The findings are thanks to NASA's Cassini spacecraft — which has been exploring the Saturn system since 2004. Today's results, published in the journal Science, add to a growing body of evidence that points to the moon's habitability. The discovery of an ocean below Enceladus' surface immediately raised the moon's profile in the early 2000s, since liquid water is crucial for life here on Earth. That excitement has been bolstered by the plumes of water the planet occasionally spits out: Cassini has found even more enticing ingredients inside those plumes, such as organic chemicals like methane — a molecule that is often associated with biological life. In fact, four of the six most important elements of Earth life — carbon, hydrogen, nitrogen, and oxygen — have been found at Enceladus; only phosphorous and sulfur haven't been seen there.
Four of the six most important elements of Earth life — carbon, hydrogen, nitrogen, and oxygen — have been found at Enceladus.
To figure out if molecular hydrogen was present on Enceladus, Cassini did a deep plunge into one of the plumes near the south pole in October 2015. It was just one of many times the spacecraft has flown through an Enceladus plume, but this flyby brought the spacecraft within 30 miles of the south pole. That's the closest Cassini has ever been to moon's surface, and the closest it will ever get.
During the dive, Cassini analyzed the plume using an instrument designed to figure out what certain particles are made of, called a mass spectrometer. Scientists had to switch the mass spectrometer to a different mode, too, in order to make sure the hydrogen they found wasn't coming from Cassini. In one mode, the plume particles can bang up against the walls of the instrument's chamber, causing chemical reactions that wind up producing hydrogen. So the scientists switched the instrument to "open source" mode — a way for the instrument to analyze the plume particles without them touching the chamber walls. "It's been quite a while [since the flyby]; you can count the months," Waite says. "But we were carefully analyzing and making sure that the hydrogen we saw wasn't coming from the instrument."
Once that matter was settled, Waite and his team got to work on figuring out where the molecules were coming from. In the end, they settled on hydrothermal activity as the most likely culprit for the hydrogen measurements. On Earth, most ocean vents don't produce that much hydrogen, though experts think they were much more common in our planet's early history. But they're not entirely gone — the Lost City in the mid-Atlantic still produces plenty of hydrogen, marking it as one of the weirdest parts of the ocean, which is already a very weird place. The sites that are rich in oxygen are known to create some of the most complex organic molecules — even when there is no life present, according to Chris German, a hydrothermal vent expert at Woods Hole Oceanographic Institution in Massachusetts who was not an author on the study.
There's a wide gap between "habitable" and "inhabited.
Though this discovery increases Enceladus' chance of being habitable, there's a wide gap between "habitable" and "inhabited." The fact that Cassini found a lot of hydrogen in its sniff test may be a strike against the possibility of inhabitants: if lots of microbes were feeding on hydrogen, there shouldn't be as much of it coming out of the plumes. "The immediate question it raises for me is how come there's all this free lunch laying around and no one's eating it?" says German.
The simple answer is that life isn't there. Waite offers another solution: maybe microbes are there, but there aren't that many of them. Perhaps they need some other kind of element that's missing from Enceladus in order to really thrive.
So life remains an open question, but one thing's for sure: the findings from Cassini are perhaps the most significant ones ever made by the spacecraft. And they came at a time when the vehicle is about to meet its end. On April 22nd, Cassini will start on its grand finale, the final few orbits it will do around Saturn. These orbits will take the spacecraft in between Saturn and its rings — the closest any vehicle has ever been to the planet. After completing 22 of these orbits, Cassini will then dive into Saturn where it will heat up and break apart. Cassini's destruction has a noble aim: its Saturn dive means there's no possibility the spacecraft will ever drift to Enceladus or Saturn's other watery moon Titan and contaminate them with Earth life.
The spacecraft's scientists argue that these latest findings mean we have to go back to Enceladus. And we need a spacecraft specifically geared to look for signs of life, preferably in one of the big cracks on the moon's surface. "Design a lander that could land in one of those tiger stripes at the south pole and sample that ice as it's coming out of those cracks," Scott Edgington, the deputy project scientist for Cassini, tells The Verge. "I would love to go there myself and drill through and see what that ocean looks like."
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