Microbial Habitability of Icy Worlds

F or most of Earth’s history, life was microscopic and, even today, microorganisms dominate the planet. Hence, we believe that, if life exists beyond Earth, it likely started as microorganisms and may remain at this stage. Much of contemporary astrobiological research focuses on the metabolic potential of microorganisms and their ability to inhabit extreme environments. The search for life beyond Earth prioritizes the search for liquid water because water on Earth is closely associated with life. However, our understanding of where liquid water can be found is changing. Moons in the outer solar system, including Europa, Ganymede, and Enceladus that orbit either Jupiter or Saturn, although covered with ice, carry subsurface oceans containing many times the volume of liquid water on Earth and may provide the greatest volume of habitable space in our solar system. On many of these worlds, water is maintained as a liquid through radiogenic decay, tidal energy dissipation, and the mere fact that ice floats and serves as a good insulator. These oceans are there today and have likely persisted for much of the history of the solar system, providing key environments in which to search for extant life beyond Earth—life from a second, independent origin. The recent discoveries of metabolically active microorganisms in subglacial environments on Earth provide useful analogues in that search. Of these worlds, Europa holds perhaps the greatest potential as a modern habitat for microbial life, and its relatively thin ice shell of about 5–15 km eases the technical challenges of trying to detect such life. Along with a liquid ocean, Europa also likely harbors a rocky seafloor that can supply energy and elements needed to drive metabolism (Fig. 1). With the goal of investigating the habitability of Europa, the National Aeronautical and Space Agency (NASA) has recently identified a mission to Europa as second only to a Mars sample-return mission. Here, we focus on the potential for microbial habitability on Europa, drawing on what we know about microbial life in Earth’s sub-ice environments.