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In Chapter 7 we were essentially concerned with the first three factors in the Drake equation that were introduced in Section 6.1.1, namely, the rate R at which suitable stars are formed, the probability pp of planets forming around a suitable star, and nE the average number of suitable planets in a habitable zone. We now move on to consider how we could determine the next factor, pl, the probability of life app earing on a suitable planet in a habitable zone.
In this chapter we concentrate on the detection of life that is based on complex carbon compounds and liquid water, i.e. carbon-liquid water life. We thus concentrate on life that resembles life on Earth. In doing so we do not assume that alien life is based on the same carbon molecules as terrestrial life. It might use mirror image isomers of some molecules used by terrestrial life, a carbon compound other than DNA to carry genetic information, or carbon compounds other than proteins to carry out the various functions performed by proteins in terrestrial life. But it is still carbon-liquid water life. Only in Chapter 9 will we free ourselves of this selfimposed (but reasonable) carbon-liquid water restriction. There, in the search for extraterrestrial intelligence (SETI) we will search for evidence of technological civilization regardless of the chemical basis of the life-forms.
In searching for signs of carbon-liquid water life, we are at least looking for something that we know to be possible. Another justification stems from fundamental chemistry. No element other than carbon has anywhere near the same facility to form compounds of sufficient complexity, diversity and versatility to supp ort the many processes of life (Section 1.1.2). Few liquids app roach water in its ability to act as both a solvent and a reactant. Ammonia is a possible alternative to water at low temperatures (at a pressure of 1 bar it is liquid from 195 K to 240 K), but it is pure speculation whether a low-temperature form of life could use ammonia in place of water. A third justification is that we know how to detect evidence of carbon-liquid water life. Apart from SETI, we have a poorer idea of how to detect life that has an entirely different chemical basis from ours, particularly as we are restricted to detection from afar.