Light emissions in the upper atmosphere from elemental lithium, potassium, and especially sodium. These alkali metals are present in the upper atmosphere at altitudes from about 50 to 62 mi (80 to 100 km) and are very efficient in resonant scattering of sunlight. The vertical column contents (number of atoms per square meter) of the alkali atoms are easily deduced from their respective emission intensities. First detected with ground-based spectrographs, the emissions were observed mainly at twilight since they tend to be overwhelmed by intense scattered sunlight present in the daytime. A chemiluminescent process gives rise to nightglow emissions at the same wavelengths. The development of lidars (laser radars) that are tuned to the resonance lines have enabled accurate resolution of the concentrations of these elements versus altitude for any time of the day. See also: Aeronomy; Chemiluminescence

Determination of the relative or absolute age of materials or objects by measurement of the degree of racemization of the amino acids present. With the exception of glycine, the amino acids found in proteins can exist in two isomeric forms called d- and l-enantiomers. Although the enantiomers of an amino acid rotate plane-polarized light in equal but opposite directions, their other chemical and physical properties are identical. Amino acid handedness or homochirality is one of the most distinctive features of terrestrial life. It was discovered by L. Pasteur around 1850 that only l-amino acids are generally found in living organisms, but scientists still have not formulated a convincing reason to explain why life on Earth is based on only l-amino acids. See also: Amino acids

A scientific discipline concerned with the chemical composition of the Earth's atmosphere. Topics falling within the field include the emission, transport, and deposition of atmospheric chemical species; the rates and mechanisms of chemical reactions taking place in the atmosphere; and the effects of atmospheric species on human health, the biosphere, and climate (Fig. 1).

The study of the cycling of chemicals between organisms and the surface environment of the Earth. The chemicals either can be taken up by organisms and used for growth and synthesis of living matter or can be processed to obtain energy. The chemical composition of plants and animals indicates which elements, known as nutrient elements, are necessary for life. The most abundant nutrient elements, carbon (C), hydrogen (H), and oxygen (O), supplied by the environment in the form of carbon dioxide (CO₂) and water (H₂O), are usually present in excess. The other nutrient elements, which are also needed for growth, may sometimes be in short supply; in this case they are referred to as limiting nutrients. The two most commonly recognized limiting nutrients are nitrogen (N; Fig. 1) and phosphorus (P).

The thin zone of living organisms and their environments at the surface of the Earth. The biosphere is the layer of life of the Earth (Fig. 1). Included in the biosphere are all environments capable of sustaining life above, on, and beneath the Earth's surface, as well as in the oceans. Consequently, the biosphere overlaps virtually the entire hydrosphere (the water portion of the Earth) and portions of the atmosphere (the gaseous layer that envelops the Earth) and outer lithosphere (the rigid outer layer of the Earth that can support stress). Moreover, in the course of its reactions, the biosphere has important influences on the outer lithosphere, hydrosphere, and atmosphere. See also: Atmosphere; Earth; Earth sciences; Ecology; Environment; Hydrosphere; Lithosphere

The average chemical and isotopic composition of the solar system is appropriately referred to as cosmic, since this elemental abundance distribution is found to be nearly the same for interstellar gas and for young stars associated with gas and dust in the spiral arms of galaxies. The Sun makes up more than 99.9% of the mass of the solar system, so the bulk chemical composition of the solar system is essentially the same as that of the Sun. The cosmic abundances of the nonvolatile elements are determined from chemical analyses of a type of meteorite known as CI chondrites, whereas the relative abundances of the volatile elements are determined from quantitative measurements of the intensities of elemental emission lines from the Sun's photosphere. In most silicate-rich meteorites and the Earth, Moon, Venus, and Mars, the most abundant elements are oxygen, magnesium, silicon, iron, aluminum, and calcium. Average solar-system composition consists of 70.7 wt % hydrogen, 27.4 wt % helium, and only 1.9 wt % of all remaining elements, lithium to uranium. Cosmic abundances are now widely referred to as standard abundances in the astrophysical literature. See also: Astronomical spectroscopy; Element (chemistry)

The science of the chemistry of the universe, particularly that beyond the Earth. As currently practiced, cosmochemistry is concerned primarily with inferences on pre-solar-system events, solar nebular processes, and early planetary processes as deduced from minerals in meteorites and from chemical and isotopic compositions of meteorites and their parts. See also: Isotope; Meteorite

A rare nuclide produced by nuclear reactions between high-energy cosmic radiation and terrestrial or extraterrestrial material. Cosmogenic nuclides may be used to examine the history of exposure to cosmic rays, and have numerous applications in earth science and archeology.

An estimate of the age of the planet Earth based on the lead isotope systematics. The first meaningful estimate was calculated from the average isotopic composition of terrestrial lead (Pb) available at the surface of the planet. The feasibility of the calculation stems principally from the unique circumstance that two isotopes of the element uranium (²³⁵U and ²³⁸U) decay to two daughters that are also isotopes of another element, lead (²⁰⁷Pb and ²⁰⁶Pb, respectively). This eliminates the requirement of determining the planetary budget of either of the two elements. The total number of ²⁰⁶Pb and ²⁰⁷Pb atoms are related to their respective parents by the equation of radioactive decay, expressed as Eq. (1) for lead-206 and Eq. (2)

(1)

(2)

The process by which water bodies, such as estuaries, coastal waters, and lakes, become enriched in dissolved nutrients, such as phosphorus and nitrogen, resulting in increased growth of algae and aquatic plants as well as the deterioration of the water’s esthetic and life-supporting qualities. The primary responses to eutrophication are seen in potentially harmful blooms of phytoplankton, or suspended algae (see illustration), oxygen-depleted zones, and fish kills. Water for consumption from eutrophic water sources must be filtered and treated. To prevent eutrophication, agricultural and waste management practices are essential, including diversion of sewage, better use of manure, erosion control, improved sewage treatment, and harvesting of the surplus aquatic crops. See also: Anoxic zones; Water conservation