Possible ecosystems and the search for life on Europa

No broadly accepted definition of life exists. Most proposed definitions (1–5) face severe objections (3, 6, 7). Nevertheless, one working definition of life has become influential in the origins-of-life community: “life is a self-sustained chemical system capable of undergoing Darwinian evolution” (8). The notion that “the origin of life is the same as the origin of evolution” is a popular corollary. But however valuable this Darwinian definition may be for guiding laboratory experiments, it is unlikely to prove useful to a remote in situ search for life (3, 6). In a search for extraterrestrial life in our solar system, we instead fall back on a less ambitious notion of “life as we know it,” meaning life based on a liquid water solvent, a suite of “biogenic” elements (most famously carbon, but others as well), and a source of free energy (7). The availability of these on a given world would suggest life to be possible, so that further exploration may be warranted.