The abundance of a chemical element measures how relatively common the element is, or how much of the element there is by comparison to all other elements. Abundance may be variously measured by mass-fraction (the same as weight fraction), or mole-fraction (fraction of atoms), or by volume fraction. Measurement by volume-fraction is a common abundance measure in mixed gases such as atmospheres, which is close to mole-fraction for ideal gas mixtures (i.e., gas mixtures at relatively low densities and pressures). The periodic table of the chemical elements A chemical element, or element for short, is a type of atom that is defined by its atomic number; that is, by the number of protons in its nucleus. ...

For example, the mass-fraction abundance of oxygen in water is about 89%, because that is the fraction of water's mass which is oxygen. However, the mole-fraction abundance of oxygen in water is only 33% because only 1 atom in 3 in water is an oxygen atom. In the universe as a whole, and in the atmospheres of gas-giant planets such as Jupiter, the mass-fraction abundances of hydrogen and helium are about 74% and 24% respectively, while the mole-fraction and volume fractions of these elements are closer to 92% and 8%.

Most abundances in this article are given as mass-fraction abundances.

Abundance of elements in the Universe

Cosmological observations suggest that about 73% of the universe consists of dark energy, 23% is composed of dark matter and only 4% corresponds to the visible baryonic matter which constitutes stars, planets and living beings. Dark matter has not yet been detected in a particle physics detector, and the nature of the dark energy is not yet understood. Observational cosmology is the study of the structure, the evolution and the origin of the universe through observation, using instruments such as telescopes and cosmic ray detectors. ... In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and has strong negative pressure. ... In astrophysics and cosmology, dark matter is matter, not directly observed and of unknown composition, that does not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence can be inferred from gravitational effects on visible matter. ... In particle physics, the baryons are a family of subatomic particles including the proton and the neutron (collectively called nucleons), as well as a number of unstable, heavier particles (called hyperons). ... This article or section does not adequately cite its references or sources. ... For alternate meanings see star (disambiguation) Hundreds of stars are visible in this image taken by the Hubble Space Telescope of the Sagittarius Star Cloud in the Milky Way Galaxy. ... A planet (from the Greek πλανήτης, planetes or wanderers) is a body of considerable mass that orbits a star and that produces very little or no energy through nuclear fusion. ... For other uses, see Life (disambiguation) and Living (disambiguation). ... Thousands of particles explode from the collision point of two relativistic (100 GeV per ion) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ...

Most standard matter is found in the form of atoms, although there are many other unusual kinds of matter.

Hydrogen is the most abundant element in the known Universe; helium is second. However, after this, the rank of abundance does not continue to correspond to the atomic number; oxygen has abundance rank 3, but atomic number 8. All others are orders of magnitude less common. General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... Universe is a word derived from the Old French univers, which in turn comes from the Latin roots unus (one) and versus (a form of vertere, to turn). Based on observations of the observable universe, physicists attempt to describe the whole of space-time, including all matter and energy and... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... It has been suggested that List of elements by atomic number be merged into this article or section. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... It has been suggested that List of elements by atomic number be merged into this article or section. ... An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. ...

Both helium-3 and helium-4 were produced in the Big Bang, during a process known as big bang nucleosynthesis. Additional helium is produced by the fusion of hydrogen inside stellar cores by a variety of processes including the proton-proton chain and the CNO cycle. According to the Big Bang theory, the universe emerged from an extremely dense and hot state (singularity). ... In cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than H-1, the normal, light hydrogen, during the early phases of the universe, shortly after the Big Bang. ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power. ... The proton-proton chain reaction is one of two fusion reactions by which stars convert hydrogen to helium, the other being the CNO cycle. ... This article does not cite its references or sources. ...

Hydrogen and helium are estimated to make up roughly 74% and 24% of all baryonic matter in the universe respectively. Despite comprising only a very small fraction of the universe, the remaining "heavy elements" can greatly influence astronomical phenomena. Only about 2% of the Milky Way galaxy's disk is comprised of heavy elements. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Galaxia Kuklos; or simply the Galaxy) is a barred spiral galaxy in the Local Group, and has special significance to humanity as the location of the solar system, which is located near the Orion...

These other elements are generated by stellar processes. In astronomy, a "metal" is any element other than hydrogen or helium. This distinction is significant because hydrogen and helium (together with trace amounts of lithium) are the only elements that occur naturally without the nuclear fusion activity of stars. Thus, the metallicity of a galaxy or other object is an indication of past stellar activity. A giant Hubble mosaic of the Crab Nebula, a supernova remnant Astronomy is the science of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earths atmosphere (such as auroras and cosmic background radiation). ... General Name, Symbol, Number lithium, Li, 3 Chemical series alkali metals Group, Period, Block 1, 2, s Appearance silvery white/grey Atomic mass 6. ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power. ... This article is about the astronomical object. ... The globular cluster M80. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 17,000 parsecs in diameter and approximately 20 million parsecs distant. ...

These are the ten most common elements in the Universe as measured in parts per million^{[citation needed]}:

See also: Stellar population General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... General Name, Symbol, Number neon, Ne, 10 Chemical series noble gases Group, Period, Block 18, 2, p Appearance colorless Atomic mass 20. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Atomic mass 55. ... General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless gas Atomic mass 14. ... It has been suggested that Silicons ranking be merged into this article or section. ... General Name, Symbol, Number magnesium, Mg, 12 Chemical series alkaline earth metals Group, Period, Block 2, 3, s Appearance silvery white Atomic mass 24. ... General Name, Symbol, Number sulfur, S, 16 Chemical series nonmetals Group, Period, Block 16, 3, p Appearance lemon yellow Atomic mass 32. ... Stars observed in our galaxy appear to group into two general types called Population I and Population II. (A hypothetical third group, Population III, does not occur in our galaxy. ...

Abundance of elements on Earth

See also: Earth#Chemical composition

The Earth formed from the same cloud of matter that formed the Sun, but the planets acquired different compositions during the formation and evolution of the solar system. The history of Earth caused parts of this planet to have differing concentrations of the elements. Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... The theories concerning the formation and evolution of the Solar System are complex and varied, interweaving various scientific disciplines, from astronomy and physics to geology and planetary science. ... The planet Earth, photographed in the year 1972. ...

Abundance of elements in Earth's crust

This graph illustrates the relative abundance of the chemical elements in Earth's upper continental crust.

Abundance (atom fraction) of the chemical elements in Earth's upper continental crust as a function of atomic number.

Many of the elements shown in the graphic are classified into (partially overlapping) categories: Image File history File links relative abundance of elements in Earths crust graphic from public domain sourcehttp://geopubs. ... Image File history File links relative abundance of elements in Earths crust graphic from public domain sourcehttp://geopubs. ...

1. rock-forming elements (major elements in green field and minor elements in light green field);
2. rare earth elements (lanthanides, La-Lu, and Y; labeled in blue);
3. major industrial metals (global production >~3×10⁷ kg/year; labeled in bold);
4. precious metals (italic);
5. the nine rarest "metals" — the six platinum group elements plus Au, Re, and Te (a metalloid).

Note that there are two breaks where the unstable elements technetium (atomic number: 43) and promethium (atomic number: 61) would be. These are very rare, as on Earth they are only produced through the fission of heavy radioactive elements (for example, uranium or thorium). Both elements have been identified spectroscopically in the atmospheres of stars, where they are produced by ongoing nucleosynthetic processes. There are also breaks where the six noble gases would be as they are found in the Earth's crust due to decay chains from radioactive elements and are therefore not included. The six very rare, highly radioactive elements (polonium, astatine, francium, radium, actinium and protactinium) are not included, as their natural abundances are too low to have been accurately measured. Rare earth ore Rare earth elements and rare earth metals are trivial names sometimes applied to a collection of 17 chemical elements in the periodic table, namely scandium, yttrium, and the lanthanides. ... A gold nugget A precious metal is a rare metallic chemical element of high economic value. ... The platinum group or platinum metals is the collective name sometimes used for six chemical elements within the periodic table. ... General Name, Symbol, Number technetium, Tc, 43 Chemical series transition metals Group, Period, Block 7, 5, d Appearance silvery gray metal Standard atomic weight [98](0) g·mol−1 Electron configuration [Kr] 4d5 5s2 Electrons per shell 2, 8, 18, 13, 2 Physical properties Phase solid Density (near r. ... General Name, Symbol, Number promethium, Pm, 61 Chemical series lanthanides Group, Period, Block n/a, 6, f Appearance metallic Atomic mass [145](0) g/mol Electron configuration [Xe] 4f5 6s2 Electrons per shell 2, 8, 18, 23, 8, 2 Physical properties Phase solid Density (near r. ... For the generation of electrical power by fission, see Nuclear power plant An induced nuclear fission event. ... The noble gases are the elements in group 18 (Group 0 IUPAC Style) of the periodic table. ... General Name, Symbol, Number polonium, Po, 84 Chemical series metalloids Group, Period, Block 16, 6, p Appearance silvery Atomic mass (209) g·mol−1 Electron configuration [Xe] 4f14 5d10 6s2 6p4 Electrons per shell 2, 8, 18, 32, 18, 6 Physical properties Phase solid Density (near r. ... General Name, Symbol, Number astatine, At, 85 Chemical series halogens Group, Period, Block 17, 6, p Appearance metallic (presumed) Atomic mass (210) g·mol−1 Electron configuration [Xe] 4f14 5d10 6s2 6p5 Electrons per shell 2, 8, 18, 32, 18, 7 Physical properties Phase solid Melting point 575 K (302... General Name, Symbol, Number francium, Fr, 87 Chemical series alkali metals Group, Period, Block 1, 7, s Appearance metallic Atomic mass (223) g·mol−1 Electron configuration [Rn] 7s1 Electrons per shell 2, 8, 18, 32, 18, 8, 1 Physical properties Phase solid Density (near r. ... General Name, Symbol, Number radium, Ra, 88 Chemical series alkaline earth metals Group, Period, Block 2, 7, s Appearance silvery white metallic Atomic mass (226) g·mol−1 Electron configuration [Rn] 7s2 Electrons per shell 2, 8, 18, 32, 18, 8, 2 Physical properties Phase solid Density (near r. ... General Name, Symbol, Number actinium, Ac, 89 Chemical series actinides Group, Period, Block 3, 7, f Appearance silvery Atomic mass (227) g·mol−1 Electron configuration [Rn] 6d1 7s2 Electrons per shell 2, 8, 18, 32, 18, 9, 2 Physical properties Phase solid Density (near r. ... General Name, Symbol, Number protactinium, Pa, 91 Chemical series actinides Group, Period, Block ?, 7, f Appearance bright, silvery metallic luster Atomic mass 231. ...

Oxygen and silicon are notably common; they form several common silicate minerals. General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... It has been suggested that Silicons ranking be merged into this article or section. ... The silicate minerals make up the largest and most important class of rock-forming minerals. ...

"Rare earth" element abundances

"Rare" earth elements is a historical misnomer; persistence of the term reflects unfamiliarity rather than true rarity. The more abundant rare earth elements are each similar in crustal concentration to commonplace industrial metals such as chromium, nickel, copper, zinc, molybdenum, tin, tungsten, or lead. Even the two least abundant rare earth elements (Tm, Lu) are nearly 200 times more common than gold. However, in contrast to ordinary base and precious metals, rare earth elements have very little tendency to become concentrated in exploitable ore deposits. Consequently, most of the world's supply of rare earth elements comes from only a handful of sources.

Differences in abundances of individual rare earth elements in the upper continental crust of Earth represent the superposition of two effects, one nuclear and one geochemical. First, rare earth elements with even atomic numbers (₅₈Ce, ₆₀Nd, ...) have greater cosmic and terrestrial abundances than adjacent rare earth elements with odd atomic numbers (₅₇La, ₅₉Pr, ...). Second, the lighter rare earth elements are more incompatible (because they have larger ionic radii) and therefore more strongly concentrated in the continental crust than the heavier rare earth elements. In most rare earth deposits, the first four rare earth elements - La, Ce, Pr, and Nd - constitute 80 to 99% of the total.

Ocean

See sea water for abundance of elements in the ocean, but note that that list is by mass - a list by molarity would look very different, especially as regards the first 4 elements; specifically, hydrogen would comprise well over half of such a list. General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... General Name, Symbol, Number sulfur, S, 16 Chemical series nonmetals Group, Period, Block 16, 3, p Appearance lemon yellow Atomic mass 32. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, Symbol, Number calcium, Ca, 20 Chemical series alkaline earth metals Group, Period, Block 2, 4, s Appearance silvery white Atomic mass 40. ... General Name, Symbol, Number chlorine, Cl, 17 Chemical series halogens Group, Period, Block 17, 3, p Appearance yellowish green Atomic mass 35. ... General Name, Symbol, Number potassium, K, 19 Chemical series alkali metals Group, Period, Block 1, 4, s Appearance silvery white Atomic mass 39. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... General Name, Symbol, Number bromine, Br, 35 Chemical series halogens Group, Period, Block 17, 4, p Appearance gas/liquid: red-brown solid: metallic luster Atomic mass 79. ... General Name, Symbol, Number magnesium, Mg, 12 Chemical series alkaline earth metals Group, Period, Block 2, 3, s Appearance silvery white Atomic mass 24. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... Sea water is water from a sea or ocean. ... This page refers to concentration in the chemical sense. ...

Atmosphere

The order of elements in the atmosphere is nitrogen (78.1%), oxygen (20.9%), argon (0.96%), followed by (in uncertain order) carbon and hydrogen. The carbon and hydrogen concentrations are variable for a number of reasons, including human activity. Sulfur, phosphorus, and all other elements are present in significantly lower proportions. Layers of Atmosphere - not to scale (NOAA)[2] Earths atmosphere is a layer of gases surrounding the planet Earth and retained by the Earths gravity. ... General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless gas Atomic mass 14. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... General Name, Symbol, Number argon, Ar, 18 Chemical series noble gases Group, Period, Block 18, 3, p Appearance colorless Atomic mass 39. ...

According to the above graphic, argon, a significant if not major component of the atmosphere, does not appear in the crust at all.

Organisms

The abundance of elements compared to carbon element* (taken from Mary K. Campbell, Shawn O. Farrell - Biochemistry)

 * number of atoms for a thousand carbon atoms. General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless gas Atomic mass 14. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... General Name, Symbol, Number magnesium, Mg, 12 Chemical series alkaline earth metals Group, Period, Block 2, 3, s Appearance silvery white Atomic mass 24. ... General Name, Symbol, Number phosphorus, P, 15 Chemical series nonmetals Group, Period, Block 15, 3, p Appearance waxy white/ red/ black/ colorless Atomic mass 30. ... General Name, Symbol, Number sulfur, S, 16 Chemical series nonmetals Group, Period, Block 16, 3, p Appearance lemon yellow Atomic mass 32. ... General Name, Symbol, Number potassium, K, 19 Chemical series alkali metals Group, Period, Block 1, 4, s Appearance silvery white Atomic mass 39. ... General Name, Symbol, Number calcium, Ca, 20 Chemical series alkaline earth metals Group, Period, Block 2, 4, s Appearance silvery white Atomic mass 40. ... General Name, Symbol, Number manganese, Mn, 25 Chemical series transition metals Group, Period, Block 7, 4, d Appearance silvery metallic Atomic mass 54. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Atomic mass 55. ... General Name, Symbol, Number zinc, Zn, 30 Chemical series transition metals Group, Period, Block 12, 4, d Appearance bluish pale gray Atomic mass 65. ...

Human body

By mass, human cells consist of 65-90% water (H₂O), and a significant portion is composed of carbon-containing organic molecules. Oxygen therefore contributes a majority of a human body's mass, followed by carbon. 99% of the mass of the human body is made up of the six elements oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus.

Chang, Raymond (2007). Chemistry, Ninth Edition. McGraw-Hill, p. 52. ISBN 0-07-110595-6.  General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless gas Atomic mass 14. ... General Name, Symbol, Number calcium, Ca, 20 Chemical series alkaline earth metals Group, Period, Block 2, 4, s Appearance silvery white Atomic mass 40. ... General Name, Symbol, Number phosphorus, P, 15 Chemical series nonmetals Group, Period, Block 15, 3, p Appearance waxy white/ red/ black/ colorless Atomic mass 30. ... General Name, Symbol, Number potassium, K, 19 Chemical series alkali metals Group, Period, Block 1, 4, s Appearance silvery white Atomic mass 39. ... General Name, Symbol, Number sulfur, S, 16 Chemical series nonmetals Group, Period, Block 16, 3, p Appearance lemon yellow Atomic mass 32. ... General Name, Symbol, Number chlorine, Cl, 17 Chemical series halogens Group, Period, Block 17, 3, p Appearance yellowish green Atomic mass 35. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... General Name, Symbol, Number magnesium, Mg, 12 Chemical series alkaline earth metals Group, Period, Block 2, 3, s Appearance silvery white Atomic mass 24. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Atomic mass 55. ... wikipedia sucks big balls For other uses, see Cobalt (disambiguation). ... General Name, Symbol, Number copper, Cu, 29 Chemical series transition metals Group, Period, Block 11, 4, d Appearance metallic pinkish red Standard atomic weight 63. ... General Name, Symbol, Number zinc, Zn, 30 Chemical series transition metals Group, Period, Block 12, 4, d Appearance bluish pale gray Atomic mass 65. ... General Name, Symbol, Number iodine, I, 53 Chemical series halogens Group, Period, Block 17, 5, p Appearance violet-dark gray, lustrous Atomic mass 126. ... Se redirects here. ... General Name, Symbol, Number fluorine, F, 9 Chemical series halogens Group, Period, Block 17, 2, p Appearance Yellowish brown gas Atomic mass 18. ...

See also

• Abundances of the elements (data page)
• Chemical composition of living beings
• Chemical Galaxy (a new periodic table)
• Chemical makeup of the human body
• Cosmochemical Periodic Table of the Elements in the Solar System
• Natural abundance (isotopic abundance)
• Periodic table

<< Chemical elements data references // Earth bulk continental crust and upper continental crust C1 — Crust: CRC Handbook C2 — Crust: Kaye & Laby C3 — Crust: Greenwood C4 — Crust: Ahrens (Taylor) C5 — Crust: Ahrens (Wänke) C6 — Crust: Ahrens (Weaver) U1 — Upper crust: Ahrens (Taylor) U2 — Upper crust: Ahrens (Shaw) Mass fraction, in kg... The living beings which inhabit the earth are all very different both in size and in form. ... The Chemical Galaxy is a new periodic table designed by Philip Stewart in November 2004 based on the cyclical nature of characteristics of the chemical elements (which depend principally on the valence electrons). ... The human body is a combination of elements and compounds. ... In cosmochemistry, the Cosmochemical Periodic Table of the Elements in the Solar System is a periodic table that lists all known chemical elements. ... Natural abundance refers to the prevalence of different isotopes of an element as found in nature. ... The periodic table of the chemical elements is a tabular method of displaying the chemical elements, first devised in 1869 by the Russian chemist Dmitri Mendeleev. ...

External links

List of elements in order of abundance in the Earth's crust: http://www.science.co.il/PTelements.asp?s=Earth

Parts of this article are taken from the public domain sources at http://geopubs.wr.usgs.gov/fact-sheet/fs087-02/ and http://imagine.gsfc.nasa.gov/docs/dict_ei.html Please update as needed.