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 nearinfrared (0.7–2.4 !m) 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 mediumaltitude 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 !m), CO2 (six moderate-strength features from 1.4 to 2.1 !m), O2 (two moderate-strength features at 0.76 and 1.26 !m), 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 nearinfrared 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. Subject headinggs: astrobiology — Earth — Moon — planetary systems Online material: color figures