Nitrogen makes up about 78% of the atmosphere by volume but the atmosphere of Mars contains less than 3% nitrogen. The element seemed so inert that Lavoisier named it azote, meaning "without life". However, its compounds are vital components of foods, fertilizers, and explosives. Nitrogen gas is colorless, odorless, and generally inert. As a liquid it is also colorless and odorless.
	It was known during the 18th century that air contains at least two gases, one of which supports combustion and life, and the other of which does not. Nitrogen was discovered by Daniel Rutherford in 1772, who called it noxious air, but Scheele, Cavendish, Priestley, and others at about the same time studied "burnt" or "dephlogisticated" air, as air without oxygen was then called.
	While about one fifth of the atmosphere is oxygen gas, the atmosphere of Mars contains only about 0.15% oxygen. Oxygen is the third most abundant element found in the sun, and it plays a part in the carbon-nitrogen cycle, one process responsible for stellar energy production. Oxygen in excited states is responsible for the bright red and yellow-green colors of the aurora. About two thirds of the human body, and nine tenths of water, is oxygen. The gas is colorless, odorless, and tasteless. Liquid and solid oxygen are pale blue (see picture above) and strongly paramagnetic (contains unpaired electrons).
	Oxygen is very reactive and oxides of most elements are known. It is essential for respiration of all plants and animals and for most types of combustion.
	Leonardo da Vinci suggested that air consists of at least two different gases. Before then, air was felt to be an element in its own right. He was also aware that one of these gases supported both flames and life. Oxygen was prepared by several workers before 1772 but these workers did not recognize it as an element. Joseph Priestley is generally credited with its discovery (who made oxygen by heating lead or mercury oxides), but Carl Wilhelm Scheele also reported it independently.
	The behavior of oxygen and nitrogen as components of air led to the advancement of the phlogiston theory of combustion, which influenced chemists for a century or so, and which delayed an understanding of the nature of air for many years.
	Ozone (O3) is another allotrope of oxygen. It is formed from electrical discharges or ultraviolet light acting on O2. It is an important component of the atmosphere (in total amounting to the equivalent of a layer about 3 mm thick at ordinary pressures and temperatures) which is vital in preventing harmful ultraviolet rays of the sun from reaching the earth's surface.  Undiluted ozone is bluish in color. Liquid ozone is bluish-black, and solid ozone is violet-black.