Nasa's Cassini mission has discovered molecular hydrogen and carbon dioxide emanating from cracks in the ice at the south pole of Saturn's brightest moon, Enceladus. The chemical imbalance detected at the moon's surface could provide an energy source for life.
The hydrogen is thought to be formed through the reaction of hot rocks at the moon's surface with liquid water, beneath the thick ice sheet. The Cassini probe measured 1.4% hydrogen by volume and 0.8% carbon dioxide in the plume.
Cassini took the measurements on its deepest dive yet into the plume, just 49 kilometres from the moon's surface, flying past at a speed of 8.5km a second. The results are published in an article published in the journal Science.
"The plume contains chemical signatures of water-rock interaction between the ocean and a rocky core," the authors write in the paper.
These hydrothermal reactions could be a source of energy for life. On Earth, bacteria at hydrothermal vents use carbon dioxide and hydrogen to create methane and water in a process called methanogenesis. This provides them with the energy to power other reactions in their cells with no need for energy from sunlight.
"The presence of molecular hydrogen in the plume of Enceladus could therefore suggest the occurrence of temperatures and chemical energy sources necessary for habitable conditions in the moon's interior," they write.
This is a significant breakthrough in the search for life beyond Earth, writes Jeffrey Seewald of Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, in a perspective piece in the same issue of the journal.
"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 fuelling life has, to this point, remained elusive," Seewald writes.
However, there are several other features that support life on Earth that Enceladus lacks. For example, methanogenesis at hydrothermal vents on Earth requires iron-rich rocks and heat supplied by magma through tectonic activity.
"Future missions to explore oceans beyond Earth will answer many of these questions and further constrain the possibility of life elsewhere in our solar system," he writes.
The Cassini spacecraft is approaching the end of its lifespan. Earlier in April it began its final series of plunges through the gap between the planet and its rings.
On 15 September, the probe is scheduled to begin its final dive into the atmosphere of Saturn itself. It will transmit its observations back to Earth for as long as the signal carries.