Seeking Extraterrestrial Life: How Do Habitability and Origin-of-Life Conditions Differ?

In recent years, research in astrobiology has increasingly recognized that the emergence of life is not solely determined by whether an environment can sustain life, but by whether it possesses the conditions necessary for life to begin. This concept is referred to as an origin of life event, and it is fundamentally distinct from the traditional notion of habitability. The former emphasizes the chemical, energetic, and environmental requirements for life to arise from non-life, whereas the latter focuses on whether life, once formed, can persist over time.

Beginning with the Viking missions in the 1970s, NASA's search for signs of life on Mars marked the first time the scientific community fully acknowledged that determining whether a world hosts life cannot rely on a single line of evidence. Although Viking did not discover definitive signs of life, it established a foundational understanding of the Martian surface and atmosphere. Subsequent exploration shifted toward a "follow the water" strategy, based on the premise that water is the universal solvent used by all known life. Missions such as Voyager 1 and 2 revealed geological features on Europa suggestive of a subsurface ocean, while the Cassini-Huygens mission detected water, hydrogen, and complex organic molecules in the plumes of Enceladus. On Mars, both ice and geological evidence of past liquid water have been identified.

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