Atmospheres of the Earth and terrestrial planets.

If you don’t know what an atmosphere is then I suggest you find a blog on fashion or similar. Perhaps, though, we think we know what an atmosphere is but do we really?

Well, just in case it’s that thin blue line that we see around the Earth when looking at pictures that the International space Station has given us. When you look at the Earth and look at the atmosphere it is a very thin layer indeed. The Earth’s atmosphere is made up roughly of about 21% oxygen and 78% nitrogen with the remainder being carbon dioxide, argon and a few other gases. As we all know (well if you live in Scotland anyway) there is that what seems a large amount of water vapour giving us clouds, rain, snow and all the other forms of precipitation. It may seem that the clouds take up a lot of the atmosphere but it is only 1% of the atmosphere.

Earth's Thin Blue line

Earth's Thin Blue line

The atmosphere is reasonably heavy at 5,000,000,000,000,000,000 kilograms and becomes thinner with increasing altitude with no real boundary between itself and space although 75 miles is where spacecraft start to feel the effects of the atmosphere as their heat shields start to work and they get buffeted around.

There are a few questions you might ask yourself about the atmosphere of the Earth and of other planets and moons. Where did it all come from is a good start? As you might expect some of the atmosphere was formed from the original creation of the solar system. When the sun had formed a solar nebula formed around it (a swirling disc of dust and gas) and some others remained as gas and formed around the rocky bodies.

The planets themselves give off gas from all sorts of sources from within the planet. The solar wind may have given us part of the atmosphere as well. The solar wind is a stream of charged particles from the atmosphere of the Sun consisting of electrons and protons. They escape the Sun because they are moving fast and they actually have a higher temperature than the core of the Sun. This is solar wind nowadays is lethal to us but we have the protection of the magnetosphere which is like a force field protecting us. In the early days of formation of the Earth may have been that this force field was non-existent and the particles from the solar wind were caught by the Earth as part of the atmosphere.

There is one thing that is important for planets and moons to enable to keep an atmosphere. The gravity is the mysterious force that keeps the molecules of the atmosphere from escaping in to space although a small amount of molecules escape into space all the time. The gases of the atmosphere actually have mass and therefore gravity can act on them and just like us gravity keeps them tied to the Earth (although, they are not as heavy and therefore find equilibrium between gravity’s pull and wanting to escape and therefore remain suspended in the air).

Well, that’s enough about the Earth what about the other planets and moons. Do they have atmospheres?

Mercury

Mercury is a poor soul being so close to the Sun and being hammered by the solar wind. The atmosphere really only has trace elements in very small quantities. Being small Mercury has a lack of gravity and lost its hold over any atmosphere that may have been around.

Venus

Venus has a similar size to Earth and is a reasonable distance from the Sun so it should have atmosphere like the Earth’s right? Unfortunately that isn’t the case. The atmosphere is much denser and hotter than that of Earth. In fact it has a pressure that is 90 times that of Earth.

Venus Atmosphere Greenhouse Effect

Venus Atmosphere Greenhouse Effect

Venus has lost the bubble really. The carbon dioxide which takes up almost 100% of the atmosphere has built up to such a level that it keeps the heat from the sun from escaping. You would think that with a cloud level such as Venus has (100% coverage) that no more radiation from the sun would reach the surface of the planet and the greenhouse effect would disappear. Unfortunately the sun’s rays still penetrate through the clouds and fuel the greenhouse effect. This is down to what scientists call the Goldilocks theory.

Before you all go to sleep this is very simple. What it boils down to is that planet must be in exactly the right distance from the Sun for the correct conditions to create an Earth like planet. The porridge must be just right! Venus is just too close and didn’t win the solar nebula lottery.

Mars
Going back to Goldilocks, Mars is just too far from the Sun and is cold. Mars has a thin atmosphere and has a low atmospheric pressure. It consists of 95% carbon dioxide, 3% nitrogen and 1.6% argon. There has recently been found traces of methane which is quite encouraging when thinking about the possibility of life.

Mars Atmosphere

Mars Atmosphere

Mars does have clouds but there is only a small percentage that is made of water vapour and of carbon dioxide. The main clouds are dust which for about once a year can cover the whole surface. When compared to Earth there are some very odd atmospheric movements on Mars. Condensation flow occurs when the face of Mars facing the Sun heats up and causes the polar carbon dioxide icecap to evaporate and rise flowing in the upper atmosphere to the southern pole. The South Pole (because it is in its winter phase) cools and carbon dioxide solidifies from the atmosphere increasing the size of the southern polar ice cap.

Titan

Is a moon circling Saturn and has given scientists quite a surprise. It has a thick atmosphere and is the only object other than the Earth for which evidence of a liquid has been found. The atmosphere is 98.4% nitrogen with the rest being 1.6% methane and other gases. Being so far away from the Sun and well out of the Goldilocks zone it shouldn’t really have an atmosphere. Titan has managed to break the rules by keeping its atmosphere cool and I mean really cool which makes it heavier and keeps it from escaping.
Titan has methane rain that has produced lakes and rivers. Because of the low gravity when it rains the drops are large and would float down to the surface like snow.

I have covered the terrestrial planets in this post but don’t worry I will cover the gas giants and other planets, the outer planets, in a Future post.

An atmosphere is essential to us although it may not be essential to some other life form living elsewhere. Everybody has heard of climate change and it may be a very good idea to keep our atmosphere in tip top condition considering we breathe it, well at least until we find another suitable planet and have built spaceships that can get there in a reasonable time.

The solar system is not the only place that has atmospheres around planets-

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