The total global environment consists of four major realms: a gaseous atmosphere, liquid hydrosphere, solid lithosphere, and living biosphere.

The Thin Blue Veil. (Click for a larger image.)

From space, Earth's atmosphere looks like a thin blue veil. This fragile, nearly transparent envelope of gases supplies the air that we breathe each day. It also regulates the global temperature and filters out dangerous levels of solar radiation. In recent years, scientific research has shown that the chemical composition of the atmosphere is changing because of both natural and human induced causes. There is growing concern over the impact of human activities. Human impact may be increasing levels of heat absorbing gases, thereby contributing to global warming and destroying ozone, the fragile atmospheric ingredient that shields the planet from ultraviolet (UV) radiation.

Historical Atmosphere

Earth is believed to have formed about 5 billion years ago. In the first 500 million years a dense atmosphere emerged from the vapor and gases that were expelled during degassing of the planet's interior. These gases may have consisted of hydrogen (H₂), water vapor, methane (CH₄), and carbon oxides. Prior to 3.5 billion years ago the atmosphere probably consisted of carbon dioxide (CO₂), carbon monoxide (CO), water (H₂O), nitrogen (N₂), and hydrogen.

The hydrosphere was formed 4 billion years ago from the condensation of water vapor, resulting in oceans of water in which sedimentation occurred.

The most important feature of the ancient environment was the absence of free oxygen. Evidence of such an anaerobic reducing atmosphere is hidden in early rock formations that contain many elements, such as iron and uranium, in their reduced states. Elements in this state are not found in the rocks of mid-Precambrian and younger ages, less than 3 billion years old.

Formation of the Ozone Layer

One billion years ago, early aquatic organisms called blue-green algae began using energy from the Sun to split molecules of H₂O and CO₂ and recombine them into organic compounds and molecular oxygen (O₂). This solar energy conversion process is known as photosynthesis. Some of the photosynthetically created oxygen combined with organic carbon to recreate CO₂ molecules. The remaining oxygen accumulated in the atmosphere, touching off a massive ecological disaster with respect to early existing anaerobic organisms. As oxygen in the atmosphere increased, CO₂ decreased.

	Ozone Creation.(Click for a larger image.)

High in the atmosphere, some oxygen (O₂) molecules absorbed energy from the Sun's ultraviolet (UV) rays and split to form single oxygen atoms. These atoms combined (27k jpeg) with remaining oxygen (O₂) to form ozone (O₃) molecules, which are very effective at absorbing UV rays. The thin layer of ozone that surrounds Earth acts as a shield, protecting the planet from irradiation by UV light.

The amount of ozone required to shield Earth from biologically lethal UV radiation, wavelengths from 200 to 300 nanometers (nm), is believed to have been in existence 600 million years ago. At this time, the oxygen level was approximately 10% of its present atmospheric concentration. Prior to this period, life was restricted to the ocean. The presence of ozone enabled organisms to develop and live on the land. Ozone played a significant role in the evolution of life on Earth, and allows life as we presently know it to exist.

Text, images and videos courtesy of Distributed Active Archive Center at NASA's Goddard Space Flight Center.