Glossary Aa

ABERRATION - Any effect in a lens or optical system which prevents a perfectly sharp image being formed. All optical systems (including electron optics) introduce aberrations and one of the key objectives of a microscope user and the microscope designer is to minimize them.

ABIOTIC - Non-biological in origin.

ABLATION1 – Gradual removal of the surface layers of a material. In the case of a meteor, this is accomplished by frictional melting during its passage through the atmosphere. The particles released ionize the surrounding atmospheric molecules, which later de-excite to produce the glowing trails commonly associated with this type of object.

ABLATION2 – Terrestrial ablation processes include reduction of a glacier by surface melting/sublimation and rock weathering by hydraulic or aeolian erosion.

ABRASION – Type of erosion in which rock fragments scrape and grind away a surface. The fragments may be carried by rivers, wind, ice, or the ocean currents.

ABSOLUTE ENTROPY Increase in the entropy of a substance as it goes from a perfectly ordered crystalline form at 0 K, where its entropy is zero, to the temperature in question.

ABSOLUTE ERROR – Difference between measured and exact values. The smaller the absolute error, the higher the accuracy.

ABSOLUTE MAGNITUDE – Magnitude an object would have if placed at a distance of exactly 10 parsecs (= 32.6 light years). A supergiant star might have an absolute magnitude of -8 whereas a dim red dwarf might have an absolute magnitude of +16. The Sun has an absolute magnitude of +4.8 – about half way between the two extremes. To convert the observed brightness of a star (the apparent magnitude, m, to an absolute magnitude, we need to know the distance in parsecs d, to the star. Alternatively, if we know the distance and the apparent magnitude of a star, we can calculate its absolute magnitude. Both calculations are made using:

The quantity m - M is known as the distance modulus.

ABSOLUTE ZERO - Lowest possible temperature, according to the kinetic theory of gases, attained when a system is at its minimum possible energy, equal to 0 K (-273.15° C and -459.67 F). Temperature is a reflection of molecular energy, thus there must be a point at which no further energy can be extracted from a system. Although it is possible to approach absolute zero, the "third law" of thermodynamics holds that it is impossible to attain absolute zero in a system.

ABSORPTION – Transfer of energy to a medium as a particle or electromagnetic radiation passes through it. Absorption of electromagnetic radiation is the combined result of Compton scattering, σ, and photoelectric absorption, τ. It may be quantified:

where, t = thickness, ρ = density, and μ = mass absorption coefficient, which combines Compton and photoelectric effects (μ = σ + τ).

ABSORPTION LINE – Narrow range of wavelengths in a spectrum that are darker (have lower intensity) than neighboring wavelengths.

Absorption lines in the spectrum of a G5IV star. Image source:

ABSORPTION SPECTRUM - Spectrum consisting of dark absorption lines superimposed over a continuum spectrum, created when a cooler gas absorbs photons from a hotter continuum source. The image shows the intricate dark Fraunhofer absorption lines superposed on the Sun’s background continuous spectrum.

Dark Fraunhofer absorption lines superposed on the Sun's background continuous spectrum. Image source:

ABUNDANCE RATIO – Logarithm of the ratio of two metallic elements in a star relative to their ratio in the Sun. This is used to quantify the relative amounts of individual elements present in a star. For example, the abundance ratio of Mg to Fe, written [Mg/Fe], is:

Chemical elements are produced through a variety of processes and abundance ratios contain useful information regarding the source of the gases making up a star. Mg (and other “light” elements such as C and O) are produced in Type II supernovae explosions; “iron peak” elements (Fe, Ni, Zn, Co, Mn, Cr) are produced in Type Ia supernovae explosions. For any given population of stars, the different types of supernovae will explode at different times with the elements produced in the different explosions incorporated into stars at different times.

ACAPULCOITE (ACA) – Primitive achondrite that belongs to a small group named after the Acapulco meteorite that was observed to fall in Mexico in 1976. Acapulcoites are made mostly of fine-grained olivine (Fo3-14), orthopyroxene(En86-97), Ca-rich pyroxene (En51Wo44), plagioclase (An12-31), Ni-Fe metal, and troilite. They are transitional between primordial chondritic matter and more differentiated rocks with mineral compositions between those of E and H chondrites. Their oxygen isotope values set them apart from all other known chondrite groups. Some acapulcoites contain relict chondrules. NWA 725 from Tissemoumine, Morocco, has abundant distinct chondrules, confirming that acapulcoites are very primitive and are better termed metachondrites.
Acapulcoites and lodranites are thought to come from the same parent body, with lodranites showing evidence of slightly more melting of a chondritic source. However, the simple bimodal classification into acapulcoites and lodranites based on   petrographic criteria is inadequate to fully describe the variability observed. These meteorites result from variable degrees of partial melting with and without accompanying melt migration. In some instances, their compositions have been changed by secondary processes on the parent body, such as metasomatism (Floss, 2000, MAPS 35:1073).

Tissemoumine. Image source:

ACCEPTOR – Element that accepts an electron from a semiconductor atom. Acceptor atoms add energy levels above the valence band. Acceptors will have one fewer valence band than the semiconductor they accept from. For example, Ga (Z=31, 3 valence electrons) could be an acceptor for a semiconductor like Si (Z=14) or Ge (Z=32), both of which have 4 valence electrons. Acceptors facilitate conduction when used to dope semiconductors, since they provide an extra hole. Semiconductors doped with acceptor atoms are called p-type semiconductor. Since semiconductors tend to fall into group IVA of the periodic table, acceptors will be found in group IIIA.

ACCRETION – Accumulation of smaller units into larger collections; the growth of planets from smaller objects by impact, one impact at a time. After formation, planets are said to have “accreted” from small objects.

ACCRETION DISK – Disk-shaped cloud of gas and solids that is accreting to a central protostar or some other object.

ACCRETIONARY LAPILLI – Pellets that form by accretion of fine ash around condensing water droplets or solid particles; particularly common in steam-rich volcanic eruptive columns, but also occurring in the turbulent explosion plume rising above an expanding excavation cavity in an impact cratering event. Accretionary lapilli exhibiting concentric internal structure have been found in deposits at the Ries impact structure and in ejecta deposits from the Chicxulub crater in Mexico.

Accretionary lapilli on the surface of the Ka'u Desert south of Kilauea caldera. Image source:

ACCURACY – How closely a measured value agrees with the correct value. In analytical work, establishing “correct” reference values, i.e., a well-characterized standard, is a significant problem.

ACHONDRITE - Rarer of the two main types of stony meteorite, representing ~9 % of meteorite falls. Achondrites lack chondrules and are made of rock that has crystallized from a molten state. They contain mostly one or more of the minerals plagioclase, pyroxene, and olivine. Most achondrites are chemically similar to basalts and are thought to result from melting on large asteroids, moons, and planets. Soon after these worlds formed, they were heated from within and partially melted. Although this process is still active on Earth, it ended ~4.4 Ga ago on asteroids, ~2.9 Ga ago on the Moon, and ~1 Ga ago on Mars.

Although the majority of achondrites are of asteroidal origin, some are known to have come from the Moon (lunar meteorites) and Mars (Martian meteorites). Angrites, a very rare achondrite type, may have originated on Mercury. The HED group, which includes howardites (HOW), eucrites (EUC) and diogenites (DIO), appears to come from the asteroid 4 Vesta. Another evolved achondrite group, which seems to derive from partially differentiated asteroid(s) other than 4 Vesta, is the aubrites (AUB).

In addition to evolved achondrites described above, there is an entire group of primitive achondrites whose members all seem to have derived from small chondritic parent bodies that only partially melted and differentiated through accretion processes or from impact events, and then rapidly cooled. Primitive achondrites vary widely in composition and fall into the following main subgroups: acapulcoites (ACA), lodranites (LOD), brachinites (BRA) winonaites (WIN), and ureilites (URE).

ACICULAR – Applied to mineral morphology: bristly, spiny, and needlelike.

ACOUSTIC FLUIDIZATION – Mechanism hypothesized to produce fluidization of rock debris by strong vibrations. This might enable the collapse of the transient crater in the modification stage of impact cratering.

ACTINIDES – Elements 90 to 103 (those after Ac, Z=89) found in the bottom row of the inner-transition elements of the periodic table. Some reference sources include Ac in this series, whereas others do not. For these elements the 5f orbital is the filling orbital. This series is a sub-series of the transition metals.

ACTIVATION COMPLEX – Unstable combination of reacting molecules that is intermediate between reactants and products.

ACTIVATION ENERGY – Amount of energy that must be placed into a system to form an activated complex in a reaction. On an energy diagram, this is the difference in energy between the reactants and the transition state. The activation energy is often perceived as an energy barrier which must be overcome for a reaction to proceed. The presence of a catalytic agent can markedly reduce activation energy.

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ACTIVE GALAXY - Galaxy characterized by certain properties: (1) high luminosity, (2) nonthermal spectra (unlike the sum of many stellar spectra), (3) luminosity in a region of the spectrum other than optical (e.g., radio, UV, Infrared), (4) bright, star-like nucleus, (5) strong emission spectral lines (most), (6) rapid variability, and sometimes (7) radio jets. The central region of an active galaxy is called an "active galactic nucleus" (AGN) where energetic activity is concentrated. Active galactic nuclei are believed to contain supermassive black holes that power the nonstellar phenomena associated with active galaxies. Examples of active galaxies include: Seyfert galaxies, Radio galaxies (image), and BL Lac objects.

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ADENINE – One of the five nitrogen-containing bases occurring in nucleotides.

ADENOSINE TRIPHOSPHATE (ATP) – Compound containing three phosphoric acids, with one low and two high energy bonds; it occurs in all cells, where it represents energy storage.

ADHESION – Attractive force between unlike molecules.

ADIABATIC – Process in which no heat enters or leaves a system. This is the case, for example, when an interstellar gas cloud expands or contracts. Adiabatic changes are usually accompanied by changes in temperature or volume.

ADIABATIC LAPSE RATE – Rate of decrease of temperature with increasing altitude in the atmosphere. If heat is neither gained nor lost from the air parcel under consideration, then the lapse rate is said to be adiabatic. The lapse rate is:

where, γ = lapse rate is given in units of temperature divided by units of altitude, T = temperature, and z = altitude, and points 1 and 2 are measurements at two different altitudes.

A rising parcel of unsaturated air loses temperature at the dry adiabatic lapse rate (DALR), which is a 9.78 °C/km (3 °C/1000 ft, or 5.37 °F/1000 ft) on Earth. The moist adiabatic lapse rate (MALR), also called the saturated adiabatic lapse rate (SALR) varies with temperature and pressure, but is usually near 4.9 °C/km (2.7 °F/1000 ft or 1.51°C/1000 ft). The reason for the difference between it and the DALR is that latent heat is released when water cools and condenses. The actual lapse rate varies with elevation. Atmosphere with an actual lapse rate greater than the DALR is unstable; with a rate lower than the DALR, it is stable.

ADSORPTION – Adhesion of a species onto the surfaces of particles

.ADVECTION – Transportation of heat or material through a medium (atmosphere, mantle) by mass movement.

.AEROBIC – Oxygen-dependent or requiring oxygen.

AEROGEL - Silicon-based solid with a porous, sponge-like structure in which 99.8 vol. % is empty space used to collect high velocity particles without damaging them. Aerogel is 1,000x less dense than glass and has many unusual properties, such as extremely low thermal conductivity, refractive index, and sound speed. When a particle hits the aerogel, it buries itself in the material, creating a carrot-shaped track up to 200x its own length. This slows it down and brings the sample to a relatively gradual stop.

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AEROLITE – Obsolete term for a stony meteorite.

AEROSOL PARTICLES – A component of atmospheric air, comprised of minute solids particles part of which is almost certainly water.

AFFINITY – Tendency of a molecule to associate with another.

AGB STARS - Stars on the Asymptotic Giant Branch, which represents a late stage of stellar evolution that all stars with initial masses < 8 Msun go through. At this late stage of stellar evolution, gas and dust are lifted off the stellar surface by massive winds that transfer material to the interstellar medium. Mass loss rates range from ~10-8 to 10-4 Msun per year.

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AGGLUTINATE - Common particle type in lunar regolith (photograph below). Agglutinates are small glassy breccias formed when micrometeorites (< 1 mm in diameter) strike the lunar regolith. During micrometeorite impacts, some of the regolith melts and some doesn't, so the final product is a glass with entrained mineral and rock fragments. The glass often shows flow features and vesicles (gas bubbles). Impacts liberate solar-wind-implanted H and He in the regolith causing bubbles in the glass. Agglutinates are typically 10s of μm to a few mm in size.

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ALABANDITE – Mn sulfide found in aubrites, EL chondrites; its formula is MnS.

ALBEDO – Ratio of the amount of light reflected by an object and the amount of incident light; used as a measure of the reflectivity or intrinsic brightness of an object (a white, perfectly reflecting surface would have an albedo of 1.0; a black perfectly absorbing surface would have an albedo of 0.0). The albedo depends upon the wavelength of the incident radiation. Bolometric Bond albedo refers to reflectivity over all wavelengths.

ALBEDO FEATURE – Dark or light marking on the surface of an object that may or may not be a geological or topographical feature.

ALKALI EARTHS – Large divalent (2+) elements in the second column (from the left) of the periodic table: Be, Mg, Ca, Sr, and Ba. These elements are less reactive than the alkali metals and have higher melting points and boiling points. They are generally white, differing by shades of color or casts and are malleable, extrudable and machinable.

ALKALI FELDSPAR – Variety of feldspar with potassium and sodium. It has three common forms with the same chemical composition, sanidine, orthoclase, and microcline. Sanidine is the high-temperature form that occurs only where quenched in volcanic rocks. Orthoclase and microcline occur in successively lower temperature plutonic rocks.

ALKALI METALS – Large univalent (1+) metals in the first column of the periodic table: Li, Na, K, Rb, Cs, and Fr. The alkali metals are silvery colored, soft, low density metals, which react readily with halogens to form ionic salts, and with water to form strongly alkaline (basic) hydroxides. They may be readily fused and volatilized; their melting and boiling points decrease with increasing atomic mass. They are the strongest electropositive metals and react vigorously, even violently, with water.

ALLENDE METEORITE - Carbonaceous chondrite that fell near the village of Pueblito de Allende in the Mexican state of Chihuahua, on Feb. 8, 1969. Several tons were collected from a 48 x 7 km area. Specimens have a fine-grained C-rich matrix studded with many chondrules; both matrix and chondrules consist predominantly of the mineral olivine. The Allende meteorite contains fine-grained, microscopic diamonds with strange isotopic signatures that point to an extrasolar origin; these interstellar grains are older than the solar system and probably were produced by a nearby supernova.

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ALLOCHTHONOUS - Material formed or introduced from somewhere other than the place it is presently found. In impact cratering this may refer to the fragmented rock thrown out of the crater during its formation that either falls back to partly fill the crater or blankets its outer flanks after the impact event.

ALLOTROPE – Different forms of the same element in the same physical state. Allotropy in crystalline materials is called polymorphism. Ozone (O3) and dioxygen (O2) are allotropes of gaseous oxygen.

ALLOY – Metal-like substance produced by mixing two or more metals or by the mixture of a metal and another substance. Ceramics can also be mixed to form alloys. A binary alloy contains two components; a ternary alloy contains three.

ALPHA DECAY – Nuclear decay by emission of an alpha particle.

ALPHA (α) PARTICLE - Term used to describe a helium nucleus (4He nucleus). Alpha particles were discovered by Ernest Rutherford (1871-1937) in 1898.

ALPHA PROCESS - Term used to describe the addition of a helium nucleus (4He) to existing nuclei with in a giant star to form heavier elements. At temperatures above 2 x 108 K, a 12C nucleus colliding with 4He nucleus can produce 16O. Although the 16O produced may fuse with other 16O nuclei at ~109 K to form 32S, it is much more probable that an 16O nucleus will capture a 4He nucleus (if one is available) to form 20Ne. Thus, as the star evolves, heavier elements tend to form through 4He-capture rather than by fusion of like nuclei.

The α process will continue up to the formation of 56Ni. However, 56Ni is unstable and decays rapidly into 56Co, then into stable 56Fe (the most stable of all nuclei). Thus, the alpha process leads inevitably to the buildup of iron in the stellar core. Because helium-capture reactions are so much more common, elements with nuclear masses of 4 units (4He, 12C, 16O,20Ne, 24Mg, and 28Si) have markedly higher cosmic abundances (below).

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AMINO ACID – Substance containing both an amine group and a carboxylic acid group. Proteins are composed of amino acids.

AMOEBOID OLIVINE AGGREGATES (AOAs) - Irregularly shaped, fine-grained objects that constitute a few volume-percent in most carbonaceous chondrite groups. They are forsterite (Mg-rich olivine) and Ca-Al-Ti mineral aggregates. The most characteristic texture of AOAs is an anorthite core (sometimes associated with spinel and augite) rimmed by Al-diopside which overgrows olivine. The photograph is a backscattered electron image of an AOA from Acfer 094 (an = anorthite; fo = forsterite; met = Ni-Fe metal; px = Al-diopside).

AMOR ASTEROIDS - Group of near-Earth asteroids (NEAs) named after asteroid 1221 Amor. These objects approach the orbit of the Earth, but do not cross it; most do cross Mar's orbit. The two moons of Mars may be captured Amor asteroids. A list of known Amor asteroids is given at:

Amor asteroid distribution in green. Image modified from:

AMORPHOUS - Material without the regular, ordered structure of crystalline solids. Amorphous substances lack a definite repeating pattern in their atomic structures (crystallinity). There may be small regions of order, but, overall there is disorder.

AMPERE – Unit of electrical current; one ampere equals one coulomb per second.

AMPHIBOLE GROUP - Complex family of hydrous double-chain silicate minerals. Amphiboles are common in terrestrial metamorphic and igneous rocks, forming in the presence of water. They are rarely found in extraterrestrial materials, having been discovered in R chondrites and graphite nodules in A type iron meteorites. Martian rocks are the main exception with kaersutite amphibole commonly reported in shergottites.

The amphibole structure consists of doubled Si4O11 chains running parallel to c-axis. These chains are bonded to octahedral strips consisting of three regular octahedral sites (M1, M2, M3) and one larger 6- to 8-fold site (M4). In addition there is an even larger 10- to 12-fold A site that is usually empty.

OH groups occur in the interiors of the rings in the double chains.

The result is an "I-beam" structure like that of pyroxene. The M123 cations are coordinated by oxygens and OH groups of the adjacent double chains, forming a TOT strip. Amphibole TOT strips are approximately twice as wide as pyroxene strips, yielding typical near 120° {110} cleavage.

Amphiboles are composed of the same cations as pyroxenes, but also have OH groups, resulting in lower densities and refractive indices than their pyroxene counterparts. Their general formula is A0-1X2Y5Z8O22(OH,F,Cl)2. The A site holds large cations such as Na+ and K+ and is commonly not completely filled. The X site (M4) holds a large to intermediate-size cations such as Ca2+, Na+, Mn2+, Fe2+, Mg2+, and Li+. The Y sites (M1, M2, M3) hold intermediate-sized to small cations such as Mn2++, Fe2+, Mg2+, Fe3+, Al3+, and Ti4+. Lastly, the tetrahedral Z sites in the chains hold the smallest cations Si4+ and Al3+. As indicated by the formula, substitution of Fand Cl can occur for HO.

The cation substitutions that produce the wide range of amphibole compositions can be complex. Multiple coupled substitutions are usually required to maintain charge balance. Simple cation substitutions include:

Coupled substitutions include:

Classification of Ca-rich amphiboles is done in a manner analogous to that for quadilateral pyroxenes by plotting the Ca, Mg, and Fe end-member components. No amphiboles plot above the above 2/7th Ca line when both M4 sites completely filled with Ca.

Cation substitutions complicate the situation. The most common amphibole, hornblende, has very variable composition owing to significant substitution of Na+ and K+ in A site and Fe3+ and Al3+ in M123 sites. Hornblende is the result of two main substitutions starting from tremolite: Si4+ + (Mg2+,Fe2+) ↔ 2Al3+, and Si4+ +  A-site ↔ Al3+ + (Na+,K+). Hornblende includes following series: magnesiohornblende-ferrohornblende, tschermakite-ferrotschermakite, edenite-ferroedenite, pargasite-ferropargasite, and magnesiohastingsite-hastingsite (mainly listed to show the spectacular mineral names). One way of classifying hornblende is to consider the A-site occupancy and the amount of Si in the T-site. In the diagram below yellow shows the range of observed composition with orange indicating the more abundant ones (Hb = hornblende (sensu stricto), Ts = tschermakite, Ed = edenite, and Pa = pargasite).

There are alkali-rich varieties in addition to the more common calcic amphiboles. Alkali amphibole occurs primarily in sodic-metamorphic rocks and in alkaline igneous rocks. Alkali amphiboles are divided into two groups based upon M4 and A site occupancies. If the M4 site contains <0.5 (Na+K) and there is no Na in the A site, the mineral is considered a low-Na alkali amphibole. If the M4 site contains >0.5 (Na+K) and there is Na in the A site, it is considered a high-Na alkali amphibole. There is no solid solution between calcic and alkali amphiboles. The two types of alkali amphoobles are further subdivided. Classification of low-Na alkali ampiboles is accomplished using the diagram below; the most common types are glaucophane and riebeckite.

Classification of the rarer high-Na alkali ampiboles is done with the diagram below.

A summary of amphibole chemistry is given below (green = calcic amphiboles; blue = alkali amphiboles).

AMPHOTERITE - Obsolete name for LL (low-low Fe content) chondrites.

ANGLE – Measured in degrees or arcminutes (denoted by a single quote) or arcseconds (denoted by a double quote) or radians. 1 radian = 180/π = 57.2958 degrees, 1 degree = 1° = 60 arcminutes = 60' = 3600 arcseconds = 3600".

ANGULAR SIZE – Apparent size of an object expressed as an angle.

ANGRITE (ANG) - Evolved achondrite composed mainly of augite (Wo>50; "fassite") with small quantities of olivine (Fa10-100), plagioclase (An100) and troilite. Angrites, named for the Angra dos Reis meteorite, which fell in Rio de Janeiro, Brazil, in early 1869, are basaltic rocks, often containing porous areas and many round vesicles (small cavities) with diameters up to 2.5 cm. Angrites are the most ancient igneous rocks known, with crystallization ages of around 4.55 Ga. They are thought to have formed on one of the earliest differentiated asteroids. By comparing the reflectance spectra of the angrites to that of several main belt asteroids, two potential parent bodies have been identified: 289 Nenetta and 3819 Robinson. Other researchers have suggested than angrites instead represent pieces of Mercury.

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ANGSTROM (Å) - Unit of length convenient for measuring wavelengths of electromagnetic radiation. Named after the Swedish physicist Anders Jonas Ångström (1814-1874), a founder of spectroscopy, 1 Å = 10-10 m. Most of the visible spectrum lies in the 3900-7500 Å range. Not an SI unit.

ANGULAR MOMENTUM – Measure of the mass, radius, and rotational velocity of a rotating or orbiting body. In the simple case of an object with mass, m, in circular orbit or radius, r, the angular momentum is mrv, where, v = orbital speed. An important principle is the “conservation of angular momentum,” which means that the angular momentum of a system remains the same as long as no external torque acts upon it.

ANGULAR MOMENTUM PROBLEM - Fact that the Sun, which contains nearly all of the mass of the solar system, accounts for just 0.3 percent of the total angular momentum of the solar system. It is possible that the solar wind, moving away from the Sun into interplanetary space, carried away much of the Sun's initial angular momentum. The early Sun probably produced more of a dense solar gale than the relatively gentle "breezes" now measured by our spacecraft. High-velocity particles leaving the Sun followed the solar magnetic field lines. As the rotating magnetic field of the Sun tried to drag those particles around with it they acted as a brake on the Sun's spin.

ANISOTROPIC - Not isotropic; i.e. having different properties in different directions. In the case of minerals, optical anisotropy is particularly marked and useful.

ANORTHOSITE- Igneous rock made up almost entirely of plagioclase feldspar. This rock type forms a major part of the lunar highlands and is represented by LUN A meteorites.

Apollo 16 anorthosite. Image source:

ANOXIC – Lacking oxygen such as the inadequate oxygenation of the blood (anoxia). In aquatic environmental chemistry, it refers to water that has become oxygen poor due to the bacterial decay of organic matter.

ANTHROPIC PRINCIPLE – Observation that, since we exist, the conditions of the universe must be such as to permit life to exist. The weak anthropic principle holds that the conditions necessary for the development of intelligent life will be met only in certain regions that are limited in space and time. That is, the region of the Universe in which we live is not necessarily representative of a purely random set of initial conditions; only conditions favorable to intelligent life would actually develop creatures who wonder what the initial conditions of the Universe were, and this process can only happen at certain times through the evolution of any given universe. The strong anthropic principle argues that  that if the laws of the Universe were not conducive to the development of intelligent creatures to ask about the initial conditions of the Universe, intelligent life would never have evolved to ask the question in the first place. In other words, the laws of the Universe are the way they are because if they weren't, no intelligent beings would be able to consider the laws of the Universe at all.

ANTIFERROMAGNETIC – Where there is no net magnetic moment because the spin magnetic moments are randomly oriented, canceling them.

ANTIMATTER - Particles with properties opposite to those of "conventional" matter. Each matter particle has a corresponding "antiparticle." The antiparticle has the same mass, but opposite electric charge as its partner. An example is the electron (negative charge) and its antimatter version the positron (positive charge). When a particle and its antiparticle collide, both are annihilated and converted into photons. Similarly two photons with sufficient energy can combine to form a particle-antiparticle pair. The universe is made almost entirely out of matter indicating that there was an excess of matter over antimatter in the Big Bang.

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ANTIPODE - Point directly on the opposite side of a planet from a reference point.

APHELION - Point farthest from the sun in an object's orbit.

APOAPSIS - Point in an object’s orbit farthest from a planetary body.

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APOGEE - Point in an object’s orbit farthest from Earth.

APOLLO ASTEROIDS - Group of near-Earth asteroids (NEAs) named after 1862 Apollo. These asteroid cross Eartyh's orbit and have orbital semi-major axes >1 AU and perihelion distances <1.017 AU. A list of know Apollo asteroids is given at:

Apollo asteroid distribution given in green. Image modified from source:

APPARENT MAGNITUDE - Magnitude determined using the brightness as observed, with no consideration given to how distance is influencing the observation.

ARCHIMEDE’S PRINCIPLE - Principle that a body submerged in a fluid is buoyed up by a force equal in magnitude to the weight of the fluid that is displaced, and directed upward along a line through the center of gravity of the displaced fluid. Applied on a crustal scale this is the principle of isostasy.

ARCMINUTE (') - Measure for small angles. 1 arcminute = 1/60 degree.

ARCSECOND (") - Measure for very small angles. 1 arcsecond = 1/60 arcminute = 1/3600 degree.

ARCUS - Term for an arc-shaped feature on a planetary surface (pl. arcus)

ARGON - Noble gas represented by the atomic symbol Ar, that has Z = 18, and an atomic weight of 39.948. It is colorless, odorless, and very inert gas, comprising ~1 % of the Earth's atmosphere.

ARMALCOLITE - Fe-Mg titanate, FeMgTi2O5, found in lunar rocks. Named after Armstrong, Aldren, and Collins, the crew of Apollo 11.

ARMORED CHONDRULE - Chondrule covered on its surface by a layer of nickel iron.

AROMATIC HYDROCARBON - Hydrocarbon compounds in which the carbon atoms are connected by a ring structure that is planar and joined by σ and π bonds between the carbon atoms. Examples of aromatic compounds include ethylene (IUPAC name: ethene), C2H4, and benzene, C6H6.

Simga (σ) and pi (π) bonds in ethene. Image source:

ASTROBLEME - Scar on the surface of Earth or other body caused by the impact of a meteoroid or asteroid; synonymous with impact crater.

ASTEROID - Metallic, rocky bodies without atmospheres that orbit the Sun but are too small to be classified as planets. Known as “minor planets,” tens of thousands of asteroids occur in the main asteroid belt, which is a doughnut-shaped ring between the orbits of Mars and Jupiter from approximately 2 to 4 AU (million to 600 million kilometers) from the Sun. Most main belt asteroids follow slightly elliptical, stable orbits, revolving in the same direction as the Earth and taking from three to six years to complete a full circuit of the Sun. Asteroids outside the main belt include the Trojan asteroids, which share Jupiter’s orbit (J), and the near-Earth asteroids (NEOs). Some of the smaller moons in the solar system appear to be captured asteroids, including the moons of Mars, and a number of the outer moons of the four gas giants. An alphabetical list of asteroid names is given at

Positions of all numbered asteroids and all numbered comets on 2009 January 1. The orbits and positions of the planets Mercury, Venus, Earth, Mars, and Jupiter are also shown. Asteroids are yellow dots and comets are symbolized by sunward-pointing wedges. Image source:

Most asteroids move in orbits somewhat more inclined and eccentric than those of the major planets (with the exception of Pluto). The orbit of an average main-belt asteroid is inclined at ~10° to the plane of the ecliptic with an eccentricity of ~0.15. However, some asteroids, such 3200 Phaethon and 944 Hidalgo, have highly inclined and/or elliptical paths, suggesting they may be defunct comet nuclei. On a plot of orbital inclination vs. semi-major axis there are obvious clusters of asteroids, as indicated by the larger number density in certain areas. These groupings are called “Hirayama Families.” Also apparent are the Kirkwood Gaps at 2 A.U., 2.5 A.U., 2.8 A.U., 2.9 A.U., and 3.3 A.U., which are a result of resonance with Jupiter. Asteroids with orbits that bring them within 1.3 AU (195 million km) of the Sun are known as Earth-approaching or near-Earth asteroids (NEAs).

The groups and families shown in this diagram collect asteroids of similar dynamic characteristics. Some of these groups may have formed from the disruption of larger "parent" asteroids (for example, the Flora familiy and Hilda group). In other cases, asteroids are concentrated in dynamically stable regions (for example, the Trojan, Hungaria, and Cybelle groups). Image source:

Asteroids range in size from a few meters to over 900 km across, and vary greatly in composition. Many can be seen with binoculars or small telescopes, including the four largest: 1 Ceres, 2 Pallas, 4 Vesta, and 10 Hygiea. Thirty known asteroids exceed 200 km in diameter. The total mass of all the asteroids, most of which is concentrated in the main belt, is about one-twentieth that of the Moon and about three times that of Ceres.

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Some asteroids, such as 1 Ceres, 2 Pallas, and 4 Vesta, are nearly spherical and probably should be termed protoplanets or planetesimals. Others, true asteroids, like 15 Eunomia, 107 Camilla, and 511 Davida, are quite elongated; still others, such as 4769 Castalia, 216 Kleopatra, and 4179 Toutatis, have bizarre shapes. There are binary asteroids, such as 90 Antiope and, possibly, 1620 Geographos, in which two components of roughly equal size orbit each other at close range. Several asteroids have been studied by passing space probes, including Ida and 951 Gaspra (by Galileo), and 253 Mathilde and 433 Eros (by NEAR-Shoemaker).

Asteroids are classified into different types according to their albedos, composition derived from spectral features in their reflected sunlight, and inferred similarities to known meteorite types. Albedos vary from just under 0.02 to over 0.5, with the majority of asteroids tending toward the lower (dark) end of this range. Low-albedo asteroids (C, P, and D types) are generally found in the outer half of the asteroid belt, while higher-albedo objects (E, S, and M types) tend to occupy the inner half.

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Most asteroids fall into three categories: (1) C-type (carbonaceous), which includes more than 75 % of known asteroids. These asteroids are very dark (albedo =  0.03–0.09) and their compositions are probably similar to that of the Sun, but depleted in H, He, and other volatiles. (2) S-type (silicaceous), which accounts for ~17 % of known asteroids. These asteroids are relatively bright (albedos = 0.10–0.22). They are composed of metallic iron mixed with Fe-Mg silicates. (3) M-type (metallic), which includes many of the rest of the known asteroids. These bodies are relatively bright (albedo = 0.10–0.18) with compositions dominated by metallic iron. Other types include E (enstatite-rich) and D and P (red color, hydrous, carbonaceous).

Asteroids are thought to be the remnants of a stillborn planet. According to this idea, the newborn Jupiter gravitationally scattered nearby large planetesimals - accreting lumps of matter in the embryonic stage of planet-formation - some of which may have been as massive as Earth is today. Some of these large planetesimals strongly perturbed the orbits of the planetesimals in the region of the asteroid belt, raising their mutual velocities to the average 5 km/s seen today. As a result, what had been mild accretionary collisions became catastrophic and only objects larger than ~500 km in diameter could have survived ~5 km/s collisions with objects of comparable size; with the exception of the largest, most present-day asteroids are either remnants or fragments of past impacts. While breaking larger asteroids into smaller ones, collisions expose deeper layers of asteroidal material. If asteroids were compositionally homogeneous, this would have no noticeable result.

However, fragmentation of differentiated bodies yields distinct meteorite types. At least one asteroid with its original basaltic surface, 4 Vesta, survives to this day. Other differentiated asteroids experience collisions that stripped away their crusts and mantles and exposed their Fe cores. Still others may have had only their crusts partially stripped away, which exposed surfaces such as those visible today on the A-, E-, and R-class asteroids.

Collisions gave rise to at least some of the NEOs. Tiny fragments from the latter enter Earth’s atmosphere to become sporadic meteors, while larger pieces survive passage through the atmosphere to end up as meteorites. The very largest produce craters such as the Barringer Crater and one may have been responsible for the extinction of the dinosaurs. Luckily, collisions of this latter sort are rare. According to current estimates, a few asteroids of 1-km diameter collide with Earth every 1 Ma. Past collisions between asteroids and the Earth appear to have played a crucial role in the evolution of life on this planet. In particular, the impact of an asteroid ~65 Ma ago caused a mass extinction in which the last of the dinosaurs were wiped out (Cretaceous-Tertiary Boundary).

ASTIGMATISM - Defect which occurs when the strength of a lens is different in different planes. An astigmatic lens is asymmetrical and has a slightly different focal length for different rays. Astigmatism can be corrected in most optical systems by applying an equal and opposite asymmetry.

ASTROBLEME - Crater formed by meteorite impact.

ASTROMETRY - Branch of astronomy entailing measurements of the positions of stars and other celestial bodies. It is the oldest search method for extrasolar planets, using precise measurements of a star's position in sky over a long period of time (years, decades) to look for shifts (“wobbles”) in position relative to other more distant stars. If a planet is present, its gravitational influence will cause the star to move in a tiny circular or elliptical orbit about their barycenter. This method is most sensitive to giant planets with large orbits. The Hubble Space Telescope was used in 2002 to make astrometric measurements to characterize the orbit of a previously discovered planet around Gliese 876b.

Gliese wobble

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ASTRONOMICAL UNIT (AU) - Semi-major axis of the elliptical orbit of the Earth. The astronomical unit is also described as the "mean" distance (average of aphelion and perihelion distances) between the Earth and the Sun. The currently accepted value for the AU is 149,597,870.66 km.

ASTROPHYSICAL JETS - Rapid outflows, launched in antiparallel directions, roughly perpendicular to an accretionary disk found circulating around a central gravitating mass. The nature of the mechanism for the formation and collimation of jets is still a matter of controversy. Many believe that strong magnetic fields generated in the accretion disk are responsible for launching and focusing the outflow.

ASTRUM – Radial-patterned features on Venus (pl. astra).

ASYMPTOTIC GIANT BRANCH - Path on the Hertzsprung-Russell diagram corresponding to the changes that a star undergoes after He burning ceases in the core. At this stage, the C core shrinks and drives the expansion of the envelope, and the star becomes a red supergiant.

ATAXITE - Rare variety of iron meteorite (designated type D) made almost entirely of taenite, a solid solution of Fe and 27 to 65% Ni. The Greek name means "without structure" and refers to the lack of a visible Widmanstätten pattern (spindles of kamacite are visible only microscopically).

Chinga ataxite Meteorite (lacks Widmanstätten beacuseof its Ni content). Image source:

ATEN ASTEROIDS - A group of near-Earth asteroids (NEAs), named after the first of the group to be discovered (2062 Aten) They have semi-major axes of less than 1 AU. A list of Aten asteroids is given at

Distribution of Aten asteroids shown in green. Image modified after source:

ATMOPHILE ELEMENT - Element in the Goldschmidt classification that tends to be concentrated in the atmosphere, e.g., H, C, N, and the noble gases.

ATMOSPHERE - Mixture of gases and traces of dust, ices, and droplets gravitationally bound to a planet.

ATTENUATION - Process by which a compound is reduced in concentration over time, through adsorption, degradation, dilution, and/or transformation. Radiologically, it is the reduction of the intensity of radiation upon passage through a medium. Attenuation is caused by absorption and scattering.

ATOM - Smallest particle of a chemical element that retains the properties of that element. Each atom consists of a compact positively charged nucleus, in which all but a tiny fraction of its total mass resides, surrounded by a cloud of negatively charged electrons. A typical atomic nucleus has a radius of ~5 x 10-15; the overall radius of an atom (i.e. radius of its electron cloud) is ~10,000 times larger (~10-10 m). The nucleus consists of at least one proton, some number of neutrons bound together by the strong nuclear interaction. The negatively charged electron cloud surrounds the positively charged nucleus, held by the electromagnetic force.

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ATOMIC MASS (A) - Mass of a neutral atom of a nuclide; also called "atomic weight." The atomic weight of an element is the weighted average of each isotope.

ATOMIC MASS UNIT (AMU) - Unit defined as exactly 1/12th the mass of a 12C atom (1.66057 x 10-24 g). Atomic mass units allow for determination of relative atomic masses of atoms for different elements.

ATOMIC NUMBER (Z) - Number of protons in the nucleus; commonly abbreviated as Z.

ATOMIC RADIUS - Radius of a neutral atom. Atomic radii show a rise-and-fall pattern of periodicity with atomic number (Z). Maxima occur for atoms of group 1A elements (Li, Na, K, Rb, Cs, Fr); the minima occur for atoms of the group 7A elements. Accurate data are not available for the group 8A elements.

ATOMIC SPECTRA - Collection of wavelengths can be emitted or absorbed by an element. Light is emitted by an atom when electrons change from a high energy state to a lower energy state, yielding an emission spectrum. Absorption of radiation can promote an electron to a higher energy level, yielding an absorption spectrum. Each element has its own characteristic set of allowed energy states and allowed transitions between energy states are unique to a particular element.

AUBRITE (AUB) - Evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8). Aubrites are named for the small Aubres meteorite that fell near Nyons, France, in 1836. Aubrite finds are rare due to their typical light-colored fusion crusts, their white interiors, and their fragile compositions. Most aubrites are witnessed falls or finds from the blue-ice fields of Antarctica. As well as large white crystals of enstatite, they contain small, varying amounts of olivine, Ni-Fe metal, troilite, and a variety of exotic accessory minerals, pointing to a magmatic origin under highly reducing conditions. Most aubrites are heavily brecciated, indicating a violent history for their parent body. Comparisons of the aubrite spectra to the spectra of asteroids have revealed striking similarities between the aubrites and the main belt asteroid 44 Nysa and other E-class objects. A small near-Earth asteroid, 3103 Eger, which is the only known E-class NEA, is suspected of being the actual parent body of the aubrites.

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AUFBAU RULES - Order in which the electrons of an atom are assigned to energy levels (each defined by quantum number). Aufbau is German for “building-up.” The rules follow. (1)  Electrons occupy the lowest-energy available orbitals; lower-energy orbitals are filled before the higher ones. (2) No more than two electrons can occupy any orbital. (3) For the lighter elements, electrons will fill orbitals of the same type only one electron at a time, so that their spins are all unpaired. They will begin to pair up only after all the orbitals are half-filled. This effect, which is a consequence of the electrostatic repulsion between electrons, is known as Hund's rule. The filling of the 3d orbitals of the first transition metals has anomalies in Cr and Cu. These are a consequence of the very small energy differences between some of the orbitals, and of the reduced electron-electron repulsion when electrons remain unpaired.

AUGER ELECTRON - Electrons emitted during radiationless inner shell ionization of an atom. Auger electrons were discovered independently by Lise Meitner (in 1923) and Pierre Auger (in 1926), but the English-speaking scientific community attached Auger's name to effect. Auger electrons are produced when a sample is bombarded with electrons and a characteristic x-ray produced by inner shell ionization is reabsorbed, ejecting an electron. For example, a Si-Kα (K-L1) x-ray (energy of 1690 eV) may be emitted from a sample or transfer its energy to the L2,3 shell (binding energy ~70 eV), ejecting a Si KL1L2,3 Auger electron (energy 1620 eV).

Auger electron production is quantified by fluorescent yield, ω, which is the fraction of inner shell ionization that produce x-rays (thus, 1 - ω gives the fraction of Auger electrons). Auger electron have energies characteristic of their atom of origin, ranging from ~280 eV (C) to 2.1 keV (S). Given these low energies, Auger electrons only escape from the surface of a sample.

AUGITE - Complex aluminous Ca-Fe-Mg pyroxene, Ca(Mg,Fe,Al,Ti)(Si,Al)2O6, that crystallizes in the monoclinic system, and occurs in many igneous rocks, particularly those of basaltic composition.

AURORA – Glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun (Latin for "dawn").

AUSTINITE - One of the allotropes of iron, also known as γ-iron. It is formed when iron is between 912° and 1,394° C and has a face-centered cubic structure.

AUSTRALITES - Tektites found in Australia that seem to be have the same origin as those from Indochina and the Philippines. Some australites are round or oval buttons that have been ablated on one side yielding a flange of melted glass around the edge.

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AUTHOGENIC – Formed or generated in place. Commonly applied to minerals formed subsequent to the formation of a rock; for example, quartz, feldspar, or calcite grown after the deposition of a sediment.

AUTUMNAL EQUINOX – Point at which on the celestial sphere that the equator and the ecliptic intersect. Generally the autumnal equinox occurs on or about September 23 in the northern hemisphere; this also signifies spring in the southern hemisphere.

AVOGADRO'S NUMBER (NA) - Number of atoms, molecules, ions, etc. in a mole of material; its value is 6.02252 x 1023 mol-1.

AWARUITE - Ni-rich Fe metal, Ni3Fe, similar to taenite found in minor amounts in CV chondrites.

AXION - Hypothetical weakly-interacting boson of mass << 200 keV. Its existence would resolve what is known as the strong-CP problem in quantum chromodynamics. Such a particle would efficiently transport energy out of stars or out of supernova cores. Axions are also prime cold dark matter candidates.