An Empirical Infrared Transit Spectrum Of Earth: Opacity Windows And Biosignatures


Mock JWST spectrum for TRAPPIST-1e, using NIRSpec and MIRI. The black line is the model, made by adapting Earth’s spectrum to the appropriate planetary and stellar radii and binning it to 0.1 µm. Mock data are normally distributed around the model. Error bars correspond to photon noise from the star, assuming 5.8 days of integration time (approximately 2 weeks of total observing time), or about 150 transits with each NIRSpec and MIRI over the course of JWST’s 10 year expected lifetime. In the near-infrared, CO2, H2O, O3, and CH4 are detectable at levels above 40σ, 20σ, 10σ, and 7σ, respectively. In the mid-infrared, O3 and CH4 can be detected at around 10σ and 5σ, respectively. Note that as the significance scales as the square root of the number of transits, it would take far fewer transits to detect most of these features at the 5σ level, as discussed in Section 5. Since NIRSpec and MIRI cannot be used in parallel, obtaining the full spectrum at this signal-to-noise would require more than JWST’s 10 year mission duration. Short-wavelength spectral features are easier to detect due to falling stellar flux at longer wavelengths. Nonetheless, the biosignatures CH4 and O3 could in principle be detected with either NIRSpec or MIRI.

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 hmin≈4 km in the 10--12μm opacity window at a spectral resolution of R=103. Nonetheless, at R=105, 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 CO2 and H2O, 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 O3 and CH4.

Evelyn J. R. Macdonald, Nicolas B. Cowan
(Submitted on 28 Aug 2019)

Comments: 9 pages, 7 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Journal reference: Monthly Notices of the Royal Astronomical Society 489 (2019) 196-204
DOI: 10.1093/mnras/stz2047
Cite as: arXiv:1908.10873 [astro-ph.EP] (or arXiv:1908.10873v1 [astro-ph.EP] for this version)
Submission history
From: Evelyn Macdonald
[v1] Wed, 28 Aug 2019 18:00:05 UTC (2,748 KB)
https://arxiv.org/abs/1908.10873
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