Surface Signatures

In addition to the rich array of spectral features associated with surface composition, such as the optical and near-infrared signatures of rock, ice or water, the Earth also displays spectral signatures due to the plant life that covers much of its surface. Chlorophyll absorbs strongly in the UV and blue (< 0.5^m) and in the red (0.6-0.7yU,m), and has slightly less absorption in the green (0.55^m). This characteristic absorption in the red end of the visible spectrum was seen by the Galileo spacecraft when it took spectra of Earth on its way to Jupiter (Sagan et al., 1993). However, the most detectable plant spectral feature is a large increase in reflectivity just beyond the visible range (> 0.7^m) due to the change in refractive index between air and the internal leaf structure. This feature, combined with the chlorophyll absorption just shortward of 0.7^m, results in a strong discontinuity in plant reflectance at — 0.7yU,m, which is known as "the red edge" (c.f. Seager et al., 2005) (Fig. 10.6). This property of plants is widely used for remote-sensing studies of the Earth via satellite, and can be used to monitor vegetation coverage over particular portions of the Earth. However, it has also been shown that it is only weakly visible in the Earth's global spectrum, by observing spectra of Earth light reflected from the dark side of the Moon (e.g. Montanes-Rodriguez, 2006; Hamdani et al., 2006). For the

Atmospheric Absorption Red Edge

Fig. 10.6. The Red Edge. Synthetic spectrum of a line of sight through the Earth's atmosphere over a conifer forest, with chlorophyll absorption and the red-edge reflectivity marked. Chlorophyll, a potentially important biosignature, has strong absorption in the UV and blue (< 0.5^m) and in the red (0.6-0.7^m marked in green), and slightly less absorption in the green (0.55^m). Due to changes in the refractive index between air and the internal leaf structure, plants are also highly reflective just beyond the visible range (> 0.7^m), resulting in a prominent discontinuity (marked in red) known as "the red

Fig. 10.6. The Red Edge. Synthetic spectrum of a line of sight through the Earth's atmosphere over a conifer forest, with chlorophyll absorption and the red-edge reflectivity marked. Chlorophyll, a potentially important biosignature, has strong absorption in the UV and blue (< 0.5^m) and in the red (0.6-0.7^m marked in green), and slightly less absorption in the green (0.55^m). Due to changes in the refractive index between air and the internal leaf structure, plants are also highly reflective just beyond the visible range (> 0.7^m), resulting in a prominent discontinuity (marked in red) known as "the red

Fig. 10.7. Photosynthetic Pigments. Since photosynthesis evolves under the influence of both the parent star's available spectrum and the planet's atmospheric composition, the pigments developed to harvest incoming radiation may be quite different on other worlds (Kiang et al., 2007). This whimsical artist's impression of an alien Earth shows what it might be like to live on a planet where alternative photosynthetic pigments dominate and plants aren't green.

Fig. 10.7. Photosynthetic Pigments. Since photosynthesis evolves under the influence of both the parent star's available spectrum and the planet's atmospheric composition, the pigments developed to harvest incoming radiation may be quite different on other worlds (Kiang et al., 2007). This whimsical artist's impression of an alien Earth shows what it might be like to live on a planet where alternative photosynthetic pigments dominate and plants aren't green.

Earth, this signature is potentially much more difficult to detect than the abundant oxygen, but it may be stronger on an extrasolar terrestrial planet with a larger fraction of visible vegetation. Indeed, the photosynthetic pigments evolved, and the spectral position of the red edge itself may be quite different for vegetation on a planet around a star of different spectral type (Kiang et al., 2007; Tinetti et al., 2006b). The important feature to look for will be a sharp, otherwise unexplained, rise in the planet's reflectivity at longer wavelengths.

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