One of the first things that Bonnie Buratti noticed about Saturn's moon Enceladus in the early 1980s was that it looked as bright as freshly fallen snow, rare for a moon orbiting a distant planet.
The images from the Voyager 2 space probe were the first closeups of the mysterious moon that had been discovered 1789 but had long been just a blip of light seen through telescopes.
"It was quite flabbergasting,"said Buratti, a senior research scientist at NASA's JPL.
"Part of the moon was heavily cratered. I mean it's old, you know, it's been subjected to all these meteoroids for so many centuries. ... But then another part of it was perfectly smooth, meaning there was something that happened there, some active geologic process."
Voyager 2 had been launched by NASA in an effort to probe the deepest reaches of our solar system. It flew by Saturn and its moon in 1981.
"We knew Enceladus was weird," Buratti said. "We knew that something good was happening there. We didn't know what."
Buratti, who was getting her Ph.D. at the time, theorized that Enceladus was as bright as it was because it was covered by some kind of crystalized ice, or basically snow. But that only led to more questions. Scientists wanted to know where the snow was coming from, how often it was falling on the planet and how, if after a few days snow gets dirty on Earth, it managed to stay so pristine on the moon.
In an effort to answer these questions and others, NASA launched the Cassini spacecraft in 1997 with Enceladus as a major focus. In 2005, it had arrived and started sending back pictures that were clearer and more detailed than those from the 1980s.
"It's this winter wonder-world," Buratti said. "If you were standing there, you could ski on it."
Scientists could see cracks in the surface, craters on the north side and a smooth snowy terrain on the south. When the infrared team measured the temperature on the moon, they came across a boiling cauldron on the south pole. And out of the cracks they saw giant plumes of liquid shooting into space.
"We had never seen anything like it before, but it did answer our questions about why Enceladus is covered with this freshly fallen snow," Buratti said.
What the researchers found surprised them. They knew that Enceladus was enveloped by an icy crust, but below that crust, it turns out there was also an ocean made of water — which was being boiled by the heat source that they'd detected. The water was being pushed up through those cracks, and shot 50 miles into space at supersonic speeds. All told, 101 massive geysers were discovered.
It's so cold that as soon as the boiling hot water hit space, it froze immediately. Some of the ice ended up in Saturn's largest feature, the E Ring. But a lot of it ended up falling back onto the moon as snow.
A major part of the puzzle was solved.
When Cassini flew through the plumes of ice, it analyzed what was in them. Scientists learned that the ocean was actually salt water, like on Earth — an indication that the sea is in contact with a rocky mantle or crust. Cassini also identified molecular hydrogen, methane and ethane in the water, all of which are potential sources of food for life.
Deep below the icy crust of Enceladus there could be hydrothermal vents. Life has survived here on Earth in such extreme conditions, meaning it's possible that it could on the far off moon, given the right conditions. However, so far, there's no evidence of life there. If NASA decides to return, the search for life on Enceladus will be a top concern.
Cassini visited Enceladus quite a few times during its 13 years at Saturn, but on Sept. 15, the spacecraft will disintegrate into the planet's atmosphere, once again bringing the veil down on the Saturn system.
"It's going to be sad to see the spacecraft die," said Buratti. "Just like people die, we feel this is a living entity. It's there. We almost feel it's an extension of our own senses, our eyes and ears. And we feel that when it finally crashes into Saturn it's like losing a close colleague."
But Cassini isn't leaving scientists empty handed. As it does increasingly risky maneuvers before plummeting to its death, it'll send back data that'll keep them occupied for years.