Earth is the third planet out from the sun. It is the planet we know best because we live here. Earth is the first planet with a moon, the only one with liquid water and a life-sustaining atmosphere. Earth is completely different from every other planet.
We tend to measure other things in terms of the earth. For instance, the distance from the earth to the sun, 92,955,807 miles (usually rounded off to 93 million), is called one Astronomical Unit, or one AU. Other planets have distances defined in terms of how many AU’s they are from the sun. You will find a table here which shows how many times something about Earth is defined as “1” and other planets are either a fraction of that or a multiple of that.
A lot has been written about the incredible combination of things that makes life on earth possible – and not only possible, but thriving. We are only going to deal with a few major items here, as the list is quite long.
Wikipedia has an excellent introduction to the atmosphere: The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).
Our atmosphere has layers.
The lowest layer, where the rain clouds are, is the troposphere. It contains about 80% of our atmosphere in terms of how many molecules are in it. It is 78% Nitrogen, 21% Oxygen, and 1% other gases. In the picture above, it looks kind of thick. It is not. Here is how thin this layer actually is compared with the size of the earth – it's that thin blue line; it only goes about 7.5 to 8 miles high:
Here is another picture to help you see where the clouds are and how low the troposphere actually is:
It is in the troposphere that we have something else that is necessary for life: the hydrologic cycle. “Hydro” means water – like a fire hydrant. Heat from the sun warms the surface of the water and plants and the earth and water evaporates. Because it is warm it also rises. As it becomes more cool, it condenses into tiny water droplets. When these droplets cling together, we see a cloud. When the cloud becomes cool enough, the water droplets start to come together (coalesce) around tiny little dust particles, and they come down as rain. If you look at the picture above, on the right hand side you will see a red line. The red line tells you the average temperature at that height, or altitude. You can see that as we go higher in the troposphere, it gets colder – so there is only so high the clouds can go before they come back down as rain or snow.
There are some clouds much higher, which look like wispy streaks to us. They are made of tiny ice crystals in the stratosphere.
The division between the troposphere and the next layer up is called the tropopause. In the third picture down from the top, you will see it just over the picture of Mt. Everest.
The next layer up in the atmosphere is the stratosphere. It contains about 19% of our atmosphere in terms of how many molecules there are. The stratosphere seems high to us, but it is still considered part of our lower atmosphere. It goes up 25 miles past the troposphere. So you can see those molecules are pretty spread out. In the stratosphere, about 12 – 19 miles above the earth we find the ozone layer. Ozone is three oxygen atoms bound together (O3). What we breathe is two oxygen atoms bound together (O2). The ozone layer bounces back a lot of the sun’s ultraviolet rays:
The stratosphere is very cold. It ‘warms’ up to freezing near the top of it. The top of the stratosphere, where it changes into the mesosphere, is called the stratopause.
The mesosphere is the last layer in the earth’s lower atmosphere. “Meso” means “middle” and it extends for another 25 miles up past the stratopause. This layer, along with the stratosphere and troposphere are called the “homosphere.” “Homo” means “same” and these three layers all have the same types of gases in them: mostly nitrogen and oxygen. This is because of winds in all three layers which mix the gases. If you added up the percentages of the atmosphere molecules in the troposphere and stratosphere, you will see they contain 99% of all our atmosphere's molecules. Not much is left for anything above them in the mesosphere. Nevertheless, it is in the mesosphere that the little bits of rock we call meteors burn up and become 'shooting stars.' The little rocks are traveling so fast -- from 7 miles a second to more than 45 miles per second -- that they are hitting enough atoms, ions, and molecules to cause them to burn up.
The Earth passes through groups of these meteors several times a year. The most impressive one happens on the night of August 12 each year. This is the Perseid Meteor shower. Take a sleeping bag and a lot of popcorn out under the stars that night and watch for all the shooting stars. Some years there are a lot and some years not so many, but there are always some that night -- mostly right after midnight. Other times we have meteor showers are in October (around the 21st -- the Orionids), in November (about the 17th -- the Leonids), in December (the 13th -- the Geminids), and some others later.
We don’t know as much about the mesosphere as we do about the other layers. This is because planes cannot fly that high and spacecraft cannot orbit that low! We have used sounding rockets to try to find out more. We have seen red sprites there, which are a cold plasma being charged electrically. They happen far above big thunderclouds. They are triggered by lightning between the cloud and the ground. They tend to occur in groups, or clusters, 30-55 miles above the earth:
The expanding disk is called an elve. And guess what it looks like? Perhaps this explains some of the ET sightings?
We also find blue jets in the mesosphere. They are also plasmas which are being electrically charged. They are also associated with the big thunderclouds, but they come off the top of them and shoot way up. Their upward speed has been measured to be about 52 miles per second. They appear to be related to strong hail activity in thunderstorms.
Blue jets and red sprites are often found together:
Here is a video of the sprites in action, taken from one of our spacecraft. Notice something interesting: people who have said they are sure they have seen spacecraft (flying saucers) have talked about how they are so fast and change directions incredibly quickly. Is this what they have seen? Here is another amazing video of sprites, elves, the blue jets and some great shots of lightning. And one more with some great footage of the sprites and jets.
Update, May 6, 2015 -- red sprite as seen from space station.
The mesosphere is also where we find noctilucent clouds. “Noctilucent” is Latin and means “night light” or “night shining.” They can only be seen when the sun is below the horizon, in the early morning or at twilight. They are made of water ice crystals and can be seen, usually in the summer, far above or below the equator in places like Canada and Siberia.
This diagram shows why we see these clouds only at these times of the day:
The top of the mesosphere is called the mesopause. It marks the boundary between the lower atmosphere and earth’s upper atmosphere.
And then comes the thermosphere. We cannot do better than this article on the web.
The picture above also helps you see why the troposphere, stratosphere and mesosphere are grouped together as the 'homosphere' or 'lower atmosphere.' The thermosphere is called the thermosphere because of the extraordinarily high temperatures found there, as the diagram shows.
The article mentioned the ionosphere. Ions are atoms which have an electron added or stripped off. Usually atoms have as many positive charges (protons) in the nucleus as they have negative charges (electrons) circling the nucleus. This means the atom is electrically neutral. However if an electron is stripped off, the atom becomes positively charged and the wandering electron has a negative charge. A lot of electrons moving together in one direction make an electrical current. A lot of positive ions moving in one direction together also makes an electrical current.
The ionosphere which surrounds the earth includes the thermosphere and extends much farther outwards from that. It reacts to the radiation and particles coming from the sun by streaming out behind the earth like a huge, gently waving tail. Every planet has an ionosphere, and the ionosphere of every planet behaves the same way. So there are times when we pass through the tail of Venus’ ionosphere and times when Mars passes through ours. It is this ionosphere which deflects the solar wind and acts like a shield around us.
The above diagram shows clearly where the solar wind, when it is strong enough, is able to enter the polar cusps, charging the ionosphere around the earth and producing the beautiful auroras, which are simply parts of our ionosphere in glow mode.
The different colors are the different elements whose ions have been charged by the solar winds. Green is oxygen, blue and red are nitrogen. Red can also be the result of hydrogen.
And we have a lot of it! Water covers almost ¾ of the earth’s surface. There is a lot of water frozen at the north and south poles and even more in the clouds in the sky. There is also water under the surface of the earth, and many of us use well water in our homes. Without water life would not be possible. This is why scientists get excited when they think they might have found water on other planets or one of their moons: it means maybe there is life there. We cannot live without water, but water does not make life. It only makes life possible.
The earth spins on its axis. We say the ‘sun rises,’ but that isn’t really happening. We are spinning to the east, so it looks like the sun is coming up in the east. The speed at which the earth spins on its axis is also important for life. At the equator, our earth is about 24,000 miles around. We spin at about 1000 miles an hour, so we have spun a full circle in one day and one night. If we were to spin more slowly, hot days would get much hotter and cold nights would get much colder. If we were to spin more quickly, the temperatures might be more evened out, but we would experience much higher winds and more active weather systems: more storms. As it is, we are spinning at a speed which keeps our winds reasonable most of the time and also keeps our days and nights from being too long, and getting too hot or cold.
The earth’s axis is tilted about 23.5 degrees. That is why the north and south poles keep their ice. This video helps explain (the video will automatically continue to others in the series which are not necessary here). When one of the poles is away from the sun, and it is winter at that pole, the sun never comes up for weeks and weeks, and so the land stays very, very cold. Some of the ice will melt in the summer, but not much of it¸ and then storms bring in more snow and it packs down, creating a new layer of ice.
It takes one year to go all the way around the sun, so in one year we have summer, autumn, winter, and spring.
The atmosphere, liquid water, and our speed of rotation and revolution around the sun are all important to keep life on earth. There are a lot more things special about the earth which allow life, but those are some of the big ones.
Like the other planets, the earth itself is made up of layers. Our dirt layer, and the rocks beneath it, are called the ‘crust.’ The crust is very thin – under the ocean it is only about five miles deep, but in other places, such as up on mountains, it can be up to 20 miles thick. Compared to how big the earth is, though, that’s not much. Look:
The mantle is hot; it is melted rock. But it gets even hotter as we go down into the core, or the middle. It is only on the surface, and a few miles down, that the earth is cool.
Are we responsible for holes in the ozone layer? Ozone is three oxygen bound together – O3. It is an unstable molecule and breaks apart easily. It makes up about 0.6 parts per million in the atmosphere. Most of it, however, is concentrated in a band around the earth in the stratosphere known as the ozone layer. It is formed when electrical energy or UV radiation breaks up a normal O2 oxygen and then three of them will slam together for a short while. It will break down again in about half an hour.
The ozone which is not in the stratosphere, but down here, is a hazard and a pollutant. Because it is unstable, it will bind to things quickly, not only causing metals to rust (oxidize) far faster than air or moisture, but it can also harm the mucous membranes of your nose and the linings of your lungs by binding to them if you breathe in too much of it. However it is also what you smell in the air after a thunderstorm where lightning and the other electrical activity have caused more ozone to form in our lower atmosphere. Because it breaks up so quickly, the smell does not last. It is the ozone which is continuously discharged from the exhausts on our vehicles and from factories which does cause a pollutant problem, however.
Tthe ozone layer in the stratosphere is not the same thickness all around the world. There is a general layer present everywhere, though, except over the Arctic and over the Antarctic. The hole in the ozone layer over the Antarctic is the bigger of the two.
Here is an amazing animation of the hole over the Antarctic and the way it changes.
This means that, under these holes, the earth lacks the protection of the ozone reflecting back a certain amount of UV radiation. Why are these holes there?
An article on the web, The Ozone Hole, states: “The ozone "hole" is really a reduction in concentrations of ozone high above the earth in the stratosphere. The ozone hole is defined geographically as the area wherein the total ozone amount is less than 220 Dobson Units. The ozone hole has steadily grown in size (up to 27 million sq. km.) and length of existence (from August through early December) over the past two decades.”
This article also tells us, “Man-made chlorines, primarily chloroflourobcarbons (CFCs), contribute to the thinning of the ozone layer and allow larger quantities of harmful ultraviolet rays to reach the earth.”
Is this true? The CFCs were used as the pressurized propellants in aerosol cans for a long time. They were also used as the refrigerant gases in our refrigerators.
“After a series of rigorous meetings and negotiations, the Montreal Protocol on Substances that Deplete the Ozone Layer was finally agreed upon on 16 september 1987 at the Headquarters of the International Civil Aviation Organization in Montreal.
First, take a look at the top three pictures in this lesson. Look at where the ozone is – about 15 miles up, on the average. The reason CFCs were used as propellants in aerosol cans was because they are inert. They do not react with other substances. They are also heavier than air. That means that they not only will not float up to reach the stratosphere but that even if any of these molecules could, they would not react with the ozone.
An article on Higher Atmosphere Basics says, “Most of the compounds released at the Earth surface do not reach the stratosphere. They are either decomposed by the main oxidants in the troposphere (OH, NO3, ozone), photolysed by sunlight, removed by dry or wet deposition or trapped in the cold tropopause. Because of the reversion in the temperature trend air exchange between the troposphere and the stratosphere is slow. The vertical exchange of the troposphere takes hours to days, the mixing of the stratosphere months to years.”
In addition, if humans are responsible for the ozone holes, why are they at the poles, where no humans live? The majority of people on earth live in the northern hemisphere, yet the larger ozone hole is over the southern pole.
This doesn’t add up to something humans have caused either. So why are the holes there?
Remember the auroras? They are there because of the magnetic poles which cause our plasma covering to dip in at those points, allowing more radiation to reach the earth. It is this radiation which breaks up the unstable ozone molecules faster than they can maintain their layer. The reason for the holes is simple, easily understood, not man-made, and certainly not politically correct.
The Earth's Interior
Now a little more on the planet Earth itself. Between the crust (the lithosphere) and the mantle is a rather thin layer called the asthenosphere. It is made up of molten and partially molten rock mixed with water. This makes it slippery, and it is on the asthenosphere that our continents and crustal plates move.
These continents today rest on cratons – massive intrusions of granitic material which act like supporting structures for the continents.
These cratons formed in the same way as the magma plume we find under Iceland.
These plumes were the result of the various impacts on the earth which happened at the time of the Late Heavy Bombardment.
NOTE: The notation of "atomic years" is made because time measured by atomic processes is not the same as time measured by how many times we go around the sun. The difference is explained in Dating Methods.