Saturn sits high in the south right now. The best time to look is around 11 p.m. when the planet is highest in the sky and less affected by the turbulent motion of Earth’s atmosphere. Look for a pale yellow point of light much brighter than any of the fainter stars that surround it.
If humans ever travel regularly among the planets, the top tourist attraction in the solar system will surely be Saturn’s rings. Even a small department-store telescope reveals them, even though the planet is currently 800 million miles away from Earth.
We can see them from Earth because of their size and reflectivity. The rings are huge — 200,000 miles from side to side. Saturn would just fit in the space between Earth and its moon. The rings are also exceedingly shiny because they are composed of a thin plane of a billion trillion hunks of ice and ice and rock mixed together. Some of the hunks are the size of grains of sand and others as big as icebergs.
The rings are thus not really rings at all but a vast collection of moons orbiting Saturn and arranged in over a thousand concentric bands.
All the gigantic gas planets — Jupiter, Uranus, Neptune, and Saturn — have some sort of ring system, but the others are pale and puny shadows of Saturn’s display. Jupiter’s ring of dusty material is barely there. Uranus has a dozen or so dark and narrow bands. Neptune’s few rings are dense over parts of their arcs and nearly invisible elsewhere.
Astronomers don’t really understand the dynamics of planetary rings very well. Why, for example, do they stay in discreet bands instead of spreading out?
A possible answer is the existence of moons near some of the rings. These so-called “shepherd moons” may provide enough gravity to keep the rings corralled. The trouble is that some of the rings don’t seem to have shepherds — at least we can’t see any. However, our lack of vision is not surprising. The shepherds are so tiny that they are invisible to Earth-based telescopes. They had to be discovered by passing spacecraft, and perhaps even those spacecraft don’t have the resolving power to see the smaller ones.
The question that has plagued astronomers since the discovery of Saturn’s rings still puzzles them today. Why do the gas planets have rings at all? Some of the rings, the ones you can¹t se in a telescope, were formed by ice geysers on some of the moons of Saturn. Others were formed by the constant bombardment of micrometeorites on those moons. These methods form nearly invisible rings around the other gas giant planets. But Saturn is clearly special. Its rings are huge and easily visible. What gives?
One theory suggests that the larger bodies, like planets and moons, formed from smaller particles that clumped together. The rings are composed of those small particles. They could not form into moons because their proximity to the enormous gravity of the nearby gas planet prevented them from coalescing.
Another possibility is that the rings are fragments of icy moons that formed farther away from the gas planets. If these moons strayed too close to their planetary brethren, they would have been torn to pieces, and the debris would have been stretched into ring-like circles around the planets. A third alternative is that the rings were created by collisions of two moons that had already formed. Or perhaps, in an even more likely scenario, a passing asteroid slammed into one of Saturn’s moon. The icy and rocky fragments were then drawn into rings by the gravity of the planet. If such cosmic train wrecks occurred in the distant past, a planet’s gravitational pull would eventually drag most of the fragments into a fiery collision with the planet’s atmosphere. Slowly, the rings would disappear. Planets like Jupiter might have had extensive ring systems billions of years ago, but time has reduced them to the remnants we see today.
Saturn’s impressive rings would be the result of more recent collisions, perhaps as late as 100 million years ago. Given enough time, they will slowly disappear. In a billion years or so, Saturn’s rings may look more like Jupiter’s, and our solar system’s most impressive tourist attraction will fade from view.
Tom Burns is the director of Ohio Wesleyan University’s Perkins Observatory, and he would be very happy to answer your questions or sell you a ticket to one of its upcoming Friday-night programs. He can be reached at email@example.com or 740–363-1257.