NASA's Cassini spacecraft has discovered hydrogen and carbon dioxide erupting from Saturns moon Enceladus – critical ingredients that sustain microbial life in extreme environments on Earth. In 2015, during the Cassini spacecraft's deepest-ever dive into a plume of spray emanating from cracks in the south polar region of the ice-covered Saturnian moon Enceladus, instruments detected the presence of hydrogen and carbon dioxide in the plume vapour. The results are reported in the journal Science by Hunter Waite and colleagues, who go on to demonstrate that the only plausible source of this hydrogen is hydrothermal reactions between hot rocks and water in the ocean beneath the moon's icy surface. On Earth, the same process provides energy for whole ecosystems around hydrothermal vents. The researchers suggest that the vapour and particle material Cassini flew through contained up to 1.4 volume percent molecular hydrogen, and up to 0.8 volume percent carbon dioxide. These are critical ingredients for a process known as methanogenesis, a reaction which produces methane from hydrogen and carbon dioxide to sustain microbial life in deep, dark undersea environments on Earth. "We didn't know we were going to do this experiment when we launched Cassini," says Hunter Waite at the Southwest Research Institute in Texas. So to look for hydrogen, Waite and his team had to put the Cassini Ion and Neutral Mass Spectrometer instrument in a new mode that measured the molecules without allowing them to touch the walls. Finally, they found the molecular hydrogen they were looking for – and a lot of it. Their findings indicated that there was too much hydrogen to be stored in tiny Enceladus's ice shell or ocean. That means it must be continuously produced there, probably by hydrothermal reactions similar to those that occur near hot vents at the bottom of Earth's oceans. The INMS saw a much higher density of volatile gases, water vapour, carbon dioxide and carbon monoxide, as well as organic materials, some 20 times denser than expected. This dramatic increase in density was evident as the spacecraft flew over the area of the plumes.
