Below the Surface, Life among the Stars

Abstract

While human missions to other stellar systems are beyond present technological capabilities, understanding the attributes of environments that can initiate and maintain life expands our ability to search and to learn. From the galactic neighborhood to the individual stars within it and the planets orbiting those stars, astrobiologists explore and define the physico-chemical extremes within which lies safe harbor for biological life. The description of the Galactic Habitable Zone predicts an astrophysical niche for “complex,” perhaps eventually intelligent, life. Focusing on planetary systems surrounding stars, the Circumstellar Habitable Zone’s main, but not only, critical feature is liquid water, life’s essential solvent. Recently, another descriptive “zone” has been added to these, the Geological Habitable Zone. The primary inputs determining the life-supporting, life-sheltering potential of a planetary subsurface are heat and pressure.

Very little of the extraordinary diversity of the Earth is yet cataloged. Exploring our own backyard and our local neighborhood, we have only scratched the surface, and what a mine of information we have found there. “Follow the water” is the mantra, for wherever we encounter liquid water on Earth we find life. As we explore our solar system, we are not only seeking evidence of life, extant or extinct, we are collecting information on the nature of the processes that permit planetary lithospheres, hydrospheres, and atmospheres to give rise to and sustain a biosphere. We will use all the tools at our disposal, from remote observations and in situ probes to sample return and human expeditions. Our solar system presents other candidates for fruitful search, including those beyond our Sun’s Circumstellar Habitable Zone. To reach for life’s signature we have to dig deep.