Space The Inner Planets Mercury, Venus, Earth And Mars Photo By NASA

Published on February 11th, 2010 | by Yellow Magpie

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The Inner Planets Of The Solar System: The Terrestrial ‘Rocky’ Planets

The Inner Planets Mercury, Venus, Earth And Mars Photo By NASA

The Solar System is, so far, the only known star and planetary system to contain life. Yet few if us are actually aware of what lies outside of our small round sphere. Fewer still  are consciously aware of the inner planets and even less try to place our own existence in a context. Without context there cannot be understanding. Without some knowledge of the bigger picture we lose sight of our true identity.

Therefore, exploring the Solar System and Space is not a trivial goal but the ultimate pursuit as we strive towards our unending search for meaning, our end destination.

The Inner Planets The Scale Of Inner And Outer Planets NASA

The Inner Planets The Scale Of Inner And Outer Planets NASA

The world outside our blue planet is vast. The human brain stalls at the complexities involved in trying to make sense of the vast reaches, of not only the Universe, but our own tiny dot, the Solar System.

Our yellow star, the Sun, is thought to contain roughly 99.86% per cent of the entire mass of the solar system. Out of the remaining percentage, the gas giants of Jupiter,  Saturn, Uranus and Neptune account for the 90%. Thus, what is left over is largely accounted by the four inner planets, of which the Earth is one.

Mercury ‘The Speedy Messenger’ And First Inner Planet

Mercury, a planet which resembles our own Moon, is the closet planet to the Sun. It takes just 88 days for Mercury to travel around the Sun. Yet, because of its close proximity to the Sun and consequently, the huge gravitational forces that it experiences, one day on Mercury lasts 58.67 days. For those who are interested in the maths, every three Mercury days takes two Mercury-years.

Unlike the other planets, which more or less have circular orbits, Mercury’s trajectory around the Sun is elliptical. At its farthest from the Sun, the Aphelion it is 70 million kilometres (43.5 million miles) away. While at its Perihelion, it is a mere 49 million kilometres (29 million miles) away.

The Inner Planets: Planet Mercury First Of The Inner Planets

The Inner Planets: Planet Mercury First Of The Inner Planets

Our Earth-Sized Neighbour, Venus

Venus is the second inner planet from the Sun. Almost the size of the Earth, Venus has a year that is only 225 Earth ‘days’. However, it takes 243 Earth days for the planet to complete one rotation. Venus orbits the Sun at a distance of 108 million kilometres ( 67 million miles).

Although this distances, in relation to the Earth, seem to be quite near, they, and the fact Venus has no Moon, has had enormous consequences and accounts for why the two planets are do dissimilar.

The Inner Planets: Venus Second Of The Inner Planets Taken By The Magellan Probe

The Inner Planets: Venus Second Of The Inner Planets Taken By The Magellan Probe

Earth, The Third ‘Rock’ From The Sun

Earth, the third ‘rock’ from the Sun is the largest of the ‘rocky’ or terrestrial planets. It takes 365.25 days for the Earth to makes its way across the Sun to complete a year. Earth orbits the Sun at a distance of roughly 150 million kilometres (93 million miles).

Although this varies, it is closest during the month of January while farthest during the month of July. The Earth is the only one of the inner planets to have a large satellite, our own Moon.

The Inner Planets: The Earth Third Of The Inner Planets

The Inner Planets: The Earth Third Of The Inner Planets

Why We Have An Affinity For The ‘Red Planet’ Mars?

Mars is the last member of the Inner Solar System and consequently the farthest inner planet from the Sun. Perhaps it is the planet that people share the most affinity with. There have been countless people who have gazed at Mars through telescopes and pondered the existence of alien life-form and indeed the nature of life itself while marvelling the beauty of the Red Planet.

A reason why people share such a close relationship with Mars could be put down to the similarities between the Red Planet and Earth. A day on Mars lasts 24 hours 38 minutes just slightly longer than our own 24 hours. While its axis tilts at a similar angle to that of the Earth’s, thus giving it similar seasons to the ones that we experience on our ocean planet.

However, there are notable differences, a year on Mars is almost twice as long lasting 687 Earth day, while it is also a lot further from the Sun. Its perihelion is 207 million kilometres (128 million miles) while its aphelion is 70 million kilometres (43.5 million miles).

The Inner Planets The Red Planet: Mars The Farthest Of The Inner Planets

The Inner Planets The Red Planet: Mars The Farthest Of The Inner Planets

The Asteroid Belt

Dividing off and separating the Inner Planets is the Asteroid Belt which lies between Mars and Jupiter. The majority of these asteroids are small, however, some are up to 1,000 kilometres (600 miles) across. Although most are confined to the Asteroid Belt between Mars and Jupiter, a good number have elliptical orbits which bring them quite close to the Earth’s orbit. These are known as NEAs (Near Earth Asteroids).

So What Formed The Inner Planets?

Billions of years ago, at least 4.8 billion years ago, when the Sun was its infancy, there were no planets. Much of the matter within the solar system was in ‘refractory material’, material which has a high melting point.

During these early stages of the Solar System there was tremendous heat present. Because of this heat, non-refractory material existed in vapour form. The vapourised material either was drawn in by the strong gravitational exertions of the Sun or was driven away by powerful Solar Winds.

The refractory particles formed a ring around the Sun, much like the rings of Saturn. Over time these particles coalesced with other particles forming sticky clusters. Over millions of years these particles coalesced into larger particles until eventually they formed planetesimals. Planetesimals were large enough to sustain their own gravitational fields.

A Sped-up Process

At this stage the entire process rapidly sped up and gathered momentum. The planetesimals coalesced and formed into planetoids, which are objects that, because of gravity, became spherical in shape. These planetoids meet other planetoids and coalesced into what we know call the present day planets. The last major impacts of planetoids lead to the formation of the Moon and to much of the surface material of Mercury being stripped.

The Late Heavy Bombardment

Approximately 4.55 million years ago much of the present planetary arrangement would be similar as it is now. However, if one was to travel back in time there would be one notable exception. The would be tremendous activity. Collisions would have been a regular occurrence as the inner solar system was littered with debris.

The final chapter in the history of these cataclysmic impacts happened 3.9 billion years ago and has been called the ‘Late Heavy Bombardment’.

The Inner Planets Lunar Cataclysm During The Late Heavy Bombardment Photo By Tim Wetherell - Australian National University

The Inner Planets Lunar Cataclysm During The Late Heavy Bombardment Photo By Tim Wetherell – Australian National University

The Late Heavy Bombardment saw unstable asteroids slamming into the terrestrial planets. Much of the appearance of the Moon’s surface was carved by such collision. The Earth would have also sustained such massive impacts.

The Energy Source Of The Inner Planets

All of the inner planets would have had there own energy source during the formation of the solar system. Of all the terrestrial planets, the Earth is the most geologically active. The next active planets are Venus and Mars. During the early stages of the planets’ lives, their sources of energy would have been derived from heat causes by the multitude of collision between other planets.

However, over time, because of the large amount of radioactive materials collected, radioactive decay has become the de facto source of energy. The amount of internal energy a planet has is largely dependent upon its size. The larger the planetary body, the more energy it will have. This is also aided by the fact that the larger a body is the less surface area it will have to lose heat from.

Mercury and the Moon have both being geologically inactive for billions of years and consequently emit next to no heat at all form their own cores.

The Asteroids

The aforementioned asteroid belt resides between Mars and Jupiter, beyond the inner planets. This so-called ‘belt’ marks the end of the Inner Solar System with that of the world of the Gas Giants, Jupiter, Saturn and Neptune. The vast majority of asteroids have changed little since their birth.

However, Vesta and other asteroids are exceptional in that they are still geologically active asteroids.

The Inner Planets Bounded By The Asteroid Belt

The Inner Planets Bounded By The Asteroid Belt

Despite the fact that the distances between the asteroids are enormous, and most people will have difficulty relating to such huge numbers, collisions occasionally do occur. Asteroids that have been pushed by other asteroids out of their normal orbit are concentrated around the path of Jupiter.

The collisions have occurred so frequently in the past that large spaces have opened up in the asteroid belt near Jupiter. These spaces are known as the ‘Kirkwood gaps’.

As already mentioned asteroids which orbit close to the Earth are known as Near Earth Asteroids or NEAs. These asteroids are further divided into three groups. These are the Apollo, Amore and Aten asteroids.

The Inner Planets Asteroid Vesta

The Inner Planets Asteroid Vesta

Asteroids And Earth

Apollo asteroids have orbits which take them closer to the Sun than to Earth. Amor asteroids have orbits, further away from the Sun which bring them closer to Mars. Finally, Aten asteroids cross the path that the Earth takes around the Sun. These asteroids are completely free of the asteroid belt. In fact the Earth, when in the aphelion period during July regularly hits asteroids.

These collisions are called meteor showers. Meteorites are meteors which have successfully managed to breach the Earth’s atmosphere. All of the inner planets have to deal with meteors and meteorites.

Venus has one of the densest of atmospheres and consequently meteors must be of enormous size to survive re-entry and become meteorites. Mars atmosphere is a good deal thinner and easy for meteors to penetrate than the Earths. However, the Moon and Mercury have no atmospheres to protect. This and the fact they are geologically inactive means that all collisions are preserved intact.


Highly Recommended

You may like to check out Yellow Magpie’s The Solar System And Beyond: A Guide To The Cosmos for more insight into our local part of the Universe.

For thus who want to pursue the subject on a deeper level, Cosmos is a marvellously well-written book with large illustrations. It is both concise and easy to understand and a most have for both space enthusiasts and for those who wish to understand the bigger picture. It also contains one of the best forewords ever written on the subject.

Amazon.co.uk
For people living in Ireland or the United Kingdom, you can access Cosmos.

Amazon.ca
For those who live in Canada, you can obtain Cosmos from here.

Amazon.de
For Germany: Cosmos.

Amazon.fr
For France: Cosmos.


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2 Responses to The Inner Planets Of The Solar System: The Terrestrial ‘Rocky’ Planets

  1. Pingback: About The Moon: Our Lunar Fascination And A Future Gateway To The Stars

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