Questions & Answers APPENDIX 1 course (perhaps in the 2nd century B.C.), the Saturn Romans became acquainted with Greek as- 1. When did we discover Saturn? tronomy and the Greek planetary names. The Romans then rendered the Greek names to fit We don’t know exactly when ancient observers their own gods. This is the origin of the names first recognized what we now call the planet Mercury, Venus, Mars, Jupiter, and Saturn, the Saturn. Many ancient civilizations were aware names still in use 2,000 years later. of five wandering points of light in the night sky, which were later named for gods of Roman mythology — Mercury, Venus, Mars, Jupiter, and Saturn. These planets were easily observed with the unaided eye. It was not until the time of Galileo, who first looked at the sky through a telescope, that we observed these wandering “stars” to be other worlds. The planets outside Saturn’s orbit — Uranus, Neptune, and Pluto — were discovered using more modern telescopes, beginning with the dis- covery of Uranus by William Herschel in 1781. The discovery of each of these outermost plan- ets generated great public interest at the time and made the discoverers famous. 2. How did Saturn get its name? Nearly all ancient human cultures created Saturn, the fearsome Roman god. names for, and stories about the Sun, the Moon, the planets and the stars. Like so many things in 3. Where is Saturn located? history, the naming of the planets happened by Saturn is the sixth planet out from the center of accident. For example, the ancient Babylonians our Solar System. It orbits the Sun at a distance recognized five specks of light moving across the of about 1.4 billion km (870 million mi). Sat- night skies and believed that these specks were urn is about 9.6 times as far from the Sun as is the images of their most important gods. Not Earth. Saturn is almost twice as far from the surprisingly, they named the moving lights after Sun as is Jupiter, the fifth planet in the Solar these gods. When the Greeks (probably the System. Pythagoreans, in the 5th century B.C.) came into contact with the Babylonian sky-lore, they The Sun around which Saturn and the other assigned to the five lights the names of those planets orbit is one of hundreds of billions of Greek gods corresponding most closely to the stars in the Milky Way galaxy. The nearest star appropriate Babylonian deities. The Greek to the Sun, called Alpha Centauri, is more than names of the five planets were: Hermes, Aphrodite, Ares, Zeus, and Kronos. In due 225 Saturn Educator Guide • Cassini Program website — http://www.jpl.nasa.gov/cassini/educatorguide • EG-1999-12-008-JPL APPENDIX 1 JUPITER URANUS EARTH MERCURY SUN NEPTUNE MARS SATURN VENUS PLUTO The Solar System (not to scale). 40 trillion km (25 trillion mi) away. Our Solar 54,000 km (33,480 mi) from its center to its System is located a little more than half way north or south pole. Its volume is almost 760 from the center of the galaxy. The whole Solar times that of Earth. System (Sun and planets) orbits the center of the galaxy once about every 225 million years. Mass: Saturn’s mass is about 95 times Earth’s The Milky Way galaxy is only one of an esti- mass. We know this by observing the orbital mated 100 billion galaxies in the known Uni- motions of Saturn’s many moons. Using verse. Each of these galaxies may have billions of Newton’s Law of Gravity and Kepler’s Laws, we stars with planets around them — perhaps even can calculate a mass figure for Saturn that some like Saturn. would create the gravitational force for the moons to move as they do. 4. How old is Saturn? Astronomers believe that the Sun and the plan- ets of our solar system first formed from a swirl- ing cloud of gas and dust about 4.6 billion years ago. The Milky Way galaxy is older still, and the Universe itself is believed to be between 10 and 20 billion years old. Astronomers predict that the Sun will burn steadily for about another 5 billion years or so. Thus, in astronomical COMPARATIVE SIZE terms, the Sun and the planets orbiting around OF EARTH it could be called “middle-aged.” 6. If Saturn is so much more massive than 5. How big is Saturn? Earth, why is it said that Saturn could float in water? Radius and Volume: Saturn is the second-largest Even though Saturn is much more massive than planet in our solar system; Jupiter is the largest. Earth, this mass is spread throughout a much Saturn measures about 60,000 km (37,200 mi) from its center to its equator, but is only 226 Saturn Educator Guide • Cassini Program website — http://www.jpl.nasa.gov/cassini/educatorguide • EG-1999-12-008-JPL larger volume than Earth and so Saturn is less 8. Could we breathe Saturn’s atmosphere? APPENDIX dense than Earth. Saturn is the least dense of all No. Saturn’s atmosphere is composed mainly of 1 the planets in the Solar System! hydrogen and helium, the same gases out of which the Sun and most stars are made. Such Saturn’s average density is only 0.69 g/cm3, an atmosphere is not poisonous to humans, but which is less than that of water (1.0 g/cm3). neither does it provide the oxygen needed to This means that a volume of water equal to the sustain human life. volume of Saturn would weigh more than Sat- urn does. So if we imagine a titanic tub of water 9. Pictures of Saturn show that it sort of big enough to hold Saturn, Saturn’s weight flattens out near the poles and is wider at the equator. Why is that? would be supported by the water and Saturn Indeed, Saturn is about 60,000 km (37,200 mi) would float! Of course, when the plug was from center to equator, but only 54,000 km pulled and the water drained away, Saturn (33,480 mi) from center to pole. The differ- might leave... a RING! ence is 6000 km (3,720 mi), which is about the radius of Earth! Saturn is thus said to be quite oblate: the greater the difference between the polar and equatorial distances, the more oblate is the shape of the planet. Saturn spins around its polar axis much faster than Earth does. Though it is much larger than Earth, Saturn rotates once in 10 hours 39 min- utes, over twice as fast as Earth rotates. This rapid rotation, combined with the planet’s gas- eous composition, forces material outward near 7. What is Saturn made of? the equator, just like a spinning merry-go-round Like the other Jovian planets (Jupiter, Uranus, can push its riders toward the outside. The ob- and Neptune), Saturn is primarily a ball of gas lateness of Saturn is actually less than would be with no solid surface. According to studies expected if it were composed only of hydrogen based on Voyager’s spectrometer, Saturn is made and helium. This has led astronomers to believe up of about 94% hydrogen, 6% helium, and that Saturn may have a massive rocky core. small amounts of methane (CH4) and ammonia 10. Why is Saturn so much larger and more (NH3). The primary components, hydrogen massive than Earth? and helium, are the same gases out of which the If we assume that the process of creation of the Sun and most stars are made. Solar System involved a spinning disk of gas and dust that slowly condensed to form the Sun Detailed analysis of Saturn’s gravitational field and the planets, the question becomes, “Why leads astronomers to believe that the deepest in- was there so much more planet-building stuff in terior of Saturn must consist of a molten rock the orbit of Saturn compared to the orbit of core about the same size as Earth, but much Earth?” One idea is that at Saturn’s distant or- more massive than Earth. This core is only a bit, the cloud of gas and dust from which the small percentage of Saturn’s total mass. planets formed was cold enough for ice to solidify out of the gas (instead of only rock). 227 Saturn Educator Guide • Cassini Program website — http://www.jpl.nasa.gov/cassini/educatorguide • EG-1999-12-008-JPL This extra ice makes a tremendous “seed” to –143 °C (–225 °F). This temperature increases APPENDIX gather in more rock and ice, and to gravitation- with depth because the gases are compressed to 1 ally attract enormously more of the light gases dramatically greater pressures at depth. Com- like hydrogen and helium to form planets and puter models predict that Saturn’s core is as hot moons. as 10,000 °C (18,000 °F). 11. Since Saturn does not have a solid sur- Saturn is about 10 times as far from the Sun as face, would I sink to the middle of the planet if I tried to walk there? Earth is, so Saturn receives only about 1/100th (1%) as much sunlight per square meter as does Saturn has an outer layer of clouds that we con- Earth. Nevertheless, Saturn is warmer than sider the “edge” of the planet. At the top of would be expected if there were a balance be- these clouds, the atmospheric pressure is the tween the solar energy absorbed and the energy same as that of air on Earth.