High School Earth Science/Inner Planets

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The four planets closest to the sun—Mercury, Venus, Earth and Mars—are the inner planets, also called the terrestrial planets because they are similar to Earth. Figure 25.7 shows the relative sizes of these four planets. All of the inner planets are small, relative to the outer planets. All of the inner planets are solid, dense, rocky planets. The inner planets either do not have moons or have just one (Earth) or two (Mars). None of the inner planets has rings. Compared to the outer planets, the inner planets have shorter orbits around the Sun, but all the inner planets spin more slowly. Venus spins the slowest of all the planets. At one time, all the inner planets have been geologically active. They are all made of cooled igneous rock with inner iron cores.

Figure 25.7: This composite shows the relative sizes of the four inner planets. From left to right, they are Mercury, Venus, Earth, and Mars.

Lesson Objectives[edit]

  • Describe key features of each of the inner planets.
  • Compare each of the inner planets to Earth and to one another.


Figure 25.8: Mariner 10 made three flybys of Mercury in 1974 and 1975.

Mercury, shown in Figure 25.7, is the planet closest to the Sun. Mercury is the smallest planet, and it has no moon. As Figure 25.7 shows, the surface of Mercury is covered with craters, like Earth's moon. The presence of impact craters that are so old means that Mercury hasn't changed much geologically for billions of years and, with only a trace of an atmosphere, 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. However, the Mariner 10 spacecraft, shown in Figure 25.8, visited Mercury in 1974–1975. In January 2008, the Messenger mission returned to Mercury and took much more detailed pictures. One of these images can be seen in Figure 25.9.

Figure 25.9: Mercury is covered with craters, like Earth's moon.

Short Year, Long Days[edit]

Mercury is named for the Roman messenger god, who could run extremely fast. Likewise, Mercury moves very fast in its orbit around the Sun. A year on Mercury—the length of time it takes to orbit the Sun—is just 88 Earth days.

Mercury has a very short year, but very long days. A day is defined as the time it takes a planet to turn on its axis. 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. In other words, on Mercury, a year is only a Mercury day and a half long!

Extreme Temperatures[edit]

Mercury is very close to the Sun, so it can get very hot. However, Mercury has virtually no atmosphere and it rotates very slowly. Because there is no atmosphere and no water to insulate the surface, temperatures on the surface of Mercury vary widely. In direct sunlight, the surface can be as hot as 427°C (801°F). On the dark side, or in the shadows inside craters, the surface can be as cold as –183°C (–297°F)! Although most of Mercury is extremely dry, scientists believe there may be a small amount of water in the form of ice at the poles of Mercury, in areas which never receive direct sunlight.

A Liquid Metal Core[edit]

Figure 25.10 shows a diagram of Mercury's interior. Mercury is one of the densest planets. Scientists believe the interior contains a relatively large, liquid core made mostly of melted iron. Mercury's core takes up about 42% of the planet's volume. Mercury's highly cratered surface is evidence that Mercury is not geologically active.

Figure 25.10: Mercury contains a thin crust (1), a mantle (2), and a large, liquid core (3) that is rich in iron.


The second planet out from the Sun, Venus, is our nearest neighbor. Not only is it closer to Earth than any other planet, but it also is the most similar to Earth in size. Named after the Roman goddess of love, it is the only planet named after a female. Venus is sometimes called Earth's "sister planet". But just how similar is Venus to Earth?

A Harsh Environment[edit]

Viewed through a telescope, Venus looks smooth and featureless. That's because Venus is covered by a thick layer of clouds, as shown in pictures of Venus taken at ultraviolet wavelengths, such as Figure 25.11. Because of the thick, cloudy atmosphere, we cannot take ordinary photos of the surface of Venus, even from spacecraft orbiting the planet. However, we can make maps of the surface using radar. Figure 25.12 shows a topographical map of Venus produced by the Magellan probe using radar.

Figure 25.11: This ultraviolet image from the Pioneer Venus Orbiter shows thick layers of clouds in the atmosphere of Venus.
Figure 25.12: This topographic map of Venus was made from radar data collected by theMagellan probe between 1990 and 1994.

Unlike clouds on Earth, Venus's clouds are not made of water vapor. They are made of carbon dioxide and sulfur dioxide—and they also contain large amounts of corrosive sulfuric acid!

The atmosphere of Venus is so thick that the atmospheric pressure on the surface of Venus is 90 times greater than the atmospheric pressure on Earth's surface. The thick atmosphere also causes a strong greenhouse effect, which traps heat from the Sun. As a result, Venus is the hottest planet, even hotter than Mercury. Temperatures at the surface reach 465°C (860°F). That's hot enough to melt lead!


Venus has more volcanoes than any other planet. Planetary scientists have estimated that Venus has up to 100,000 or even a million volcanoes. Although these volcanoes contributed carbon dioxide in the past, most of the volcanoes are now dead. Venus doesn't seem to have tectonic plates like the Earth's. Its surface is covered with dead volcanoes and ancient craters.

Orbiting spacecraft have used radar to reveal mountains, valleys, and canyons on Venus. Most of the surface, however, has large areas of volcanoes surrounded by plains of lava. Figure 25.13 is an image made by a computer using radar data. It shows a volcano called Maat Mons, with lava beds in the foreground. The reddish-orange color is close to what scientists think the color of sunlight would look like on the surface of Venus.

Figure 25.13: This image of Maat Mons was generated from radar data. The surface of Venus has many mountains, volcanoes, and plains of lava.

Motion and Appearance[edit]

Venus is the only planet that rotates clockwise as viewed from its North pole, in a direction opposite to the direction it orbits the Sun. It turns slowly in the reverse direction, making one turn every 243 days. This is longer than a year on Venus—it takes Venus only 224 days to orbit the Sun.

Because the orbit of Venus is inside Earth's orbit, Venus always appears close to the Sun. When Venus rises early in the morning, just before the Sun rises, it is sometimes called "the morning star". When it sets in the evening, just after the Sun sets, it may be called "the evening star". Venus' clouds reflect sunlight very well. As a result, Venus is very bright. When it is visible, Venus is the brightest object in the sky besides the sun and the Moon.

Like Mercury, Venus has no moon.


The third planet out from the Sun is shown in Figure 25.14. Does it look familiar? It's Earth! Because it is our home planet, we know a lot more about Earth than we do about any other planet. But what are key features of Earth when viewed as a member of our solar system?

Figure 25.14: This famous image of Earth was taken during the Apollo 17 mission to the moon.

Oceans and Atmosphere[edit]

As you can see in Figure 25.14, Earth has vast oceans of liquid water, large masses of land, ice covering the poles, and a dynamic atmosphere with clouds of water vapor. Earth's average surface temperature is 14°C (57°F). As you know, water is a liquid at this temperature. The oceans and the atmosphere help keep Earth's surface temperatures fairly steady.

Earth is the only planet known to have life. The conditions on Earth, especially the presence of liquid water, are ideal for life as we know it. The atmosphere filters out radiation that would be harmful to life, such as ultraviolet radiation and X rays. The presence of life has changed Earth's atmosphere, so it has much more oxygen than the atmospheres of other planets.

Plate Tectonics[edit]

The top layer of Earth's interior—the crust—contains numerous plates, known as tectonic plates. These plates move on the convecting mantle below, so they slowly move around on the surface. Movement of the plates causes other geological activity, such as earthquakes, volcanoes, and the formation of mountains. Earth is the only planet known to have plate tectonics.

Earth's Motions and Moon[edit]

Earth rotates on its axis once per day. In fact, the time of this rotation is how people have defined a day. Earth orbits the Sun once every 365.24 days, which is also how we have defined a year. Earth has one large moon, which orbits Earth once every 29.5 days, a period known as a month.

Earth's moon is the only large moon around a terrestrial planet in the solar system. The Moon is covered with craters, and also has large plains of lava. There is evidence that the Moon formed when a very large object—perhaps as large as the planet Mars—struck Earth in the distant past.


Mars, shown in Figure 25.15, is the fourth planet from the Sun, and the first planet beyond Earth's orbit. The Martian atmosphere is thin relative to Earth's and with much lower atmospheric pressure. Unlike Earth’s neighbor on the side nearer the sun, Mars has only a weak greenhouse effect, which raises its temperature only slightly above what it would be if the planet did not have an atmosphere.

Figure 25.15: This image of Mars, taken by the Hubble Space Telescope in August, 2003, shows the planet's reddish color and a prominent polar ice cap.

Although Mars is not the closest planet to Earth, it is the easiest to observe. Therefore, Mars has been studied more thoroughly than any other planet besides Earth. Humans have sent many space probes to Mars. Currently, there are three scientific satellites in orbit around Mars, and two functioning rovers on the surface. No humans have ever set foot on Mars. However, both NASA and the European Space Agency have set goals of sending people to Mars sometime between 2030 and 2040.

A Red Planet[edit]

Viewed from Earth, Mars is reddish in color. The ancient Greeks and Romans named the planet after the god of war. They may have associated the planet with war because its red color reminded them of blood. Mars appears red because the surface of the planet really is a reddish-orange rust color, due to large amounts of iron in the soil. Mars has only a very thin atmosphere, made up mostly of carbon dioxide.

Surface Features[edit]

Mars is home to the largest mountain in the solar system—Olympus Mons, shown in Figure 25.16. Olympus Mons is a shield volcano, similar to the volcanoes that make up the Hawaiian islands on Earth. Olympus Mons is about 27 km (16.7 miles/88,580 ft) above the normal Martian surface level. That makes it more than three times taller than Mount Everest. At its base, Olympus Mons is about the size of the entire state of Arizona!

Figure 25.16: The Martian volcano Olympus Mons is the largest mountain in the solar system.

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, as long as Europe is wide, and one-fifth the circumference of Mars. The canyon is 7 km (4.3 miles) deep. By comparison, the Grand Canyon on Earth is only 446 km (277 miles) long and about 2 km (1.2 miles) deep.

Figure 25.17: The Martian canyon Valles Marineris is the largest canyon in the solar system.

Although Mars has mountains, canyons, and other features similar to Earth, it doesn't 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, though fewer than on the Moon.

Is There Water on Mars?[edit]

Water cannot stay in liquid form on Mars because the pressure of the atmosphere is too low. However, there is a lot of water in the form of ice. Figure 25.15 shows a prominent ice cap at the south pole of Mars. Scientists also believe there is also a lot of ice water present just under the Martian surface. This ice can melt when volcanoes erupt, and water can flow across the surface temporarily.

Scientists have reason to think that water once flowed over the surface of Mars because they can see surface features that look like water-eroded canyons, and the Mars rover collected round clumps of crystals that, on Earth, usually form in water. The presence of water on Mars, even though it is now frozen as ice, suggests that it might have been possible for life to exist on Mars in the past.

Two Martian Moons[edit]

Mars has two very small moons, Phobos and Deimos. As you can see in Figure 25.18, these moons are not spherical in shape, but instead just look like irregular rocks. Phobos and Deimos were discovered in 1877. They are named after characters in Greek mythology—the two sons of Ares, who followed their father into war. Ares is equivalent to the Roman god Mars.

Figure 25.18: Mars has two small moons, Phobos (left) and Deimos (right).

Lesson Summary[edit]

  • The four inner planets, or terrestrial planets, have solid, rocky surfaces.
  • Mercury is the smallest planet and the closest to the Sun. It has an extremely thin atmosphere, with surface temperatures ranging from very hot to very cold. Like the Moon, it is covered with craters.
  • Venus is the second planet from the Sun and the closest planet to Earth, in distance and in size. It has a very thick, corrosive atmosphere, and the surface temperature is extremely high.
  • Radar maps of Venus show that it has mountainous areas, as well as volcanoes surrounded by plains of lava.
  • Venus rotates slowly in a direction opposite to the direction of its orbit.
  • Earth is the third planet from the Sun. It is the only planet with large amounts of liquid water, and the only planet known to support life. Earth has a large moon, the only large moon around a terrestrial planet.
  • Mars is the fourth planet from the Sun. It has two small moons. Mars is reddish in color because of oxidized iron (rust) in its soil. Mars has the largest mountain and the largest canyon in the solar system.
  • There is a lot of water ice in the polar ice caps and under the surface of Mars.

Review Questions[edit]

  1. Name the inner planets in order from the Sun outward. Then name them from smallest to largest.
  2. Why do the temperatures on Mercury vary widely?
  3. Venus is farther from the Sun than Mercury. Why does Venus have higher temperatures than Mercury?
  4. How are maps of Venus made?
  5. Name two major ways in which Earth is unlike any other planets.
  6. Why does Mars appear to be red?
  7. Suppose you are planning a mission to Mars. Identify two places where you might be able to get water on the planet.


The time it takes a planet to rotate once on its axis.
inner planets
The four planets inside the asteroid belt of our solar system; Mercury, Venus, Earth and Mars.
terrestrial planets
The solid, dense, rocky planets that are like Earth.
The time it takes for a planet to orbit the Sun.

Points to Consider[edit]

  • We are planning to send humans to Mars sometime in the next few decades. What do you think it would be like to be on Mars? Why do you think we are going to Mars instead of Mercury or Venus?
  • Why do you think the four inner planets are also called terrestrial planets? What might a planet be like if it weren't a terrestrial planet?

Introduction to the Solar System · Outer Planets