Lesson Objectives

Describe vital features of every of the inner planets.Compare every of the within planets to Earth and to one another.

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What proof do planetary geologists have to go on to recognize the geology the the within planets? ~ above Earth, scientists have the right to collect and analyze the chemistry that samples, perform radiometric date to determine their ages, and also look at satellite images to see large-scale features. Rovers have actually landed on Mars and also sent earlier enormous amounts of information however much the the remainder of what is known around the inside planets is native satellite images.

The within Planets

The inner planets, or terrestrial planets, room the four planets closest to the Sun: Mercury, Venus, Earth, and also Mars. Figure below shows the family member sizes of these 4 inner planets.


This composite shows the family member sizes the the 4 inner planets. Indigenous left come right, they are Mercury, Venus, Earth, and also Mars.

Unlike the external planets, i beg your pardon have plenty of of satellites, Mercury and also Venus perform not have actually moons, planet has one, and also Mars has actually two. Of course, the within planets have much shorter orbits about the Sun, and also they all spin more slowly. Geologically, the within planets are all made of cooled igneous rock v iron cores, and also all have actually been geologically active, in ~ least early on in your history. Nobody of the within planets has actually rings.


Although planet is the 3rd planet the end from the sun this class will begin here. We understand a lot much more about Earth, therefore what us know can be offered for comparison v the other planets.


What space Earth’s many distinctive features? This famed image of planet was taken during the Apollo 17 mission come the moon. Can you discover a hurricane? A storm spinning in the contrary direction from the hurricane?

Earth’s Surface and Life

As you can see in (Figure above), earth has large oceans of fluid water, huge masses that exposed land, and a dynamic setting with clouds that water vapor. Earth additionally has ice extending its polar regions. Earth’s mean surface temperature is 14°C (57°F). Water is a fluid at this temperature, however the planet likewise has water in its other two states, solid and also gas. The oceans and the atmosphere assist keep Earth’s surface ar temperatures fairly steady.

As yet earth is the only planet well-known to have actually life. The existence of liquid water, the ability of the atmosphere to filter the end harmful radiation, and many other functions make the planet uniquely suitable to harbor life. Life and also Earth now influence each other; because that example, the development of plants permitted oxygen to go into the environment in large enough quantities for pets to evolve. Back life has not been found elsewhere in the solar system, various other planets or satellites may harbor primitive life forms. Life may additionally be uncovered elsewhere in the universe.

Structure and Plate Tectonics

The warmth that stayed from the planet’s accretion, gravitational compression, and radioactive decay permitted the earth to melt, probably an ext than once. Together it ultimately cooled, gravity pulled metal into the center to create the core. More heavier rocks created the mantle and lighter rocks created the crust.

Earth’s late is divided into tectonic plates, i m sorry move about on the surface because of the convecting mantle below. Movement of the plates reasons other geological activity, such together earthquakes, volcanoes, and the formation of mountains. The areas of these functions are greatly related to present or former plate boundaries. Planet is the only planet known to have plate tectonics.

Earth’s Motions and Satellites

Earth rotates top top its axis once per day, through definition. Planet orbits the Sun as soon as every 365.24 days, i m sorry is defined as a year. Planet has one big moon, i beg your pardon orbits earth once every 29.5 days, a duration known as a month.

Earth’s moon is the only huge moon orbiting a terrestrial planet in the solar system. The Moon is covered with craters; it also has big plains of lava. The huge variety of craters suggests that Moon’s surface ar is ancient. Over there is proof that the Moon formed when a large object — possibly as large as the earth Mars — struck earth in the far-off past (Figure below).


Besides the Moon, earth is orbited through a an excellent deal of room debris, the continues to be of satellites, and rocket stages.


The smallest planet, Mercury, is the world closest come the Sun. Because Mercury is so close come the Sun, that is complicated to watch from Earth, even with a telescope. However, the Mariner 10 spacecraft, displayed in Figure below, went to Mercury indigenous 1974 come 1975. The MESSENGER spacecraft has been examining Mercury in detail since 2005. The craft is right now in orbit around the planet, where it is producing detailed maps. MESSENGER means Mercury Surface, space Environment, Geochemistry and also Ranging.


(a) Mariner 10 made 3 flybys that Mercury in 1974 and also 1975. (b) A 2008 photo of compiled native a flyby by MESSENGER.

As Figure below shows, the surface ar of Mercury is covered with craters, choose Earth’s moon. Ancient impact craters method that because that billions of year Mercury hasn’t readjusted much geologically. Also, through very tiny atmosphere, the procedures of weathering and erosion carry out not wear down frameworks on the planet.


Mercury is spanned with craters, favor Earth’s moon. MESSENGER has taken extremely comprehensive pictures the the planet’s surface.

There are numerous images, movie and activities on the MESSENGER site: http://messenger.jhuapl.edu/index.php.

Short Year, lengthy Days

Mercury is named for the roman messenger god, who can run exceptionally quickly, just as the world moves really quickly in its orbit about the Sun. A year top top Mercury — the size of time it takes to orbit the sunlight — is simply 88 earth days.

Despite its an extremely short years, Mercury has very long days. A work is defined as the time it bring away a world to rotate on that is axis. Mercury rotates gradually on the axis, turning exactly 3 times for every two times that orbits the Sun. Therefore, every day on Mercury is 57 earth days long. In other words, top top Mercury, a year is just a Mercury day and a half long!

Extreme Temperatures

Mercury is close to the Sun, for this reason it have the right to get an extremely 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 straight sunlight, the surface have the right to be as hot as 427°C (801°F). Top top the dark side, or in the shadows within craters, the surface have the right to be together cold as -183°C (-297°F)! Although many of Mercury is extremely dry, researchers think there may be a small amount that water in the kind of ice cream at the poles of Mercury, in locations that never receive straight sunlight.

A Liquid metal Core

Figure below shows a chart of Mercury’s interior. Mercury is among the densest planets. It’s fairly large, fluid core, made mostly of melted iron, take away up around 42% the the planet’s volume.


Mercury consists of a thin crust, a mantle, and also a large, liquid main point that is well-off in iron.


Named ~ the roman inn goddess the love, Venus is the just planet called after a female. Venus’ thick clouds reflect sunlight well for this reason Venus is an extremely bright. When it is visible, Venus is the brightest object in the sky besides the Sun and also the Moon. Since the orbit of Venus is inside Earth’s orbit, Venus constantly appears close come the Sun. Once Venus rises just prior to the sunlight 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 planet in size and also density. Venus is likewise our nearest neighbor. The planet’s inner structure is similar to Earth’s with a big iron core and also a silicate mantle (Figure below). But the resemblance in between the 2 inner planets end there.

Find out much more about Venus in ~ the adhering to link: http://www.nasa.gov/worldbook/venus_worldbook.html.


Venus’s inner is similar to Earth’s.


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

Extreme Atmosphere

Venus is spanned by a thick layer the 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 environment of Venus.

Venus’ clouds room not make of water vapor favor Earth’s clouds. Clouds ~ above Venus space made mainly of carbon dioxide with a bit of sulfur dioxide — and also they likewise contain corrosive sulfuric acid. Since carbon dioxide is greenhouse gas, the atmosphere traps warm from the Sun and creates a an effective greenhouse effect. Even though Venus is further from the sunlight than Mercury, the greenhouse impact makes Venus the sexy planet. Temperatures at the surface reach 465°C (860°F). That’s hot sufficient to melt lead.

The setting of Venus is full of acid, its pressure is crushing, and the huge amount the carbon dioxide causes runaway greenhouse effect(4d): http://www.youtube.com/watch?v=HqFVxWfVtoo (2:05).

The setting of Venus is so thick that the atmospheric pressure on the planet’s surface is 90 times better 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 can not see with the special atmosphere, radar is offered to map Venus’ surface. Plenty of features discovered on the surface ar are similar to Earth and yet are an extremely different. Figure below shows a topographical map the Venus created by the Magellan probe utilizing radar.


This false color image the Venus to be made indigenous radar data collected by the Magellan probe in between 1990 and 1994. What functions can friend identify?

Orbiting spacecraft have actually used radar to expose mountains, valleys, and canyons. Many of the surface has huge areas the volcanoes surrounded by levels of lava. In fact, Venus has many an ext volcanoes than any other world in the solar system and also some of those volcanoes are really large.

Most that the volcanoes room no longer active, but scientists have discovered evidence the there is some active volcanism (Figure below). Think around what friend know about the geology the Earth and also what produces volcanoes. What walk the presence of volcanoes suggest around the geology of Venus? What evidence would girlfriend look for to find the causes of volcanism on Venus?


This photo of the Maat Mons volcano with lava bed in the foreground was produced by a computer system from radar data. The reddish-orange color is close to what scientists think the color of sunlight would look favor on the surface ar of Venus.

Venus also has very couple of impact craters compared with Mercury and also the Moon. What is the meaning of this? earth has fewer affect craters than Mercury and the Moon too. Is this because that the same factor that Venus has actually fewer affect craters?

It’s daunting for researchers to figure out the geological history of Venus. The atmosphere is also harsh because that a rover to go there. That is also more complicated for college student to figure out the geological history of a far-off planet based upon the information given here. Still we have the right to piece together a few things.

On Earth, volcanism is generated due to the fact that the planet’s interior is hot. Lot of the volcanic task is brought about by key tectonic activity. However on Venus, there is no evidence of bowl boundaries and also volcanic attributes do no line up the method they carry out at bowl boundaries.

Because the density of affect craters can be used to determine how old a planet’s surface ar is, the small number of impact craters method that Venus’ surface ar is young. Scientists think that there is frequent, planet-wide resurfacing that Venus through volcanism occurring in many locations. The cause is warm that build up listed below the surface ar that has no escape until finally it destroys the crust and also results in volcanoes.


Mars is the 4th planet indigenous the Sun, and the first planet past Earth’s orbit (Figure below). Mars is a quite different from Earth and also yet an ext similar than any type of other planet. Mars is smaller, colder, drier, and also appears to have no life, however volcanoes are usual to both planets and also Mars has many.

Mars is easy to observe so Mars has been studied more thoroughly than any type of other extraterrestrial planet. Room probes, rovers, and also orbiting satellites have all yielded details to planetary geologists. Back no humans have actually ever set foot ~ above Mars, both NASA and the European an are Agency have set goals that sending people to Mars sometime in between 2030 and also 2040.

Find the end all you desire to know around Mars at http://mars.jpl.nasa.gov/extreme/.


This picture of Mars, take away by the Hubble an are Telescope in October, 2005, shows the planet’s red color, a small ice lid on the southern pole, and also a dust storm.

A Red Planet

Viewed native Earth, Mars is red in color. The ancient Greeks and also Romans called the earth after the god that war. However the surface ar is no red native blood however from big amounts of steel oxide in the soil.

The Martian atmosphere is very thin relative to Earth’s and also has much reduced atmospheric pressure. Although the atmosphere is consisted of mostly that carbon dioxide, the planet has only a weak greenhouse impact so temperature are just slightly higher than if the planet had actually no atmosphere.

Surface Features

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


Olympus Mons is about 27 km (16.7 miles/88,580 ft) above the Martian surface, much more than 3 times taller 보다 Mount Everest. The volcano’s base is about the size of the state of Arizona.


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

Mars has much more impact craters than Earth, though fewer 보다 the Moon. A video comparing geologic features on Mars and Earth is checked out here: Mars tectonics video http://news.discovery.com/videos/space-3-questions-mars-tectonics.html

Is there Water ~ above Mars?

Water cannot continue to be in liquid type on Mars due to the fact that the atmospheric push is also low. However, over there is a most water in the form of ice and even significant ice cap (Figure below). Scientists additionally think the there is a lot of water ice current just under the Martian surface. This ice have the right to melt once volcanoes erupt, and water deserve to flow across the surface temporarily.


The north polar ice cap on Mars.

Scientists think that water when flowed over the Martian surface because there space surface features that look favor water-eroded canyons (Figure below). The presence of water top top Mars, also though the is currently frozen as ice, argues that it might have been possible for life to exist on Mars in the past.


The Mars rover built up rounded clumps of crystals that, on Earth, are known to type in water.

A video clip of the top 5 Phoenix Lander web page on Mars is viewed here: http://news.discovery.com/videos/space-top-5-mars-phoenix-lander-images.html.

Two Martian Moons

Mars has two very little moons that are irregular rocky bodies (Figure below). Phobos and also Deimos are called after personalities in Greek mythology — the 2 sons the Ares, who complied with their father right into war. Ares is identical to the roman inn god Mars.


Mars has actually two small moons, Phobos (left) and also Deimos (right). Both were uncovered in 1877 and are thought to be caught asteroids.

An computer animation of the moons orbiting Mars is watched here: http://commons.wikimedia.org/wiki/File:Orbits_of_Phobos_and_Deimos.gif.

KQED: searching for Life on Mars

The Mars scientific research Laboratory was released on November 26, 2011 and also will search for any evidence the the Red planet was once qualified of supporting life. Curiosity is a car-sized rover the is scouring the red planet for clues; it landing in august 2012. Learn much more at:http://science.kqed.org/quest/video/searching-for-life-on-mars/.

Lesson Summary

The four inner planets, or terrestrial planets, have solid, rocky surfaces.Earth, the 3rd planet from the Sun, is the just planet with large amounts of liquid water, and also the just planet well-known to assistance life. Earth has a huge round moon.Mercury is the the smallest planet and also is the closest come the Sun. Through its incredibly thin atmosphere, Mercury has actually a large temperature range. Choose the Moon, the is covered with craters.Venus is the second planet indigenous the Sun and also the closest planet to Earth, in distance and also in size. V its thick, corrosive atmosphere, the surface ar temperature is extremely high.Venus has actually mountainous areas, as well as volcanoes surrounding by levels of lava.Mars is the 4th planet native the Sun. Mars is reddish in color and also has the biggest mountain and also the largest canyon in the solar system. It has actually two little moons.Water ice cream is discovered in the polar ice cream caps and under the surface ar of Mars.

Review Questions

1. Surname the inner planets native the sunlight outward. Then name them from smallest to largest.

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2. Why carry out the temperatures on part planets vary widely? Why are some temperatures much less variable?

3. Why does Venus have greater temperatures than Mercury?

4. How are maps that Venus made?

5. Name two significant ways in which earth is unlike any other planet.

6. Why is Mars red?

7. Expect you room planning a mission to Mars. Identify two locations where you might have the ability to get water on the planet. Why is this important?

Further analysis / Supplemental Links

Points come Consider

The first humans might reach Mars at some time in the next couple of decades. What conditions will they face? Why execute you think we space going to Mars rather of Mercury or Venus?Why space the four inner planets called terrestrial planets? What could a earth be choose if it weren’t a terrestrial planet?