Astrobiology and the Search for Life in the Universe

Abstract

Astrobiology is the search for the origin, evolution, distribution, and future of life in the universe. This chapter examines the requirements for life as we know it. With these constraints as guidelines, the chapter then examines potential habitats for life in our solar system and, finally, elaborates on a strategy to search for life in the universe. Environmental requirements and limitations for active life on Earth have been discussed in the chapter. Most life forms on Earth (including most bacteria, all fungi, and all animals) are heterotrophs that live off energy captured by autotrophic organisms. Salinity affects biological activity, in part, because it controls water availability. The two solar system bodies beyond Earth that have elicited the most interest as potential habitats for life are Mars and Europa because both have clearly been subjected to aqueous processes. The chapter examines these two solar-system bodies in detail. In principle, there are three potential habitats for active life: the surface of planetary bodies, its subsurface, and its atmosphere. Advantages for subsurface life include stable temperatures and vapor pressures and protection from damaging radiation and meteoritic impacts. In the search for life in our solar system, important resources for life (liquid water, readily available energy, and organic compounds), constraints for life, biosignatures, and geosignatures are appropriate criteria. In the search for life beyond our solar system, attention should focus on (i) energy (especially solar) sources, (ii) liquid water, (iii) complex organic chemistry, (iv) the presence of an atmosphere, and (v) O2 (O3) disequilibrium.