By Paul VoosenApr. 13, 2017 , 2:00 PM
In 2005, NASA’s Cassini spacecraft spied jets of water ice and vapor erupting into space from fissures on Enceladus, evidence of a salty ocean beneath the saturnian moon’s placid icy surface. Now, it turns out that the jets contain hydrogen gas, a sign of ongoing reactions on the floor of that alien sea. Because such chemistry provides energy for microbial life on Earth, the discovery makes Enceladus the top candidate for hosting life elsewhere in the solar system—besting even Jupiter’s Europa, another icy moon with an ocean. “We didn’t see microbes,” says Hunter Waite, a planetary scientist at the Southwest Research Institute in San Antonio, Texas, and the lead author of a study published this week in Science. “But we saw their food.”
In previous passes through the plumes, the Cassini team found organic molecules like methane and formaldehyde. A final pass in October 2015, just 48 kilometers above the surface, revealed something even more tantalizing: abundant hydrogen gas (H2). The authors argue that it comes from hydrothermal activity on the ocean floor, perhaps seafloor vents like those on Earth that spew H2 and support rich microbial life. It could be produced by water reacting with iron, or similar reactions baking out primitive organic molecules trapped in the rocky core.
Waite and his colleagues were previously blind to hydrogen in the plumes because of surges of it created in their own instrument when water reacted with its titanium components. All the potential biases seem accounted for now, says Gabriel Tobie, a planetary scientist at the University of Nantes in France who was unaffiliated with the study. The high volume of hydrogen is a puzzle, as are spikes in its abundance, but it is not the only evidence of seafloor activity on Enceladus. In 2015, Cassini scientists studied particles from Saturn’s E ring, known to be fed by Enceladus’s plumes, and found tiny glass beads that could have only come from hydrothermal reactions. “These converging pieces of evidence tell us that something special is happening inside Enceladus,” Tobie says.
Currently, NASA is pinning its hopes for life on Europa, with the $2 billion Europa Clipper planned for launch in the next decade. The Hubble telescope has seen signs of a plume on Europa erupting twice from the same spot, according to a study in this week’s issue of The Astrophysical Journal. But the signal is contested, and there’s no evidence yet of hydrothermal activity.
“I’m not trying to second guess targets at all,” says Waite, who leads an instrument on the Clipper. But he thinks Enceladus is just begging to be sampled with a mission designed for the task.