Temporal Signatures

The Earth's biomass is largely supported either directly or indirectly by photosynthesis, and cycles in the life processes on our planet are tied to the diurnal or seasonal cycles of sunlight. Consequently, a third type of biosignature is a temporal signature, the time-varying behavior in photometric brightness or spectral features. For example, a 'snapshot' spectrum of the Earth would show the presence of CO2 and CH4. If not seen in the presence of oxygen or ozone, it would be hard to conclude that these gases are biologically produced, since photochemistry and geological processes also generate them. However, sensitive spectroscopic observations of the Earth taken over a period of time would reveal periodic variations in the atmospheric CO2 and CH4 abundance. This behavior could be shown to be correlated with season, which would be unlikely for a geological process. However a photochemical model, and an understanding of the planet's environment, including the parent star's spectrum and the atmospheric composition, would be required to preclude the possibility that these variations were simply photochemically produced. On Earth the observed seasonal cycling of CO2 and CH4 (Fig. 10.8) is known to be linked to seasonal variations in the amount and photosynthesis of land plants (Tucker et al., 1986), and can be traced to a surface source, rather than a photochemical product. However, these seasonal variations are very small, and would require a very sensitive instrument to detect them, making temporal variability of atmospheric constituents potentially the hardest type of biosignature to detect for a truly Earth-like planet. This is perhaps beyond the ability of the first generation of planet detection and characterization missions. Another time-variable sign of life might be vegetation coverage as a function of season, which might be detected spectrally or photometrically. One must be cautious, however. Not all time-variable surface signatures are due to life. Numerous astronomers from the late 19th and early 20th century attributed seasonal albedo variations on Mars to variations in vegetation, when the true cause was the seasonal cycle of dust storm activity.

Fig. 10.8. Temporal Biosignatures. This 3-D plot generated by the NOAA-CMDL, represents the measured temporal and spatial variability in the concentration of methane in the Earth's atmosphere over a span of almost 10 yrs. The periodic ripples in the concentration are due to seasonal cycles in the methane, and the sharp step function in the plot from northern to southern latitudes shows the disparity in CH4 concentration in the land-dominated Northern Hemisphere, and the ocean-dominated Southern Hemisphere.

Global Distribution of Atmospheric Methane

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Fig. 10.8. Temporal Biosignatures. This 3-D plot generated by the NOAA-CMDL, represents the measured temporal and spatial variability in the concentration of methane in the Earth's atmosphere over a span of almost 10 yrs. The periodic ripples in the concentration are due to seasonal cycles in the methane, and the sharp step function in the plot from northern to southern latitudes shows the disparity in CH4 concentration in the land-dominated Northern Hemisphere, and the ocean-dominated Southern Hemisphere.

NOAA-CMDL

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