Where space stops and Earth begins

"So Albert when do we reach this chaotic planet?"

That depends on where you decide the Earth begins.

"Don't be daft, there is space and then there is this great lump of rock. I'm sure we'll notice when we reach it."

Around the Earth is a very unsolid atmosphere and that’s a very important part of the Earth. That’s where all this weather and climate stuff happens.

"OK, when do we reach the edge of the atmosphere then?"

The atmosphere only has one well defined edge and that's where it meets solid ground or the ocean.

"The outer edge I mean. Everything that has a beginning must have an end."

Not everything. The Earth's atmosphere is one of those fading away sort of things. It gets thinner and thinner and then eventually blends in with the solar wind over a thousand miles above the surface.

“The winds from the sun reach the earth?”

Solar wind is the rather romantic name given to the stream of particles like electrons and protons coming out of the sun. The outermost layer of the atmosphere is called the magnetosphere. This is really a portion of the solar wind trapped by the Earth’s magnetic field rather than anything you would think of as an atmosphere. The magnetosphere sits around the Earth like a giant ring doughnut. Near the north and south poles some of the charged particles from the solar wind can get through this magnetic field. As they fall earthwards they produce an eerie swirling light; the aurora borealis or northern lights.

“Wow, one day I’ll see that for real.”

One of the many things on this planet, everyone should try and see once.

“Excuse me for asking but is the atmosphere a dangerous place for photons?"

Why do you ask?

"Because some of our fellow photons seem to be disappearing as we speak."

Don't worry about them, they are just ultraviolet photons. They’re the high energy photons that are like an invisible type of very blue light. Most of them don’t get past the first bit of atmosphere they meet. The outer atmosphere has a thin smattering of a particularly nasty type of oxygen called ozone.

"I thought things that destroyed ozone were 'environmentally unfriendly'. That should make ozone environmentally friendly and nice rather than nasty."

It rather depends where it is. Ozone or O₃, is just three oxygen atoms stuck together. When photons from the sun collide with normal oxygen molecules, which are two oxygen atoms stuck together or O2, they can be split up into single oxygen atoms. Some of these free oxygen atoms join back up with other oxygen atoms to remake normal oxygen gas and others stick together in a different way to make ozone. Ozone absorbs ultraviolet photons and prevents most of the ultraviolet photons from the sun from reaching the Earth's surface which is a good thing as these ultraviolet photons can cause a lot of damage.

"Ultraviolent?"

Well yes, but more high energy than violent. UV light is certainly ultraviolent to one thing that is very important to living things, DNA. You remember the stuff that contains all the genetic codes or instructions about how everything works in living things. If you are a single celled organism then damaging your DNA is a death sentence. If, like humans, you are composed of trillions of cells then it is only the cells on the outside that are damaged. This is less drastic, but as any sunburn sufferer will tell you, it's no fun. More serious damage to the DNA in human skin can produce something far more dangerous than sunburn - skin cancer. Cancer is a type of disease where cells start to grow out of control. Damage to the DNA from things like ultraviolet light can cause random changes, or mutations, to some of the letters in the genetic code. Some of these mutations don’t cause any problems, but some affect how a cell grows and behaves. Without the normal brakes on their growth these mutated cells keep on growing and spread into other parts of the body causing damage as they go.

“So ozone is like sunscreen for the planet?”

I suppose it is but for all the talk about ozone, there is surprisingly little of it in the atmosphere. Ozone is found high up in atmosphere, in a layer called the stratosphere about 15 miles up. At this height the atmosphere is very thin and wispy. Down at ground level, where the atmosphere is much thicker than in the stratosphere, all the ozone that exists would only make a layer a few centimetres thick. Although ozone is being made all the time in the upper atmosphere, chemicals that used to be used in aerosol cans and old fridges can spread up to the stratosphere and destroy ozone. Each of these molecules which are called Chloroflurocarbons or CFC’s can destroy thousands and thousands of molecules of ozone. Pollution like this has created a hole in the ozone layer that appears every spring at the South Pole. It was first recognised in 1984 but looking back it was happening in the 1970’s.

“Why does it appear only in the south pole?”

Ozone is only destroyed when the temperature gets below minus 80^oC. At that temperature clouds form high in the atmosphere with ice crystals covered in nitric acid. It’s the combination of these clouds and CFC’s that destroy ozone. Ozone at the North Pole hasn’t had the same problem because it’s a little warmer than the South Pole so only a small dimple in ozone appears in the artic spring at the North Pole.

“And why in spring? It is surely colder in the middle of winter at the South Pole?”

“It is colder in winter, but the damage to ozone needs light from the sun. Remember we talked about how the Earth is tilted? Well because of that tilt the sun doesn’t rise at all in the South Pole in winter. It is only when the sun reappears in spring that ozone starts to be destroyed.

“Is anyone doing anything about it?”

This is one time when countries have managed to agree to do something together. Twenty years ago the Montreal protocol was agreed and the use of these CFC chemicals has been reduced dramatically. Unfortunately as these chemicals can last 50 years or more in the atmosphere it will be a long time before they are gone.

“And the ozone hole?”

Well it stopped getting bigger. 2003 was a very bad year but overall things have stopped getting worse which is a good thing. But ozone is not always a good thing. Ozone can also be made closer to the surface where it is not such a pleasant thing to have around. Environmentalists may not like CFC's because they are bad for the ozone layer but that doesn't mean that they like ozone everywhere. Photochemical smog, the kind that Los Angeles is famous for, is caused by the effect of sunlight on the pollution coming out of car exhausts, contains ozone. Ozone near the ground is very nasty stuff. Even at minuscule concentrations it can start to kill plants. At even lower concentrations it can cause breathing problem for some people. Ozone is also produced by electric sparks. It is that slightly sharp, acrid smell that comes off electric train sets. Fortunately there aren't that many electric train sets in the world and they are not all turned on at the same time. The only truly ecologically safe pastime left seems to be sitting on your compost heap and slowly decomposing. But I don't suppose that is as much fun as pretending to be the Flying Scotsman.

“What would happen if all the ozone was destroyed?”

If the ozone layer was seriously damaged it would be disastrous for life on Earth. If all the ozone disappeared overnight I'm afraid it would be dinosaur time all over again for most creatures. Humans can wear sunscreen but the food supply would start to arrive pre-cooked and then would disappear altogether. The good news for us is that being a nice, non controversial, middle of the road sort of wavelength, ozone doesn't really bother us. Since we are part of the band of visible light, we should breeze through.

"It's lucky for us that the atmosphere lets visible light through."

Well it’s a sort of the other way around. If a certain type of light didn't get through the atmosphere, then there wouldn't be any point in life forms evolving eyes that could see that wavelength. And if there aren't any eyes that can see it then...

"It isn't visible light."

Precisely.