A Search Strategy for Life in the Universe

In this chapter, we will first discuss a search strategy for life in the Universe and then discuss the future of life in the Universe. 

In the search for extraterrestrial signatures of life, we need to distinguish among Solar System sites, extrasolar system sites, and the search for intelligent life. It is likely that within the 21st century most potential life-bearing 

Radiation Microbes up to 4000 x more tolerant than humans 

A plausibility scale for life beyond Earth was developed by Irwin and Schulze-Makuch (2001) (Table 6.2). Earth-like planetary bodies with liquid water, readily available energy, and organic compounds were listed as Plausibility of Life (POL) Category I (high). At the other extreme are solar system bodies such as the Sun and Moon with conditions or compositions so harsh that life is unrealistic, which is Category V (remote verging on utterly impossible). The previously discussed planetary bodies, Mars and Europa, are Category II (favorable), which is why so much attention has been placed on these two Solar System bodies. 

In the search for life in our Solar System, all of the above important resources for life, constraints for life, biosignatures, and geosignatures are appropriate criteria. In the search for life beyond our Solar System, attention should focus on (1) energy (especially solar) sources, (2) liquid water, (3) 

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