Life in ice-covered oceans.

Recent evidence for salt-water oceans beneath the ice crusts of Europa and Callisto (1, 2) has bolstered speculation that these satellites of Jupiter may harbor life (3,4). Attention has focused on Europa. The geology and young age of its surface (5,6) and the predicted heat flow from radiogenic decay and tidal dissipation (7,8) suggest a geologically active interior and liquid water at shallow depths. To assess the plausibility of life on Europa and suggest which kinds of organisms are most likely to inhibit this ocean, we must look at the fundamental requirements for life on Earth. In addition to liquid water and a supply of suitable chemical elements, organisms on Earth require a source of energy for metabolism, growth, maintenance, and reproduction. This energy is derived primarily from sunlight through photosynthesis (in certain bacteria or the bacterially-derived chloroplasts in plants) or photorespiration (in archaea living in salty environments); a very small proportion is derived from geothermally driven chemical disequilibria. The chemical products of this activity span a range of oxidation potentials that drive a cascade of intermediate oxidation-reduction (redox) reaction pairs (see the figure). Organisms on Earth exploit these thermodynamically-favored reactions as energy sources, and combinations of oxidants and reductants support various metabolic lifestyles. Before the evolution of photosynthesis, organisms could not use sunlight directly, and life on Earth depended on abiotic sources of chemical energy in the form of disequilibrium concentrations of redox reactants driven by hydrothermal activity, solar ultraviolet radiation, electrical discharges, and impacts (9). All these redox pairs are depleted by abiotic reactions and biological activity, and without an external energy source such as sunlight (or internal geothermal energy), chemical equilibration will ultimately extinguish all life. This thermodynamic reality imposes severe constraints on any biota on Europa that is based on chemical energy. If an ocean exists on Europa, it is physically and chemically isolated by an ice crust 10-100 km in thickness (1). We have to go back far into Earth's climatic history to find a possible analogy for this situation. Kilometer__________________________________ E.J. Gaidos and J.L. Kirschvink are in the Geological and Planetary Sciences Division, California Institute of Technology, Pasadena, CA 91125,USA. E-mail: [email protected]. K.H. Nealson is in the Center for Life Detection, Jet Propulsion Laboratory, Pasadena, CA