Essay about terrestrial planets in the Solar System.

Terrestrial planets in our Solar System are four in number: the largest is the Earth, then Venus, Mars and Mercury. All this has important similarities because they have rocks, iron cores, have mantles inside and crust on the outside. What we learn about these planets will teach us more about our own.

Mercury Credit: NASA/Messenger

We start with the closest planet from the Sun and the smallest of the terrestrial planets: Mercury. Because it is so small, telescope observers are often disappointed with the results, not only for its size but also because it is very close to the Sun and from the Earth is seen always on the horizon in the evening or morning, so this is a challenge for relief view of the planet.

Being so small and so close to the Sun, Mercury's atmosphere is very thin and it results from the atoms from the sun hitting the ground of the planet, and this in turn eliminates atoms, but this atmosphere is almost non-existent, and has no effect, and that there is no erosion, so that the impact craters remained intact after bombardment, which has been subjected to the planet. Therefore Mercury reminds us of the month when we look at its surface imagery spacecraft that orbited the planet in the 60s.

Mercury is a small body, about 38% of Earth's radius and only 5% by mass. What makes it unique is the enormous core from its center, despite its size.
Mercury has a weak magnetic field which means that the nucleus is slightly liquid.
It takes 88 days to orbit the Sun and the rotation around its axis is exactly 2/3 of the orbital period, 59 days.

This planet is hostile to visitors. The nights and days are long and temperatures goes from one extreme to another in a very dramatic way. During the day the temperature rises unbearable. Even if you are protected by a suit it would melt at 100 degrees, and on the unlit side it goes to the other extreme of very low temperatures.
Although it seems hard to believe, Mercury has ice at the poles, even if it is so close to the Sun because there are places where the sunlight never reaches.

Venus. Credit: NASA/Ricardo Nunes.

The second planet away from the sun is Venus, also called Earth's twin, because this is almost the same size. It is about 95% of the Earth's radius and 80% by mass. Therefore if you stepped on Venus's gravity you will feel it very similar to ours.

If you turn the telescope toward Venus, you will see it large but matte, with no feature on it because it is constantly wrapped by clouds. If you were on Venus with a telescope, however, considering that it would not be shrouded by clouds, and you point it towards Earth, you could visually detect oceans and continents even if our planet have clouds, but it is not entirely covered by them, there are gaps. Without these gaps we could not see stars and planets from Earth or other galaxies. From Venus, the Earth would be about the same size observed through a telescope.
What I find fascinating is that this planet is rotating in the wrong direction. If Venus had clouds, we would see the Sun rising from the west and setting in the east.

Although it is much farther from the Sun than Mercury, is equally hot, but this phenomenon is due to the thick clouds.
On the surface, the atmospheric pressure is 100 x bigger than on our planet. You would simply be crushed trying to walk on the surface. This atmosphere contains 96% CO2, unlike the Earth which is only 1%. The very high temperature is blocked by the thick clouds, and so whether it is day or night, it is the same everywhere. The clouds are composed of sulfuric acid.

The surface is full of volcanoes as it shows radar images taken by the Magellan probe, which photographed by radio waves the whole the planet.
Venus is closer to the Sun than we are and therefore the temperature is very high. Water could not exist in liquid form because it would boil and evaporate, unlike Earth which is farther from the Sun and water is abundant for supporting life.

Mars. Credit: NASA/ESA

We pass the Earth and reach Mars. This planet is surprisingly similar to our planet than Venus for many reasons. We see its surface. Very few clouds are in the planet's atmosphere and so we can see it even with ground telescopes on Earth. Mars even has ice at the poles that looks very much like on the Earth. Tilting axis of the planet is 23 degrees which means that there are seasons. In winter the northern polar cap extends as on Earth. The rotation axis is a little more than 24 hours, and the Sun rises in the east, but is smaller, and so it's cold, but there still is a lot like the landscape of our planet.

What is different is the atmosphere on Mars because it has no oxygen, but carbon dioxide, like Venus, the rest being nitrogen. At those temperatures, CO2 freezes, forming dry ice. Some caps are made of water ice, however.
An unusual phenomenon on Mars are dust storms which in spring time are reaching planetary proportions, such as sandstorms in the Sahara desert, only that they cover the entire planet.

The entire northern hemisphere is full of volcanoes and the southern is cratered. Still, it is not known why this difference. On the southern hemisphere are also valleys where water flowed in the past and now are completely dried up.
Currently atmospheric pressure on Mars is not good enough for the existence of liquid water. Neither on Earth we can not have liquid CO2. This goes from solid directly into gaseous state, not liquid, but there are serious indications that on Mars has flowed once water through those valleys and that it had a good atmosphere, but from an unknown reason lost it, possibly because it is not massive enough to hold it. Even if we start terraforming the planet would lose it again for sure because its gravity is not strong enough.

What we have learned from examining these terrestrial planets, is that Venus and Mercury are too close to the Sun and too hot and Mars is too far, too cold to support life, and it seems that our planet called Earth, had the chance to orbit the sun exactly at that distance which allows it. Are there planets like ours in other solar systems? We do not know. We have discovered the existence of planets similar to Jupiter, but more massive. The only place we found life remains the third body orbiting our Sun, our beautiful and precious Earth.

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