Transmission spectroscopy with the ACE-FTS infrared spectral atlas of Earth: A model validation and feasibility study

Abstract

Infrared solar occultation measurements are well established for remote sensing of Earth's atmosphere, and the corresponding primary transit spectroscopy has turned out to be valuable for characterization of extrasolar planets. Our objective is an assessment of the detectability of molecular signatures in Earth's transit spectra.To this end, we take a limb sequence of representative cloud-free transmission spectra recorded by the space-borne ACE-FTS Earth observation mission (Hughes et al., ACE infrared spectral atlases of the Earth's atmosphere, JQSRT 2014) and combine these spectra to the effective height of the atmosphere. These data are compared to spectra modeled with an atmospheric radiative transfer line-by-line infrared code to study the impact of individual molecules, spectral resolution, the choice of auxiliary data, and numerical approximations. Moreover, the study serves as a validation of our infrared radiative transfer code.The largest impact is due to water, carbon dioxide, ozone, methane, nitrous oxide, nitrogen, nitric acid, oxygen, and some chlorofluorocarbons (CFC11 and CFC12). The effect of further molecules considered in the modeling is either marginal or absent. The best matching model has a mean residuum of 0.4 km and a maximum difference of 2 km to the measured effective height. For a quantitative estimate of visibility and detectability we consider the maximum change of the residual spectrum, the relative change of the residual norm, the additional transit depth, and signal-to-noise ratios for a JWST setup. In conclusion, our study provides a list of molecules that are relevant for modeling transmission spectra of Earth-like exoplanets and discusses the feasibility of retrieval.

Detectability of atmospheric features of Earth-like planets in the habitable zone around M dwarfs

“…We calculated the effective height of the Earth around the Sun for comparison. Dominant absorption features are H 2 O at 1.4 µm and 6.3 µm, CO 2 at 2.8 µm, and 4.3 µm and O 3 at 9.6 µm, comparable to observed and modelled transmission spectra of the Earth (Pallé et al 2009;Kaltenegger & Traub 2009;Vidal-Madjar et al 2010;Bétrémieux & Kaltenegger 2013;Yan et al 2015;Schreier et al 2018). We do not consider clouds and hazes when calculating the transmissivity of the atmosphere.…”

“…We note that not all of the species are relevant for transmission spectroscopy of Earth-like atmospheres (see e.g. Schreier et al 2018). The radius of a planet with an atmosphere is wavelength dependent.…”

“…The Generic Atmospheric Radiation Line-by-line Infrared Code (GARLIC) is used to calculate synthetic spectra between 0.4 µm and 12 µm (Schreier et al 2014(Schreier et al , 2018. The GARLIC model is based on the Fortran 77 code MIRART-SQuIRRL (Schreier & Schimpf 2001), which has been used for radiative transfer modelling in previous studies like Rauer et al (2011) and Hedelt et al (2013).…”

An empirical infrared transit spectrum of Earth: opacity windows and biosignatures

“…where H is the atmospheric scale height and F λ is a wavelength-dependent atmospheric contribution. On the other hand, Kaltenegger & Traub (2009) and Schreier et al (2018) calculate the effective thickness assuming a planeparallel atmosphere:…”

“…These spectra represent globally-averaged spectra (Section 2) and the spectral resolution has been reduced to 0.02 µm. Molecules are labelled as in Kaltenegger & Traub (2009) and Schreier et al (2018). The simultaneous presence of O 3 and CH 4 is a biosignature (Sagan et al 1993;Segura et al 2005).…”