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.
In the first 500 million years, a dense atmosphere formed from the vapor and gases that were expelled during degassing of the planet's interior...
The hydrosphere was formed 4 billion years ago from the condensation of water vapor, which made sedimentation build up in the oceans.
Once upon a time, one billion years ago, early aquatic organisms called blue-green algae began using energy from me to split molecules of H2O and CO2 (carbon dioxide) and recombine them into organic compounds and molecular oxygen (O2).
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
High in the atmosphere, some oxygen (O2) molecules absorbed energy from my ultraviolet (UV) rays and split to form single oxygen atoms. These atoms combined with remaining oxygen (O2) to form ozone (O3) molecules, which are very effective at absorbing my UV rays. The thin layer of ozone that surrounds Earth acts as a shield, protecting the planet from irradiation by my UV light.