[Perspective] Detecting molecular hydrogen on Enceladus


Planetary bodies with global oceans are prime targets in the search for life beyond Earth owing to the essential role of liquid water in biochemical reactions that sustain living organisms. In addition to water, life requires energy and a source of essential chemical elements (C, H, N, O, P, and S). Although there is compelling evidence for liquid water and many of the essential elements on several ice-covered planetary bodies in our solar system and beyond, direct observation of energy sources capable of fueling life has, to this point, remained elusive. On page 155 of this issue, Waite et al. (1) report that recent flybys of the ice-covered saturnian moon Enceladus by the Cassini spacecraft reveal the presence of molecular hydrogen (H2) in jets of vapor and particles ejected from a liquid water ocean through cracks in the ice shell. The abundance of H2 along with previously observed carbonate species suggests a state of chemical disequilibria in the Enceladus ocean that represents a chemical energy source capable of supporting life.