James F. Kasting
Department of Geosciences
Pennsylvania State University
NASA's proposed Terrestrial Planet Finder (TPF) mission (and the European Space Agency's [ESA's] Darwin mission) may eventually provide the capability of obtaining low-resolution thermal-infrared spectra of extrasolar planet atmospheres. One of the primary goals of these missions is to determine whether such spectra provide indirect evidence for life. A TPF-like interferometer looking at Earth from a distance would be able to detect CO2, H2O, and O3. O3 (ozone) is a potential indicator for life, since it is formed photochemically from O2, and most of Earth's O2 comes from photosynthesis. Exceptions to this rule (i.e., planets with high abiotically produced O2 levels) are possible and must be considered. CH4 (methane) is a potential bioindicator in atmospheres resembling that of early Earth, prior to the rise of O2. Methane is an ambiguous indicator of biological activity, however, because it could also have significant abiotic sources from impacts and volcanism. Hence, other indicators of life on early-Earth-type planets need to be identified.
A possible strategy for detecting life spectroscopically is to look for examples of extreme disequilibrium in planetary atmospheres. Sagan et al. credit Lederberg with suggesting this idea originally. 40,41 Lovelock made the concept more concrete by pointing out that in Earth's atmosphere, O2 is many orders of magnitude out of thermodynamic equilibrium with reduced gases like CH4 and N2O.42 All three of these gases are produced either exclusively or primarily by the biota. Lovelock argued that biological fluxes are needed to maintain this degree of disequilibrium. One has to be careful with this argument as planetary atmospheres are always out of thermodynamic equilibrium because they are being constantly irradiated by high-energy ultraviolet photons from their parent stars. Mars, for example, has 0.1 percent O2 in its atmosphere, which is not in equilibrium with CO2 and N2, and Venus has clouds of sulfuric acid, which are not in equilibrium with SO2 and H2O. The idea is still useful, however. It just needs to be backed by detailed photochemical modeling to verify that certain atmospheric compositions are truly indicative of biological forcing.
If one were to look at our own solar system from a great distance using an interferometer such as those proposed for TPF or Darwin, one would see a great difference between the atmospheres of Venus, Earth, and Mars. At low spectral resolution, Venus and Mars show only the strong 15-µm band of CO2. By contrast, Earth exhibits bands of both CO2 and H2O, along with a pronounced absorption band at 9.6 µm caused by O3. As argued by several authors,43−45 O3 is formed photochemically from O2, and most of our O2 comes from photosynthesis. Thus, under most circumstances, detection of O3 by itself is fairly strong evidence for life. Two possible exceptions to this conclusion have been identified.46 The first is a runaway greenhouse planet like Venus, where rapid loss of hydrogen from a water-rich atmosphere could result in abiotic O2 buildup. The second is a frozen planet like Mars that also lacks volcanism. The absence of liquid water at the surface would inhibit oxygen loss by weathering, whereas the lack of volcanism would eliminate oxygen loss by reaction with reduced volcanic gases. Oxygen left behind by H2O photodissociation followed by hydrogen escape could therefore accumulate indefinitely in such a planet's atmosphere. As mentioned previously, Mars itself has 0.1 percent O2 in its atmosphere as a result of this process. Mars might have even more O2 if the planet were slightly larger, so that it did not lose oxygen to space, but still not so large that it would have active volcanism. Despite assertions to the contrary,47 there is no theoretical upper limit to the amount of O2 that could build up in this manner.