Saturn's thick rings, which make it so unique in the Solar System, are not permanent. According to data first gathered from Earth-based observations, a phenomenon called “ring rain” pulls material from Saturn's rings into the planet. Subsequent data from the Cassini spacecraft shows even more inflow going from the rings to the planet. Combined these mean Saturn's rings could be gone in as little as 100 million years: not long in terms of the Solar System.
Ring rain is caused by Saturn's gravitational pull on its rings. One theory of why this is that relatively recently a comet may have destabilized the rings, making them more vulnerable to being pulled in. Another is that the innermost ring, called the D ring, pulls in material from the outer rings, allowing it to exist for so long despite the rapid rate of its depletion from ring rain. While ring rain was originally thought to be made of mostly water, like the rings are themselves, data from the Cassini spacecraft puzzlingly shows that water makes up only 24 percent of the falling rain. Other molecules in the rain include organic material, methane, carbon monoxide, and dinitrogen. A lot about ring rain is not yet certain.
The observations which were used to measure ring rain were looking at the specific form of hydrogen found in the phenomenon. This type of hydrogen glows in infrared light, which is what made it possible to observe from the ground. These observations, taken in 2011 in Hawaii, were later extremely useful in tracking ring rain. The results showed between 925 and 6,000 pounds of ring rain going into the planet every second. This rate lead researchers to approximate that the rings would last another 300 million years; when considering the second type of inflow seen from Cassini, that number appears to be closer to 100 million years.
Because of the large variability in the observed rate of ring rain, any approximations for how long the rings have left are far from exact. Still, one thing is certain: Saturn's rings are disappearing because of the planet's own gravity and will one day be gone entirely.
[Sources: Space.com; ScienceNews]