Universe and Solar System

2.5 Terrestrial 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 are from satellite images.

The inner planets, or terrestrial planets, are the four planets closest to the Sun: Mercury, Venus, Earth, and Mars. Unlike the outer planets, which have many 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. (Inner Planets | Earth Science, n.d.)

Terrestrial planets are substantially different from gas giants, which might not have solid surfaces and are composed mostly of hydrogen, helium, and water existing in various physical states. Terrestrial planets all have roughly the same structure: a central metallic core, mostly iron, with a surrounding silicate mantle. Terrestrial planets have canyons, craters, mountains, volcanoes, and secondary atmospheres.


The smallest planet, Mercury, is the planet closest to the Sun. Because of Mercury’s proximity to the Sun, it is difficult to observe from Earth, even with a telescope. However, the Mariner 10 spacecraft visited Mercury from 1974 to 1975. The MESSENGER spacecraft, which stands for Mercury Surface, Space Environment, Geochemistry, and Ranging, 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.

The surface of Mercury is covered with craters. Ancient impact craters mean that for billions of years, Mercury has not changed much geologically. Also, with minimal atmosphere, the processes of weathering and erosion do not wear down structures on the planet. Mercury is one of the solar system’s densest planets. It is relatively large, liquid core, made mostly of melted iron, takes up about 42 percent of the planet’s volume.

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 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 precisely 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!

Mercury is close to the Sun so that it can get scorching. However, Mercury has virtually no atmosphere, no water to insulate the surface, and rotates very slowly. For these reasons, temperatures on the surface of Mercury vary widely. Indirect sunlight, the surface can be as hot as 427 degrees Celsius (801 degrees Fahrenheit). On the dark side, or in the shadows inside craters, the surface can be as cold as -183 degrees Celsius (-297 degrees Fahrenheit). 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.


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, making it the brightest object in the sky beside 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. However, the resemblance between the two inner planets ends there.

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, where it only takes Venus 224 days to orbit the Sun.

A thick layer of clouds covers 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 corrosive sulfuric acid. Because carbon dioxide is a 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 degrees Celsius (860 degrees Fahrenheit), which is hot enough to melt lead.

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 obscures the surface of Venus, even from spacecraft orbiting the planet.

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. 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. 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 that there is some active volcanism.

It is 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. Scientists can still piece information together to begin t understand Venus’s past.

On Earth, volcanism is generated because the planet’s interior is hot. Much of the volcanic activity is caused by plate tectonic activity. However, 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 still relatively young. Scientists think that there is frequent, planet-wide resurfacing of Venus with volcanism taking place in many locations. The cause is the heat that builds up below the surface that has no es-cape until finally it destroys the crust and results in volcanoes.

In September 2020, NASA reported that phosphine was discovered in the clouds of Venus. Phosphine is considered a “biosignature,” meaning that some form of life on the planet may be creating the gas. From our current understanding regarding phosphine on earth, it is only produced by industrial emissions and microbes. That is why scientists are interested in studying Venus more because of the potential of microscopic life in the atmosphere of the planet.


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 degrees Celsius (57 degrees Fahrenheit). 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 reasonably steady.

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Earth is the only planet known to have known life. The presence of liquid water, the atmosphere’s ability 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.

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. The plates’ movement causes other geological activity, such as earthquakes, volcanoes, and 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 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.

The Moon

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 vast plains of lava. The considerable number of craters suggests that the 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.

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Lunar Eclipses

Solar Eclipses


Mars is the fourth planet from the Sun and the first planet beyond Earth’s orbit. Mars is 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 the planet has been studied more thoroughly than any other extraterrestrial planet in our solar system. 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.

Viewed from Earth, Mars is reddish. The ancient Greeks and Romans named the planet after the god of war. However, the surface is not red from blood but significant 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.

Mars has mountains, canyons, and other features similar to Earth, and some of these surface features are exceptional for their size. Olympus Mons is a shield volcano, similar to the volcanoes that make up the Hawaiian Islands. However, Olympus Mons is also the largest mountain in the solar system. Mars also has the largest canyon in the solar system, Valles Marineris.

Mars has more impact craters than Earth, though fewer than the moon. Water cannot stay in liquid form on Mars because the atmospheric pressure is too low. However, there is much water in the form of ice and even prominent ice caps. 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.

Scientists think that water once flowed over the Martian surface because surface features look like water-eroded canyons. 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.

Mars has two tiny moons that are irregular rocky bodies. Phobos and Deimos are named after characters in Greek mythology; the two sons of Ares followed their father into war. Ares is equivalent to the Roman god Mars.


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Physical Geography and Natural Disasters Copyright © 2020 by R. Adam Dastrup, MA, GISP is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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