures from transmission spectra of exoplanets with general compositions. In this study, we identify what can unambiguously be determined about the atmospheres of exoplanets from their transmission spectra by applying the retrieval method to synthetic observations of the super-Earth GJ 1214b. Our approach to inferring constraints on atmospheric parameters is to compute their joint and marginal posterior probability distributions using the Markov Chain Monte Carlo technique in a parallel tempering scheme. A new atmospheric parameterization is introduced that is applicable to general atmospheres in which the main constituent is not known a priori and clouds may be present. Our main finding is that a unique constraint of the mixing ratios of the absorbers and two spectrally inactive gases (such as N2 and primordial H2+ He) is possible if the observations are sufficient to quantify both (1) the broadband transit depths in at least one absorption feature for each absorber and (2) the slope and strength of the molecular Rayleigh scattering signature. A second finding is that the surface pressure or cloud-top pressure can be quantified if a surface or cloud deck is present at low optical depth. A third finding is that the mean molecular mass can be constrained by measuring either the Rayleigh scattering slope or the shapes of the absorption features, thus enabling one to distinguish between cloudy hydrogen-rich atmospheres and high mean molecular mass atmospheres. We conclude, however, that without the signature of molecular Rayleigh scattering—even with robustly detected infrared absorption features (10σ)—there is no reliable way to tell from the transmission spectrum whether the absorber is a main constituent of the atmosphere or just a minor species with a mixing ratio of X abs 0.1%. The retrieval method leads us to a conceptual picture of which details in transmission spectra are essential for unique characterizations of well-mixed exoplanet atmospheres.
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机译：我们提出了一种基于贝叶斯分析的检索方法，可以从系外行星具有一般组成的透射光谱中推断出大气成分以及地表或云顶压力。在这项研究中，我们通过将检索方法应用于超地球GJ 1214b的合成观测中，确定从系外行星的透射光谱中可以明确确定的内容。我们推断大气参数约束的方法是在平行回火方案中使用马尔可夫链蒙特卡罗技术计算它们的联合和边际后验概率分布。引入了一种新的大气参数化方法，该参数化方法适用于先验未知且可能存在云的一般大气。我们的主要发现是，如果观测值足以量化两个方面的吸收率和光谱非活性气体（例如N2和原始H2 + He）的混合比，则可能存在独特的约束条件（1）至少一个方向的宽带传输深度每个吸收体的吸收特征和（2）分子瑞利散射特征的斜率和强度。第二个发现是，如果在低光学深度处存在表面或云层板，则可以量化表面压力或云层顶压力。第三个发现是，可以通过测量瑞利散射斜率或吸收特征的形状来限制平均分子量，从而使人们能够区分浑浊的富氢气氛和高平均分子量气氛。然而，我们得出的结论是，即使没有分子瑞利散射的特征-即使具有可靠检测到的红外吸收特征（>10σ），也没有可靠的方法可以从透射光谱中判断吸收剂是大气的主要成分还是仅仅是大气的主要成分。 X abs <0.1％的混合比例的次要物种。检索方法使我们得到一张概念图，其中透射光谱中的细节对于充分混合的系外行星大气的独特表征至关重要。
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