tunnels. This system allows lower temperatures to be employed in the incinerator section, where the solids are volumetrically reduced. Origin of life The Atmosphere of early Earth is widely speculated to have been reducing. The Miller-Urey experiment, related to some hypotheses for the origin of life, entailed reactions in a reducing atmosphere composed of a mixed atmosphere of methane, ammonia, and hydrogen sulfide. Some hypotheses for the origin of life invoke a reducing atmosphere consisting of hydrogen cyanide (HCN). Experiments show that HCN can polymerize in the presence of ammonia to give a variety of products including amino acids. The same principle applies to Mars, Venus and Titan. This would have proven to be a good environment for cyanobacteria to evolve the first photosynthetic metabolic pathways, which gradually increased the oxygen portion of the atmosphere, changing it to what is known as an oxidizing atmosphere. With increased levels of oxygen, evolution of the more efficient aerobic respiration might have been enabled, allowing animal life to evolve and thrive. In contrast to the hypothesized early reducing atmosphere, evidence exists that Hadean atmospheric oxygen levels were similar to those of today. These results suggests prebiotic building blocks were delivered from elsewhere in the galaxy. The results however do not run contrary to existing theories on life's journey from anaerobic to aerobic organisms. The results quantify the nature of gas molecules containing carbon, hydrogen, and sulphur in the earliest atmosphere, but they shed no light on the much later rise of free oxygen in the air. See also Notes Metallurgy Planetary science Pottery Redox