Most people have run into a beautiful photo of the extravagant rings of Saturn, large and visually pleasing. Saturn’s rings are in fact one of the only ways astronomers can get the general public to even show up on public observatory nights. However, many don’t know that it is not just Saturn but all of the Gas Giant planets that have rings, only much thinner and less visible. Where did these rings come from? Essentially, they are just debris, either from the formation of that planet’s moons or from collisions in which material from the moons got thrown out. That is why scientists pretty much always find a moon in each ring’s orbit. Some have interesting stories behind them.
Jupiter – It wasn’t until 1979, by a spacecraft specifically designed to find planetary rings (Voyager 1), that the discovery of a ring system was made around the largest planet in our solar system. Like many planetary rings, Jupiter’s is mostly dust. Voyager 1 brought back information that the largest of the ring’s particles are dark, rough, and red, matching Jupiter’s moons. However, because of Jupiter’s extremely strong magnetosphere causing volatile actions and collisions, its dust particles are estimated to have a lifespan of only 100 to 1,000 years. The ring system is defined to have three components – the Halo Ring followed by the Main Ring and then the Gossamer Ring, which is actually made up of two parts itself. The Halo and Main rings are found in the orbits of the moons Adrastea and Metis, closest to the planet, and so are thought to have formed from high-velocity impacts that these moons have undergone. Images from a passing spacecraft reveal that the Halo Ring is blue in color and contains very fine particles, while the rest of the rings are reddish under visible light. The first Gossamer ring lies interior to the moon Amalthea’s orbit while the second one lies interior to the next moon out, Thebe. Compared to the thin, narrow Main Ring (4,000 miles wide) and only a slightly larger Halo Ring, the Gossamer Rings stretch an impressive 60,000 to 90,000 miles wide, respectively. Unfortunately, the thinness makes it near impossible to catch a glimpse of Jupiter’s rings from an Earth-based telescope.
Saturn – Unlike the dusty rings of Jupiter, Saturn’s rings are made up almost entirely of water ice. Even more, the size of the particles is in such a wide range that some are as small as pebbles while others are as gigantic as mountains! Because of this icy characteristic, Saturn’s rings are thought to have formed from pieces of comets, as well as asteroids and/or moons that fell apart but didn’t crash all the way down to the planet. The entire width of the rings is 175,000 miles, almost the entire distance from the Earth to our Moon! One of the most notable feature of this enormous ring system is the 3,000-mile-wide Cassini Division, the bigger of two gaps in the ring system. These gaps exist because Saturn’s moons Encke and Keeler are currently residing in those orbits, clearing the space around them. The rings themselves, however, don’t have remarkable names; they are simply named A-F in the order of which they were found.
Uranus – Although they weren’t discovered until 1977, the rings of Uranus have a unique feature; they don’t begin until almost 24,000 miles off the center of the planet and are only about 37,000 miles wide. The whole of it is comprised of 13 currently known rings and is close to Saturn’s rings in complexity except that the rings of Uranus are extremely dark and faint. That is why astronomers believe that, unlike Saturn, the rings of Uranus are probably not composed of water ice. The fact that the rings are made up of large boulders instead of small dust particles (which reflect light better) is also an indication as to why they are so dark. Like the strange planet itself, which lies on its side instead of right-side-up (according to the position of its axis), its rings are also not standard – they are more elliptical than circular and don’t even match up entirely to the plane of the planet’s equator. However, a more notable feature is its brightest ring, the Epsilon Ring, which is believed to have formed from two nearby moons, Ophelia and Cordelia. This is actually the only ring of the entire ring system to have a corresponding moon. Unlike Jupiter and Saturn, the rest of Uranus’s rings don’t have a nearby moon from which they could have been formed. That is why this ring system is not yet fully understood.
Neptune – The farthest official planet out is in turn the one with the smallest of all four ring systems in our solar system, a measly 20,000 miles wide at best. It was first caught on camera fairly recently in astronomical history, in 1989, when the spacecraft Voyager 2 passed by for a look and noticed five rings – Galle, Le Verrier, Lassell, Arago, and Adams (in order of increasing distance from the planet). Although dark, the rings are made up of mainly dust. The outermost ring, the Adams Ring, is famous for its curious arcs (named Liberty, Equality, and Fraternity), where the material from the ring clumps together in one area, apparently defying a law of physics that would require the material in those arcs to be spread evenly into a ring. Astronomers are perplexed by this and so have to succumb to only knowing that the moon Galatea is the most probable puppeteer of these arcs.
And there we have it – four very different ring systems in our little solar system. From Saturn’s complexity to Neptune’s mystery, the rings of planets will continue to provide astronomers with an adventurous journey.