When NASA researchers first realized in 2005 that Saturn's moon Enceladus has liquid water under its surface, they were stunned. "Tremendously exciting," Cassini project scientist Linda Spilker told Popular Mechanics. "I sort of call it jaw-dropping because we were so certain that Enceladus was too small to support activity like this. We expected it to be frozen solid."

Since then, the Cassini spacecraft orbiting Saturn has sent back amazing visuals and data about this small watery moon, now considered one of the best places to look for life beyond Earth. And today, NASA made the next big announcement about this promising place: The Cassini team found evidence of hydrothermal vents on Enceladus's ocean floor, which they document in a new study inScience.

A Moon Gets More Amazing

Saturn's sixth largest moon, Enceladus. NASA/JPL-Caltech/Space Science Institute

These hydrothermal vents are fissures in the rocky core of Enceladus. They release water that has been heated by geothermal activity, and the jets of hot water carry minerals and nutrients that could support life. Some of those compounds were detected by the Cassini spacecraft when it flew through the water geysers that erupt from Enceladus's surface. Microbes, algae, tube worms, crabs, and even fish swim around hydrothermal vents in Earth's oceans. Maybe a similar alien ecosystem exists on the seafloor of Enceladus.

"We know from looking at the hydrothermal vents on the Earth—there's no sunlight there—and yet if you look near a hydrothermal vent that has the heat energy and the nutrients that you need in the liquid water, you find a very thriving community of life," says Spilker. "You find tiny crabs and tube worms and a very thriving community around these hydrothermal vents well away from sunlight. So we wonder, could life have begun in the same way in the Enceladus ocean as well?"

Cutaway view of Enceladus's subsurface ocean, rocky core, and hydrothermal activity. Around the north pole, there are many impact craters from collisions with other objects, but in the south, geologic activity wipes away the craters, and watery geysers erupt from the surface of the moon. NASA/JPL-Caltech/Space Science Institute

Around 2015, after more than a decade of collecting data in the Saturn system, the Cassini team concluded that what they were seeing on Enceladus was not just a liquid water reservoir under the surface ice, but an entire subsurface ocean that surrounds the whole moon. This was determined based on inconsistencies measured in Enceladus's libration, or wobbling in the moon's rotation as it orbits Saturn. Once they knew there was a large ocean and likely a rocky core, the team speculated that if the moon is warm enough for liquid water, then it might have enough geologic activity for hydrothermal vents. And if there are vents, then there could be life—even out in the distant solar system almost a billion miles from the sun.

Now we have reason to believe there are indeed hydrothermal vents on the watery moon Enceladus. "What we have is this chain of evidence, not just one thing but a number of things that point toward the very real possibility of these hydrothermal vents," says Spilker.

Most of that evidence came from the flybys Cassini made through Enceladus's geysers. The craft used two science instruments, the Cosmic Dust Analyzer (CDA) and Ion and Neutral Mass Spectrometer (INMS), to analyze samples from the plumes. Sure enough, the spacecraft picked up clues that point to hydrothermal vents.

Enceladus is only 314 miles (505 km) across, small enough to fit within the length of the United Kingdom. NASA/JPL-Caltech/Space Science Institute

"You have the tiny nano-silica grains detected by the Cosmic Dust Analyzer that had come out with some of the ice grains [in the plumes]," says Spilker. "These nano-silica grains could only form in water that's near the boiling point. So we think that the water goes into the seafloor, into the rocky core of Enceladus, it gets heated up, picks up things like silica, and then as the water comes back out and hits the cold water, those silica grains condense."

On Earth, when the silica grains of a hydrothermal vent condense upon hitting the colder water, we can see it as a white plume of material called a white smoker. Compounds associated with these white smokers, such as an excess of hydrogen and methane, were also detected by Cassini. (Black smokers, which are also common on Earth, get their color from sulfur-rich material.)

The Enceladus Clipper

Plumes of water erupting from Enceladus, captured by the Cassini Spacecraft in April 2013. NASA/JPL-Caltech/Space Science Institute

Cassini is running dry on fuel, and on September 15, 2017, the spacecraft fly into Saturn to burn up in the thick atmosphere. NASA will dispose of the spacecraft this way to protect Saturn's pristine moons like Enceladus and Titan from a potential impact with the dead craft. But even if Cassini could continue studying the Saturn system for longer, we still need a new spacecraft if we want to search for life.

The Cassini team did not initially plan to fly the spacecraft through the geysers of Enceladus—they didn't even know Enceladus had active geysers when the spacecraft launched in October 1997. So the instruments used to analyze the material in the plumes were intended for other purposes. The INMS was primarily to measure Titan's atmosphere, and the CDA was to measure particles in the rings of Saturn itself.

"Here we are getting these free samples out of the geysers of Enceladus," says Spilker. "Cassini has flown through those, but we just don't have the instrumentation to look for things like fatty acids, amino acids—the heavier molecules that might be indicators of life. So I'm working on a proposal that would carry the kinds of instruments that you would need to better characterize the ocean and to start to answer the question: Is there life in Enceladus's ocean?"

Enceladus viewed floating above the rings of Saturn. NASA/JPL-Caltech/Space Science Institute

That proposal will be submitted to NASA's New Frontiers program by the end of the month. If all goes well, Spilker will then have the opportunity to write a step two proposal, and if that's approved, we could see a mission launch specifically to search Enceladus for signs of life, similar to the Europa Clipper mission that will visit the icy moon with a large subsurface ocean that orbits Jupiter.

Assuming everything is approved by NASA, a new Enceladus spacecraft would take five to six years to build, says Spilker, and eight to ten years to fly all the way to Saturn. Best-case scenario would be a launch in the mid-2020s, and perhaps later, which is all the more reason to get started now.

"You could orbit Enceladus or another way is just to have repeated flybys though the plume," says Spilker. "The nice thing about Enceladus is you don't have the harsh radiation environment that Europa Clipper has to face, so you wouldn't have to worry so much about building a vault or something for the electronics and shielding the instruments."

Close up image of Enceladus's north pole taken by the Cassini spacecraft in October 2015. NASA/JPL-Caltech/Space Science Institute

In September of 2016, we called Europa "the best bet for alien life in the solar system," largely based on the fact that the orange streaks on Europa's surface are a likely indication of briny conditions that would be favorable to life. Now that we know Enceladus has not only a salty ocean but also direct evidence of hydrothermal vents and large geysers that provide easy access to samples from the moon's seafloor, it may be time to transfer the title to Saturn's small, watery moon.

Spilker, who worked on the Voyager missions before spending three decades on Cassini, cannot wait to get back to Enceladus.

"For me personally, to go back, follow up on Cassini's discoveries, and perhaps answer the question: Is there life on Enceladus? The implications are profound."

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