The planet Saturn and its remarkable rings will be visible in Michigan’s southern sky after dark for the next three months, according to astronomer Richard Teske.
“Saturn provides a thrilling view when seen through a small telescope,” Teske says. “The smooth, cream-colored shapes of planet and rings seem toylike and unreal against the surrounding darkness of deep space. You can easily see the flattening of the planet caused by its rapid rotation, the different brightnesses of its rings, and the dark gaps between them.”
Everything about Saturn and its rings is huge, according to Teske. The planet is almost 10 times farther across than Earth and its ring-span equals 22 Earths. The planet weighs in at almost one hundred times Earth’s mass. Despite its great size, Saturn rotates at 23,000 miles per hour and its day is only a little longer than 10 Earth hours.
Saturn’s rings are made of small chunks of icy material that circle the planet in independent gravitational orbits. “Each piece is actually a small moon of Saturn,” Teske explains. “They all circle the planet in the direction of its rotation, just as our own moon orbits Earth in the direction of its west-to-east rotation.”
The moon fragments farthest out from the planet take a long time to go once around it, while the pieces nearest to Saturn make the trip in less than 10 hours, Teske says. About halfway out, the ring pieces orbit Saturn with exactly the same period as the length of its day.
“An astronaut watching the rings from Saturn would see the outer ring fragments rise in the east, cross the sky, and set in the west, like our own moon does,” Teske said. “But the inner fragments, moving faster than the planet’s rotation, would seem to rise in the west and set in the east. At a place in between, the little moonlets would cruise along with the rotating planet and hang motionless in Saturn’s sky.”
Astronomers learn about the sizes and compositions of ring fragments by observing how the rings transmit and reflect light and radio waves of different wavelengths. Stars can be seen through the rings, and spacecraft passing Saturn have sent radio signals back to Earth right through them. Study of the transmitted radiation gives important information to astronomers. Radar waves from Earth have been reflected off the ring material to add to knowledge gained from examination of reflected sunlight.
“From these observations, scientists have learned that most of the pieces are about the size of marbles and ping-pong balls. A few may be the size of watermelons, and an occasional one the size of an automobile,” Teske says. “This kind of size distribution is a result of many collisions among the ring fragments; they are actually broken-up pieces of one another. The chunks have been grinding away against their neighbors for a very long time, but where they came from in the first place isn’t known. Guesses range from a broken-up moon to a comet that smashed into a moon to debris left over from Saturn’s formation.”
It only took a small amount of material to make Saturn’s rings, according to Teske. Scientists estimate the total amount is less than 1/2,000th of Earth’s moon, despite the huge ring diameter of 22 Earths. “We could provide the same amount of material by scraping off the top one-mile of the surface of the United States,” he says.
Even though Saturn’s rings don’t contain much material, they reflect sunlight strongly and they are bright. Their high reflectivity is due to their composition of mostly water ice, with some flecks of sand-like material containing sulfur or iron.
“At Saturn’s distance from the sun of almost a billion miles, sunlight is weak and the climate very cold,” Teske notes. “Laboratory experiments demonstrate that at the low temperatures prevailing there, ice is very stable and can exist for billions of years without evaporating.”
