The Rings of Saturn

Of all the wonders of the solar system, none are quite as amazing as the rings of Saturn. Saturn, the second largest planet in our solar system, is not the only world with rings - Jupiter, Uranus and Neptune have some too - but Saturn's are by far the most visible and spectacular.

Discovery

Though man had watched the sky for thousands of years, it wasn't until 1610 A.D. that Italian scientist Galileo Galilei first noticed Saturn's rings while using a new invention called the telescope. At first Galileo wasn't sure of what he was seeing and thought that Saturn wasn't just a single object but one with two smaller objects on either side. The "planet Saturn is not alone, but is composed of three, which almost touch one another and never move nor change with respect to one another," he wrote. Later he described the planet as having "ears". Even more puzzling was when he looked at it again in 1612; the "ears" had disappeared only to reappear again to him in 1613.

It took another scientist, Christiaan Huygens, using a more powerful telescope in 1655 to figure out that what Galileo had seen were not two objects on either side of the planet, but a ring surrounding it. Why had it disappeared in 1612? As Saturn orbits the sun, its angle changes when observed from earth. During 1612 the rings, which are very narrow, were facing Earth edge on, and disappeared from sight. They reappeared in 1613 as the angle of the planet continued to change when it moved further along in its orbit. Today we know that we can expect the rings of Saturn to vanish from view as they turn edge on about every 14 years.

In 1675 astronomer Giovanni Domenico Cassini noticed that the ring wasn't solid, but composed of multiple rings with gaps. The largest of these separations was named the Cassini gap in his honor. James Clerk Maxwell calculated in 1859 that the rings could not be solid objects because the gravitational forces imposed on them would rip them apart. He speculated they were actually composed of millions of small particles, all in their own orbit around Saturn. This theory was confirmed in 1895 by spectroscopic studies of the rings through the telescope at the Allegheny Observatory which is part of the University of Pittsburgh.

Size and Composition

Observations done with modern telescopes and space probes show that there are at least a dozen concentric rings circling Saturn. Most of them are designated by letters in the order in which they were discovered (For example, the Cassini gap is bordered by the A and B rings). The portion of the ring system easily visible from Earth's telescopes starts at the D ring about 41,570 miles (66,900 km) from Saturn's center and runs to the F ring at 87,104 miles (140,180km). This is a distance of 45,534 miles (73,280km) or about 5½ times the diameter of the earth. Diffuse and invisible parts of the rings composed mainly of dust, however, can be detected as far away as 8 million miles (13,000,000km) from the planet's center.

While the visible part of the rings has a diameter of 174,208 miles (280360 km) which is about ¾ the distance between the earth and the moon, the width of the rings only ranges between 30 feet (10m) and 3,200 feet (1km). Despite the huge area of the rings, they are not very dense and the amount of actual material they are composed of is relatively small. If it was all gathered together in one place it is estimated it might be the volume of one of Saturn's medium-sized moons like Mimas, which has a diameter of 246 miles (396km).

Analysis of the rings show that they are about 99.9 percent composed of ice with just a trace of minerals. The particles that make up the rings range from the size of a pebble to the size of a small house. Photos from space probes have shown that within the rings themselves complex patterns can appear that make it look like a ring is braided. The rings and patterns in the rings are probably caused by the gravitational interaction of Saturn and its many moons. Several tiny "Shepherd moons" orbit within the rings creating some of the gaps. For example, the F ring is maintained by the moons Prometheus on its inner edge and Pandora on its outer edge.

Origin

There are several theories about how the rings came to be. In the 19th century French astronomer and mathematician Édouard Roche proposed that the rings were the remains of a large moon of Saturn's that had been ripped apart. Using math, he calculated how close a moon could get to its mother planet before the planet's gravitational tidal forces would tear it to pieces. This rule, now known as the Roche limit, can be calculated for any celestial object. The rings fall within Saturn's Roche limit.

Roche's idea that an existing moon had been drawn close to Saturn, then been broken up was an early theory, but not the only one. Another theory is that the rings are composed of matter left over from original material from which Saturn formed. Material outside the Roche limit was eventually drawn together under its own gravity into clumps that formed Saturn's moons, while material inside the limit spread out to make rings. Another variation of Roche's theory is that an existing moon was struck by a large comet or asteroid, which caused it to break apart into small pieces and form the rings.

Until the 1980's everything astronomers knew about the rings was learned from Earth-based telescopes. During that decade, however, two space probes, Voyager 1 and 2, swept by Saturn and took a number of high resolution photos of the rings. More recently, in 2005, the Cassini-Huygens probe (named after two 17th century astronomers) actually passed though the rings at the gap between the F and G rings. It continues to operate in the vicinity of Saturn, returning photos and other observations about Saturn, its moons, and rings to scientists on Earth.

Scientists aren't exactly sure how long the rings have been circling Saturn. One recent estimate, however, based on data from the Cassini probe, puts their age at around 4 billion years, about the same age as the rest of the solar system.

Although we don't know exactly how they formed or how old they are, one thing about the rings is certain. They are of surpassing beauty and a wonder of our solar system.

Copyright 2013 Lee Krystek. All Rights Reserved.