Some of Cassini’s best discoveries were serendipitous. What Cassini found at Saturn prompted scientists to rethink their understanding of the solar system.
You can only get to know a planet so well with remote and sporadic observations. To truly understand the dynamics of a place as complicated and interesting as Saturn, you have to go there and stay to explore.
Towering jets of ice and water vapor pouring out of a moon as tiny as Enceladus were a huge surprise (explaining why Voyager flybys in the early 1980s saw that the moon had a young surface), as was the later finding that the moon has an ocean under its icy crust. Scientists also had not expected to find Saturn’s magnetosphere -- the region around the planet strongly influenced by Saturn’s magnetic field -- to be filled with an electrically excited gas, or plasma, of oxygen. It turned out this was another surprise from Enceladus, as the water vapor from its plume is broken apart by sunlight and the liberated oxygen spreads out through Saturn’s magnetic bubble. Cassini detected this oxygen on approach to Saturn, but its origin was perplexing at first.
No one knew for sure what kind of environment ESA’s Huygens probe would find when it came to rest on Titan’s surface, so Huygens was built either to land on hard ground or float, if need be. Cassini later showed scientists that most of the moon’s lakes and seas were near the north pole, and most of the moon’s landscape was more like the Arizona desert. Cassini also observed a surprisingly rich variety of complex, organic chemicals forming in Titan’s atmosphere.
Another unexpected finding -- which endures as a mystery -- is the irregularity of Saturn’s day (how long the planet takes to make one rotation on its axis). At Jupiter, a beacon-like burst of radio waves known as “kilometric radiation” beams out with clock-like regularity once a day. But Saturn’s kilometric radiation isn’t consistent. It’s somewhere between 10.6 and 10.8 hours. That might not seem like a big discrepancy, but for such a fundamental property as the planet’s rotation period, it’s frustratingly imprecise for scientists. They hope to settle the score by the time the mission ends by flying Cassini close enough to the planet to tease out the true answer from the magnetic field.