http://www.planetary.org/blog/article/00002084/ wrote:
Happy Saturn ring plane crossing day! By Emily Lakdawalla | Sep. 4, 2009
<<During the Saturn ring plane crossing of 1995, astronomers discovered that Prometheus, one of the two tiny moons that shepherd Saturn's F ring, was a full thirty minutes behind where it was predicted to be, twenty degrees off from its expected orbital position. That's the sort of puzzling observation that really gets astronomers thinking. Eventually they figured out that Prometheus is in a chaotic dance with Pandora, the other shepherd of the F ring; the two moons perturb each other's orbits, slowing each other down and speeding each other up.
Ring plane crossings are also excellent times for Earth-based astronomers to study Saturn's fainter rings. In fact, it was during the crossing of 1966 that astronomers discovered the E ring. The E ring has very little in common with the dazzlingly bright A and B rings that are visible in your backyard telescope. The E ring is incredibly faint, but it's also vertically thick, extending thousands of kilometers above and below the ring plane. And it stretches from the edge of the rest of the ring system for hundreds of thousands of kilometers, including the orbits of the moons Mimas, Enceladus, Tethys, Dione, and Rhea. Astronomers found that the E ring was brightest at the orbital position of Enceladus, giving the first clue that tiny Enceladus might be geologically active. We now know that Enceladus has geysers that spew tiny ice crystals into Saturn orbit, and is responsible for creating the vast E ring.
Astronomers have used ring plane crossings to try to determine just how thick the rings are. But it was surprisingly difficult to settle on that number for Saturn. Most estimates came in at about a kilometer thick. Recently, the Cassini orbiter has measured the thickness of Saturn's main A and B rings and found they are much thinner than that, only a few tens of meters thick. Why don't Earth-based astronomers get the same answer? The problem is the F ring, the thin, braided structure that orbits just outside the main A and B rings. It's ordinarily faint and difficult to spot from Earth. But near an equinox, when the Sun strikes the rings from the side, the sparse, dusty particles within the F ring scatter light in all directions, with a goodly fraction of that light bouncing back to observers on Earth. When astronomers tried to measure the thickness of Saturn's rings, they were really measuring the thickness of the F ring.
Some ring plane crossing events are better than others. Sometimes, Earth crosses the ring plane just once, but sometimes there are actually three ring plane crossings in a row, spaced about three months apart. When Earth crosses the ring plane only once, Saturn is usually on the opposite side of the Sun from us and very difficult to observe. But triple crossings happen near opposition, when Saturn is very high in our sky and easy to see.
Ring plane crossings give us a chance to see something that's almost never visible from Earth: the dark side of the rings. Earth is usually on the sunlit side of the rings, since Earth is always within 6 degrees of the Sun as seen from Saturn. This year, when the equinox happened on August eleven, the Sun went to the other side of the rings, so from Earth we were looking upon their unlit face. Today, we pass to the sunlit side again, where we'll be for the next sixteen years. But we can't really enjoy the view very much because this year is one of the duds, a single crossing where Saturn is lost in the Sun's glare.
1995 was a different story. Earth crossed the ring plane in May and spent the next three months on the shadowed side of Saturn's rings. The dense A and B rings were particularly dark, making it much easier to study the smaller moons and the faint C, D, E, and F rings. In August Earth passed back through the plane to the sunlit side. But then Saturn's equinox happened in November, bringing darkness to the rings again for another three months, until Earth passed through the ring plane for the third and final time.
Ring plane crossing events are rare because they depend upon the stately pace of the giant planets' orbits around the Sun, and happen only twice each orbit.
Jupiter's are the most frequent because it has the shortest year, only twelve Earth years long;
the most recent [Jupiter] ring plane crossing was just two months ago.
Saturn ring plane crossings happen every fourteen to seventeen Earth years, but the next good triple crossing isn't until 2038.
Uranus ring plane crossings happen only every forty-two years;
the last one was two years ago, when Hubble observations showed that the ring system looked radically different from its appearance to Voyager 2 back in 1986.
Finally, there's distant Neptune, which takes a 165 Earth years to travel around the Sun. The last time we had a chance to observe a Neptune ring plane crossing from Earth, it was 1964, and the charged-coupled device that forms the basis of most modern digital cameras hadn't yet been invented. Our next chance is still another forty years away.>>