Astronomy and Astrophysics – Astronomy
Scientific paper
May 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007aas...210.0906w&link_type=abstract
American Astronomical Society Meeting 210, #09.06; Bulletin of the American Astronomical Society, Vol. 39, p.106
Astronomy and Astrophysics
Astronomy
Scientific paper
Quasi-specular reflection of sunlight at the waxy surface of a leaf yields appreciably polarized light especially near the Brewster angle. Sunlight entering the leaf is either absorbed by photosynthetic pigments or, after reflection at internal surfaces, emerges as a diffuse component with a non-zero but low linear polarization and a yet smaller level of circular polarization. The net polarization varies with the leaf's surface roughness, internal architecture and health and is difficult to model. The polarization of a forest canopy is complex and depends especially on the leaf orientation distribution. The global polarization and reflectance properties of Planet Earth have been measured by the POLDER satellite: at 443nm atmospheric Rayleigh scattering dominates, but at 865nm the average surface properties of ocean, vegetation, desert and snow can be estimated. For cloud-free surfaces at 865nm and 90 degree phase angle the percentage polarization,p, and reflectance,R, are respectively [55%,9%](ocean), [7%,23%](vegetation), [6%,40%](desert) and [3%,80%](snow). Note that the values for clear and cloudy ocean are very different, viz [55%,9%] and [4%,45%] respectively. Allowing for the fractional global areas of each component and a global cloud cover of 55% yields p=7.3% for a pale-blue-dot Earth. pR is greatest for oceans and least for vegetation and hence the prospects for detecting pR from vegetation on an Earth-like planet are poor unless >50% is covered in vegetation. However prospects of using the phase angle and wavelength dependence of a pale-blue-dot planet to deduce its properties as it rotates and orbits are more encouraging: the main obstacle will be to overcome the difficulty of correcting for the unknown and variable cloud cover. The possible application of a circular polarization signal that is unique to vegetation remains an intriguing possibility for remote sensing that requires further study.
Breon F.
Tranter G.
Wolstencroft Ramon D.
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