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Lesson 4-02 Terrestrial Planets

Page history last edited by Julie McShea 11 years ago


Lesson 4.02 – Terrestrial Planets

Standard:  ES1.a Students know how the differences and similarities among the sun, the terrestrial planets, and the gas planets may have been established during the formation of the solar system.




Earth is part of a solar system, which consists of the sun, eight (possibly nine) planets and their moons, asteroids, meteoroids and comets.  In this lesson you will learn about the group of planets which scientists call the Terrestrial Planets of which Earth is one.  (Earth will be covered extensively in later lessons so it is not included in this lesson.)




In January 2004, the space rover, Spirit, bounced onto the rock-littered surface of Mars, known as the Red Planet. Shown in Figure 5, Spirit and its companion rover, Opportunity, were on the Red Planet to study minerals and geological processes, both past and present. They also searched for signs of the liquid water—such as eroded rocks or dry stream channels on Mars’s surface. For the next few months, the rovers sent back to Earth numerous images and chemical analysis of Mars’s surface. Much of what we learn about the planets has been gathered by rovers, such as Spirit, or space probes that travel to the far reaches of the solar system, such as Voyager. In this section, we’ll explore three terrestrial planets—Mercury, Venus, and Mars—and see how they compare with the fourth terrestrial planet, Earth.


Figure 5 Spirit roved the surface of Mars and gathered data about the Red Planet’s geologic past and present.



Mercury: The Innermost Planet

Mercury, the innermost and smallest planet, is hardly larger than Earth’s moon and is smaller than three other moons in the solar system. Like our own moon, it absorbs most of the sunlight that strikes it and reflects only 6 percent of sunlight back into space. This low percentage of reflection is characteristic of terrestrial bodies that have no atmosphere. Earth, on the other hand, reflects about 30 percent of the light that strikes it. Most of this reflection is from clouds.



Surface Features

Mercury has cratered highlands, much like the moon, and some smooth terrains that resemble maria. Unlike the moon, however, Mercury is a very dense planet, which implies that it contains a large iron core for its size. Also, Mercury has very long scarps (deep slopes) that cut across the plains and craters alike. These scarps may have resulted from crustal changes as the planet cooled and shrank.



Surface Temperature

Mercury, shown in Figure 6, revolves around the sun quickly, but it rotates slowly. One full day-night cycle on Earth takes 24 hours. On Mercury, one rotation requires 59 Earth-days. Thus, a night on Mercury lasts for about three months and is followed by three months of daylight. Nighttime temperatures drop as low as −173°C, and noontime temperatures exceed 427°C—hot enough to melt lead. Mercury has the greatest temperature extremes of any planet. The odds of life as we know it existing on Mercury are almost nonexistent.





Figure 6 Mercury’s surface looks somewhat similar to the far side of Earth’s moon.





Venus: The Veiled Planet

Venus, second only to the moon in brilliance in the night sky, is named for the goddess of love and beauty.



It orbits the sun in a nearly perfect circle once every 255 Earth-days.

Venus is similar to Earth in size, density, mass, and location in the solar system. Thus, it has been referred to as “Earth’s twin.”



Because of these similarities, it is hoped that a detailed study of Venus will provide geologists with a better understanding of Earth’s history.



Surface Features

Venus is covered in thick clouds that visible light cannot penetrate. Nevertheless, radar mapping by the uncrewed Magellan spacecraft and by instruments on Earth have revealed a varied topography with features somewhat between those of Earth and Mars, as shown in Figure 7.



Figure 7 Venus This global view of the surface of Venus is computer generated from two years of Magellan Project radar mapping. The twisting bright features that cross the planet are highly fractured mountains and canyons of the eastern Aphrodite highland.



About 80 percent of Venus’s surface consists of plains covered by volcanic flows. Some lava channels extend hundreds of kilometers—one is 6800 kilometers long. Scientists have identified thousands of volcanic structures. Most are small shield volcanoes, although more than 1500 volcanoes greater than 20 kilometers across have been mapped. Figure 8 shows two of these volcanoes—one is Sapas Mons, 400 kilometers across and 1.5 kilometers high. Flows from this volcano mostly erupted from its flanks rather than its summit, in the manner of Hawaiian shield volcanoes.


Figure 8 Sapas Mons and Maat Mons In this computer-generated image from Venus, Maat Mons, a large volcano, is near the horizon. Sapas Mons is the bright feature in the foreground.

Comparing And Contrasting What features on Venus are similar to those on Earth? What features are different?



Only 8 percent of Venus’s surface consists of highlands that may be similar to continental areas on Earth.



Surface Temperature

Before the advent of spacecraft, Venus was considered to be a possible habitat for living things. However, evidence from space probes indicates otherwise. The surface temperature of Venus reaches 475°C, and its atmosphere is 97 percent carbon dioxide.

Only small amounts of water vapor and nitrogen have been detected. Venus’s atmosphere contains a cloud layer about 25 kilometers thick. The atmospheric pressure is 90 times that at Earth’s surface. This hostile environment makes it unlikely that life as we know it exists on Venus.



Mars: The Red Planet

Mars has evoked greater interest than any other planet.  Mars is easy to observe, which may explain why so many people are fascinated by it. The surfaces of all other planets within telescopic range are hidden by clouds—except for Mercury, whose nearness to the sun makes viewing it difficult. Mars is known as the Red Planet because it appears as a reddish ball when viewed through a telescope. Mars also has some dark regions that change intensity during the Martian year. The most prominent telescopic features of Mars are its brilliant white polar caps.



The Martian Atmosphere

The Martian atmosphere has only 1 percent the density of Earth’s. It is made up primarily of carbon dioxide with tiny amounts of water vapor. Data from Mars probes confirm that the polar caps of Mars are made of water ice, covered by a thin layer of frozen carbon dioxide. As winter nears in either hemisphere, temperatures drop to −125°C, and additional carbon dioxide is deposited.



Although the atmosphere of Mars is very thin, extensive dust storms occur and may cause the color changes observed from Earth. Hurricane-force winds up to 270 kilometers per hour can persist for weeks. As shown in Figure 9, images from Spirit reveal a Martian landscape remarkably similar to a rocky desert on Earth, with abundant sand dunes and impact craters partially filled with dust.




Figure 9 Many parts of Mars’s landscape resemble desert areas on Earth.



Surface Features

Mariner 9, the first spacecraft to orbit another planet, reached Mars in 1971 amid a raging dust storm. When the dust cleared, images of Mars’ northern hemisphere revealed numerous large volcanoes. The biggest, Olympus Mons, is the size of Ohio and is 23 kilometers high—over two and a half times higher than Mount Everest. This gigantic volcano and others resemble Hawaiian shield volcanoes on Earth.



Most Martian surface features are old by Earth standards. The highly cratered southern hemisphere is probably 3.5 billion to 4.5 billion years old. Even the relatively “fresh” volcanic features of the northern hemisphere may be older than 1 billion years.



Another surprising find made by Mariner 9 was the existence of several canyons that are much larger than Earth’s Grand Canyon. The largest, Valles Marineris, is shown in Figure 10. It is thought to have formed by slippage of material along huge faults in the crustal layer. In this respect, it would be comparable to the rift valleys of Africa.


Figure 10 Valles Marineris Mars’s Valles Marineris canyon system is more than 5000 kilometers long and up to 8 kilometers deep. The dark spots on the left edge of the image are huge volcanoes.



Water on Mars

Some areas of Mars exhibit drainage patterns similar to those created by streams on Earth. The rover Opportunity, for example, found evidence of evaporite minerals and geologic formations associated with liquid water, as shown in Figure 11. In addition, Viking images have revealed ancient islands in what is now a dry streambed. When these stream-like channels were first discovered, some observers speculated that a thick water-laden atmosphere capable of generating torrential downpours once existed on Mars. If so, what happened to this water? The present Martian atmosphere contains only traces of water.



Many scientists do not accept the theory that Mars once had an active water cycle similar to Earth’s. Rather, they believe that most of the large stream-like valleys were created by the collapse of surface material caused by the slow melting of subsurface ice. Data from Opportunity, however, indicate that some areas were “drenched” in water. It will take scientists many months, if not years, to analyze the data gathered by the latest Mars mission. Because water is an essential ingredient for life, scientists and nonscientists alike are enthusiastic about exploring this phenomenon.


Figure 11 The composition and markings of some Martian rocks indicate that liquid water was once present on Mars’s surface. The marking shown in the center of the rock, however, was created by a NASA rover during chemical analysis.




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