The year is 2005, and the Cassini spacecraft is closing in on Saturn's moon Enceladus for the very first time. Both NASA's Voyager probes had visited the distant moon about two decades earlier, and from those observations, NASA was convinced that Enceladus was nothing more than a solid ball of ice.
On February 17, Cassini's magnetometer instrument noticed something rather odd. Enceladus lies within the Saturn's magnetic field, and the magnetometer detected a disturbance in the planet's magnetosphere around Enceladus's south pole. The perturbations in the magnetic field were similar to what astronomers have observed around objects with thin atmospheres. It seemed that something was restoring the thin air around Enceladus—something was moving underneath the ice.
"Enceladus was so exciting that, instead of just three close flybys planned for our four-year primary mission, we added 20 more, including seven that went right through the geysers at the south pole," Linda Spilker, Cassini project scientist at JPL, said in a press release.
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Those twenty flybys proved crucial to developing an understanding the mysterious icy world. In 2006, Cassini was able to prove that liquid water reservoirs were feeding the plumes of vapor that shoot out from the surface, and in 2008, a direct sample of one of the plumes showed a dense "organic brew" described as "carbonated water with an essence of natural gas." Geologic activity, including Earth-like tectonics, was occurring on Enceladus. In 2014, scientists began to understand that it was not just an isolated source of salt water they were dealing with, but an entire ocean. The next year, the science team realized the subterranean ocean enveloped the entire moon.
Cassini is going to plunge into Saturn on September 15, but just because the mission is ending doesn't mean that we can't continue to learn about this enticing world. In fact, many scientists are now arguing that the moon's abundance of briny water and geysers makes it worthy of its own unique craft with tools specifically designed to search for life.
"Half the excitement of doing science is that you sometimes find yourself going in a totally different direction than you expected, which can lead to amazing discoveries," says Spilker. "That little anomaly in Cassini's magnetometer signal was unusual enough that it eventually led us to an ocean world."
Source: JPL
David Grossman is a staff writer for PopularMechanics.com. He's previously written for The Verge, Rolling Stone, The New Republic and several other publications. He's based out of Brooklyn.
