Lesson Objectives

Describe an essential features of every of the inner planets.Compare every of the within planets come Earth and also to one another.

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What proof do planetary geologists need to go ~ above to recognize the geology of the inner planets? on Earth, scientists deserve to collect and also analyze the chemistry of samples, perform radiometric date to identify their ages, and look in ~ satellite images to see large features. Rovers have landed top top Mars and also sent earlier enormous quantities of information but much of the rest of what is known about the within planets is from satellite images.

The within Planets

The inner planets, or terrestrial planets, are the four planets closest to the Sun: Mercury, Venus, Earth, and Mars. Figure below shows the loved one sizes that these 4 inner planets.


This composite mirrors the loved one sizes the the four inner planets. Indigenous left to right, they space Mercury, Venus, Earth, and also Mars.

Unlike the outer planets, i beg your pardon have countless of satellites, Mercury and also Venus do not have moons, earth has one, and Mars has actually two. That course, the inner planets have much shorter orbits roughly the Sun, and they all spin much more slowly. Geologically, the within planets space all made of cooled igneous rock v iron cores, and all have been geologically active, in ~ least early on in your history. Nobody of the inner planets has actually rings.


Although planet is the third planet the end from the sun this lesson will begin here. We know a lot much more about Earth, for this reason what we know can be offered for comparison with the various other planets.


What are Earth’s most distinctive features? This well known image of earth was taken during the Apollo 17 mission to the moon. Deserve to you uncover a hurricane? A storm turn in opposing direction from the hurricane?

Earth’s Surface and Life

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

As yet earth is the just planet known to have life. The existence of fluid water, the capacity of the environment to filter out harmful radiation, and many other functions make the world uniquely suited to harbor life. Life and also Earth now affect each other; because that example, the advancement of plants permitted oxygen to enter the setting in big enough quantities for animals to evolve. Although life has not been found elsewhere in the solar system, various other planets or satellites might harbor primitive life forms. Life may additionally be discovered elsewhere in the universe.

Structure and also Plate Tectonics

The warmth that continued to be from the planet’s accretion, gravitational compression, and radioactive decay enabled the earth to melt, probably an ext than once. As it subsequently cooled, gravity pulled metal right into the center to produce the core. More heavier rocks formed the mantle and lighter rocks created the crust.

Earth’s crust is divided into tectonic plates, i m sorry move approximately on the surface due to the fact that of the convecting mantle below. Movement of the plates causes other geology activity, such together earthquakes, volcanoes, and the formation of mountains. The locations of these functions are largely related to existing or previous plate boundaries. Earth is the just planet recognized to have plate tectonics.

Earth’s Motions and also Satellites

Earth rotates ~ above its axis as soon as per day, by definition. Planet orbits the Sun once every 365.24 days, i m sorry is identified as a year. Earth has one large moon, i beg your pardon orbits planet once every 29.5 days, a period known together a month.

Earth’s moon is the only large moon orbiting a terrestrial planet in the solar system. The Moon is spanned with craters; it additionally has big plains of lava. The huge variety of craters argues that Moon’s surface is ancient. There is proof that the Moon created when a large object — possibly as huge as the earth Mars — struck planet in the remote past (Figure below).


Besides the Moon, planet is orbited through a great deal of space debris, the stays of satellites, and also rocket stages.


The smallest planet, Mercury, is the planet closest to the Sun. Due to the fact that Mercury is therefore close to the Sun, that is daunting to watch 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 researching Mercury in detail since 2005. The handmade is right now in orbit approximately the planet, whereby it is producing detailed maps. MESSENGER means Mercury Surface, space Environment, Geochemistry and also Ranging.


(a) Mariner 10 made 3 flybys the Mercury in 1974 and 1975. (b) A 2008 picture of compiled from a flyby through MESSENGER.

As Figure below shows, the surface of Mercury is spanned with craters, prefer Earth’s moon. Ancient affect craters method that because that billions of years Mercury hasn’t readjusted much geologically. Also, v very little atmosphere, the procedures of weathering and erosion execute not wear down frameworks on the planet.


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

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

Short Year, long Days

Mercury is called for the roman messenger god, who could run very quickly, simply as the earth moves very quickly in the orbit about the Sun. A year ~ above Mercury — the length of time it takes come orbit the sun — is just 88 planet days.

Despite its very short years, Mercury has very long days. A job is characterized as the moment it take away a world to turn on that axis. Mercury rotates slowly on its axis, transforming exactly three times for every two times the orbits the Sun. Therefore, every day ~ above Mercury is 57 earth days long. In various other words, ~ above Mercury, a year is just a Mercury day and also a half long!

Extreme Temperatures

Mercury is close come the Sun, therefore it can get very hot. However, Mercury has actually virtually no atmosphere, no water come insulate the surface, and it rotates really slowly. Because that these reasons, temperature on the surface ar of Mercury vary widely. In direct sunlight, the surface can be as warm as 427°C (801°F). ~ above the dark side, or in the shadows within craters, the surface deserve to be as cold as -183°C (-297°F)! Although most of Mercury is extremely dry, researchers think there might be a small amount of water in the type of ice cream at the poles that Mercury, in locations that never ever receive direct sunlight.

A Liquid steel Core

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


Mercury includes a slim crust, a mantle, and also a large, liquid core that is rich in iron.


Named after ~ the roman inn goddess that love, Venus is the just planet named after a female. Venus’ thick clouds reflect sunlight well so Venus is very bright. Once it is visible, Venus is the brightest object in the sky besides the Sun and also the Moon. Because the orbit the Venus is within Earth’s orbit, Venus constantly appears close to the Sun. When Venus rises just prior to the sunlight rises, the bright object is called the morning star. Once it sets just after the sunlight sets, it is the evening star.

Of the planets, Venus is most similar to earth in size and density. Venus is likewise our nearest neighbor. The planet’s inner structure is similar to Earth’s through a big iron core and a silicate mantle (Figure below). However the resemblance between the two inner planets end there.

Find out an ext about Venus at the complying with link: http://www.nasa.gov/worldbook/venus_worldbook.html.


Venus’s internal is similar to Earth’s.


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

Extreme Atmosphere

Venus is extended by a thick layer of clouds, as presented in images of Venus taken at ultraviolet wavelength (Figure below).


This ultraviolet photo from the Pioneer Venus Orbiter reflects thick class of clouds in the setting of Venus.

Venus’ clouds room not make of water vapor like Earth’s clouds. Clouds on Venus space made greatly of carbon dioxide through a little of sulfur dioxide — and also they additionally contain corrosive sulfuric acid. Because carbon dioxide is greenhouse gas, the environment traps heat from the Sun and creates a an effective greenhouse effect. Also though Venus is more from the sunlight than Mercury, the greenhouse effect makes Venus the sexty planet. Temperatures at the surface ar reach 465°C (860°F). It is hot enough to melt lead.

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

The setting of Venus is for this reason thick that the atmospheric push on the planet’s surface is 90 times greater than the atmospheric press on earth surface. The thick atmosphere totally obscures the surface of Venus, also from spacecraft orbiting the planet.

Venus’s Surface

Since spacecraft cannot see v the special atmosphere, radar is provided to map Venus’ surface. Many features uncovered on the surface ar are comparable to Earth and yet are very different. Figure below shows a topographical map the Venus created by the Magellan probe using radar.


This false shade image that Venus was made native radar data gathered by the Magellan probe in between 1990 and 1994. What attributes can you identify?

Orbiting spacecraft have actually used radar to disclose mountains, valleys, and canyons. Many of the surface ar has huge areas the volcanoes surrounded by plains of lava. In fact, Venus has many much more volcanoes than any kind of other world in the solar system and some of those volcanoes are very large.

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


This photo of the Maat Mons volcano through lava beds in the foreground was generated by a computer system from radar data. The reddish-orange shade is close come what scientists think the shade of sunlight would certainly look choose on the surface of Venus.

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

It’s difficult for scientists to figure out the geological history of Venus. The environment is too harsh for a rover to walk there. That is even more complicated for student to figure out the geological background of a distant planet based upon the information given here. Tho we can piece together a few things.

On Earth, volcanism is generated since the planet’s internal is hot. Much of the volcanic activity is led to by bowl tectonic activity. However on Venus, there is no proof of bowl boundaries and also volcanic attributes do not line up the way they carry out at bowl boundaries.

Because the thickness of influence craters can be used to determine just how old a planet’s surface is, the small variety of impact craters means that Venus’ surface ar is young. Scientists think the there is frequent, planet-wide resurfacing that Venus v volcanism following in many locations. The reason is warm that builds up below the surface that has actually no escape until lastly it destroys the crust and also results in volcanoes.


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

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

Find out all you want to know about Mars at http://mars.jpl.nasa.gov/extreme/.


This photo of Mars, bring away by the Hubble space Telescope in October, 2005, mirrors the planet’s red color, a small ice lid on the southern pole, and also a dust storm.

A Red Planet

Viewed from Earth, Mars is reddish in color. The ancient Greeks and Romans named the earth after the god the war. However the surface ar is no red from blood but from big amounts of steel oxide in the soil.

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

Surface Features

Mars has mountains, canyons, and other features comparable to Earth. Several of these surface functions are exceptional for your size! Olympus Mons is a shield volcano, similar to the volcanoes that comprise the Hawaiian Islands. But Olympus Mons is also the biggest mountain in the solar device (Figure below).


Olympus Mons is around 27 kilometres (16.7 miles/88,580 ft) above the Martian surface, more than 3 times taller than Mount Everest. The volcano’s basic is around the size of the state that Arizona.


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

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

Is over there Water top top Mars?

Water cannot remain in liquid type on Mars due to the fact that the atmospheric push is also low. However, there is a the majority of water in the form of ice and even influential ice caps (Figure below). Scientists additionally think the there is a most water ice current just under the Martian surface. This ice deserve to melt once volcanoes erupt, and also water deserve to flow throughout the surface temporarily.


The phibìc polar ice cap ~ above Mars.

Scientists think the water as soon as flowed over the Martian surface due to the fact that there room surface features that look like water-eroded canyons (Figure below). The visibility of water ~ above Mars, also though the is now frozen as ice, says that it can have been feasible for life come exist on Mars in the past.


The Mars rover gathered rounded clumps the crystals that, top top Earth, are well-known to kind in water.

A video clip of the top 5 Phoenix Lander web page on Mars is seen 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 that Ares, who adhered to their father into war. Ares is identical to the roman inn god Mars.


Mars has two little moons, Phobos (left) and Deimos (right). Both were uncovered in 1877 and also are assumed to be recorded asteroids.

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

KQED: searching for Life top top Mars

The Mars scientific research Laboratory was launched on November 26, 2011 and also will find for any evidence the the Red planet was once capable of sustaining life. Curiosity is a car-sized rover that is scouring the red planet for clues; it landed in respectable 2012. Learn more at:http://science.kqed.org/quest/video/searching-for-life-on-mars/.

Lesson Summary

The 4 inner planets, or terrestrial planets, have actually solid, rocky surfaces.Earth, the third planet indigenous the Sun, is the just planet with huge amounts of liquid water, and the just planet well-known to support life. Earth has a large round moon.Mercury is the smallest planet and is the closest come the Sun. Through its very thin atmosphere, Mercury has actually a big temperature range. Favor the Moon, that is extended with craters.Venus is the 2nd planet indigenous the Sun and also the closest planet to Earth, in distance and also in size. V its thick, corrosive atmosphere, the surface temperature is very high.Venus has actually mountainous areas, and volcanoes surrounded by levels of lava.Mars is the 4th planet from the Sun. Mars is red in color and also has the biggest mountain and the largest canyon in the solar system. It has actually two little moons.Water ice is discovered in the polar ice caps and under the surface ar of Mars.

Review Questions

1. Surname the inner planets indigenous the sunlight outward. Then surname them from smallest to largest.

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

3. Why go Venus have greater temperatures than Mercury?

4. Just how are maps of Venus made?

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

6. Why is Mars red?

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

Further analysis / Supplemental Links

Points to Consider

The first humans might reach Mars sometime in the next few decades. What conditions will castle face? Why do you think we are going to Mars instead of Mercury or Venus?Why room the four inner planets referred to as terrestrial planets? What can a planet be choose if it weren’t a terrestrial planet?