First up on my top 10 favorite astronomical objects list, the ringed giant. Crown jewel of the backyard astronomer, Saturn’s rings were visible through the first telescope ever turned to the night sky and are often the target of choice when introducing new folks to small telescopes. One guy at an observing night I was helping with was so stunned by the view though a 16” scope that he was initially inclined to think we had played a trick on him with a sticker on the eyepiece!
Of course the best views these days are right here at your computer, beamed back by the Cassini probe from a minimum of 1.2 billion kilometers away. There is perhaps no more jaw-droppingly awesome image than the one above of Saturn perfectly backlit by the Sun in 2006. The rings lit up so spectacularly that whole new ones were discovered for the first time. And if that wasn’t enough, take a closer look at the little dot on the left, just above the bright main rings. That’s here. That’s home. Zoom in and you can even make out the Moon.
It’s their composition that makes the rings so dazzling – 99.9% good ol’ H2O. Despite being only about 50 billionths of Saturn’s mass, the ice crystals in the rings are spread out over such a massive surface area that they reflect a tremendous amount of sunlight. Alternating light and dark rings create the illusion of gaps, but there are only a few true breaks in the disk. The animation in the upper left shows how a uniform disk of material might develop into a “corrugated” pattern if it becomes a bit titled relative to Saturn’s equatorial plane.
For all that we’ve learned about Saturn’s rings, much still remains unknown. One of the most tantalizing of these open questions is the origin of the mysterious “spokes” in the B-ring, the largest in brightest of the bunch. The spokes were first noticed during the Voyager flyby in 1980 and have thrown scientists for a loop ever since. It was 25 years before Cassini arrived on the scene and observed them again, revealing the spokes to be a seasonal effect that peaks near an equinox and fades by the solstices. The leading idea so far is that they consist of microscopic dust particles suspended away from the main rings by electrostatic repulsion, the same effect that causes hair to stand up when rubbed with a balloon or under the influence of a Vandergraph generator.
While it’s true that the rings may often overshadow the ringed, there are plenty of wonders to behold on Saturn proper. The gas giant is composed of about 96% hydrogen, 3% helium, and a smattering of trace molecules (with a probable rocky core), which leads to a number of interesting atmospheric dynamics. One of the most interesting is the unusual hexagon that rotates around the north pole with a period of about 10.5 hours. Polygons don’t often crop up in astronomical images, or nature in general, so astronomers were more than a bit surprised when Voyager first sent back images of one with six straight sides, each with length greater than the diameter of Earth. Most researchers believe the hexagon to be a standing-wave pattern, and a recent laboratory experiment producing a similar shape using spinning buckets of fluid seems to corroborate this idea.
Saturn is also host to a fascinating array of over 60 moons, most of which are captured asteroids that ventured too close to the giant planet and never escaped. Two of the larger moons, Enceladus and Titan, are of particular interest. The former is notable because it is one of very few objects in the solar system where active eruptions have been observed. But instead of magma, Enceladus’ “ice volcanoes” spew water and other volatiles such as ammonia and methane, possible evidence for subsurface oceans. This, along with complex hydrocarbons identified in the plumes, has led some scientists to point to Enceladus as one of the best places to look for life elsewhere in the solar system.
But as fascinating as Enceladus is, the largest of Saturn’s moons, Titan, steals the show. Larger in volume than the planet Mercury, Titan is the only moon with an appreciable atmosphere and the only other other object in the solar system besides Earth where stable bodies of liquid are known to exist on the surface. Many researchers believe the atmosphere of Titan to be similar to that of the early Earth but stuck in a deep freeze because of its distance from the Sun. There have been a number of experiments that have used spectroscopic data to cook up their own Titan atmospheres in the lab in attempt to see what types of interesting chemistry may develop. These experiments have revealed what I think is one of the most amazing discoveries ever made: When UV radiation (like from the Sun) is applied, the resulting reactions include the formation of nucleotide bases and amino acids – the essential building blocks of life. I’m sure your imagination can take it from there.