Terrestrial Planets

A Terrestrial planet is a celestial body which is composed of rocks, mainly silicate rocks, and has a well-defined, solid surface. It is also called ‘inner planet‘ or the rocky planet. Terrestrial planets are different from the gaseous planets as gaseous planets are mainly composed of hydrogen, helium, dust etc.

Interesting Terrestrial Planets Facts

  • A Terrestrial planet is a celestial body which is composed of rocks, mainly silicate rocks, and has a well-defined, solid surface. They are also called ‘inner planets’.
  • The word ‘terrestrial’ comes from the Latin word Terra which means ‘earth’.
  • They are all named after figures from Roman mythology.
  • In order of their increasing distance from the sun, they are named as Mercury, Venus, Earth, and Mars.
  • Mercury is the first planet of the solar system. It is the closest to the sun and the smallest of all the eight planets.
  • Venus is the second closest planet to the sun. After the moon, it is the brightest object in the night sky.
  • Venus is known as the morning star and evening star because it is visible from the Earth during sunrise and during sunset as well.
  • Earth is the third planet from the sun. It is about 150 million kilometers away from the Sun.
  • Mars is the fourth planet from the sun and the last of the terrestrial planets in the solar system. It has been named after the Roman God of War.
  • Mars is the second smallest planet in the solar system after Mercury.

What are the Terrestrial planets?

A Terrestrial planet is a celestial body which is composed of rocks, mainly silicate rocks, and has a well-defined, solid surface. They are also called ‘inner planets’. They are different from the gaseous planets as gaseous planets are mainly composed of hydrogen, helium, dust, etc.

The word ‘terrestrial’ comes from the Latin word Terra which means ‘earth’. Hence, the term terrestrial means ‘earth-like’ or similar to the Earth as they are similar to the Earth in terms of structure and composition.

All the terrestrial planets have the same basic structure. They have a central core which is mostly made up of iron. Then, there is a silicate mantle that surrounds the core. There are also many common surface features like mountains, basins, canyons, craters, volcanoes, etc. They also have an atmosphere which is generated by the activities of comets or volcanoes over the course of millions of years.

Terrestrial planets also have very few or no moons, unlike gas giants which have numerous. Also, terrestrial planets have no ring system whereas all the gas planets have some kind of a ring system. Terrestrial planets are closer to the sun than gas giants and hence have relatively lower temperatures. They are also smaller in size.

There are four terrestrial planets in the solar system which are separated from the gas giants by a disc-like region called the asteroid belt. They are all named after figures from Roman mythology. In order of their increasing distance from the sun, they are named as Mercury, Venus, Earth, and Mars.

Some scientists have proposed that there are four types of terrestrial planets i.e. silicate planet, iron planet, carbon planet, and the coreless planet.

Silicate Planets

Silicate planets are the standard kind of planets i.e. the terrestrial planets found in the solar system.

Carbon Planets

Carbon planets or diamond planets are the planets that are mainly composed of carbon compounds. There are no carbon planets in the solar system but there are carbonaceous asteroids.

Iron Planets

Iron planets are the dense planets that are very rich in iron content or are almost entirely composed of iron. Mercury is the closest to be an iron planet as 60-70% of it is made up of carbon.

Coreless Planets

Coreless planets, as the name suggests, do not have an iron core. However, they can have silicate rocks. They are the exact opposite of iron planets. There are no coreless planets in the solar system but some asteroids are known to be coreless.

Composition of Terrestrial Planets

Mercury

Mercury is the first planet of the solar system. It is the closest to the sun and the smallest of all the eight planets. It is also smaller than two moons which are Ganymede moon of Jupiter’s and Titan a moon of Saturn’s.

Appearance and Rotation

When seen through telescope mercury is a metallic grey or silverish in color. It has been named after the Roman messenger of God. This name is apt as the Roman messenger of God was known to be swift-footed and Mercury is the fastest revolving planet in the solar system due to its small orbit.

It travels faster than any other planet in the solar system, traveling at the speed of 180,000 km/h. It is almost 58 million kilometers away. A day on Mercury lasts for 59 Earth days while a year on the planet lasts for almost 88 Earth days. It does not have any moons.

Discovery

Mercury has been known to humanity since ancient times as it can be seen without the help of advanced telescopes. Its date of discovery is not known. However, Galileo Galilei was the first person to observe it through a telescope.

Size

It has a radius of 2440 km with an equatorial circumference of 15,329 km. It has a surface temperature that varies from -180°C to 440°C. The reason for this huge variation in temperatures is the fact that Mercury has a very thin atmosphere.

Surface and Composition

About 60 to 70% of the planet is composed of iron and nickel. Even though mercury is the closest planet to the sun, it is not the hottest. Mercury is about one third the size of the earth.

There are a lot of craters present on the surface of Mercury. Because of its close proximity to the sun, Mercury may not ever harbor life. It is technically very difficult for any spacecraft to reach Mercury.

Exploration

This is due to the fact that mercury is too close to the sun. Hence, Mercury has only been visited by two spacecrafts. They are Mariner 10 and MESSENGER, both by NASA. Mariner 10 visited mercury in 1974 and 1975 while MESSENGER visited mercury in 2005. The word MESSENGER is an acronym for Mercury Surface, Space Environment, Geochemistry, and Ranging. Both spacecrafts provided great details about the surface, temperature, atmosphere, and the general characteristics of the planet. The planet has also found a place in pop culture on earth as it has been mentioned in many books, movies, and TV series. 

Venus

Venus is the second closest planet to the sun. After the moon, it is the brightest object in the night sky. It is known as the morning star and evening star because it is visible from the Earth during sunrise and during sunset as well.

Appearance

Venus appears bright orange or red in color when observed through a telescope. Venus is named after the Roman goddess of love and beauty. It is the only planet to be named after a female figure. Venus is a hundred and eight million kilometers away from the Sun.

Rotation

Although Mercury is the closest planet to the sun Venus is the hottest planet in the solar system. Venus is one of the only two planets in the solar system which revolves around the sun in a clockwise direction.

All the other planets, except Uranus, revolve around the sun in an anticlockwise direction. Venus revolves around the sun in approximately 225 Earth days. Its period of revolution is around 243 Earth days making is the slowest rotating planet of the solar system. This means that a day on Venus is longer than a year. Hence, Venus is a unique planet.

Size

The planet has a radius of 6052 km. The temperature on the surface of Venus can go up to 462°C making Venus the hottest planet. Venus is the closest planet to the earth. Venus is quite similar to the earth in terms of size, mass and distance from the sun and. The Earth and Venus are sometimes also called twin planets.

Surface

Venus also has craters, volcanoes, mountains and other physical features. It has an atmosphere which is mostly composed of carbon dioxide. This makes Venus the densest planet. The highest mountain on Venus is Maxwell Montes which is more than 20,000 ft or 8.8 km high.

Exploration and Life

Venus is a very difficult planet to observe or explore. This is because of its high temperatures and dense atmosphere. Still, about 50 attempts have been made to send spacecraft’s to Venus out of which 38 have been at least partially successful.

Venus has no moons making it one of the only two planets of the solar system along with Mercury to have no moons. The temperatures and the extreme conditions on the planet make it almost impossible for it to harbor any kind of life. In pop culture, Venus is considered as a significant figure that represents women, feminism, and empowerment.

Earth

Earth is the third planet from the sun. It is about 150 million kilometers away from the Sun. The distance between the Earth and the Sun is referred to as one astronomical unit or 1 AU. The light of the sun takes approximately 8 minutes to reach the earth.

Rotation

The Earth takes 365 days to revolve around the sun once and it takes 24 hours to rotate about its axis. It is the only planet in the solar system which has not been named after a god. The earth is the only planet known to have life on it.

Surface

About 70% of the Earth’s surface is covered by water in the forms of rivers, oceans at, etc. The diameter of the planet is 12,756 km. In ancient times it was believed that the Earth was the center of the universe and all the other planets and stars including the sun revolved around it.

Appearance

From outer space, the earth appears to be blue in color because of the large amount of water present on its surface. It also appears to have green patches which are actually the Earth’s continents.

Atmosphere

The earth has an envelope of air around it which is called the atmosphere. The earth’s atmosphere is composed of 78% Nitrogen, 21% Oxygen and the remaining 1% of carbon dioxide, methane, etc.

The atmosphere performs vital functions like maintaining the planet’s temperature and protecting it from the sun’s harmful ultraviolet radiation. The atmosphere of the Earth is one of the factors that make the planet Earth ideal to support life.

Composition

The earth is the largest of the terrestrial planets. The earth has a central core which mainly consists of molten iron and nickel. The inner core can have a temperature of up to 10,000 °C. Then there is a mantle which is the thickest layer of the Earth’s structure. Finally, there is a crust that is only a few kilometers thick.

Moon

It has one natural satellite that is the moon. The earth is the only planet in the solar system to have one moon. The moon revolves around the planet once every 27 days. There are other man-made satellites that are continuously revolving around the earth.

The satellites are used for the purposes of communication broadcasting military etc. Over the course of time the earth’s natural resources are being overexploited and its air, water, soil and etc. are been contaminated or polluted. Humans need to conserve these resources and use them judiciously to continue living on the Earth.

Mars

Mars is the fourth planet from the sun and the last of the terrestrial planets in the solar system. It has been named after the Roman God of War. Mars is the second smallest planet in the solar system after Mercury.

Distance, Rotation, and Appearance

Mass is nearly 228 million kilometers away from the Sun. Mars takes about 687 days to revolve around the sun once. It has a diameter of roughly 9000 km. Mars can be seen from the Earth easily.

Mars appears to be a brownish red in cooler. It is sometimes also called the red planet because of it. The red cooler of Mars comes from the presence of excess iron oxide on its surface. The total mass of Mars is about 10% of that of the earth.

Composition

Just like all the other Terrestrial planet Mars has a core, a mantle, and a surface. Mars has physical features like craters, canyons, and volcanoes, etc. some of which are the largest in the entire solar system. About 96% of the atmosphere of Mars consists of carbon dioxide.

Moons

Mars has two moons called Phobos and Deimos. However, these moons are more like asteroids rather than natural satellites. The larger of the two moons is Phobos which is about 22 km in diameter while the Deimos is only 12 kilometers in diameter.

Exploration and Life

Mass is the only planet whose surface can be clearly seen and observed from the Earth. Scientists considered it as the most habitable planet in the solar system other than the earth. Various speculations exist about the fact that Mars could sustain life.

No evidence of life existing on the planet in the past or in the present has yet been found. Scientists are trying to find water on Mars. No human being has ever visited Mars but numerous spacecrafts have been sent to the planet by many countries.

The first successful Mars mission was NASA’s Mariner 4 which was launched on 24th February 1969. The first and the only country to have succeeded in a Mars mission in its first attempt was India when it launched its Mars Orbital Mission (MOM) which was also called ‘Mangalyaan’ mission in 2013.

Currently, two rovers called Opportunity and Curiosity are exploring the surface of the planet Mars. Both of them have been sent by NASA. In popular culture, Mars is said to be inhabited by alien creatures called the Martians. Martians have been featured in many books, movies, and TV shows. The day Tuesday is named after mass in almost all cultures. 

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