The Terrestrial Planets 7: Touring Our Solar System

Venus: Surface Features 2

About 80 percent of Venus's surface consists of plains covered by volcanic flows. Some lava channels extend hundreds of kilometers—one is 6,800 kilometers long. Scientists have identified thousands of now inactive volcanic structures. Most are small shield volcanoes, although more than 1,500 volcanoes greater than 20 kilometers across have been mapped. Two of these volcanoes are 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. Only 8 percent of Venus's surface consists of highlands that may be similar to continental areas on Earth. Tectonic activity on Venus seems to be driven by the upwelling and downwelling of material in the planet's interior.

Several characteristics of Mercury—such as its size, surface features, temperature range, and lack of atmosphere—make Mercury most like _____.

Earth's moon

Water on Mars 2

Images from the Mars Global Surveyor indicate that groundwater has recently migrated to the surface. These spring-like seeps have created gullies where they emerge from valley and crater walls. Some of the escaping water may have initially frozen due to the average Martian temperatures that range between -70°C and -100°C. Eventually, however, it seeped out as a slurry of sediment, ice, and liquid that formed the gullies.

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Introduction

In January 2004, the space rover Spirit bounced onto the rocky surface of Mars, known as the Red Planet. 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 analyses of Mars's surface. Much of what we learn about the planets have been gathered by rovers such as Spirit, or space probes such as Voyager, that travel to the far reaches of the solar system. In this lesson, you'll explore three terrestrial planets—Mercury, Venus, and Mars—and see how they compare with Earth.

Water on Mars 3

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.

Mars: 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's northern hemisphere revealed numerous large inactive volcanoes. The biggest, Olympus Mons, is the size of Ohio and is 23 kilometers high—more than 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 the figure. It is thought to have formed by slippage of material along huge faults in the crustal layer.

Mars: The Red Planet

Mars has evoked great interest throughout history. Mars is easy to observe, which may explain why so many people are fascinated by it. 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.

Mercury: 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.

Mercury: Surface Temperature

Mercury 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. 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.

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.

Venus: Surface Temperature

On Venus, the greenhouse effect has heated the planet's atmosphere to 475°C—hot enough to melt lead. Several factors contribute to what scientists have called Venus's runaway greenhouse effect. The main reason for the runaway greenhouse effect on Venus is that its atmosphere is 97 percent carbon dioxide, a greenhouse gas. Venus lacks oceans in which carbon dioxide gas could dissolve and remove it from the atmosphere. Scientists think that oceans on Venus may have evaporated early in its history. Water vapor in the atmosphere then accelerated the greenhouse effect. But the atmosphere eventually lost most of its water vapor. The sun's ultraviolet radiation broke down water molecules into hydrogen and oxygen. These gases then escaped into space.

How have scientists mapped the surface features of Venus?

Scientists have sent radar pulses to Venus's surface. The time it takes the pulses to return enables scientists to map topography.

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. 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.

Which statement best describes the role of water on Mars?

Some features suggest liquid water once was plentiful on Mars, but it is gone now.

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 Martian 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.

The Martian Atmosphere 2

The composition of Mars's atmosphere is similar to that of Venus. But Mars is very cold. Why doesn't the greenhouse effect warm Mars's atmosphere? The reason is that Mars's atmosphere is extremely thin compared with the atmosphere of Venus (or Earth). Scientists think that, early in its history, Mars had a thick atmosphere warmed by the greenhouse effect. But Mars's gravity was too low for the planet to keep its atmosphere. Most of the gases escaped into space, and the planet cooled.

What is the composition of the polar caps of Mars?

The polar caps on Mars consist of water ice and are covered by a thin layer of frozen carbon dioxide.

Venus: Surface Features

Venus is covered in thick clouds that hide its surface from view. 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 the figure. To map Venus, radar pulses are sent toward the planet's surface, and the heights of plateaus and mountains are measured by timing the return of the radar echo. Data have confirmed that basaltic volcanism and tectonic activity have shaped Venus's surface. Based on the low density of impact craters, these forces must have been very active during the recent geologic past.

Venus: The Veiled Planet

Venus is second only to the moon in brilliance in the night sky. It orbits the sun 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.

Which processes most shaped the surface of Venus during the recent geologic past?

basaltic volcanism and tectonic activity

Most of Mars appears red through a telescope, but brilliant white regions may be observed too. What are these white regions?

polar ice caps

Venus has been described as Earth's twin. One reason that Venus and Earth are alike is their similar _____.

size and mass

Among the eight planets, Mercury has the greatest _____.

temperature extremes