TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 2
Figure 2-3. Simulated mid-infrared spectrum of an Earth-like planet showing the
effects of viewing angle (Giovanna Tinetti, Institut d’Astrophysique de Paris).
2.4.1 Biomarker Signatures in the Mid Infrared
In the mid-IR, the classical signature of biological activity is the combined detection of the 9.6-μm O 3
band, the 15-μm CO 2 band, and the 6.3-μm H 2 O band or its rotational band that extends from 12 μm out
into the microwave region (Selsis and Despois 2002). Because oxygen is a chemically reactive gas, it
follows that reduced gases and oxygen have to be produced concurrently to be detectable in the
atmosphere, as they react rapidly with each other. The oxygen and ozone absorption features in the visible
and thermal infrared, respectively, could indicate the presence of photosynthetic biological activity on
Earth any time during the past 50% of the age of the Solar System. In the Earth’s atmosphere, the 9.6-μm
O 3 band is a poor quantitative indicator of the O 2 amount, but an excellent qualitative indicator for the
existence of even traces of O 2 . The Ozone 9.6-μm band is a very nonlinear indicator of O 2 for two
reasons. First, for the present atmosphere, low-resolution spectra of this band show little change with the
O 3 abundance because it is strongly saturated. Second, the apparent depth of this band remains nearly
constant as O 2 increases from 0.01 times the present atmosphere level (PAL) of O 2 to 1 PAL (Segura et al.
2003). The primary reason for this is that the stratospheric ozone increases that accompanied the O 2
buildup lead to additional ultraviolet heating of the stratosphere. At these higher temperatures, the
stratospheric emission from this band partially masks the absorption of upwelling thermal radiation from
the surface.
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