OZONE: What is it?

Ozone (O3) is a gas which has chemical properties similar to hydrogen peroxide (H2O2). It is created and distroyed in the Earth's atmosphere by delicate balance between the rate of formation and the rate of destruction, both of which vary with increasing altitude above the Earth's surface. The Ozone layer extends from a height of 20 kilometers to 60 kilometers above the Earth's surface. The air is very thin at these altitudes. If all of the Ozone in the Earth's atmosphere were compressed into a single layer at the Earth's surface, it would only be 3 millimeters thick! Ozone is important because it is the only atmospheric gas which absorbs light in the B region of the ultraviolet spectrum (UVB).

WHY DO WE NEED TO LOOK AT THE EARTH FROM SPACE?

Measuring the composition and structure of the Earth's atmosphere from the ground is like trying to measure the contents of a test-tube, from the inside, without knowing what is on the outside. There is always something between you and the outside world, so you can only make relative measurements.

For example, one way to measure how much Ozone is in the Earth's atmosphere is by measuring how much sunlight is the Ozone absorbs as the sunlight travels through the atmosphere. The amount of light absorbed is directly proportional to how much Ozone is between you, the observer, and the sun. Unfortunately, to do this you need to know how much light sunlight there was before the ozone absorbed some. This is tough to do from the surface of the Earth, because the Ozone is always between you and the sun!

We can measure how much sunlight reaches the Earth's atmosphere before it passes through the Ozone by riding the Space Shuttle as it orbits above the ozone layer. Now we can measure the initial amount of light, but now because we are above the Ozone, how do we measure the amount of light absorbed by theOzone ? We do this by following the sun as it passes into the Earth's shadow, taking a series of measurements as the sun is occulted by the Earth's atmosphere . This is called a solar occultation. While the sun is above the Ozone layer, we measure the initial intensity of the light, as the ozone layer moves between the sun and our point of view on the shuttle more and more light is removed, so we measure how much light the Ozone absorbs which tells us how much ozone is in the Earth's atmosphere.

The sun is much to bright for our GLO instrument, so we use very hot stars which are bright in the ultraviolet to measure how much Ozone is in the Earth's Atmosphere. This plot shows how light is absorbed by Ozone as a double star (alpha and beta cruces) is passes behind the Earth. The lines on the plot show a ratio between the intensity of the light at a given altitude and the intensity of the light measured from the stars well above the Ozone layer. The ratio is used because it cancels out things that are common to both measurements, accentuating the differences between them.

The plot shows the part of the solar spectrum screened by Ozone and how effective Ozone is at protecting us from UVB radiation. The line marked "Center of O3 layer" shows a dip between 2400 and 3000 Angstroms which absorbs the light from the star almost completely. This is done by a layer of gas less than 1/4 inch thick!