ASTROBIOLOGY Volume 2, Number 1, 2002 © Mary Ann Liebert, Inc. Research Paper Energy Cycling and Hypothetical Organisms in Europa’s Ocean DIRK SCHULZE-MAKUCH 1 and LOUIS N. IRWIN 2 ABSTRACT While Europa has emerged as a leading candidate for harboring extraterrestrial life, the ap- parent lack of a source of free energy for sustaining living systems has been argued. In this theoretical analysis, we have quantified the amount of energy that could in principle be ob- tained from chemical cycling, heat, osmotic gradients, kinetic motion, magnetic fields, and gravity in Europa’s subsurface ocean. Using reasonable assumptions based on known organ- isms on Earth, our calculations suggest that chemical oxidation–reduction cycles in Europa’s subsurface ocean could support life. Osmotic and thermal gradients, as well as the kinetic en- ergy of convection currents, also represent plausible alternative sources of energy for living systems at Europa. Organisms thriving on these gradients could interact with each other to form the complex energy cycling necessary for establishing a stable ecosystem. Key Words: Europa—Planetary ocean—Energy sources—Ecosystem—Extraterrestrial life. Astrobiology 2, 105–121. 105 INTRODUCTION A S ROBOTIC EXPLORATION of the Solar System brings new information on the physical, chemical, and dynamic properties of other worlds, the search for extraterrestrial life depends in part on the identification of energy sources that can be transformed into the free energy used by life-sustaining biochemical processes. The history of life on Earth provides a prime example of how light and chemical energy, individually and in concert, have been harvested to power biological systems. On other worlds, these energy sources may not be available, and alternative energy sources may be more favorable. The ocean now widely assumed to exist be- neath Europa’s surface ice (Carr et al., 1998; Geissler et al., 1998; Khurana et al., 1998; Kargel et al., 2000; Kivelson et al., 2000) may have a chem- ical composition conducive for yielding free en- ergy useful to living organisms. Light, however, cannot penetrate to the depth where liquid water could exist (Reynolds et al., 1983). Gaidos et al. (1999) argue that geochemical cycling under con- ditions likely to prevail on Europa might well be insufficient to support life using any metabolic process known on Earth. We agree with the strat- egy of relating metabolic processes known on Earth to plausible conditions on other worlds, but propose that the problem be viewed in the larger context of evolutionary history under different conditions and the potential interaction of a va- Departments of 1 Geological Sciences and 2 Biological Sciences, University of Texas at El Paso, El Paso, TX.