The existence of ample amounts of hydrogen in the subsurface ocean of Enceladus indicates that microbes--if any exist there--could use it to obtain energy by mixing with carbon dioxide dissolved in water. This particular chemical reaction, termed methanogenesis, because it manufactures methane as a byproduct, may have been of critical importance in the emergence of life on our planet.
"This is the closest we've come, so far, to identifying a place with some of the ingredients needed for a habitable environment. These results demonstrate the interconnected nature of NASA's science missions that are getting us closer to answering whether we are indeed alone or not," commented Dr. Thomas Zurbuchen in an April 13, 2017 NASA Press Release. Dr. Zurbuchen is associate administrator for NASA's Science Mission Directorate at Headquarters in Washington D.C.
Ganymede is larger than Mercury, which is the innermost--and smallest--major planet in our Solar System. The surface area of Ganymede is more than half that of the land area of Earth, and it provides scientists with a wealth of data concerning a great variety of surface features.