The Icy Plumes Shooting Out of Saturn’s Moon Just Got Way More Interesting

Hydrogen in the jets suggests that Earth-like processes may be operating deep in Enceladus’s ice-capped ocean.

Cassini / JPL / NASA

Earlier this week, Sarah Hörst, a planetary scientist and professor at Johns Hopkins University, realized the lecture she planned to give her students on Thursday would be out of date about a half hour after they sat down.

Hörst’s class on the introduction to the solar system was scheduled to learn about Enceladus, an icy moon of Saturn, on the same afternoon NASA was going to hold a big press conference to reveal new findings about it. Cassini, the spacecraft that has been orbiting Saturn for 13 years, has detected for the first time molecular hydrogen in the plumes of water vapor shooting out of Enceladus, scientists announced Thursday. Astronomers say the hydrogen is produced by hydrothermal reactions between hot rock and liquid water under Enceladus’s surface, through similar chemical processes that create bustling ecosystems around seafloor vents on Earth.

“Planetary science isn’t just a bunch of facts that you memorize in a book … we’re constantly learning new things and our understanding changes at the pace where textbooks are instantly out of date,” Hörst said, after she tweaked her lesson plans to account for the news. “Which is really frustrating as a professor, but really exciting as a scientist.”

What’s exciting about the Enceladus news, Hörst said, is that it contributes to a growing body of evidence that suggests that while the moon might look like an inhospitable ice ball, it may harbor under its surface the chemicals and processes capable of sparking and supporting life. In 2005, a young Cassini observed jets of water vapor and ice coming from cracks near the moon’s south pole, signs of a potential underground ocean. Cassini detected traces of carbon dioxide, methane, and nitrogen during the flyby. In 2014, with the help of a network of massive radio antenna back on Earth, Cassini confirmed the existence of a large ocean of liquid water beneath the ice. The heat keeping the ocean liquid, scientists believe, comes from the stretching and pulling Enceladus experiences from Saturn’s gravity, a phenomenon known as tidal heating.

The hydrogen announced Thursday was detected in plumes at the moon’s south pole, during a flyby in 2015. The findings, from astronomers at the Southwest Research Institute, are published in the journal Science. It’s difficult to surmise the composition of an entire ocean world using hints of chemical signatures, but such measurements help scientists better understand what might be going on inside. The researchers say the likeliest source of the hydrogen is chemical reactions between liquid water and the silicate rock sitting at the center of Enceladus. Cassini also picked up carbon dioxide during the 2015 flyby. Together, carbon dioxide and hydrogen are key ingredients for a process called methanogenesis, a process by which microbes, including those near ocean vents, produce methane on Earth.

The Enceladus news is another reminder of the vast trove of information that Cassini has provided scientists about Saturn and its moons. In September, the spacecraft will end its mission by plunging into Saturn’s atmosphere and disintegrating, out of fuel after a 20-year journey from Earth. But researchers will be poring over Cassini data long after the spacecraft is gone, Hörst said, and could find more signs of what’s hiding underneath Enceladus’s icy crust. Hörst hopes humans won’t be away from Enceladus for too long, pointing to a NASA proposal, currently under consideration, for a life-hunting mission to Enceladus that would search the same plumes Cassini did, looking for the gases that give rise to life as we know it.

“We know enough about the ocean that we really need to go back and see if there’s life there,” she said.