Remote Sensing of Planetary Properties and Biosignatures on Extrasolar Terrestrial Planets

Abstract

The major goals of NASA's Terrestrial Planet Finder (TPF) and the European Space Agency's Darwin missions are to detect terrestrial-sized extrasolar planets directly and to seek spectroscopic evidence of habitable conditions and life. Here we recommend wavelength ranges and spectral features for these missions. We assess known spectroscopic molecular band features of Earth, Venus, and Mars in the context of putative extrasolar analogs. The preferred wavelength ranges are 7-25 μm in the mid-IR and 0.5 to ∼1.1 μm in the visible to near-IR. Detection of O₂ or its photolytic product O₃ merits highest priority. Liquid H₂O is not a bioindicator, but it is considered essential to life. Substantial CO₂ indicates an atmosphere and oxidation state typical of a terrestrial planet. Abundant CH₄ might require a biological source, yet abundant CH₄ also can arise from a crust and upper mantle more reduced than that of Earth. The range of characteristics of extrasolar rocky planets might far exceed that of the Solar System. Planetary size and mass are very important indicators of habitability and can be estimated in the mid-IR and potentially also in the visible to near-IR. Additional spectroscopic features merit study, for example, features created by other biosignature compounds in the atmosphere or on the surface and features due to Rayleigh scattering. In summary, we find that both the mid-IR and the visible to near-IR wavelength ranges offer valuable information regarding biosignatures and planetary properties; therefore both merit serious scientific consideration for TPF and Darwin.