During one of its last flybys of Enceladus on October 28, 2015, the spacecraft grazed the moon’s southern pole at 8.5 kilometers per second, just 49 kilometers above the surface. It crossed the active region where jets spew material from the ocean that we know is located below the icy surface. On three previous flybys, scientists had managed to measure the composition of the jets’ material, and detected molecules of water, carbon dioxide, methane, and ammonia. During the October 2015 flyby, they used the instrument in a mode they hoped would allow them to measure the content of hydrogen molecules in the gas of the vents.



They succeeded—Cassini detected molecular hydrogen.



This is important because the gas is used by microorganisms, known as methanogens, to produce methane from carbon dioxide. Thriving ecosystems seen in the deep oceans of our planet near the volcanic hydrothermal vents of the mid-Atlantic ridge, for instance, depend on the production of energy using this chemistry.



The scientists are very careful when discussing the origin of this molecular hydrogen. They show that the high concentrations measured are not compatible with a geological origin—in other words, such a large amount of molecular hydrogen couldn’t have been stored in the ice shell or in the ocean. Similarly, the scientists are confident that strong radiation on the surface of Enceladus can’t be the source of this molecular hydrogen. They conclude its source is probably hydrothermal reactions between water and rock, emerging out of active volcanism, as it happens in submarine hydrothermal systems on Earth. The source of this volcanism on Enceladus is still not fully understood, but it is probably related to tidal dissipation in the moon’s core, which is squeezed and warmed as the satellite orbits the gas giant Saturn. As with Europa, a moon of Jupiter, this heat warms up the interior, creating an ocean with hydrothermal activity and surface fractures from which materials can escape in space.



It must be emphasized that the scientists did NOT report the detection of life in Enceladus’ ocean, but rather the detection of molecular hydrogen—the final piece needed to infer the presence of methanogenesis. A model including the characteristics of the ocean (temperature, pH, mixing ratio and composition) supports the idea that methanogenic life could survive in this environment. But thermodynamic models alone are not enough to claim that life is indeed present on Enceladus. In other words, “habitable” does not mean “inhabited,” and this distinction is important for astrobiologists.