Atmospheric Biosignatures

An atmospheric signature of life could either be the detection of a single atmospheric constituent in a sufficiently large quantity that it cannot be explained as a product of a non-biological process, or a combination of atmospheric constituents whose simultaneous presence, or inferred abundance ratio, are again unlikely via non-biological processes. For example, in our own atmosphere, the abundant oxygen (20.95% of the total atmosphere) is largely produced by photosynthetic organisms such as bacteria and vegetation and so is considered to be a global biosignature (Leger et al., 1994). On a planet with liquid water on its surface, this amount would be very difficult to produce via geological or photochemical processes, even in the presence of high stellar UV and a lack of volcanic activity (Segura et al., 2007).

Oxygen can be detected most readily in the Earth's spectrum at visible wavelengths near 0.76 ¡m, the so-called oxygen 'A-band'. This spectral feature is routinely seen from space, and via disk-averaged observations of the Earth obtained from Earthlight scattered from the Moon (e.g. (Montanes-Rodriguez, 2006)). In the mid-infrared, molecular oxygen has no prominent spectral features, and the presence of O2 must be inferred from detection of ozone at 9.6^m (Fig. 10.5). Significant concentrations of ozone can be formed even at relatively low oxygen levels (Kasting & Donahue, 1980) making ozone a more sensitive indicator of oxygen then oxygen itself. Once O3 is detected, O2 concentration can, in principle, be inferred from atmospheric chemistry models that combine information on the parent star's spectrum and the amount of O3 observed. However, inferring O2 abundance this way may not be straightforward in atmospheres with different chemical composition and incoming solar flux to our own. Like O2, O3 is considered a stronger biosignature when seen in the presence of water (Leger et al., 1999; Selsis et al., 2002).

It is also important to remember that oxygen as a large fraction of the total atmosphere has only been characteristic of the Earth's spectrum since the Pro-terozoic, i.e. for about half the time that the Earth has supported life (Holland, 1994; Farquhar et al., 2000). Prior to that time, the dominant microbial life-forms may have produced different gas products, such as methane, or sulfur compounds, such as methanethiol (Pilcher, 2003). Remote-sensing biosignatures detectable before oxygen became a major constituent of our atmosphere are currently not well understood, and it should not be assumed that all planets with life have evolved down the same path taken by our Earth.

Another type of atmospheric biosignature is the simultaneous presence of strongly oxidized and reduced gases that are not in chemical equilibrium e.g. O2 and CH4 in the Earth's atmosphere (Margulis and Lovelock, 1974). This type of biosignature is thought to be robust for many different kinds of planetary atmospheres. However, to be interpreted correctly, some understanding of the planetary environment is required to set the different components in context. This is especially important because on another planet, or on Earth earlier in its history, the biosignature pair may not necessarily be O2 and CH4, but other combinations of gases that must be assessed relative to the rest of the atmosphere to determine the equilibrium state. Also, these robust two-component indicators are generally

Fig. 10.5. Atmospheric Biosignatures. Synthetic spectrum of a the Earth's atmosphere taken in the mid-infrared. The biosignatures, O3, CH4 and N2O are indicated. CO2 and H2O are not biosignatures, but instead serve as "habitability markers" indicating the presence of an atmosphere, the possibility of liquid water on the surface, and in the temperature inversion seen in the CO2 band profile (the central emission seen in the absorption band), the likelihood that there is a UV absorber present high in the atmosphere that shields surface life.

Wavelength (fjm)

Fig. 10.5. Atmospheric Biosignatures. Synthetic spectrum of a the Earth's atmosphere taken in the mid-infrared. The biosignatures, O3, CH4 and N2O are indicated. CO2 and H2O are not biosignatures, but instead serve as "habitability markers" indicating the presence of an atmosphere, the possibility of liquid water on the surface, and in the temperature inversion seen in the CO2 band profile (the central emission seen in the absorption band), the likelihood that there is a UV absorber present high in the atmosphere that shields surface life.

much harder to detect via remote-sensing techniques. In the case of the Earth, the oxygen is relatively readily detectable, at least in the visible, but the methane is at much lower concentration and is spectrally most active at near-infrared wavelengths near 2.2^m (a wavelength region that is not currently being considered for the first generation planet finding instruments) and at thermal wavelengths near 7-8^m, a spectral range that includes strong water vapor bands. Consequently the methane is much harder to detect, and requires higher sensitivity and spectral resolution to disentangle its contribution to the spectrum from that of water vapor.

Other potential biosignatures, especially when seen in the presence of oxygen, include the products of biomass burning, such as CH3 Cl, and N2O, which have low abundances and are difficult to detect in the Earth's spectrum, but which may build up to detectable levels on Earth-like planets around cooler stars (Segura et al., 2005) or on planets near the outer edges of the habitable zone (Grenfell et al., 2007) Even ammonia, when seen on a terrestrial planet in the presence of oxygen, may be considered a biosignature.

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