Spectrum of a Habitable World: Earthshine in the Near-Infrared

To characterize the spectrum of Earth viewed as an extrasolar planet, we observed the spatially integrated near-infrared (0.7-2.4 μm) reflection spectrum of Earth via the dark side of the Moon (earthshine). After contributions from the Sun, Moon, and local atmosphere were removed, the resulting spectrum was fitted with a simple model of the reflectivity of Earth. The best model fit is dominated by the reflection spectrum of the atmosphere above medium-altitude water clouds, with lesser contributions from high-altitude ice clouds and from the ground. The spectral features seen are H₂O (six strong band structures from 0.7 to 2.0 μm), CO₂ (six moderate-strength features from 1.4 to 2.1 μm), O₂ (two moderate-strength features at 0.76 and 1.26 μm), and several weak CH₄ features. Interpreted as a spectrum of an extrasolar planet, we would confidently conclude that this is a habitable planet, based on the presence of strong water bands. Furthermore, the simultaneous presence of oxygen and methane is a strong indicator of biological activity. We might also conclude that the planet is geologically active, based on the presence of CO₂, water, and a dynamic atmosphere (inferred from cirrus clouds, cumulus clouds, and clear-air fractions in our model fit). This suggests that it would be valuable for the Terrestrial Planet Finder-Coronagraph (TPF-C) mission to include near-infrared spectroscopy capability. On the basis of the present work, we suggest that future long-term monitoring of the earthshine would allow us to discern how the globally integrated spectrum changes with planet rotation, cloud cover, and seasons.