Biology
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmsa52a..06f&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #SA52A-06
Biology
0406 Astrobiology And Extraterrestrial Materials, 0424 Biosignatures And Proxies, 5200 Planetary Sciences: Astrobiology, 5464 Remote Sensing
Scientific paper
Earth's deciduous plants have a sharp order-of-magnitude increase in leaf reflectance between approximately 700 and 750 nm wavelength. This strong reflectance of Earth's vegetation suggests that surface biosignatures with sharp spectral features might be detectable in the spectrum of scattered light from a spatially unresolved extrasolar terrestrial planet. We assess the potential of Earth's step-function-like spectroscopic feature, referred to as the ``red edge,'' as a tool for astrobiology. First, we review the basic characteristics and physical origin of the red edge. Then, we discuss the challenges involved in detecting the red edge in Earthshine (i.e., a spatially integrated scattered light spectrum of the Earth), as evidenced by recent attempts to detect the red edge using spectroscopic observations of the dark side of the moon (which is illuminated by Eartshine). We present Earthshine observations from Apache Point Observatory (New Mexico) to emphasize that time variability is key to detecting weak surface biosignatures such as the vegetation red edge. We briefly discuss the evolutionary advantages of vegetation's red edge reflectance, and speculate that while extraterrestrial ``light-harvesting organisms'' have no compelling reason to display the exact same red edge feature as terrestrial vegetation, they might have similar spectroscopic features at different wavelengths than terrestrial vegetation. This implies that future terrestrial-planet characterizing space missions should obtain data that allow time-varying, sharp spectral features at unknown wavelengths to be identified. We caution that some mineral reflectance edges are similar in slope and strength to vegetation's red edge (albeit at different wavelengths). Due to the small amplitude of the terrestrial red edge, the temporal variability of atmospheric water vapor, and the potential for similar mineralogical features, we conclude that great care must be taken in the interpretation of any reflectance edges observed in the spectra of extrasolar planets.
Ford Eric B.
Seager Sara
Turner Edwin L.
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