Extraterrestrial microbiology is the study of microbiological processes that could occur outside of the boundaries of Earth, or on other bodies in the solar system. While such microorganisms have not yet been found, recent findings of living bacteria in very inhospitable environments on Earth, combined with the existence of water on planets such as Mars, have buttressed the possibility that life in similar conditions on other planets is not inconceivable.
The scientific search for extraterrestrial life began in 1860, when the microbiologist Louis Pasteur attempted and failed to culture bacteria from the Orgueil meteorite.
The search for extraterrestrial life has always been one of the curiosities that has pulled man into the exploration of space. As the chemistries of the planets in our solar system became clearer, the possibilities for human-like life faded. However, at about the same time, the diversity of microbial life on Earth became more apparent. In particular, a type of evolutionarily ancient microorganism known as archaebacteria was isolated from extremely harsh environments, such as hot springs, thermal hot vents on the ocean floor, and from deep in the subsurface of the planet. In contrast to life forms that require oxygen and organic carbon, archaebacteria live on hydrogen and carbon dioxide. Planetary bodies such as Mars and Europa contain atmospheres of hydrogen and carbon dioxide. Thus, theoretically, archaebacteria could find such planets hospitable. Moreover, the finding of bacterial life below the Earth's surface makes the probability of similar life elsewhere greater. Other worlds are more likely to have, or have had, hot and oxygen-limited conditions similar to thermal vents or the subsurface, rather than the sunlight, oxygen-rich atmosphere of Earth's surface. Furthermore, the now prevailing view that archaebacteria are very ancient indicates that life on Earth may have arisen from environments now considered inhospitable. The environment on other solar bodies may be similar to what Earth experienced when microbial life first arose.
Unmanned probes have explored a variety of bodies in our solar system. One such stellar body, Europa, has so far not proved to be a source of life. Probes sent to scan the planet's surface found only lifeless slush. However, two of the moons, which orbit the gas planet Saturn, are of interest. Enceladus has visual signs and chemical signals consistent with the presence of liquid water. The other moon, Titan, is icy and spectral monitoring of the surface has detected signals indicative of organic compounds.
By far the bulk of interest in extraterrestrial microbiology has centered on the planet Mars. Interest in Mars as a potential supporter of microbiological life prompted the Viking mission that occurred in 1976. The, in two separate missions, proves landed on different regions of the planet ad conducted experiments designed to detect signature molecules of microbiological activity. One experiment, the gas exchange experiment, sought to detect alterations in the composition of gases in a test chamber. The alterations would presumable be due to microbial decomposition of nutrients, with the consumption of some gases and the release of others. The results were equivocal at first, but with examination were thought to be the result of abiological activity, specifically solar ultraviolet radiation. In a second experiment, radioactive nutrient was released into wetted Martian soil. Bacterial metabolism would be evident by the appearance of different radioactive compounds. Again the results were equivocal, and may have indicated microbiological activity. A third experiment that looked for the presence of organic compounds in the soil was negative. In the final experiment, soil was examined using an instrument called a gas chromatograph-mass spectrometer for chemical signatures of biological activity. The test revealed a great deal of water but little else.
These results have been the subject of debate, and have not proven to be conclusive for the absence of microbiological life. For example, at the time of the Viking missions, the full extent of the diversity of microbiological life on Earth, specifically the existence of living bacteria far below the surface in regions where organic material was virtually absent, was not known. With the discovery of archaebacteria, the possibility that microbiological life could exist in the subsurface layers of a planet like Mars has warranted a reassessment of the possibility of Martian life. Furthermore, high resolution photographic surveys of the planet by orbiting probes in the 1990s revealed geological features that are the same as dried rive valleys and floodplains on the Earth. These observations have bolstered the view that Mars was once an abundantly moist planet, capable of sustaining microbiological life.
To definitively address the issue of microbiological life on Mars, the European Space Agency is scheduled to launch the so-called Mars Express in June 2003. The mission will have a two-fold purpose. An orbiting satellite will analyze the planet from high altitude, while a surface probe will sample the planet's surface. The analytical equipment aboard the probe is designed to detect minute amounts of carbon, and all metabolic forms of the atom. For example, experiments will look for the presence of methane, such as would be produced by methanogenic bacteria.
In addition to extraterrestrial microbial life, interest has arisen over the possibility that extraterrestrial microorganisms could find their way to Earth. Transport of microorganisms via meteorites and on material ejected from the solar body by a meteorite impact has been proposed. In the 1990s, the electron microscopic examination of meteorite ALH84001, which originated from Mars, found bacteria-like objects. Their shape, size and chemistry were at the time consistent with a biological origin. However, further study negated this possibility and no other such observations have been made.
See also Anaerobes and anaerobic infections; Biogeochemical cycles; Extremophiles