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.
ABLATION - Gradual removal of the surface layers of a material. In the case of a meteorite, this is accomplished by frictional melting during its passage through the atmosphere. Terrestrial ablation processes include reduction of a glacier by surface melting/sublimation, and rock weathering by hydraulic or aeolian erosion.
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 - A more or less narrow range of wavelengths in a spectrum that is darker than neighboring wavelengths.
ABSORPTION SPECTRA - Spectrum consisting of dark absorption lines superimposed over a continuum spectrum, created when a cooler gas absorbs photons from a hotter continuum source.
ACAPULCOITES (ACA)- Primitive achondrites that belong 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 (Fa3-14), 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).
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 raising 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.
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).
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. According to Melosh and Ivanov (1999), this process that greatly degrades the strength of the material in the vicinity of
an impact with strength degradation evidently transient, lasting little more
than the few minutes required for craters in the 10 to 100 km range to collapse.
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.
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.
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.
AEROLITE - Obsolete term for a stony meteorite.
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.
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.
ALABANDITE - Mn sulfide, MnS, found in aubrites and EL chondrites.
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).
ALKALI EARTHS - Large divalent (2+) elements: Be, Mg, Ca, Sr, and Ba.
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 - Larger univalent (1+) elements: Li, Na, K, Rb, and Cs.
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.
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.
ALLOTROPY - Ability of a substance to exist in more than one phase in the solid (or indeed, liquid and gaseous) state. Allotropy in crystalline materials is called polymorphism. Ozone (O3) and dioxygen (O2) are allotropes of 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.
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).
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.
AMPHIBOLE - Complex family of hydrous silicate minerals with similar physical features and a general formula A0-1X2Y5(Si,Al)8O22(OH,F,CL)2, where A = Ca, Na, K; X = Ca, Fe, Mg, Mn, Na; Y = Al, Fe, Mg, Mn, Ti. Amphiboles form in both igneous and metamorphic environments in the presence of water.
AMPHOTERITE - Obsolete name for LL (low-low Fe content) chondrites.
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.
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 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.
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.
APHELION - Point farthest from the sun in an object's orbit.
ASTEROID - Rocky or metallic object, smaller than a planet but bigger than a meteoroid, that orbits the Sun or another star; also known as a minor planet. More than 20,000 asteroids have been given official designations, most of them in the Asteroid Belt between the orbits of Mars and Jupiter. Asteroids outside this belt include the Trojans, which share Jupiter's orbit, 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.
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.
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).
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." It is also fairly apparent from this image that there is a resonance at 2 A.U., 2.5 A.U., 2.8 A.U., 2.9 A.U., and 3.3 A.U., since these are distances at which the number density of asteroids is almost zero.
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. This fact stems from compositional differences, which in turn are related to how far from the Sun asteroids of different types formed (E = enstatite-rich; S = stony; M = metallic; C = carbonaceous; D and P = red 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.
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 opposite 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.
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).
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.
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 PRINCIPLE - Order in which the electrons of an atom are assigned to energy levels (each defined by quantum number). Shells are filled in order of increasing energy beginning with 1s, then 2s, then 2p, then 3s, etc. "Aufbau" is German for "building-up".
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.
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.
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.