Hydrogen on Enceladus: Interesting, But Not Conclusive for Life

Vents below the icy moon’s surface may power a biosphere, although there are other possibilities, too.

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Enceladus and its geysers, as seen by Cassini.

Today’s announcement of new findings by the Cassini spacecraft at Saturn’s moon Enceladus strengthens the argument that there may be a habitat for life deep below the ocean world’s frozen surface. Hunter Waite of the Southwest Research Institute and his colleagues found a relatively high amount of molecular hydrogen in the vapor plume spraying upward from cracks in the ice. Hydrogen is the starting molecule for many chemical pathways, including biological pathways.

This discovery, combined with earlier detections of carbon dioxide in the Enceladus plume, means that both gases needed for a process called methanogenesis are present, where hydrogen and carbon dioxide combine to form methane and water. Not only is this an important metabolic pathway for many microbes living at hydrothermal vents, it has been proposed as possibly the first metabolic pathway for life on Earth.

Waite and his colleagues argue that the most plausible source of the detected hydrogen is ongoing hydrothermal reactions of rocks containing both inorganic and organic material. That makes sense, and may very well be correct, but it doesn’t mean life is present on Enceladus. There also are inorganic processes at hydrothermal vents that can form organic molecules. One famous example is known as the Fischer-Tropsch synthesis, a chemical reaction recognized since 1925.

We also have to keep in mind that geological and environmental conditions in the outer Solar System are not the same as what we observe on Earth, and different chemical processes and reactions may be at work, some of which we may not even be aware. For example, vast amounts of methane and a slew of organic compounds are found on Saturn’s largest moon, Titan, but we don’t know how they formed.

The new findings are startling, no doubt, and bolster the case for further study of the so-called ocean moons. Luckily, samples from Enceladus are relatively easy to come by, since they spray up from the surface where a spacecraft could collect them. But my vote still would be for a more comprehensive mission to investigate both Enceladus and Saturn’s other intriguing world, Titan.

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