FIGURE 1. Earth's visible and near-infrared transmission and reflection spectra.

The Earth's transmission spectrum is a proxy for Earth observations during a primary transit as seen beyond the Solar System, while the reflection spectrum is a proxy for the observations of Earth as an exoplanet by direct observation after removal of the Sun's spectral features. a, The transmission spectrum, with some of the major atmospheric constituents marked. The spectrum has a resolution of 0.00068 m in the optical (resolving power, R  960) and 0.0013–0.0024 m in the near-infrared (R  920). A detailed atlas of the transmission spectrum with identifications of the main atomic and molecular absorption features is available (Supplementary Fig. 3). b, A comparison between the Earth's transmission (black) and reflection (blue) spectra. Both spectra have been degraded to a spectral resolution of 0.02 m and normalized at the same flux value at around 1.2 m. It is readily seen from the figure that the reflection spectrum shows increased Rayleigh reflectance in the blue. It is also noticeable how most of the molecular spectral bands are weaker, and some non-existent, in the reflection spectrum. In a and b, the noise (r.m.s.) of the spectra, which takes into account the corrections for the strongest local telluric features, is plotted point-per-point (in grey for black lines, in light blue for dark blue line) along with the spectra, although, for most spectral regions, the size of the error bars is comparable to the width of the line. The quality of the transmission data are measured in terms of signal-to-noise ratio, which goes from 100 (at the deepest absorptions features and the blue optical wavelengths) up to 400 (at the largest values of the relative fluxes).

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