Astrobiology, a term coined for the study of all life anywhere in the universe (including Earth), has replaced exobiology, the study of extraterrestrial life exclusively and therefore criticizable as “a science that lacks a subject matter.” Unlike exobiology, astrobiology respects the scientific possibility that life beyond Earth may never be found. Indeed, no evidence for life beyond Earth has been adduced. However, the design of astrobiological experiments forces critical examination of the generality of assumptions derived from Earth life.

There is an entire spectrum of possibilities for life on another planet. A planet may be lifeless and lack any vestiges of organic matter or fossils. Alternatively, it may be lifeless but contain organic matter or fossils. There may be life having simple or quite complex biochemistry, physiology, and behaviour. Even intelligent life with a technical civilization may be found. Confirmation of any of these possibilities would be of great scientific importance.

The search for extraterrestrial life is most clearly grasped by imagining the reverse situation. For example, if humans were on Mars, examination of Earth for life with the full armory of contemporary scientific instrumentation and knowledge would be illuminating. Both remote and in situ testing might be attempted. In remote testing, light of any wavelength reflected from or emitted by the target planet can be examined. Remote-sensing methods seek thermodynamic disequilibrium, especially in the fluid phases (atmosphere and hydrosphere) of the planet. With in situ studies, samples of a planet must be acquired by instrumentation that lands there and performs experiments.

Chemical, mechanical, or spectral disequilibria may also be sought. Earth’s atmosphere contains large amounts of molecular oxygen and about 1.7–2 parts per million (106) of methane, but in thermodynamic equilibrium the abundance of methane should be less than one part in 1035. This huge discrepancy implies that some process continuously and rapidly generates methane on Earth such that methane increases to a very large steady-state abundance before it can be oxidized. Although the methane disequilibrium mechanism need not be biological (e.g., relatively stable aromatic hydrocarbons could have been produced nonbiologically early in Earth’s history, and slow degradation may then have led to a continuous loss of methane from the planetary subsurface), a biological explanation seems more likely. As seen from Mars, the methane discrepancy could be considered as a preliminary positive test for life on Earth. Indeed, the methane abundance on Earth is due to bacteria. Some methanogenic bacteria live in wetlands (hence the term marsh gas for methane), and others live in the intestinal tracts of cows and other ruminants. Similarly, the large amount of free oxygen gas might be considered a sign of life. The possibility that the photodissociation of water and the subsequent escape to space of hydrogen are the source of oxygen would need to be excluded. Also, spectroscopic detection of such relatively complex reduced organic molecules as terpenes (hydrocarbons given off by plants and found over forests) could be used as a test for life.By contrast, photographic observations of the daytime Earth from Mars would not necessarily detect life. Even at a resolution of 100 meters (330 feet)—that is, an ability to discriminate fine detail at high contrast only if its components are more than 100 meters apart—cities, canals, bridges, the Great Wall of China (long erroneously believed to be visible from the Moon), highways, and other large-scale accoutrements of Earth’s technical civilization would be extremely difficult to discern. As resolution progressively improves, it becomes increasingly easy to distinguish the regular geometric patterns of cultivated fields, highways, and airports. However, these are products of recent civilization; thus, only 100,000 years ago no clear sign of life would have been visible with remote-sensing techniques. Today lights of the largest cities are detectable from Mars, as are seasonal changes in the color of plants.

Scanning of the electromagnetic spectrum offers another technique for detecting life. Domestic television transmissions, the high-frequency end of the AM broadcast band, and radar defense networks make up some of the enormous amount of energy put out by Earth into space at certain radio frequencies. According to an estimate made by the Russian astrophysicist Iosif S. Shklovskii, if this radiation were to be interpreted as ordinary thermal emission, the implied temperature of Earth would be hundreds of millions of degrees. This radio “brightness temperature” of Earth would have steadily increased over the last several decades. The frequency and the time variation of these signals are not purely random noise.

In situ studies by vehicles that enter Earth’s atmosphere and land on the surface could detect life at many places on Earth. However, there are many other places where large organisms are infrequent such that life-detection attempts based solely on television searches for large animals would be inconclusive. Of course, if such an experiment were successful—for example, if the camera recorded a cavorting dolphin, a strolling camel, or a flying peacock—it would provide quite convincing evidence of life.

Although the open ocean, the Gobi Desert, and Antarctica are relatively devoid of large life-forms, they are—like other, less-barren ecosystems—replete with microorganisms. A television camera coupled to an optical or electron microscope might be an optimal life detector. The 17th-century Dutch microscopist Antonie van Leeuwenhoek had no difficulty in identifying as alive the little “animalcules” he found in a drop of water, even though nothing similar had been seen before in human history.

Metabolic and chemical criteria might be used for detecting life with in situ studies. The fixation of gas (such as carbon dioxide) when illuminated might be due to photosynthesis or chemosynthesis. Direct tests of soil or water for optical activity might be made. Organic molecules could certainly be sought with gas chromatography, mass spectrometry, or remote analytic chemistry. The detection of organic matter would then lead to experiments that would determine if it was biological in origin.

In general, many tests for life are intrinsically ambiguous. There remains the omnipresent problem of contamination. Any spacecraft might carry living organisms from the home planet and report them as detected on the target planet. Great care must be taken to ensure that the spacecraft is rigorously sterilized and travels without life from home.

Even the detection of significant quantities of extraterrestrial organic matter can be misleading. Carbonaceous chondrite meteorites fall on Earth from the asteroid belt. They contain up to 4 percent organic matter by mass. This matter has been ascertained to be of nonbiological origin. Microscopic inclusions also have been detected. The most abundant of these inclusions are mineralogical in origin. Highly structured inclusions, such as filaments or microspheres with central dots, are rare and sometimes the result of obvious contamination (one inclusion contained ragweed pollen). Claims of the extraction of viable microorganisms from the interiors of carbonaceous chondrites were not supported by subsequent evidence. These meteorites are porous and “breathe” air in and out during their entry into the atmosphere and during their storage prior to study. Significant opportunities exist for contamination after their arrival on Earth because of the ubiquity of microorganisms. Some bacteria extracted from a meteorite were facultative aerobes. As no planet in the solar system except Earth harbors significant quantities of oxygen gas, it is unlikely that the electron-transfer multienzyme pathways required for oxygen respiration evolved in the asteroid belt. Nevertheless, the large amounts of organic matter found in carbonaceous chondrites suggest that organic molecule production occurs with great efficiency in certain extraterrestrial locations. This production may serve as a natural precursor to life elsewhere.No single unambiguous “life detector” exists. Instruments of great generality that make few ambiguous assumptions about the nature of extraterrestrial life require luck (e.g., an animal or protist must walk or swim by during the operating lifetime of the camera) or the solution of difficult instrumental problems (e.g., the acquisition and preparation of samples for remote electron microscopic examination). Highly sensitive instruments, such as metabolism detectors, are directed at organisms presumably vastly more abundant than animals. These instruments critically depend on assumptions that are basically informed guesses (e.g., that extraterrestrial organisms eat sugars). Therefore, an array of both very general and very specific instruments is recommended to establish, or preclude, the existence of extraterrestrial life in the solar system.