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In this chapter, explore the various bodies of the solar system, including planets, moons, asteroids, comets, and meteoroids. Learn about measuring interplanetary distances, the nine planets, and the unique characteristics of Mercury and Venus.
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CHAPTER 20 A Family of Planets
Chapter 20 In this chapter you will learn about all of the bodies of the solar system – the planets and their moons, asteroids, comets and meteoroids. You will also learn how space missions are making discoveries about our solar system that expand our knowledge of the solar neighborhood in which we live.
Measuring InterplanetaryDistances One way scientists measure distance in space is by using the astronomical unit. The astronomical unit (AU)is the average distance between the Earth and the sun. Another way to measure distances in space is by the distance light travels in a given amount of time.
Light Speed Light travels at about 300,000 km per second in space. This means that in 1 second, light travels a distance of 300,000 km --- or about the distance you would cover if you traveled around the Earth 7.5 times! In 1 minute, light travels nearly 18,000,000 km! This distance is also called 1 light-minute.The distance from the sun to the Earth is 8.3 light minutes.
Distances Distances within the solar system can be measured in light-minutes and light-hours, but distances between the stars are measured in light-years.
The Nine Planets Ancient people knew about the existence of planets and could predict their motions. But it wasn’t until the seventeenth century, when Galileo used the telescope to study planets and stars, that we began our first exploration of these alien worlds. Since the former Soviet Union launched Sputnik 1 --- the first artificial satellite --- in 1957, over 150 successful missions have been launched to moons, planets, comets and asteroids.
The Planets The solar system is divided into two groups of planets --- the inner planets and the outer planets. The inner planets are more closely spaced than the outer planets. Other differences are their sizes and the materials of which they are made. The inner planets are called terrestrial planetsbecause they are like Earth --- small, dense and rocky. The outer planets, except for icy Pluto, are much larger and are made mostly of gasses.
Mercury-Closest to the Sun If you were to visit the planet Mercury, you would find a very strange world. For one thing, on Mercury you would weigh only 38 percent of what you weigh on Earth. The weight you experience on Earth is due to surface gravity, which is less on massive planets. Also, a day on Mercury is almost 59 Earth days long! This is because Mercury spins on its axis much more slowly than Earth does. The spin of an object in space is called rotation. The amount of time it takes for an object to rotate once is called its period of rotation.
Mercury-Closest to the Sun (cont) Another curious thing about Mercury is that its year is only 88 Earth days long. As you know, a year is the time it takes for a planet to go around the sun once. The motion of a body as it orbits another body in space is called revolution. Every 88 Earth days, or 1.5 Mercurial days, Mercury completes one revolution around the sun.
BRAIN FOOD The density value given in statistics boxes is the bulk density for each planet. Different layers of planets have different densities. The cores are denser than the surface layers, for example. Although Mercury is about 29 percent smaller than Mars, its surface gravity is about the same. What does this tell us about the two planets? Because the force of gravity depends on how massive an object is and because Mercury is smaller than Mars, Mercury must have a higher density than Mars. Astronomers speculate that Mercury has more iron, a relatively dense element, in its core than Mars does.
ENVIRONMENTAL SCIENCE The greenhouse effect caused by the concentration of carbon dioxide in Venus’s atmosphere creates a surface temperature hot enough to melt lead. Could an increased greenhouse effect cause Earth’s atmosphere to become more like Venus’s? Earth’s current atmosphere is primarily nitrogen and oxygen. Deforestation and fossil fuel use have caused CO2 levels in our atmosphere to rise steadily since the industrial revolution. Scientists have already noticed possible global warming trends, and they are concerned that it could have far-reaching effects on Earth’s biological systems.
Venus; Earth’s Twin? In many ways Venus is more similar to Earth than any other planet—they have about the same size, mass, and density. But in other ways Venus is very different from Earth. Unlike on Earth, on Venus the sun rises in the west and sets in the east. This is because Venus rotates in the opposite direction that Earth rotates. Earth is said to have prograde rotation, because when viewed from above its north pole, Earth appears to spin in a counterclockwisedirection. If a planet spins in a clockwise direction, it is said to have a retrograde rotation.
The Atmosphere of Venus At 90 times the pressure of Earth’s atmosphere, the atmosphere of Venus is the densest of the terrestrial planets. It consists mostly of carbon dioxide, but it also contains some of the most corrosive acids known. The carbon dioxide in the atmosphere traps thermal energy from sunlight in a process known as the greenhouse effect. This is why the surface temperature is so high. With an average temperature of 4640 C, Venus has the hottest surface of any planet in the solar system.
IS THAT A FACT!! Venus is the second-brightest object in the night sky, after the moon. Venus is often called the morning star or the evening star because it always rises and sets with the sun. Also, the planet rotated much more slowly than Earth. A Venusian day is longer than a Venusian year!
Earth-An Oasis in Space As viewed from space, Earth is like a sparkling blue oasis suspended in a black sea. Constantly changing weather patterns create the swirls of clouds that blanket the blue and brown sphere we call home. Why did Earth have such good fortune while its two neighbors, Venus and Mars, are unsuitable for life as we know it?
Water on Earth Earth is fortunate enough to have formed at just the right distance from the sun. The temperatures are warm enough to prevent most of its water from freezing but cool enough to keep it from boiling away. Liquid water was the key to the development of life on Earth. Water provides a means for much of the chemistry that living things depend on for survival.
The Earth from Space You might think the only goal of space exploration is to make discoveries beyond Earth. But NASA has a program to study Earth by using satellites—just as we study other planets. The goal of this project, called the Earth Science Enterprise, is to study the Earth as a system and to determine the effects humans have in changing the global environment. By studying Earth from space, we hope to understand how different parts of the global system—such as weather, climate, and pollution—interact.
WEIRD SCIENCE The earth and its moon revolve around the sun like a “double planet”. They can be thought of as the unequal ends of a weighted barbell. The center of gravity for the Earth-moon system, called the barycenter, is actually 1,700 km below the Earth’s surface. This barycenter is what follows the curved line of Earth’s Orbit. Around this center of gravity, the moon and Earth wobble as they circle the sun.
MISCONCEPTION ALERT Earth is not a perfect sphere. The diameter of Earth as measured from the North Pole to the South Pole is 44 km less than the diameter as measured at the equator. None of the other planets or stars are perfectly spherical either. Therefore, all of the planetary diameters given in this chapter are equatorialdiameters
Mars—The Red Planet Other than Earth, Mars is perhaps the most studied planet in the solar system. Much of our knowledge of Mars has come from information gathered by the Viking 1 and Viking 2 spacecraft that landed on Mars in 1976 and from the Pathfinder spacecraft that landed on Mars in 1997.
The Atmosphere of Mars Because of its thin atmosphere and its great distance from the sun, Mars is a cold planet. Mid-summer temperatures recorded by the Pathfinder-lander ranged from -130 C to -770 C. The atmosphere of Mars is so thin that the air pressure at the planet’s surface is roughly equal to the pressure 30 km above the Earth’s surface—about three times higher than most planes fly. The pressure is so low that any liquid water would quickly boil away. The only water you’ll find on Mars is in the form of ice.
Water on Mars Even though liquid water cannot exist on Mar’s surface today, there is strong evidence that it did exist there in the past. Mars has two polar icecaps that contain both frozen water and frozen carbon dioxide, but this cannot account for all the water. Looking closely at the walls of some Martian craters, scientists have found that the debris surrounding the craters looks as if it were made by a mud flow rather than by the movement of dry material, Where does this suggest some of the “lost” Martian water went? Many scientists think it is frozen beneath the Martian soil.
Martian Volcanoes Mars has a rich volcanic history. Unlike on Earth, where volcanoes occur in many places, Mars has only two large volcanic systems. The largest, the Tharsis region, stretches 8,000 km across the planet. The largest mountain in the solar system, Olympus Mons, is an extinct shield volcano similar to Mauna Kea, on the island of Hawaii. Mars is not only smaller and cooler than Earth, but is also has a slightly different chemical composition. Those factors may have prevented the Martian crust from moving around as Earth’s crust has, so the volcanoes kept building up in the same spot.
BRAIN FOOD Some astronomers think of Jupiter as a small star that never reached maturity. In fact, the Galileo probe mentioned in the text, found that the relative amounts of hydrogen and helium in Jupiter’s atmosphere are very similar to those in the sun. Jupiter’s 30,0000 C core temperature is not high enough to initiate the fusion reactions that occur in the sun’s core however. Had Jupiter been about 75 times more massive, it would have been hot enough to initiate nuclear reactions and become a second star in our solar system.
The Outer Planets The outer planets differ significantly in composition and size from the inner planets. All of the outer planets, except Pluto, are gas giants. Gas giants are very large planets that don’t have any known solid surfaces—their atmospheres blend smoothly into the denser layers of their interiors, very deep beneath the outer layers.
Jupiter-A Giant Among Giants Like the sun, Jupiter is made primarily of hydrogen and helium. The outer part of Jupiter’s atmosphere is made of layered clouds of water, methane, and ammonia. Another striking feature of Jupiter is the Great Red Spot, which is a long-lasting storm system that has a diameter of about one and a half times that of Earth! At a depth of about 10,000 km, the pressure is high enough to change the hydrogen gas into a liquid. Deeper still, the pressure changes the liquid hydrogen into a metallic liquid state. Unlike most planets, Jupiter radiates much more energy into space than it receives from the sun. This is because energy is continuously transported from Jupiter’s interior to its outer atmospheric layers, where it is radiated into space.
METEOROLOGY CONNECTION All planets with atmospheres have weather. Jupiter’s Great Red Spot appears to be very similar to a hurricane system on Earth, but it has lasted for centuries, driven by the planet’s internal energy.
IS THAT A FACT!! The solar system has two main bodies; the Sun and Jupiter. In terms of mass, the rest of the solar system is insignificant! Jupiter has one-thousandth the mass of the sun but is roughly 317 times more massive than Earth, and Jupiter’s volume is 1,321 times that of Earth.
Saturn-Still Forming Saturn, the second largest planet in the solar system, has roughly 764 times the volume of Earth and is 95 times more massive. Its overall composition, like Jupiter’s is mostly hydrogen and helium, with methane, ammonia, and ethane in the upper atmosphere. Saturn’s interior is probably very similar to that of Jupiter. Like Jupiter, Saturn gives off a lot more energy than it receives from the sun. Scientists believe that, in Saturn’s case, the extra energy is caused by helium raining out of the atmosphere and sinking into the core. In essence, Saturn is still forming!
The Rings of Saturn Although all of the gas giants have rings, Saturn’s rings are the largest. Saturn’s rings start near the top of Saturn’s atmosphere and extend out 136,000 km, yet they are only a few hundred meters thick. The rings consist of icy particles that range in size from a few centimeters to several meters across.
IS THAT A FACT!! Saturn has the most violent winds of any planet in our solar system. At Saturn’s equator, the wind blows at nearly 1,700 km/h—not exactly good weather for playing outside.
WEIRD SCIENCE Although Saturn has a similar composition to Jupiter, it appears less colorful. This is because its colder atmosphere causes thick, white ammonia clouds to condense, blocking our view.
Uranus-A Small Giant Uranus (YOOR uh nuhs), was discovered by the English amateur astronomer William Hershel in 1781. Viewed through a telescope, Uranus looks like a featureless blue-green disk. The atmosphere is mainly hydrogen and methane gas, which absorbs the red part of sunlight very strongly. Uranus and Neptune are much smaller than Jupiter and Saturn, and yet they have similar densities. This suggests that they have lower percentages of light elements and more water in their interior.
A Tilted Planet Uranus has about 63 times the volume of Earth and is nearly 15 times as massive. One especially unusual quality of Uranus is that it is tipped-over on its side—the axis of rotation is tilted by almost 90 degrees and lies almost in the plane of its orbit. For part of a Uranus year, one pole points towards the sun while the other pole is in darkness. At the other end of Uranus’s orbit the poles are reversed. Scientists suggest that early in its history, Uranus got hit by a massive object that tipped the planet over.
SCIENCE BLOOPERS Uranus was discovered in 1781 by an English music teacher named William Herschel. Herschel originally named the planet Georgium Sidus, Latin for George’s Star, after England’s King George III. No one outside of England liked the name. A few years later, an astronomer named J. E. Bode suggested the name Uranus because it would continue the tradition of naming the planets after Greek or Roman Gods.
Neptune-The Blue World Irregularities in the orbit of Uranus suggested to early astronomers that there must be another planet beyond Uranus whose gravitational force causes Uranus to move off its predicted path. By using the predictions of the new planet’s orbit, astronomers discovered the planet Neptune in 1846.
The Atmosphere of Neptune The Voyager 2 spacecraft sent back images that gave us much new information about the nature of Neptune’s atmosphere. Although the composition of Neptune’s atmosphere is nearly the same as that of Uranus’s atmosphere, Neptune’s atmosphere contains belts of clouds that are much more visible. At the time of Voyager 2’s visit, Neptune had a Great Dark Spot, similar to the Great Red Spot on Jupiter. And like the interiors of Jupiter and Saturn, Neptune’s interior releases energy to its outer layers. This helps the warm gases rise and the cool gases sink, setting up the wind patterns in the atmosphere that create the Belts of clouds.
ENVIRONMENTAL SCIENCE CONNECTION Since the Voyager spacecraft passed Neptune’s moon Triton in 1989, scientists have noticed an interesting trend in Triton’s atmosphere. Images from the Hubble Space telescope taken in 1998 indicate that Triton is going through a rapid period of global warming. As Triton warms, Frozen nitrogen on its surface melts and contributes nitrogen gas to its thin atmosphere. This process happened so rapidly that the atmospheric pressure of Triton doubled in less than 10 years! Scientists hope to use the global warming trends on Triton to understand warming patterns on Earth. Because Triton is a much similar world than Earth—with a thinner atmosphere, a surface of frozen nitrogen, and no oceans—it is a good place to study environmental change.
QUIZ 1. Which planets have retrograde rotation? 2. Which planet has the lowest density? 3. Which planet has the greatest number of moons? 4. Which planet has the shortest period of rotation? 5. Which planet has the largest known volcanoes? • Venus, Uranus and Pluto • Saturn • Uranus • Jupiter • 5. Mars
