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

Abstract

The Atmospheric Chemistry Experiment's Fourier Transform Spectrometer on the SCISAT satellite has been measuring infrared transmission spectra of Earth during Solar occultations since 2004. We use these data to build an infrared transit spectrum of Earth. Regions of low atmospheric opacity, known as windows, are of particular interest, as they permit observations of the planet's lower atmosphere. Even in the absence of clouds or refraction, imperfect transmittance leads to a minimum effective thickness of h min ≈ 4 km in the 10-12 µm opacity window at a spectral resolution of R = 10 3 . Nonetheless, at R = 10 5 , the maximum transmittance at the surface is around 70 %. In principle, one can probe the troposphere of an Earth-like planet via high-dispersion transit spectroscopy in the mid-infrared; in practice aerosols and/or refraction likely make this impossible. We simulate the transit spectrum of an Earth-like planet in the TRAPPIST-1 system. We find that a long-term near-infrared campaign with JWST could readily detect CO 2 and H 2 O, establishing the presence of an atmosphere. A mid-IR campaign or longer NIR campaign would be more challenging, but in principle could detect the biosignatures O 3 and CH 4 .

EarthShine: Observing our world as an exoplanet from the surface of the Moon

“…These data can be converted into exoplanet-like transit spectra, as described by Robinson et al 41 and demonstrated by Macdonald and Cowan. 42 Measurements of Earth occultations of stars with a variety of stellar types would yield further detail on Earth's atmosphere and allow us to build a database of star-planet observations to compare with exoplanet transmission spectroscopy.…”

The Fundamental Connections between the Solar System and Exoplanetary Science

“…However, correct interpretation of all of these data relies upon models of planetary processes that are best developed and validated using in situ or remote‐sensing observations of Solar System bodies (Fujii et al., 2014). Similarly, observations of Solar System bodies, even very fundamental ones like phase‐dependent photometry of the Jovian planets (Mayorga et al., 2016), or simulated transmission observations of Titan (Robinson, Maltagliati, et al., 2014) or the Earth (Macdonald & Cowan, 2019) can help inform planning and interpretation of exoplanet observations and help us train predictive (forward) and retrieval (inverse) models for exoplanets.…”

Transmission Spectroscopy of the Earth-Sun System to Inform the Search for Extrasolar Life

“…Transmission spectra for the Earth have generally been achieved through indirect means, by observing solar occultations (e.g. Macdonald & Cowan 2019) or via a tertiary body reflecting the filtered starlight (e.g. Pallé et al 2009;Vidal-Madjar et al 2010;Ugolnikov et al 2013;Arnold et al 2014;Yan et al 2015;Kawauchi et al 2018;Youngblood et al 2020).…”