Reducing atmosphere

A reducing atmosphere, also known as a reduction atmosphere, is an atmosphere without significant amounts of free oxygen (O2) and other oxidising gases or vapours thus preventing oxidation. Chemists refer to an atmosphere with abundance of hydrogen atoms, or other substances that readily provides electrons, and a scarcity of oxygen and nitrogen atoms as "reduced". A reducing atmosphere contains reductants, or molecules saturated with hydrogen atoms, such as ammonia (NH3) and methane (CH4), which are able to reduce other molecules. So carbon, for example, appears in its most reduced form (CH4) but not in an oxidized form (CO) or a fully oxidized form (CO2). The discussion of the degree of oxidation of Earth's early atmosphere is one of the fundamental topics of the origin of life models.

The primitive atmosphere
Theories of the origin of life initially assumed that the primitive atmosphere should be a reducing one in order to prevent oxidative destruction of the components of the "primordial soup". So the evolutionists theorize an early atmosphere without oxygen. Oparin believed that the Earth's early atmosphere was composed of hydrogen (H2), methane (CH4), ammonia (NH3) and water vapor (H2O), containing no oxygen. Haldane postulated the same theory and his most important conclusions were presented in a short article for the Rationalist Annual. Other scientists who corroborated this model of atmosphere were Miller and Urey. They reasoned that a small but significant amount of H2 remained in the primordial atmosphere and reacted with atoms of carbon, nitrogen or oxigen forming this kind of atmosphere. By sparking together hydrogen, methane, ammonia, and water vapor, they produced several amino acids, the building blocks of organic life.

Nowadays there is almost universal agreement among specialists that the primitive atmosphere contained no reducing gases like methane, ammonia or hydrogen. Instead, it is that the atmosphere contained carbon dioxide and nitrogen. According Hubert Yockey, the modern view regarding the atmosphere of the early Earth is that it was neutral and composed of nitrogen (N2), carbon dioxide (CO2), water (H2O) and maybe some ammonia (NH3).

An atmosphere without oxygen carries some difficulties regarding the problem of the origin of life. Michael Denton points out that with an atmosphere without oxygen there would be no ozone layer in the upper atmosphere and the ultraviolet flux reaching the Earth would be sufficient to break down organic compounds as rapidly as they were produced. Some scientists latterly, (e.g. R. T. Brickmann and J. H. Carver) believe that the earth´s primordial atmosphere was not so strongly reducing and probably contained some amount of oxygen produced by photodissociation of water.