OBSERVING
saturn
OBSERVING
saturn
CONTENTS discovery
. . . . . . . . . . . . . . .1
the planet
. . . . . . . . . . . . . . .
5
the rings
. . . . . . . . . . . . . . .
11
many moons
finding saturn
. . . . . . . . . . . . . . . 17 . . . . . . . . . . . . . . .
21
DISCOVERING SATURN
Saturn was thought to be a star for many years. A few years after Huygens’ discoveries, an Italian–French astronomer by the name of Jean-Dominique Cassini discovered four other major moons of Saturn: Iapetus, Rhea, Tethys, and Dione. In 1675, Cassini discovered a narrow gap that splits Saturn’s ring system into two parts, and the gap has since been known as the “Cassini Division.” Because of his numerous contributions to our knowledge about the planet Saturn, Cassini was chosen as the name of the spacecraft flying to Saturn.
Galileo Galilei was the first to observe Saturn with a telescope in 1610. Because of the crudeness of his telescope, he couldn’t determine what the rings were. He incorrectly guessed that there were two large moons on either side of Saturn. Two years later when he viewed Saturn again, the “moons” had disappeared. We know now this is because Galileo was viewing the rings edge– on so that they were invisible, but at the time it was very confusing to him. After another two years, Galileo viewed Saturn again and found that the “moons” had returned. He concluded that the rings were “arms” of some sort.
Other major discoveries came later. During the 19th century, J. E. Keeler showed that the ring system is not a uniform sheet but actually comprised of small particles. Most recently, the Voyager spacecrafts (visiting in 1980–1981) made discoveries about the composition and interaction of the rings.
Many years later, in 1659, a Dutch astronomer named Christiaan Huygens solved the mystery of Saturn’s “arms.” Because of improved telescope optics, he correctly deduced that the “arms” were actually a ring system. Huygens also discovered Saturn’s moon, Titan, and for this reason, the probe exploring Titan is named after him.
2
Above: Cassini approaches Saturn, carrying a passenger: the European Huygens probe, the first man–made object to land on a world in the distant outer solar system.
made its final approach to the giant planet Saturn. But this encounter was like no other. This time, Cassini dived into the planet’s atmosphere, sending science data for as long as its small thrusters could keep the spacecraft’s antenna pointed at Earth. Soon after, Cassini burned up and disintegrated like a meteor. And although the spacecraft may be gone, its enormous collection of data about Saturn—the giant planet itself, its magnetosphere, rings, and moons—will continue to yield new discoveries for decades.
We are still discovering new things about Saturn in the present era. Launched on October 15, 1997, the Cassini spacecraft entered orbit around Saturn on June 30, 2004, carrying the European Huygens probe. Its key discoveries included a global ocean with indications of hydrothermal activity within Enceladus, and liquid methane seas on Titan (two of Saturn’s moons). During a series of twenty– two orbits that each passed between the planet and its rings, Cassini observed Saturn and simultaneously sent data back to Earth. On September 15, 2017, the spacecraft
3
THE PLANET
The sixth planet is arguably the most striking. Saturn is the sixth planet from the sun and the second largest planet in our solar system. Adorned with thousands of beautiful rings, Saturn is unique among the planets. It is not the only planet to have rings—made of chunks of ice and rock—but none are as spectacular or as complicated as Saturn’s. Like fellow gas giant Jupiter, Saturn is a massive ball made mostly of hydrogen and helium. Surrounded by fifty–three confirmed and nine provisional moons, Saturn is home to some of the most fascinating
landscapes in our solar system. From the jets of Enceladus to the methane lakes on smoggy Titan, the Saturn system is a rich source of scientific discovery and still holds many mysteries. The farthest planet from Earth discovered by the unaided human eye, Saturn has been known since ancient times and is named for the Roman god of agriculture and wealth. The Greek equivalent was Cronos, the father of Zeus/Jupiter.
chapter cover image Cassini captured the cover image for this chapter from the back side of Saturn; it is like a solar eclipse. With the sun behind the planet, this sight is something we could never see from Earth. This is similar to the solar eclipse we experience, where the sun appears to be behind the moon.
by the numbers The grand size of Saturn is more easily understood when compared to our own planet. Saturn takes 29.461 Earth years or 10,760 days to orbit the Sun. Even though its diameter is the length of nine Earths, it would float! As the lightest planet, Saturn is less dense than water. Wind at the equator reaches speeds of 1100 mph. A giant storm located at its north pole is the shape of a hexagon, and this storm is over twice the size of Earth.
7
THE RINGS
Saturn’s rings are its crowning feature. Certain forces keep the rings in relatively the same shape. Collisions among ring particles logically would cause the rings to spread outward. Instead, shepherd satellites gravitationally usher straying particles back into the rings.
Saturn has the most spectacular ring system, with seven rings and several gaps and divisions between them. Although they appear to be one smooth structure, the rings are made up of billions of particles, ranging in size from microscopic specks to chunks bigger than a mountain. Each particle orbits Saturn in its own circular orbit. Sometimes waves occur that sweep through the rings, creating hundreds of ringlets.
Now we know that the rings are made of particles instead of one piece; in addition, they are not as uniform as they appear. Ripples in the rings, caused by waves, can be hundreds of meters high, but the sheet of particles may be only a dozen or so meters thick. That is why the rings “vanish” when seen edge–on.
The rings are divided into different regions: A, B, and C Rings, Encke’s division, and Cassini’s division. The C Ring is also known as the Crepe Ring in reference to the black, semitransparent cloth of the same name.
12
in perspective Saturn’s rings are 4.5 Earths wide and 2/3 mile thick!
the importance of moons The rings of Saturn and the other Jovian moons are created by and controlled by the planet’s moons. No moon? No rings. Small moons near the rings can shepherd the ring particles and confine them. Moons also create variation. For example, if a particle in Cassini’s division orbits Saturn twice in the time the moon Mimas takes to orbit once, then on every other orbit the particle feels a gravitational tug from Mimas. Eventually, it is pulled out of its circular orbit into an elliptical orbit.
13
formation The rings were not primordial (made of material left over from the formation of the planet), heat from formation would have vaporized the leftover materials. Also, the distended atmosphere would have caused friction, and any leftover particles would have moved so slowly that they would just fall off. Therefore, the rings are not as old as Saturn. Saturn’s rings may have been produced within the last 100,000 years, and there are a few different theories about their formation. One idea is that an icy planetesimal came within Saturn’s Roche limit and tides pulled it apart: some debris would settle into the ring plane. Another idea is that a comet struck one of Saturn’s moons (comets and moons are icy); the collision would produce icy debris. Bright planetary rings such as Saturn’s may be a temporary phenomena, produced when fresh ice is scattered into the rings and then wasting away as the ice is gradually lost. Even today, the rings must be replenished now and then by impacts on Saturn’s icy moons.
optical illusion Twice every twenty–nine and a half years the great planet Saturn appears ringless. We cannot see Saturn’s rings when we view the rings edge–on from Earth. Even with powerful telescopes, they are barely visible at this angle.
14
swim with saturn An astronaut could swim through the rings—particles orbit Saturn at a high velocity, but all particles at the same distance from the planet orbit at about the same speed, so they collide gently at low velocities. We could push our way from one icy particle to the next.
15
MANY MOONS
Saturn’s many moons are vital to its appearance. Greco-Roman Titans and descendants of the Titans. But as many new moons were discovered, scientists began selecting names from different mythologies including Gallic, Inuit, and Norse stories.
Saturn has over sixty moons of various sizes and terrains. Many are small, such as Enceladus, which is about as wide as Arizona; Titan, the largest, is almost as wide as Canada. While the larger moons are spherical, others are shaped like a sweet potato, a regular potato, a meatball, and even a sponge (see Hyperion at right). Some have a gnarled, irregular shape and texture (see Epimetheus at right). One object observed in the rings (and unofficially called Peggy) may be a moon forming or disintegrating.
Four spacecraft have visited the Saturn system, but Cassini alone actually orbited the ringed planet. Doing so bought Cassini time—more than a decade—to linger and watch Saturn’s exotic zoo of sixty-plus moons like no spacecraft before. Cassini looked, listened, sniffed, and even tasted Saturn’s moons, collecting extraordinary data.
Saturn’s moons originally came from Greek and Roman mythology. The first moons discovered were named after
18
Titan (back) and Rhea
Mimas
Hyperion
Epimetheus
Iapetus
Enceladus
Above: Just a few of Saturn’s moons are shown here. They vary greatly in size and shape. These images do not show relative scale.
19
Phoebe
Pan
FINDING SATURN
Saturn does not have to be so far away. typically appear to have a yellowish–golden hue and won’t twinkle like stars do. Since it doesn’t shimmer, it may not be as bright or instantly noticeable as some stars. Use the nearest constellation from your research as a point of reference and look for a color difference. You will not be able to see the rings without a telescope, but a simple telescope will yield a beautiful view of the planet, its rings, and even some of its larger moons depending on the strength.
Can you find our Earth and Moon in this chapter’s cover image and in the image to the right? Cassini was able to view Earth from Saturn, and you can see Saturn from Earth! While stars and constellations have a steady and easily–trackable path, planets are slightly more difficult to find because they seem to wander in comparison to stars. To find Saturn in the night sky, research what constellation it is near on the date you want to view it. Go outside on a clear night, away from city lights, and look for a golden color shining steadily. Saturn will
22
Above: Earth as seen from Saturn.
23
Saturn is stunning, enigmatic, and lovely. Its subtly marked disk and its delicate rings are both massive and graceful. Its extensive system of moons only adds to the mystery of this beautiful and curious world.
25
“ When I look at your
heavens, the work of your fingers, the moon and the stars, which you have set in place, what is man that you are mindful of him, and the son of man that you care for him?
�
acknowledgments https://www.nationalgeographic.com/science/space/solar–system/saturn/ https://attic.gsfc.nasa.gov/huygensgcms/Shistory.htm https://saturn.jpl.nasa.gov/ https://solarsystem.nasa.gov/ Foundations of Astronomy, 7th Edition, by Michael A. Seeds https://www.wikihow.com/Find–Saturn
TAKE ANOTHER LOOK AT SATURN THROUGH THE EYES OF CASSINI For more than a decade, NASA’s Cassini spacecraft shared the wonders of Saturn and its family of icy moons. It revealed in great detail the true wonders of Saturn, a giant world ruled by raging storms and delicate harmonies of gravity. Cassini carried a passenger to the Saturn system, the European Huygens probe—the first man–made object to land on a world in the distant outer solar system. After twenty years in space—thirteen of those years exploring Saturn— Cassini exhausted its fuel supply. After a series of nearly two dozen nail–biting dives between the planet and its icy rings, Cassini plunged into Saturn’s atmosphere on September 15, 2017, returning scientific data to the very end.
See what amazing things we have learned about Saturn as well as the mysteries that are left to be answered.
OBSERVING
saturn