life can be gauged only after extensive development. However, nanotechnologies constitute a very active area of research, and more than one novel approach to single molecule detection is under development.1
While most of the efforts to search for extraterrestrial life are currently focused on either sample return or in situ experiments on Mars, Europa, and Titan (see Chapter 2), there is growing interest in the possibility of detecting habitable planets around nearby stars. One of the strategies for detecting habitable planets is to obtain low- to moderate-resolution thermal-infrared spectra of their atmospheres (see the Session 1 paper by Kasting).
A spectroscopic examination of Earth's atmosphere would reveal the presence of CO2, H2O, and O3 (ozone), the last as a proxy for the spectroscopically inactive O2. The analyses would also show that atmospheric O2 is orders of magnitude out of thermodynamic equilibrium with reduced gases such as CH4 and N2O. Extreme disequilibrium in these gases may signify the presence of living organisms and therefore could be useful for detecting life on extrasolar planets.
The presence of ozone in the atmosphere of an extrasolar planet is a particularly interesting bioindicator since it could signify the presence of O2 at concentrations indicative of photosynthesis. However, it is conceivable that there are planets inhabited only by anaerobic microorganisms or that O2 produced by photosynthesis is titrated in situ by reduced metals such as iron. The geologic record tells us that Earth's atmosphere had a very low O2 content during the first 2 billion years. Thus, for almost half of its history, Earth would have appeared lifeless by the “ozone criterion,” even though it supported life. Furthermore, a planet in the process of rapidly losing its water by atmospheric escape, as Venus might have early in its history, would show a strong signature of ozone even though life might not be present.
Methane could also be a bioindicator, particularly if found in high concentrations. Anaerobic microorganisms produce most of the methane on Earth. These methanogens are strict anaerobes and are believed to be evolutionarily ancient. There are caveats for using methane as a bioindicator. It is produced by abiotic sources as well, and very little is known about the compositions and chemistry of early Earth-type anoxic atmospheres.
Another technique that could be used to detect evidence of extraterrestrial life outside the solar system is radio astronomy. With this method, many organic molecules have already been detected and their abundances determined in interstellar and circumstellar gas. The bases of identification are the unique rotational spectra of these chemical compounds. With the continuing improvement in detector sensitivity, more complicated species, including isotopic and isomeric variances, may be detected in the interstellar medium that could be bioindicators.
Deeper understanding of the evolution of the planets in our own solar system, particularly Mars and Venus, will provide some ground truth on the possible evolutionary paths that planets may take away from habitability, and the consequent spectroscopic signatures.
1. H. Craighead, “Separation and Analysis of DNA in Nanofluidic Systems,” AAAS Annual Meeting, San Francisco, 2001.