ATMOSPHERIC RETRIEVAL FOR SUPER-EARTHS: UNIQUELY CONSTRAINING THE ATMOSPHERIC COMPOSITION WITH TRANSMISSION SPECTROSCOPY

Abstract

We present a retrieval method based on Bayesian analysis to infer the\natmospheric compositions and surface or cloud-top pressures from transmission\nspectra of exoplanets with general compositions. In this study, we identify\nwhat can unambiguously be determined about the atmospheres of exoplanets from\ntheir transmission spectra by applying the retrieval method to synthetic\nobservations of the super-Earth GJ 1214b. Our approach to infer constraints on\natmospheric parameters is to compute their joint and marginal posterior\nprobability distributions using the MCMC technique in a parallel tempering\nscheme. A new atmospheric parameterization is introduced that is applicable to\ngeneral atmospheres in which the main constituent is not known a priori and\nclouds may be present. Our main finding is that a unique constraint of the\nmixing ratios of the absorbers and up to two spectrally inactive gases (such as\nN2 and primordial H2+He) is possible if the observations are sufficient to\nquantify both (1) the broadband transit depths in at least one absorption\nfeature for each absorber and (2) the slope and strength of the molecular\nRayleigh scattering signature. The surface or cloud-top pressure can be\nquantified if a surface or cloud deck is present. The mean molecular mass can\nbe constrained from the Rayleigh slope or the shapes of absorption features,\nthus enabling to distinguish between cloudy hydrogen-rich atmospheres and high\nmean molecular mass atmospheres. We conclude, however, that without the\nsignature of Rayleigh scattering--even with robustly detected infrared\nabsorption features--there is no reliable way to tell if the absorber is the\nmain constituent of the atmosphere or just a minor species with a mixing ratio\nof <0.1%. The retrieval method leads us to a conceptual picture of which\ndetails in transmission spectra are essential for unique characterizations of\nwell-mixed atmospheres.\n