Inner Planets

Lesson Objectives

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


  • day
  • inner planets
  • terrestrial planets
  • year


What evidence do planetary geologists have to go on to determine the geology of the inner planets? On Earth, scientists can collect and analyze the chemistry of samples, do radiometric dating to determine their ages, and look at satellite images to see large-scale features. Rovers have landed on Mars and sent back enormous amounts of information but much of the rest of what is known about the inner planets is from satellite images.

The Inner Planets

The inner planets, or terrestrial planets, are the four planets closest to the Sun: Mercury, Venus, Earth, and Mars. Figure  below shows the relative sizes of these four inner planets.

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

Unlike the outer planets, which have many of satellites, Mercury and Venus do not have moons, Earth has one, and Mars has two. Of course, the inner planets have shorter orbits around the Sun, and they all spin more slowly. Geologically, the inner planets are all made of cooled igneous rock with iron cores, and all have been geologically active, at least early in their history. None of the inner planets has rings.


Although Earth is the third planet out from the Sun this lesson will start here. We know a lot more about Earth, so what we know can be used for comparison with the other planets.

What are Earth’s most distinctive features? This famous image of Earth was taken during the Apollo 17 mission to the moon. Can you find a hurricane? A storm spinning in the opposite direction from the hurricane?

Earth’s Surface and Life

As you can see in (Figure above), Earth has vast oceans of liquid water, large masses of exposed land, and a dynamic atmosphere with clouds of water vapor. Earth also has ice covering its polar regions. Earth’s average surface temperature is 14°C (57°F). Water is a liquid at this temperature, but the planet also has water in its other two states, solid and gas. The oceans and the atmosphere help keep Earth’s surface temperatures fairly steady.

As yet Earth is the only planet known to have life. The presence of liquid water, the ability of the atmosphere to filter out harmful radiation, and many other features make the planet uniquely suited to harbor life. Life and Earth now affect each other; for example, the evolution of plants allowed oxygen to enter the atmosphere in large enough quantities for animals to evolve. Although life has not been found elsewhere in the solar system, other planets or satellites may harbor primitive life forms. Life may also be found elsewhere in the universe.

Structure and Plate Tectonics

The heat that remained from the planet’s accretion, gravitational compression, and radioactive decay allowed the Earth to melt, probably more than once. As it subsequently cooled, gravity pulled metal into the center to create the core. Heavier rocks formed the mantle and lighter rocks formed the crust.

Earth’s crust is divided into tectonic plates, which move around on the surface because of the convecting mantle below. Movement of the plates causes other geological activity, such as earthquakes, volcanoes, and the formation of mountains. The locations of these features are mostly related to current or former plate boundaries. Earth is the only planet known to have plate tectonics.

Earth’s Motions and Satellites

Earth rotates on its axis once per day, by definition. Earth orbits the Sun once every 365.24 days, which is defined as 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 orbiting a terrestrial planet in the solar system. The Moon is covered with craters; it also has large plains of lava. The huge number of craters suggests that Moon’s surface is ancient. There is evidence that the Moon formed when a large object — perhaps as large as the planet Mars — struck Earth in the distant past (Figure  below).

Besides its Moon, Earth is orbited by a great deal of space debris, the remains of satellites, and rocket stages.


The smallest planet, Mercury, is the planet closest to the Sun. 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  below, visited Mercury from 1974 to 1975. The MESSENGER spacecraft has been studying Mercury in detail since 2005. The craft is currently in orbit around the planet, where it is creating detailed maps. MESSENGER stands for Mercury Surface, Space Environment, Geochemistry and Ranging.

(a) Mariner 10 made three flybys of Mercury in 1974 and 1975. (b) A 2008 image of compiled from a flyby by MESSENGER.

As Figure  below shows, the surface of Mercury is covered with craters, like Earth’s moon. Ancient impact craters means that for billions of years Mercury hasn’t changed much geologically. Also, with very little atmosphere, the processes of weathering and erosion do not wear down structures on the planet.

Mercury is covered with craters, like Earth’s moon. MESSENGER has taken extremely detailed pictures of the planet’s surface.

There are many images, movies and activities on the MESSENGER site:

Short Year, Long Days

Mercury is named for the Roman messenger god, who could run extremely quickly, just as the planet moves very quickly 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.

Despite its very short years, Mercury has 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 57 Earth days long. In other words, on Mercury, a year is only a Mercury day and a half long!

Extreme Temperatures

Mercury is close to the Sun, so it can get very hot. However, Mercury has virtually no atmosphere, no water to insulate the surface, and it rotates very slowly. For these reasons, 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 think there may be a small amount of water in the form of ice at the poles of Mercury, in areas that never receive direct sunlight.

A Liquid Metal Core

Figure  below shows a diagram of Mercury’s interior. Mercury is one of the densest planets. It’s relatively large, liquid core, made mostly of melted iron, takes up about 42% of the planet’s volume.

Mercury contains a thin crust, a mantle, and a large, liquid core that is rich in iron.


Named after the Roman goddess of love, Venus is the only planet named after a female. Venus’ thick clouds reflect sunlight well so Venus is very bright. When it is visible, Venus is the brightest object in the sky besides the Sun and the Moon. Because the orbit of Venus is inside Earth’s orbit, Venus always appears close to the Sun. When Venus rises just before the Sun rises, the bright object is called the morning star. When it sets just after the Sun sets, it is the evening star.

Of the planets, Venus is most similar to Earth in size and density. Venus is also our nearest neighbor. The planet’s interior structure is similar to Earth’s with a large iron core and a silicate mantle (Figure  below). But the resemblance between the two inner planets ends there.

Find out more about Venus at the following link:

Venus’s interior is similar to Earth’s.


Venus rotates in a direction opposite the other planets and opposite to the direction it orbits the Sun. This rotation is extremely slow, only one turn every 243 days. This is longer than a year on Venus—it takes Venus only 224 days to orbit the Sun.

Extreme Atmosphere

Venus is covered by a thick layer of clouds, as shown in pictures of Venus taken at ultraviolet wavelengths (Figure  below).

This ultraviolet image from the Pioneer Venus Orbiter shows thick layers of clouds in the atmosphere of Venus.

Venus’ clouds are not made of water vapor like Earth’s clouds. Clouds on Venus are made mostly of carbon dioxide with a bit of sulfur dioxide — and they also contain corrosive sulfuric acid. Because carbon dioxide is greenhouse gas, the atmosphere traps heat from the Sun and creates a powerful greenhouse effect. Even though Venus is further from the Sun than Mercury, the greenhouse effect makes Venus the hottest planet. Temperatures at the surface reach 465°C (860°F). That’s hot enough to melt lead.

The atmosphere of Venus is full of acid, its pressure is crushing, and the enormous amount of carbon dioxide causes runaway greenhouse effect(4d) (2:05).

The atmosphere of Venus is so thick that the atmospheric pressure on the planet’s surface is 90 times greater than the atmospheric pressure on Earth’s surface. The dense atmosphere totally obscures the surface of Venus, even from spacecraft orbiting the planet.

Venus’s Surface

Since spacecraft cannot see through the thick atmosphere, radar is used to map Venus’ surface. Many features found on the surface are similar to Earth and yet are very different. Figure  below shows a topographical map of Venus produced by the Magellan probe using radar.

This false color image of Venus was made from radar data collected by the Magellan probe between 1990 and 1994. What features can you identify?

Orbiting spacecraft have used radar to reveal mountains, valleys, and canyons. Most of the surface has large areas of volcanoes surrounded by plains of lava. In fact, Venus has many more volcanoes than any other planet in the solar system and some of those volcanoes are very large.

Most of the volcanoes are no longer active, but scientists have found evidence that there is some active volcanism (Figure  below). Think about what you know about the geology of Earth and what produces volcanoes. What does the presence of volcanoes suggest about the geology of Venus? What evidence would you look for to find the causes of volcanism on Venus?

This image of the Maat Mons volcano with lava beds in the foreground was generated by a computer from radar data. The reddish-orange color is close to what scientists think the color of sunlight would look like on the surface of Venus.

Venus also has very few impact craters compared with Mercury and the Moon. What is the significance of this? Earth has fewer impact craters than Mercury and the Moon too. Is this for the same reason that Venus has fewer impact craters?

It’s difficult for scientists to figure out the geological history of Venus. The environment is too harsh for a rover to go there. It is even more difficult for students to figure out the geological history of a distant planet based on the information given here. Still we can piece together a few things.

On Earth, volcanism is generated because the planet’s interior is hot. Much of the volcanic activity is caused by plate tectonic activity. But on Venus, there is no evidence of plate boundaries and volcanic features do not line up the way they do at plate boundaries.

Because the density of impact craters can be used to determine how old a planet’s surface is, the small number of impact craters means that Venus’ surface is young. Scientists think that there is frequent, planet-wide resurfacing of Venus with volcanism taking place in many locations. The cause is heat that builds up below the surface that has no escape until finally it destroys the crust and results in volcanoes.


Mars is the fourth planet from the Sun, and the first planet beyond Earth’s orbit (Figure  below). Mars is a quite different from Earth and yet more similar than any other planet. Mars is smaller, colder, drier, and appears to have no life, but volcanoes are common to both planets and Mars has many.

Mars is easy to observe so Mars has been studied more thoroughly than any other extraterrestrial planet. Space probes, rovers, and orbiting satellites have all yielded information to planetary geologists. Although no humans have ever set foot on Mars, both NASA and the European Space Agency have set goals of sending people to Mars sometime between 2030 and 2040.

Find out all you want to know about Mars at

This image of Mars, taken by the Hubble Space Telescope in October, 2005, shows the planet’s red color, a small ice cap on the south pole, and a dust storm.

A Red Planet

Viewed from Earth, Mars is reddish in color. The ancient Greeks and Romans named the planet after the god of war. But the surface is not red from blood but from large amounts of iron oxide in the soil.

The Martian atmosphere is very thin relative to Earth’s and has much lower atmospheric pressure. Although the atmosphere is made up mostly of carbon dioxide, the planet has only a weak greenhouse effect so temperatures are only slightly higher than if the planet had no atmosphere.

Surface Features

Mars has mountains, canyons, and other features similar to Earth. Some of these surface features are amazing for their size! Olympus Mons is a shield volcano, similar to the volcanoes that make up the Hawaiian Islands. But Olympus Mons is also the largest mountain in the solar system (Figure  below).

Olympus Mons is about 27 km (16.7 miles/88,580 ft) above the Martian surface, more than three times taller than Mount Everest. The volcano’s base is about the size of the state of Arizona.

Mars also has the largest canyon in the solar system, Valles Marineris (Figure  below).

Valles Marineris is 4,000 km (2,500 mi) long, as long as Europe is wide, and one-fifth the circumference of Mars. The canyon is 7 km (4.3 mi) deep. By comparison, the Grand Canyon on Earth is only 446 km (277 mi) long and about 2 km (1.2 mi) deep.

Mars has more impact craters than Earth, though fewer than the Moon. A video comparing geologic features on Mars and Earth is seen here: Mars tectonics video

Is There Water on Mars?

Water cannot stay in liquid form on Mars because the atmospheric pressure is too low. However, there is a lot of water in the form of ice and even prominent ice caps (Figure  below). Scientists also think that there is a lot of water ice present just under the Martian surface. This ice can melt when volcanoes erupt, and water can flow across the surface temporarily.

The north polar ice cap on Mars.

Scientists think that water once flowed over the Martian surface because there are surface features that look like water-eroded canyons (Figure below). 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.

The Mars rover collected rounded clumps of crystals that, on Earth, are known to form in water.

A video of the top five Phoenix Lander sites on Mars is seen here:

Two Martian Moons

Mars has two very small moons that are irregular rocky bodies (Figure  below). Phobos and Deimos 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.

Mars has two small moons, Phobos (left) and Deimos (right). Both were discovered in 1877 and are thought to be captured asteroids.

An animation of the moons orbiting Mars is seen here:

KQED: Searching for Life on Mars

The Mars Science Laboratory was launched on November 26, 2011 and will search for any evidence that the Red Planet was once capable of supporting life. Curiosity is a car-sized rover that is scouring the red planet for clues; it landed in August 2012. Learn more at:

Lesson Summary

  • The four inner planets, or terrestrial planets, have solid, rocky surfaces.
  • Earth, the third planet from the Sun, is the only planet with large amounts of liquid water, and the only planet known to support life. Earth has a large round moon.
  • Mercury is the smallest planet and is the closest to the Sun. With its extremely thin atmosphere, Mercury has a large temperature range. 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. With its thick, corrosive atmosphere, the surface temperature is extremely high.
  • Venus has mountainous areas, as well as volcanoes surrounded by plains of lava.
  • Mars is the fourth planet from the Sun. Mars is reddish in color and has the largest mountain and the largest canyon in the solar system. It has two small moons.
  • Water ice is found in the polar ice caps and under the surface of Mars.

Review Questions

1. Name the inner planets from the Sun outward. Then name them from smallest to largest.

2. Why do the temperatures on some planets vary widely? Why are some temperatures much less variable?

3. 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 planet.

6. Why is Mars red?

7. Suppose you are planning a mission to Mars. Identify two places where you might be able to get water on the planet. Why is this important?

Further Reading / Supplemental Links

Points to Consider

  • The first humans may reach Mars sometime in the next few decades. What conditions will they face? Why do you think we are going to Mars instead of Mercury or Venus?
  • Why are the four inner planets called terrestrial planets? What might a planet be like if it weren’t a terrestrial planet?