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L_0054 | earths moon | T_0534 | When you look at the Moon from Earth, you notice dark and light areas. The maria are dark, solid, flat areas of lava. Maria covers around 16% of the Moons surface, mostly on the near side. The maria formed about 3.0 to 3.5 billion years ago, when the Moon was continually bombarded by meteorites (Figure 24.15). Large meteorites broke through the Moons newly formed surface. This caused magma to flow out and fill the craters. Scientists estimate volcanic activity on the Moon ended about 1.2 billion years ago. The lighter parts on the Moon are called terrae, or highlands (Figure 24.15). They are higher than the maria and include several high mountain ranges. The rock that makes up the highlands is lighter in color and crystallized more slowly than the maria. The rock looks light because it reflects more of the Suns light. | text | null |
L_0054 | earths moon | T_0534 | When you look at the Moon from Earth, you notice dark and light areas. The maria are dark, solid, flat areas of lava. Maria covers around 16% of the Moons surface, mostly on the near side. The maria formed about 3.0 to 3.5 billion years ago, when the Moon was continually bombarded by meteorites (Figure 24.15). Large meteorites broke through the Moons newly formed surface. This caused magma to flow out and fill the craters. Scientists estimate volcanic activity on the Moon ended about 1.2 billion years ago. The lighter parts on the Moon are called terrae, or highlands (Figure 24.15). They are higher than the maria and include several high mountain ranges. The rock that makes up the highlands is lighter in color and crystallized more slowly than the maria. The rock looks light because it reflects more of the Suns light. | text | null |
L_0054 | earths moon | T_0535 | There are no lakes, rivers, or even small puddles anywhere to be found on the Moons surface. So there is no running water and no atmosphere. This means that there is no erosion. Natural processes continually alter the Earths surface. Without these processes, our planets surface would be covered with meteorite craters just like the Moon. Many moons in our solar system have cratered surfaces. NASA scientists have discovered a large number of water molecules mixed in with lunar dirt. There is also surface water ice. Even though there is a very small amount of water, there is no atmosphere. Temperatures are extreme. So it comes as no surprise that there has not been evidence of life on the Moon. | text | null |
L_0054 | earths moon | T_0536 | Like Earth, the Moon has a distinct crust, mantle, and core. The crust is composed of igneous rock. This rock is rich in the elements oxygen, silicon, magnesium, and aluminum. On the near side, the Moons crust is about 60 kilometers thick. On the far side, the crust is about 100 kilometers thick. The mantle is made of rock like Earths mantle. The Moon has a small metallic core, perhaps 600 to 800 kilometers in diameter. The composition of the core is probably mostly iron with some sulfur and nickel. We learned this both from the rock samples gathered by astronauts and from spacecraft sent to the Moon. | text | null |
L_0055 | the sun | T_0537 | The Sun is made almost entirely of the elements hydrogen and helium. The Sun has no solid material. Most atoms in the Sun exist as plasma. Plasma is superheated gas with an electrical charge. Because the Sun is made of gases, it does not have a defined outer boundary. Like Earth, the Sun has an internal structure. The inner three layers make up what we would actually call the Sun. | text | null |
L_0055 | the sun | T_0538 | The core is the Suns innermost layer. The core is plasma. It has a temperature of around 15 million degrees Celsius (C). Nuclear fusion reactions create the immense temperature. In these reactions, hydrogen atoms fuse to form helium. This releases vast amounts of energy. The energy moves towards the outer layers of the Sun. Energy from the Suns core powers most of the solar system. | text | null |
L_0055 | the sun | T_0539 | The radiative zone is the next layer out. It has a temperature of about 4 million degrees C. Energy from the core travels through the radiative zone. The rate the energy travels is extremely slow. Light particles, called photons, can only travel a few millimeters before they hit another particle. The particles are absorbed and then released again. It may take 50 million years for a photon to travel all the way through the radiative zone. | text | null |
L_0055 | the sun | T_0540 | The convection zone surrounds the radiative zone. In the convection zone, hot material from near the Suns center rises. This material cools at the surface, and then plunges back downward. The material then receives more heat from the radiative zone. | text | null |
L_0055 | the sun | T_0541 | The three outer layers of the Sun are its atmosphere. | text | null |
L_0055 | the sun | T_0542 | The photosphere is the visible surface of the Sun (Figure 24.17). Its the part that we see shining. Surprisingly, the photosphere is also one of the coolest layers of the Sun. It is only about 6000 degrees C. | text | null |
L_0055 | the sun | T_0543 | The chromosphere lies above the photosphere. It is about 2,000 km thick. The thin chromosphere is heated by energy from the photosphere. Temperatures range from about 4000 degrees C to about 10,000 degrees C. The chromosphere is not as hot as other parts of the Sun, and it glows red. Jets of gas sometimes fly up through the chromosphere. With speeds up to 72,000 km per hour, the jets can fly as high as 10,000 kilometers. | text | null |
L_0055 | the sun | T_0544 | The corona is the outermost part of the Suns atmosphere. It is the Suns halo, or crown. With a temperature of 1 to 3 million K, the corona is much hotter than the photosphere. The corona extends millions of kilometers into space. Sometime you should try to see a total solar eclipse. If you do you will see the Suns corona shining out into space. | text | null |
L_0055 | the sun | T_0545 | The Sun has many incredible surface features. Dont try to look at them though! Looking directly at the Sun can cause blindness. Find the appropriate filters for a pair of binoculars or a telescope and enjoy! | text | null |
L_0055 | the sun | T_0546 | The most noticeable magnetic activity of the Sun is the appearance of sunspots. Sunspots are cooler, darker areas on the Suns surface (Figure 24.18). Sunspots occur in an 11 year cycle. The number of sunspots begins at a minimum. The number gradually increases to the maximum. Then the number returns to a minimum again. Sunspots form because loops of the Suns magnetic field break through the surface. Sunspots usually occur in pairs. The loop breaks through the surface where it comes out of the Sun. It breaks through again where it goes back into the Sun. Sunspots disrupt the transfer of heat from the Suns lower layers. | text | null |
L_0055 | the sun | T_0547 | A loop of the Suns magnetic field may break. This creates solar flares. Solar flares are violent explosions that release huge amounts of energy (Figure 24.19). The streams of high energy particles they emit make up the solar wind. Solar wind is dangerous to spacecraft and astronauts. Solar flares can even cause damage on Earth. They have knocked out entire power grids and can disturb radio, satellite, and cell phone communications. | text | null |
L_0055 | the sun | T_0548 | Another amazing feature on the Sun is solar prominences. Plasma flows along the loop that connects sunspots. This plasma forms a glowing arch. The arch is a solar prominence. Solar prominences can reach thousands of kilometers into the Suns atmosphere. Prominences can last for a day to several months. Prominences can be seen during a total solar eclipse. NASAs Solar Dynamics Observatory (SDO) was launched on February 11, 2010. SDO is studying the Suns magnetic field. This includes how the Sun affects Earths atmosphere and climate. SDO provides extremely high resolution images. The craft gathers data faster than anything that ever studied the Sun. To learn more about the SDO mission, visit: http://sdo.gsfc.nasa.gov To find these videos for download, check out: There are other ways to connect with NASA. Subscribe to NASAs Goddard Shorts HD podcast (http://svs.gsfc.nasa | text | null |
L_0056 | the sun and the earthmoon system | T_0549 | When a new moon passes directly between the Earth and the Sun, it causes a solar eclipse (Figure 24.20). The Moon casts a shadow on the Earth and blocks our view of the Sun. This happens only all three are lined up and in the same plane. This plane is called the ecliptic. The ecliptic is the plane of Earths orbit around the Sun. The Moons shadow has two distinct parts. The umbra is the inner, cone-shaped part of the shadow. It is the part in which all of the light has been blocked. The penumbra is the outer part of Moons shadow. It is where the light is only partially blocked. When the Moons shadow completely blocks the Sun, it is a total solar eclipse (Figure 24.21). If only part of the Sun is out of view, it is a partial solar eclipse. Solar eclipses are rare events. They usually only last a few minutes. That is because the Moons shadow only covers a very small area on Earth and Earth is turning very rapidly. Solar eclipses are amazing to experience. It appears like night only strange. Birds may sing as they do at dusk. Stars become visible in the sky and it gets colder outside. Unlike at night, the Sun is out. So during a solar eclipse, its easy to see the Suns corona and solar prominences. This NASA page will inform you on when solar eclipses are expected: http://eclipse.gsfc.nasa.gov/solar.html | text | null |
L_0056 | the sun and the earthmoon system | T_0549 | When a new moon passes directly between the Earth and the Sun, it causes a solar eclipse (Figure 24.20). The Moon casts a shadow on the Earth and blocks our view of the Sun. This happens only all three are lined up and in the same plane. This plane is called the ecliptic. The ecliptic is the plane of Earths orbit around the Sun. The Moons shadow has two distinct parts. The umbra is the inner, cone-shaped part of the shadow. It is the part in which all of the light has been blocked. The penumbra is the outer part of Moons shadow. It is where the light is only partially blocked. When the Moons shadow completely blocks the Sun, it is a total solar eclipse (Figure 24.21). If only part of the Sun is out of view, it is a partial solar eclipse. Solar eclipses are rare events. They usually only last a few minutes. That is because the Moons shadow only covers a very small area on Earth and Earth is turning very rapidly. Solar eclipses are amazing to experience. It appears like night only strange. Birds may sing as they do at dusk. Stars become visible in the sky and it gets colder outside. Unlike at night, the Sun is out. So during a solar eclipse, its easy to see the Suns corona and solar prominences. This NASA page will inform you on when solar eclipses are expected: http://eclipse.gsfc.nasa.gov/solar.html | text | null |
L_0056 | the sun and the earthmoon system | T_0550 | Sometimes a full moon moves through Earths shadow. This is a lunar eclipse (Figure 24.22). During a total lunar eclipse, the Moon travels completely in Earths umbra. During a partial lunar eclipse, only a portion of the Moon enters Earths umbra. When the Moon passes through Earths penumbra, it is a penumbral eclipse. Since Earths shadow is large, a lunar eclipse lasts for hours. Anyone with a view of the Moon can see a lunar eclipse. Partial lunar eclipses occur at least twice a year, but total lunar eclipses are less common. The Moon glows with a dull red coloring during a total lunar eclipse. | text | null |
L_0056 | the sun and the earthmoon system | T_0551 | The Moon does not produce any light of its own. It only reflects light from the Sun. As the Moon moves around the Earth, we see different parts of the Moon lit up by the Sun. This causes the phases of the Moon. As the Moon revolves around Earth, it changes from fully lit to completely dark and back again. A full moon occurs when the whole side facing Earth is lit. This happens when Earth is between the Moon and the Sun. About one week later, the Moon enters the quarter-moon phase. Only half of the Moons lit surface is visible from Earth, so it appears as a half circle. When the Moon moves between Earth and the Sun, the side facing Earth is completely dark. This is called the new moon phase. Sometimes you can just barely make out the outline of the new moon in the sky. This is because some sunlight reflects off the Earth and hits the Moon. Before and after the quarter-moon phases are the gibbous and crescent phases. During the crescent moon phase, the Moon is less than half lit. It is seen as only a sliver or crescent shape. During the gibbous moon phase, the Moon is more than half lit. It is not full. The Moon undergoes a complete cycle of phases about every 29.5 days. | text | null |
L_0057 | introduction to the solar system | T_0553 | The Sun and all the objects that are held by the Suns gravity are known as the solar system. These objects all revolve around the Sun. The ancient Greeks recognized five planets. These lights in the night sky changed their position against the background of stars. They appeared to wander. In fact, the word planet comes from a Greek word meaning wanderer. These objects were thought to be important, so they named them after gods from their mythology. The names for the planets Mercury, Venus, Mars, Jupiter, and Saturn came from the names of gods and a goddess. | text | null |
L_0057 | introduction to the solar system | T_0554 | The ancient Greeks thought that Earth was at the center of the universe, as shown in Figure 25.1. The sky had a set of spheres layered on top of one another. Each object in the sky was attached to one of these spheres. The object moved around Earth as that sphere rotated. These spheres contained the Moon, the Sun, and the five planets they recognized: Mercury, Venus, Mars, Jupiter, and Saturn. An outer sphere contained all the stars. The planets appear to move much faster than the stars, so the Greeks placed them closer to Earth. Ptolemy published this model of the solar system around 150 AD. | text | null |
L_0057 | introduction to the solar system | T_0555 | About 1,500 years after Ptolemy, Copernicus proposed a startling idea. He suggested that the Sun is at the center of the universe. Copernicus developed his model because it better explained the motions of the planets. Figure 25.2 shows both the Earth-centered and Sun-centered models. Copernicus did not publish his new model until his death. He knew that it was heresy to say that Earth was not the center of the universe. It wasnt until Galileo developed his telescope that people would take the Copernican | text | null |
L_0057 | introduction to the solar system | T_0556 | Today we know that we have eight planets, five dwarf planets, over 165 moons, and many, many asteroids and other small objects in our solar system. We also know that the Sun is not the center of the universe. But it is the center of the solar system. Figure 25.3 shows our solar system. The planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Table 25.1 gives some data on the mass and diameter of the Sun and planets relative to Earth. Object Mass (Relative to Earth) Sun Mercury Venus Earth Mars Jupiter Saturn Uranus 333,000 Earths mass 0.06 Earths mass 0.82 Earths mass 1.00 Earths mass 0.11 Earths mass 317.8 Earths mass 95.2 Earths mass 14.6 Earths mass Diameter of Planet (Relative to Earth) 109.2 Earths diameter 0.39 Earths diameter 0.95 Earths diameter 1.00 Earths diameter 0.53 Earths diameter 11.21 Earths diameter 9.41 Earths diameter 3.98 Earths diameter Neptune 17.2 Earths mass | text | null |
L_0057 | introduction to the solar system | T_0557 | Youve probably heard about Pluto. When it was discovered in 1930, Pluto was called the ninth planet. Astronomers later found out that Pluto was not like other planets. For one thing, what they were calling Pluto was not a single object. They were actually seeing Pluto and its moon, Charon. In older telescopes, they looked like one object. This one object looked big enough to be a planet. Alone, Pluto was not very big. Astronomers also discovered many objects like Pluto. They were rocky and icy and there were a whole lot of them. Astronomers were faced with a problem. They needed to call these other objects planets. Or they needed to decide that Pluto was something else. In 2006, these scientists decided what a planet is. According to the new definition, a planet must: Orbit a star. Be big enough that its own gravity causes it to be round. Be small enough that it isnt a star itself. Have cleared the area of its orbit of smaller objects. If the first three are true but not the fourth, then that object is a dwarf planet. We now call Pluto a dwarf planet. There are other dwarf planets in the solar system. They are Eris, Ceres, Makemake and Haumea. There are many other reasons why Pluto does not fit with the other planets in our solar system. | text | null |
L_0057 | introduction to the solar system | T_0558 | Figure 25.4 shows the Sun and planets with the correct sizes. The distances between them are way too small. In general, the farther away from the Sun, the greater the distance from one planets orbit to the next. In Figure 25.5, you can see that the orbits of the planets are nearly circular. Plutos orbit is a much longer ellipse. Some astronomers think Pluto was dragged into its orbit by Neptune. Distances in the solar system are often measured in astronomical units (AU). One astronomical unit is defined as the distance from Earth to the Sun. 1 AU equals about 150 million km (93 million miles). Table 25.2 shows the distance from the Sun to each planet in AU. The table shows how long it takes each planet to spin on its axis. It also shows how long it takes each planet to complete an orbit. Notice how slowly Venus rotates! A day on Venus is actually longer than a year on Venus! Planet Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Average Distance from Sun (AU) 0.39 AU 0.72 1.00 1.52 5.20 9.54 19.22 30.06 Length of Day (In Earth Days) 56.84 days 243.02 1.00 1.03 0.41 0.43 0.72 0.67 Length of Year Earth Years) 0.24 years 0.62 1.00 1.88 11.86 29.46 84.01 164.8 (In | text | null |
L_0057 | introduction to the solar system | T_0558 | Figure 25.4 shows the Sun and planets with the correct sizes. The distances between them are way too small. In general, the farther away from the Sun, the greater the distance from one planets orbit to the next. In Figure 25.5, you can see that the orbits of the planets are nearly circular. Plutos orbit is a much longer ellipse. Some astronomers think Pluto was dragged into its orbit by Neptune. Distances in the solar system are often measured in astronomical units (AU). One astronomical unit is defined as the distance from Earth to the Sun. 1 AU equals about 150 million km (93 million miles). Table 25.2 shows the distance from the Sun to each planet in AU. The table shows how long it takes each planet to spin on its axis. It also shows how long it takes each planet to complete an orbit. Notice how slowly Venus rotates! A day on Venus is actually longer than a year on Venus! Planet Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Average Distance from Sun (AU) 0.39 AU 0.72 1.00 1.52 5.20 9.54 19.22 30.06 Length of Day (In Earth Days) 56.84 days 243.02 1.00 1.03 0.41 0.43 0.72 0.67 Length of Year Earth Years) 0.24 years 0.62 1.00 1.88 11.86 29.46 84.01 164.8 (In | text | null |
L_0057 | introduction to the solar system | T_0559 | Planets are held in their orbits by the force of gravity. What would happen without gravity? Imagine that you are swinging a ball on a string in a circular motion. Now let go of the string. The ball will fly away from you in a straight line. It was the string pulling on the ball that kept the ball moving in a circle. The motion of a planet is very similar to the ball on a string. The force pulling the planet is the pull of gravity between the planet and the Sun. Every object is attracted to every other object by gravity. The force of gravity between two objects depends on the mass of the objects. It also depends on how far apart the objects are. When you are sitting next to your dog, there is a gravitational force between the two of you. That force is far too weak for you to notice. You can feel the force of gravity between you and Earth because Earth has a lot of mass. The force of gravity between the Sun and planets is also very large. This is because the Sun and the planets are very large objects. Gravity is great enough to hold the planets to the Sun even though the distances between them are enormous. Gravity also holds moons in orbit around planets. | text | null |
L_0057 | introduction to the solar system | T_0560 | Since the early 1990s, astronomers have discovered other solar systems. A solar system has one or more planets orbiting one or more stars. We call these planets extrasolar planets, or exoplanets. They are called exoplanets because they orbit a star other than the Sun. As of June 2013, 891 exoplanets have been found. More exoplanets are found all the time. You can check out how many we have found at http://planetquest.jpl.nasa.gov/. We have been able to take pictures of only a few exoplanets. Most are discovered because of some tell-tale signs. One sign is a very slight motion of a star that must be caused by the pull of a planet. Another sign is the partial dimming of a stars light as the planet passes in front of it. | text | null |
L_0057 | introduction to the solar system | T_0561 | To figure out how the solar system formed, we need to put together what we have learned. There are two other important features to consider. First, all the planets orbit in nearly the same flat, disk-like region. Second, all the planets orbit in the same direction around the Sun. These two features are clues to how the solar system formed. | text | null |
L_0057 | introduction to the solar system | T_0562 | Scientists think the solar system formed from a big cloud of gas and dust, called a nebula. This is the solar nebula hypothesis. The nebula was made mostly of hydrogen and helium. There were heavier elements too. Gravity caused the nebula to contract (Figure 25.6). As the nebula contracted, it started to spin. As it got smaller and smaller, it spun faster and faster. This is what happens when an ice skater pulls her arms to her sides during a spin move. She spins faster. The spinning caused the nebula to form into a disk shape. This model explains why all the planets are found in the flat, disk-shaped region. It also explains why all the planets revolve in the same direction. The solar system formed from the nebula about 4.6 billion years ago | text | null |
L_0057 | introduction to the solar system | T_0563 | The Sun was the first object to form in the solar system. Gravity pulled matter together to the center of the disk. Density and pressure increased tremendously. Nuclear fusion reactions begin. In these reactions, the nuclei of atoms come together to form new, heavier chemical elements. Fusion reactions release huge amounts of nuclear energy. From these reactions a star was born, the Sun. Meanwhile, the outer parts of the disk were cooling off. Small pieces of dust started clumping together. These clumps collided and combined with other clumps. Larger clumps attracted smaller clumps with their gravity. Eventually, all these pieces grew into the planets and moons that we find in our solar system today. The outer planets Jupiter, Saturn, Uranus, and Neptune condensed from lighter materials. Hydrogen, helium, water, ammonia, and methane were among them. Its so cold by Jupiter and beyond that these materials can form solid particles. Closer to the Sun, they are gases. Since the gases can escape, the inner planets Mercury, Venus, Earth, and Mars formed from denser elements. These elements are solid even when close to the Sun. | text | null |
L_0058 | inner planets | T_0564 | Mercury is the smallest planet. It has no moon. The planet is also closest to the Sun and appears in Figure 25.7. As Figure 25.8 shows, the surface of Mercury is covered with craters, like Earths Moon. The presence of impact craters that are so old means that Mercury hasnt changed much geologically for billions of years. With only a trace of an atmosphere, it has no weather to wear down the ancient craters. Because Mercury is so close to the Sun, it is difficult to observe from Earth, even with a telescope. The Mariner 10 spacecraft did a flyby of Mercury in 19741975, which was the best data from the planet for decades. In 2004, the MESSENGER mission left Earth. On its way to Mercury it did one flyby of Earth, two of Venus and three of Mercury. In March 2011, MESSENGER became the first spacecraft to enter an orbit around Mercury. During its year-long mission, the craft will map the planets surface and conduct other studies. One of these images can be seen in Figure 25.9. | text | null |
L_0058 | inner planets | T_0564 | Mercury is the smallest planet. It has no moon. The planet is also closest to the Sun and appears in Figure 25.7. As Figure 25.8 shows, the surface of Mercury is covered with craters, like Earths Moon. The presence of impact craters that are so old means that Mercury hasnt changed much geologically for billions of years. With only a trace of an atmosphere, it has no weather to wear down the ancient craters. Because Mercury is so close to the Sun, it is difficult to observe from Earth, even with a telescope. The Mariner 10 spacecraft did a flyby of Mercury in 19741975, which was the best data from the planet for decades. In 2004, the MESSENGER mission left Earth. On its way to Mercury it did one flyby of Earth, two of Venus and three of Mercury. In March 2011, MESSENGER became the first spacecraft to enter an orbit around Mercury. During its year-long mission, the craft will map the planets surface and conduct other studies. One of these images can be seen in Figure 25.9. | text | null |
L_0058 | inner planets | T_0565 | Mercury is named for the Roman messenger god. Mercury was a messenger because he could run extremely fast. The Greeks gave the planet this name because Mercury moves very quickly in its orbit around the Sun. Mercury orbits the Sun in just 88 Earth days. Mercury has a very short year, but it also has very long days. Mercury rotates slowly on its axis, turning exactly three times for every two times it orbits the Sun. Therefore, each day on Mercury is 58 Earth days long. | text | null |
L_0058 | inner planets | T_0566 | Mercury is very close to the Sun, so it can get very hot. Mercury also has virtually no atmosphere. As the planet rotates very slowly, the temperature varies tremendously. In direct sunlight, the surface can be as hot as 427C (801F). On the dark side, the surface can be as cold as 183C (297F)! The coldest temperatures may be on the insides of craters. Most of Mercury is extremely dry. Scientists think that there may be a small amount of water, in the form of ice, at the planets poles. The poles never receive direct sunlight. | text | null |
L_0058 | inner planets | T_0567 | Figure 25.10 shows a diagram of Mercurys interior. Mercury is one of the densest planets. Scientists think that the interior contains a large core made mostly of melted iron. Mercurys core takes up about 42% of the planets volume. Mercurys highly cratered surface is evidence that Mercury is not geologically active. | text | null |
L_0058 | inner planets | T_0568 | Named after the Roman goddess of love, Venus is the only planet named after a female. Venus is sometimes called Earths sister planet. But just how similar is Venus to Earth? Venus is our nearest neighbor. Venus is most like Earth in size. | text | null |
L_0058 | inner planets | T_0569 | Viewed through a telescope, Venus looks smooth and featureless. The planet is covered by a thick layer of clouds. You can see the clouds in pictures of Venus, such as Figure 25.11. We make maps of the surface using radar, because the thick clouds wont allow us to take photographs of the surface of Venus. Figure 25.12 shows the topographical features of Venus. The image was produced by the Magellan probe on a flyby. Radar waves sent by the spacecraft reveal mountains, valleys, vast lava plains, and canyons. Like Mercury, Venus does not have a moon. Clouds on Earth are made of water vapor. Venuss clouds are a lot less pleasant. They are made of carbon dioxide, sulfur dioxide and large amounts of corrosive sulfuric acid! The atmosphere of Venus is so thick that the pressure on the surface of Venus is very high. In fact, it is 90 times greater than the pressure at Earths surface! The thick atmosphere causes a strong greenhouse effect. As a result, Venus is the hottest planet. Even though it is farther from the Sun, Venus is much hotter even than Mercury. Temperatures at the surface reach 465C (860F). Thats hot enough to melt lead! | text | null |
L_0058 | inner planets | T_0570 | Venus has more volcanoes than any other planet. There are between 100,000 and one million volcanoes on Venus! Most of the volcanoes are now inactive. There are also a large number of craters. This means that Venus doesnt have tectonic plates. Plate tectonics on Earth erases features over time. Figure 25.13 is an image made using radar data. The volcano is Maat Mons. Lava beds are in the foreground. Scientists think the color of sunlight on Venus is | text | null |
L_0058 | inner planets | T_0570 | Venus has more volcanoes than any other planet. There are between 100,000 and one million volcanoes on Venus! Most of the volcanoes are now inactive. There are also a large number of craters. This means that Venus doesnt have tectonic plates. Plate tectonics on Earth erases features over time. Figure 25.13 is an image made using radar data. The volcano is Maat Mons. Lava beds are in the foreground. Scientists think the color of sunlight on Venus is | text | null |
L_0058 | inner planets | T_0570 | Venus has more volcanoes than any other planet. There are between 100,000 and one million volcanoes on Venus! Most of the volcanoes are now inactive. There are also a large number of craters. This means that Venus doesnt have tectonic plates. Plate tectonics on Earth erases features over time. Figure 25.13 is an image made using radar data. The volcano is Maat Mons. Lava beds are in the foreground. Scientists think the color of sunlight on Venus is | text | null |
L_0058 | inner planets | T_0571 | Venus is the only planet that rotates clockwise as viewed from its North Pole. All of the other planets rotate counterclockwise. Venus turns slowly, making only one turn every 243 days. This is longer than a year on Venus! It takes Venus only 225 days to orbit the Sun. Because the orbit of Venus is inside Earths orbit, Venus always appears close to the Sun. You can see Venus rising early in the morning, just before the Sun rises. For this reason, Venus is sometimes called the morning star. When it sets in the evening, just after the Sun sets, it may be called the evening star. Since planets only reflect the Suns light, Venus should not be called a star at all! Venus is very bright because its clouds reflect sunlight very well. Venus is the brightest object in the sky besides the Sun and the Moon. | text | null |
L_0058 | inner planets | T_0572 | Earth is the third planet out from the Sun, shown in Figure 25.14. Because it is our planet, we know a lot more about Earth than we do about any other planet. What are main features of Earth? | text | null |
L_0058 | inner planets | T_0573 | Earth is a very diverse planet, seen in Figure 25.14. Water appears as vast oceans of liquid. Water is also seen as ice at the poles or as clouds of vapor. Earth also has large masses of land. Earths average surface temperature is 14C (57F). At this temperature, water is a liquid. The oceans and the atmosphere help keep Earths surface temperatures fairly steady. Earth is the only planet known to have life. Conditions on Earth are ideal for life! The atmosphere filters out harmful radiation. Water is abundant. Carbon dioxide was available for early life forms. The evolution of plants introduced more oxygen for animals. | text | null |
L_0058 | inner planets | T_0574 | The Earth is divided into many plates. These plates move around on the surface. The plates collide or slide past each other. One may even plunge beneath another. Plate motions cause most geological activity. This activity includes earthquakes, volcanoes, and the buildup of mountains. The reason for plate movement is convection in the mantle. Earth is the only planet that we know has plate tectonics. | text | null |
L_0058 | inner planets | T_0575 | Earth rotates on its axis once every 24 hours. This is the length of an Earth day. Earth orbits the Sun once every 365.24 days. This is the length of an Earth year. Earth has one large moon. This satellite orbits Earth once every 29.5 days. This moon is covered with craters, and also has large plains of lava. The Moon came into being from material that flew into space after Earth and a giant asteroid collided. This moon is not a captured asteroid like other moons in the solar system. | text | null |
L_0058 | inner planets | T_0576 | Mars, shown in Figure 25.15, is the fourth planet from the Sun. The Red Planet is the first planet beyond Earths orbit. Mars atmosphere is thin compared to Earths. This means that there is much lower pressure at the surface. Mars also has a weak greenhouse effect, so temperatures are only slightly higher than they would be if the planet did not have an atmosphere. Mars is the easiest planet to observe. As a result, it has been studied more than any other planet besides Earth. People can stand on Earth and observe the planet through a telescope. We have also sent many space probes to Mars. In April 2011, there were three scientific satellites in orbit around Mars. The rover, Opportunity, was still moving around on the surface. No humans have ever set foot on Mars. NASA and the European Space Agency have plans to send people to Mars. The goal is to do it sometime between 2030 and 2040. The expense and danger of these missions are phenomenal. | text | null |
L_0058 | inner planets | T_0577 | Viewed from Earth, Mars is red. This is due to large amounts of iron in the soil. The ancient Greeks and Romans named the planet Mars after the god of war. The planets red color reminded them of blood. Mars has only a very thin atmosphere, made up mostly of carbon dioxide. | text | null |
L_0058 | inner planets | T_0578 | Mars is home to the largest volcano in the solar system. Olympus Mons is shown in Figure 25.16. Olympus Mons is a shield volcano. The volcano is similar to the volcanoes of the Hawaiian Islands. But Olympus Mons is a giant, about 27 km (16.7 miles/88,580 ft) tall. Thats three times taller than Mount Everest! At its base, Olympus Mons is about the size of the entire state of Arizona. Mars also has the largest canyon in the solar system, Valles Marineris (Figure 25.17). This canyon is 4,000 km (2,500 miles) long. Thats as long as Europe is wide! One-fifth of the circumference of Mars is covered by the canyon. Valles Marineris is 7 km (4.3 miles) deep. How about Earths Grand Canyon? Earths most famous canyon is only 446 km (277 miles) long and about 2 km (1.2 miles) deep. Mars has mountains, canyons, and other features similar to Earth. But it doesnt have as much geological activity as Earth. There is no evidence of plate tectonics on Mars. There are also more craters on Mars than on Earth. Buy there are fewer craters than on the Moon. What does this suggest to you regarding Mars plate tectonic history? | text | null |
L_0058 | inner planets | T_0578 | Mars is home to the largest volcano in the solar system. Olympus Mons is shown in Figure 25.16. Olympus Mons is a shield volcano. The volcano is similar to the volcanoes of the Hawaiian Islands. But Olympus Mons is a giant, about 27 km (16.7 miles/88,580 ft) tall. Thats three times taller than Mount Everest! At its base, Olympus Mons is about the size of the entire state of Arizona. Mars also has the largest canyon in the solar system, Valles Marineris (Figure 25.17). This canyon is 4,000 km (2,500 miles) long. Thats as long as Europe is wide! One-fifth of the circumference of Mars is covered by the canyon. Valles Marineris is 7 km (4.3 miles) deep. How about Earths Grand Canyon? Earths most famous canyon is only 446 km (277 miles) long and about 2 km (1.2 miles) deep. Mars has mountains, canyons, and other features similar to Earth. But it doesnt have as much geological activity as Earth. There is no evidence of plate tectonics on Mars. There are also more craters on Mars than on Earth. Buy there are fewer craters than on the Moon. What does this suggest to you regarding Mars plate tectonic history? | text | null |
L_0058 | inner planets | T_0579 | Water on Mars cant be a liquid. This is because the pressure of the atmosphere is too low. The planet does have a lot of water; it is in the form of ice. The south pole of Mars has a very visible ice cap. Scientists also have evidence that there is also a lot of ice just under the Martian surface. The ice melts when volcanoes erupt. At this times liquid water flows across the surface. Scientists think that there was once liquid water on the planet. There are many surface features that look like water- eroded canyons. The Mars rover collected round clumps of crystals that, on Earth, usually form in water. If there was liquid water on Mars, life might have existed there in the past. | text | null |
L_0058 | inner planets | T_0580 | Mars has two very small, irregular moons, Phobos (seen in Figure 25.18) and Deimos. These moons were discovered in 1877. They are named after the two sons of Ares, who followed their father into war. The moons were probably asteroids that were captured by Martian gravity. | text | null |
L_0059 | outer planets | T_0581 | Jupiter, shown in Figure 25.19, is the largest planet in our solar system. Jupiter is named for the king of the gods in Roman mythology. Jupiter is truly a giant! The planet has 318 times the mass of Earth, and over 1,300 times Earths volume. So Jupiter is much less dense than Earth. Because Jupiter is so large, it reflects a lot of sunlight. When it is visible, it is the brightest object in the night sky besides the Moon and Venus. Jupiter is quite far from the Earth. The planet is more than five times as far from Earth as the Sun. It takes Jupiter about 12 Earth years to orbit once around the Sun. | text | null |
L_0059 | outer planets | T_0582 | Since Jupiter is a gas giant, could a spacecraft land on its surface? The answer is no. There is no solid surface at all! Jupiter is made mostly of hydrogen, with some helium, and small amounts of other elements. The outer layers of the planet are gas. Deeper within the planet, the intense pressure condenses the gases into a liquid. Jupiter may have a small rocky core at its center. | text | null |
L_0059 | outer planets | T_0583 | Jupiters atmosphere is unlike any other in the solar system! The upper layer contains clouds of ammonia. The ammonia is different colored bands. These bands rotate around the planet. The ammonia also swirls around in tremendous storms. The Great Red Spot, shown in Figure 25.20, is Jupiters most noticeable feature. The spot is an enormous, oval-shaped storm. It is more than three times as wide as the entire Earth! Clouds in the storm rotate counterclockwise. They make one complete turn every six days or so. The Great Red Spot has been on Jupiter for at least 300 years. It may have been observed as early as 1664. It is possible that this storm is a permanent feature on Jupiter. No one knows for sure. | text | null |
L_0059 | outer planets | T_0584 | Jupiter has lots of moons. As of 2011, we have discovered over 60 natural satellites of Jupiter. Four are big enough and bright enough to be seen from Earth using a pair of binoculars. These four moons were first discovered by Galileo in 1610. They are called the Galilean moons. Figure 25.21 shows the four Galilean moons and their sizes relative to Jupiters Great Red Spot. These moons are named Io, Europa, Ganymede, and Callisto. The Galilean moons are larger than even the biggest dwarf planets, Pluto and Eris. Ganymede is the biggest moon in the solar system. It is even larger than the planet Mercury! Scientists think that Europa is a good place to look for extraterrestrial life. Europa is the smallest of the Galilean moons. The moons surface is a smooth layer of ice. Scientists think that the ice may sit on top of an ocean of liquid water. How could Europa have liquid water when it is so far from the Sun? Europa is heated by Jupiter. Jupiters tidal forces are so great that they stretch and squash its moon. This could produce enough heat for there to be liquid water. Numerous missions have been planned to explore Europa, including plans to drill through the ice and send a probe into the ocean. However, no such mission has yet been attempted. In 1979, two spacecrafts, Voyager 1 and Voyager 2, visited Jupiter and its moons. Photos from the Voyager missions | text | null |
L_0059 | outer planets | T_0584 | Jupiter has lots of moons. As of 2011, we have discovered over 60 natural satellites of Jupiter. Four are big enough and bright enough to be seen from Earth using a pair of binoculars. These four moons were first discovered by Galileo in 1610. They are called the Galilean moons. Figure 25.21 shows the four Galilean moons and their sizes relative to Jupiters Great Red Spot. These moons are named Io, Europa, Ganymede, and Callisto. The Galilean moons are larger than even the biggest dwarf planets, Pluto and Eris. Ganymede is the biggest moon in the solar system. It is even larger than the planet Mercury! Scientists think that Europa is a good place to look for extraterrestrial life. Europa is the smallest of the Galilean moons. The moons surface is a smooth layer of ice. Scientists think that the ice may sit on top of an ocean of liquid water. How could Europa have liquid water when it is so far from the Sun? Europa is heated by Jupiter. Jupiters tidal forces are so great that they stretch and squash its moon. This could produce enough heat for there to be liquid water. Numerous missions have been planned to explore Europa, including plans to drill through the ice and send a probe into the ocean. However, no such mission has yet been attempted. In 1979, two spacecrafts, Voyager 1 and Voyager 2, visited Jupiter and its moons. Photos from the Voyager missions | text | null |
L_0059 | outer planets | T_0585 | Saturn, shown in Figure 25.22, is famous for its beautiful rings. Saturn is the second largest planet in the solar system. Saturns mass is about 95 times Earths mass. The gas giant is 755 times Earths volume. Despite its large size, Saturn is the least dense planet in our solar system. Saturn is actually less dense than water. This means that if there were a bathtub big enough, Saturn would float! In Roman mythology, Saturn was the father of Jupiter. Saturn orbits the Sun once about every 30 Earth years. Saturns composition is similar to Jupiters. The planet is made mostly of hydrogen and helium. These elements are gases in the outer layers and liquids in the deeper layers. Saturn may also have a small solid core. Saturns upper atmosphere has clouds in bands of different colors. These clouds rotate rapidly around the planet. But Saturn has fewer storms than Jupiter. Thunder and lightning have been seen in the storms on Saturn (Figure 25.23). | text | null |
L_0059 | outer planets | T_0586 | There is a strange feature at Saturns north pole. The clouds form a hexagonal pattern, as shown in the infrared image in Figure 25.24. This hexagon was viewed by Voyager 1 in the 1980s. It was still there when the Cassini Orbiter visited in 2006. No one is sure why the clouds form this pattern. | text | null |
L_0059 | outer planets | T_0587 | Saturns rings were first observed by Galileo in 1610. He didnt know they were rings and thought that they were two large moons. One moon was on either side of the planet. In 1659, the Dutch astronomer Christiaan Huygens realized that they were rings circling Saturns equator. The rings appear tilted. This is because Saturn is tilted about 27 degrees to its side. The Voyager 1 spacecraft visited Saturn in 1980. Voyager 2 followed in 1981. These probes sent back detailed pictures of Saturn, its rings, and some of its moons. From the Voyager data, we learned that Saturns rings are made of particles of water and ice with a little bit of dust. There are several gaps in the rings. These gaps were cleared out by moons within the rings. Ring dust and gas are attracted to the moon by its gravity. This leaves a gap in the rings. Other gaps in the rings are caused by the competing forces of Saturn and its moons outside the rings. | text | null |
L_0059 | outer planets | T_0587 | Saturns rings were first observed by Galileo in 1610. He didnt know they were rings and thought that they were two large moons. One moon was on either side of the planet. In 1659, the Dutch astronomer Christiaan Huygens realized that they were rings circling Saturns equator. The rings appear tilted. This is because Saturn is tilted about 27 degrees to its side. The Voyager 1 spacecraft visited Saturn in 1980. Voyager 2 followed in 1981. These probes sent back detailed pictures of Saturn, its rings, and some of its moons. From the Voyager data, we learned that Saturns rings are made of particles of water and ice with a little bit of dust. There are several gaps in the rings. These gaps were cleared out by moons within the rings. Ring dust and gas are attracted to the moon by its gravity. This leaves a gap in the rings. Other gaps in the rings are caused by the competing forces of Saturn and its moons outside the rings. | text | null |
L_0059 | outer planets | T_0588 | As of 2011, over 60 moons have been identified around Saturn. Only seven of Saturns moons are round. All but one is smaller than Earths Moon. Some of the very small moons are found within the rings. All the particles in the rings are like little moons, because they orbit around Saturn. Someone must decide which ones are large enough to call moons. Saturns largest moon, Titan, is about one and a half times the size of Earths Moon. Titan is even larger than the planet Mercury. Figure 25.25 compares the size of Titan to Earth. Scientists are very interested in Titan. The moon has an atmosphere that is thought to be like Earths first atmosphere. This atmosphere was around before life developed on Earth. Like Jupiters moon, Europa, Titan may have a layer of liquid water under a layer of ice. Scientists now think that there are lakes on Titans surface. Dont take a dip, though. These lakes contain liquid methane and ethane instead of water! Methane and ethane are compounds found in natural gas. | text | null |
L_0059 | outer planets | T_0589 | Uranus, shown in Figure 25.26, is named for the Greek god of the sky, the father of Saturn. Astronomers pronounce the name YOOR-uh-nuhs. Uranus was not known to ancient observers. The planet was first discovered with a telescope by the astronomer William Herschel in 1781. Uranus is faint because it is very far away. Its distance from the Sun is 2.8 billion kilometers (1.8 billion miles). A photon from the Sun takes about 2 hours and 40 minutes to reach Uranus. Uranus orbits the Sun once about every 84 Earth years. | text | null |
L_0059 | outer planets | T_0590 | Uranus is a lot like Jupiter and Saturn. The planet is composed mainly of hydrogen and helium. There is a thick layer of gas on the outside. Further on the inside is liquid. But Uranus has a higher percentage of icy materials than Jupiter and Saturn. These materials include water, ammonia, and methane. Uranus is also different because of its blue-green color. Clouds of methane filter out red light. This leaves a blue-green color. The atmosphere of Uranus has bands of clouds. These clouds are hard to see in normal light. The result is that the planet looks like a plain blue ball. Uranus is the least massive outer planet. Its mass is only about 14 times the mass of Earth. Like all of the outer planets, Uranus is much less dense than Earth. Gravity is actually weaker than on Earths surface. If you were at the top of the clouds on Uranus, you would weigh about 10 percent less than what you weigh on Earth. | text | null |
L_0059 | outer planets | T_0591 | All of the planets rotate on their axes in the same direction that they move around the Sun. Except for Uranus. Uranus is tilted on its side. Its axis is almost parallel to its orbit. So Uranus rolls along like a bowling ball as it revolves around the Sun. How did Uranus get this way? Scientists think that the planet was struck and knocked over by another planet-sized object. This collision probably took place billions of years ago. | text | null |
L_0059 | outer planets | T_0592 | Uranus has a faint system of rings, as shown in Figure 25.27. The rings circle the planets equator. However, Uranus is tilted on its side. So the rings are almost perpendicular to the planets orbit. We have discovered 27 moons around Uranus. All but a few are named for characters from the plays of William Shakespeare. The five biggest moons of Uranus, Miranda, Ariel, Umbriel, Titania, and Oberon, are shown in Figure | text | null |
L_0059 | outer planets | T_0592 | Uranus has a faint system of rings, as shown in Figure 25.27. The rings circle the planets equator. However, Uranus is tilted on its side. So the rings are almost perpendicular to the planets orbit. We have discovered 27 moons around Uranus. All but a few are named for characters from the plays of William Shakespeare. The five biggest moons of Uranus, Miranda, Ariel, Umbriel, Titania, and Oberon, are shown in Figure | text | null |
L_0059 | outer planets | T_0593 | Neptune is shown in Figure 25.29. It is the eighth planet from the Sun. Neptune is so far away you need a telescope to see it from Earth. Neptune is the most distant planet in our solar system. It is nearly 4.5 billion kilometers (2.8 billion miles) from the Sun. One orbit around the Sun takes Neptune 165 Earth years. Scientists guessed Neptunes existence before it was discovered. Uranus did not always appear exactly where it should. They said this was because a planet beyond Uranus was pulling on it. This gravitational pull was affecting its orbit. Neptune was discovered in 1846. It was just where scientists predicted it would be! Due to its blue color, the planet was named Neptune for the Roman god of the sea. Uranus and Neptune are often considered sister planets. They are very similar to each other. Neptune has slightly more mass than Uranus, but it is slightly smaller in size. | text | null |
L_0059 | outer planets | T_0594 | Like Uranus, Neptune is blue. The blue color is caused by gases in its atmosphere, including methane. Neptune is not a smooth looking ball like Uranus. The planet has a few darker and lighter spots. When Voyager 2 visited Neptune in 1986, there was a large dark-blue spot south of the equator. This spot was called the Great Dark Spot. When the Hubble Space Telescope photographed Neptune in 1994, the Great Dark Spot had disappeared. Another dark spot had appeared north of the equator. Astronomers believe that both of these spots represent gaps in the methane clouds on Neptune. Neptunes appearance changes due to its turbulent atmosphere. Winds are stronger than on any other planet in the solar system. Wind speeds can reach 1,100 km/h (700 mph). This is close to the speed of sound! The rapid winds surprised astronomers. This is because Neptune receives little energy from the Sun to power weather systems. It is not surprising that Neptune is one of the coldest places in the solar system. Temperatures at the top of the clouds are about 218C (360F). | text | null |
L_0059 | outer planets | T_0595 | Like the other outer planets, Neptune has rings of ice and dust. These rings are much thinner and fainter than Saturns. Neptunes rings may be unstable. They may change or disappear in a relatively short time. Neptune has 13 known moons. Only Triton, shown in Figure 25.30, has enough mass to be round. Triton orbits in the direction opposite to Neptunes orbit. Scientists think Triton did not form around Neptune. The satellite was captured by Neptunes gravity as it passed by. | text | null |
L_0059 | outer planets | T_0596 | Pluto was once considered one of the outer planets, but when the definition of a planet was changed in 2006, Pluto became one of the dwarf planets. It is one of the largest and brightest objects that make up this group. Look for Pluto in the next lesson, in the discussion of dwarf planets. | text | null |
L_0060 | other objects in the solar system | T_0597 | Asteroids are very small, irregularly shaped, rocky bodies. Asteroids orbit the Sun, but they are more like giant rocks than planets. Since they are small, they do not have enough gravity to become round. They are too small to have an atmosphere. With no internal heat, they are not geologically active. An asteroid can only change due to a collision. A collision may cause the asteroid to break up. It may create craters on the asteroids surface. An asteroid may strike a planet if it comes near enough to be pulled in by its gravity. Figure 25.31 shows a typical asteroid. | text | null |
L_0060 | other objects in the solar system | T_0598 | Hundreds of thousands of asteroids have been found in our solar system. They are still being discovered at a rate of about 5,000 new asteroids per month! The majority are located in between the orbits of Mars and Jupiter. This region is called the asteroid belt, as shown in Figure 25.32. There are many thousands of asteroids in the asteroid belt. Still, their total mass adds up to only about 4 percent of Earths Moon. Asteroids formed at the same time as the rest of the solar system. Although there are many in the asteroid belt, they were never were able to form into a planet. Jupiters gravity kept them apart. | text | null |
L_0060 | other objects in the solar system | T_0598 | Hundreds of thousands of asteroids have been found in our solar system. They are still being discovered at a rate of about 5,000 new asteroids per month! The majority are located in between the orbits of Mars and Jupiter. This region is called the asteroid belt, as shown in Figure 25.32. There are many thousands of asteroids in the asteroid belt. Still, their total mass adds up to only about 4 percent of Earths Moon. Asteroids formed at the same time as the rest of the solar system. Although there are many in the asteroid belt, they were never were able to form into a planet. Jupiters gravity kept them apart. | text | null |
L_0060 | other objects in the solar system | T_0599 | Near-Earth asteroids have orbits that cross Earths orbit. This means that they can collide with Earth. There are over 4,500 known near-Earth asteroids. Small asteroids do sometimes collide with Earth. An asteroid about 510 m in diameter hits about once per year. Five hundred to a thousand of the known near-Earth asteroids are much bigger. They are over 1 kilometer in diameter. When large asteroids hit Earth in the past, many organisms died. At times, many species became extinct. Astronomers keep looking for near-Earth asteroids. They hope to predict a possible collision early so they can to try to stop it. | text | null |
L_0060 | other objects in the solar system | T_0600 | Scientists are very interested in asteroids. Most are composed of material that has not changed since early in the solar system. Scientists can learn a lot from them about how the solar system formed. Asteroids may be important for space travel. They could be mined for rare minerals or for construction projects in space. Scientists have sent spacecraft to study asteroids. In 1997, the NEAR Shoemaker probe orbited the asteroid 433 Eros. The craft finally landed on its surface in 2001. The Japanese Hayabusa probe returned to Earth with samples of a small near-earth asteroid in 2010. The U.S. Dawn mission will visit Vesta in 2011 and Ceres in 2015. | text | null |
L_0060 | other objects in the solar system | T_0601 | If you look at the sky on a dark night, you may see a meteor, like in Figure 25.33. A meteor forms a streak of light across the sky. People call them shooting stars because thats what they look like. But meteors are not stars at all. The light you see comes from a small piece of matter burning up as it flies through Earths atmosphere. | text | null |
L_0060 | other objects in the solar system | T_0602 | Before these small pieces of matter enter Earths atmosphere, they are called meteoroids. Meteoroids are as large as boulders or as small as tiny sand grains. Larger objects are called asteroids; smaller objects are interplanetary dust. Meteoroids sometimes cluster together in long trails. They are the debris left behind by comets. When Earth passes through a comet trail, there is a meteor shower. During a meteor shower, there are many more meteors than normal for a night or two. | text | null |
L_0060 | other objects in the solar system | T_0603 | A meteoroid is dragged towards Earth by gravity and enters the atmosphere. Friction with the atmosphere heats the object quickly, so it starts to vaporize. As it flies through the atmosphere, it leaves a trail of glowing gases. The object is now a meteor. Most meteors vaporize in the atmosphere. They never reach Earths surface. Large meteoroids may not burn up entirely in the atmosphere. A small core may remain and hit the Earths surface. This is called a meteorite. Meteorites provide clues about our solar system. Many were formed in the early solar system (Figure 25.34). Some are from asteroids that have split apart. A few are rocks from nearby bodies like Mars. For this to happen, an asteroid smashed into Mars and sent up debris. A bit of the debris entered Earths atmosphere as a meteor. | text | null |
L_0060 | other objects in the solar system | T_0604 | Comets are small, icy objects that orbit the Sun. Comets have highly elliptical orbits. Their orbits carry them from close to the Sun to the solar systems outer edges. When a comet gets close to the Sun, its outer layers of ice melt and evaporate. The vaporized gas and dust forms an atmosphere around the comet. This atmosphere is called a coma. Radiation and particles streaming from the Sun push some of this gas and dust into a long tail. A comets tail always points away from the Sun, no matter which way the comet is moving. Why do you think that is? Figure Gases in the coma and tail of a comet reflect light from the Sun. Comets are very hard to see except when they have comas and tails. That is why they appear only when they are near the Sun. They disappear again as they move back to the outer solar system. The time between one visit from a comet and the next is called the comets period. The first comet whose period was known was Halleys Comet. Its period is 75 years. Halleys Comet last traveled through the inner solar system in 1986. The comet will appear again in 2061. Who will look up at it? | text | null |
L_0060 | other objects in the solar system | T_0605 | Some comets have periods of 200 years or less. They are called short-period comets. Short-period comets are from a region beyond the orbit of Neptune called the Kuiper Belt. Kuiper is pronounced KI-per, rhyming with viper. The Kuiper Belt is home to comets, asteroids, and at least two dwarf planets. Some comets have periods of thousands or even millions of years. Most long-period comets come from a very distant region of the solar system. This region is called the Oort cloud. The Oort cloud is about 50,000100,000 times the distance from the Sun to Earth. Comets carry materials in from the outer solar system. Comets may have brought water into the early Earth. Other substances could also have come from comets. | text | null |
L_0060 | other objects in the solar system | T_0606 | For several decades, Pluto was a planet. But new solar system objects were discovered that were just as planet-like as Pluto. Astronomers figured out that they were like planets except for one thing. These objects had not cleared their orbits of smaller objects. They didnt have enough gravity to do so. Astronomers made a category called dwarf planets. There are five dwarf planets in our solar system: Ceres, Pluto, Makemake, Haumea and Eris. Figure 25.36 shows Ceres. Ceres is a rocky body that orbits the Sun and is not a star. It could be an asteroid or a planet. Before 2006, Ceres was thought to be the largest asteroid. Is it an asteroid? Ceres is in the asteroid belt. But it is by far the largest object in the belt. Ceres has such high gravity that it is spherical. Is it a planet? Ceres only has about 1.3% of the mass of the Earths Moon. Its orbit is full of other smaller bodies. Its gravity was not high enough to clear its orbit. Ceres fails the fourth criterion for being a planet. Ceres is now considered a dwarf planet along with Pluto. | text | null |
L_0060 | other objects in the solar system | T_0607 | For decades Pluto was a planet. But even then, scientists knew it was an unusual planet. The other outer planets are all gas giants. Pluto is small, icy and rocky. With a diameter of about 2400 kilometers, it has only about 1/5 the mass of Earths Moon. The other planets orbit in a plane. Plutos orbit is tilted. The shape of the orbit is like a long, narrow ellipse. Plutos orbit is so elliptical that sometimes it is inside the orbit of Neptune. Plutos orbit is in the Kuiper belt. We have discovered more than 200 million Kuiper belt objects. Pluto has 3 moons of its own. The largest, Charon, is big. Some scientists think that Pluto-Charon system is a double dwarf planet (Figure 25.37). Two smaller moons, Nix and Hydra, were discovered in 2005. | text | null |
L_0060 | other objects in the solar system | T_0607 | For decades Pluto was a planet. But even then, scientists knew it was an unusual planet. The other outer planets are all gas giants. Pluto is small, icy and rocky. With a diameter of about 2400 kilometers, it has only about 1/5 the mass of Earths Moon. The other planets orbit in a plane. Plutos orbit is tilted. The shape of the orbit is like a long, narrow ellipse. Plutos orbit is so elliptical that sometimes it is inside the orbit of Neptune. Plutos orbit is in the Kuiper belt. We have discovered more than 200 million Kuiper belt objects. Pluto has 3 moons of its own. The largest, Charon, is big. Some scientists think that Pluto-Charon system is a double dwarf planet (Figure 25.37). Two smaller moons, Nix and Hydra, were discovered in 2005. | text | null |
L_0060 | other objects in the solar system | T_0608 | Haumea was named a dwarf planet in 2008. It is an unusual dwarf planet. The body is shaped like an oval! Haumeas longest axis is about the same as Plutos diameter, and its shortest axis is about half as long. The bodys orbit is tilted 28. Haumea is so far from the Sun that it takes 283 years to make one orbit (Figure 25.38). Haumea is the third-brightest Kuiper Belt object. It was named for the Hawaiian goddess of childbirth. Haumea has two moons, Hiiaka and Namaka, the names of the goddess Haumeas daughters. Haumeas odd oval shape is probably caused by its extremely rapid rotation. It rotates in just less than 4 hours! Like other Kuiper belt objects, Haumea is covered by ice. Its density is similar to Earths Moon, at 2.6 3.3 g/cm3 . This means that most of Haumea is rocky. Haumea is part of a collisional family. This is a group of astronomical objects that formed from an impact. This family has Haumea, its two moons, and five more objects. All of these objects are thought to have formed from a collision very early in the formation of the solar system. | text | null |
L_0060 | other objects in the solar system | T_0609 | Makemake is the third-largest and second-brightest dwarf planet we have discovered so far (Figure 25.39). Make- make is only 75 percent the size of Pluto. Its diameter is between 1300 and 1900 kilometers. The name comes from the mythology of the Eastern Islanders. Makemake was the god that created humanity. At a distance between 38.5 to 53 AU, this dwarf planet orbits the Sun in 310 years. Makemake is made of methane, ethane, and nitrogen ices. | text | null |
L_0060 | other objects in the solar system | T_0610 | Eris is the largest known dwarf planet in the solar system. It is 27 percent larger than Pluto (Figure 25.40). Like Pluto and Makemake, Eris is in the Kuiper belt. But Eris is about 3 times farther from the Sun than Pluto. Because of its distance, Eris was not discovered until 2005. Early on, it was thought that Eris might be the tenth planet. Its discovery helped astronomers realize that they needed a new definition of planet. Eris has a small moon, Dysnomia. Its moon orbits Eris once about every 16 days. Astronomers know there may be other dwarf planets far out in the solar system. Look for Quaoar, Varuna and Orcus to be possibly added to the list of dwarf planets in the future. We still have a lot to discover and explore! | text | null |
L_0061 | stars | T_0611 | The stars that make up a constellation appear close to each other from Earth. In reality, they may be very distant from one another. Constellations were important to people, like the Ancient Greeks. People who spent a lot of time outdoors at night, like shepherds, named them and told stories about them. Figure 26.1 shows one of the most easily recognized constellations. The ancient Greeks thought this group of stars looked like a hunter. They named it Orion, after a great hunter in Greek mythology. The constellations stay the same night after night. The patterns of the stars never change. However, each night the constellations move across the sky. They move because Earth is spinning on its axis. The constellations also move with the seasons. This is because Earth revolves around the Sun. Different constellations are up in the winter than in the summer. For example, Orion is high up in the winter sky. In the summer, its only up in the early morning. | text | null |
L_0061 | stars | T_0612 | Only a tiny bit of the Suns light reaches Earth. But that light supplies most of the energy at the surface. The Sun is just an ordinary star, but it appears much bigger and brighter than any of the other stars. Of course, this is just because it is very close. Some other stars produce much more energy than the Sun. How do stars generate so much energy? | text | null |
L_0061 | stars | T_0613 | Stars shine because of nuclear fusion. Fusion reactions in the Suns core keep our nearest star burning. Stars are made mostly of hydrogen and helium. Both are very lightweight gases. A star contains so much hydrogen and helium that the weight of these gases is enormous. The pressure at the center of a star is great enough to heat the gases. This causes nuclear fusion reactions. A nuclear fusion reaction is named that because the nuclei (center) of two atoms fuse (join) together. In stars like our Sun, two hydrogen atoms join together to create a helium atom. Nuclear fusion reactions need a lot of energy to get started. Once they begin, they produce even more energy. | text | null |
L_0061 | stars | T_0614 | Scientists have built machines called particle accelerators. These amazing tools smash particles that are smaller than atoms into each other head-on. This creates new particles. Scientists use particle accelerators to learn about nuclear fusion in stars. They can also learn about how atoms came together in the early universe. Two well-known accelerators are SLAC, in California, and CERN, in Switzerland. | text | null |
L_0061 | stars | T_0615 | Stars shine in many different colors. The color relates to a stars temperature and often its size. | text | null |
L_0061 | stars | T_0616 | Think about the coil of an electric stove as it heats up. The coil changes in color as its temperature rises. When you first turn on the heat, the coil looks black. The air a few inches above the coil begins to feel warm. As the coil gets hotter, it starts to glow a dull red. As it gets even hotter, it becomes a brighter red. Next it turns orange. If it gets extremely hot, it might look yellow-white, or even blue-white. Like a coil on a stove, a stars color is determined by the temperature of the stars surface. Relatively cool stars are red. Warmer stars are orange or yellow. Extremely hot stars are blue or blue-white. | text | null |
L_0061 | stars | T_0617 | The most common way of classifying stars is by color as shown, in Table 26.1. Each class of star is given a letter, a color, and a range of temperatures. The letters dont match the color names because stars were first grouped as A through O. It wasnt until later that their order was corrected to go by increasing temperature. When you try to remember the order, you can use this phrase: Oh Be A Fine Good Kid, Man. Class O Color Blue Temperature range 30,000 K or more Sample Star An artists depiction of the O class star Zeta Pup- pis. B Blue-white 10,00030,000 K An artists depiction of Rigel, a Class B star. Class A Color White Temperature range 7,50010,000 K Sample Star Sirius A is the brightest star that we see in the night sky. The dot on the right, Sirius B, is a white dwarf. F Yellowish-white 6,0007,500 K There are two F class stars in this image, the super- giant Polaris A and Po- laris B. What we see in the night sky as the single star Polaris, we also known as the North Star. G Yellow 5,5006,000 K Our Sun: the most im- portant G class star in the Universe, at least for hu- mans. Class K M Color Orange Red Temperature range 3,5005,000 K 2,0003,500 K Sample Star Arcturus is a Class K star that looks like the Sun but is much larger. There are two types of Class M stars: red dwarfs and red giants. An artists concept of a red dwarf star. Most stars are red dwarfs. The red supergiant Betel- geuse is seen near Orions belt. The blue star in the lower right is the Class B star Rigel. The surface temperature of most stars is due to its size. Bigger stars produce more energy, so their surfaces are hotter. But some very small stars are very hot. Some very big stars are cool. | text | null |
L_0061 | stars | T_0618 | We could say that stars are born, change over time, and eventually die. Most stars change in size, color, and class at least once during their lifetime. | text | null |
L_0061 | stars | T_0619 | Stars are born in clouds of gas and dust called nebulas. Our Sun and solar system formed out of a nebula. A nebula is shown in Figure 26.2. In Figure 26.1, the fuzzy area beneath the central three stars contains the Orion nebula. For a star to form, gravity pulls gas and dust into the center of the nebula. As the material becomes denser, the pressure and the temperature increase. When the temperature of the center becomes hot enough, nuclear fusion begins. The ball of gas has become a star! | text | null |
L_0061 | stars | T_0620 | For most of a stars life, hydrogen atoms fuse to form helium atoms. A star like this is a main sequence star. The hotter a main sequence star is, the brighter it is. A star remains on the main sequence as long as it is fusing hydrogen to form helium. Our Sun has been a main sequence star for about 5 billion years. As a medium-sized star, it will continue to shine for about 5 billion more years. Large stars burn through their supply of hydrogen very quickly. These stars live fast and die young! A very large star may only be on the main sequence for 10 million years. A very small star may be on the main sequence for tens to hundreds of billions of years. | text | null |
L_0061 | stars | T_0621 | A star like our Sun will become a red giant in its next stage. When a star uses up its hydrogen, it begins to fuse helium atoms. Helium fuses into heavier atoms like carbon. At this time the stars core starts to collapse inward. The stars outer layers spread out and cool. The result is a larger star that is cooler on the surface, and red in color. Eventually a red giant burns up all of the helium in its core. What happens next depends on the stars mass. A star like the Sun stops fusion and shrinks into a white dwarf star. A white dwarf is a hot, white, glowing object about the size of Earth. Eventually, a white dwarf cools down and its light fades out. | text | null |
L_0061 | stars | T_0622 | A more massive star ends its life in a more dramatic way. Very massive stars become red supergiants, like Betelgeuse. In a red supergiant, fusion does not stop. Lighter atoms fuse into heavier atoms. Eventually iron atoms form. When there is nothing left to fuse, the stars iron core explodes violently. This is called a supernova explosion. The incredible energy released fuses heavy atoms together. Gold, silver, uranium and the other heavy elements can only form in a supernova explosion. A supernova can shine as brightly as an entire galaxy, but only for a short time, as shown in Figure 26.3. | text | null |
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