Non-biological in origin, or not derived from living organisms ... Read More


Gradual removal of the successive surface layers of a material through various processes.
  1. The gradual removal and loss of ... Read More


Type of erosion process in which rocks and/or rock fragments scrape and grind away each other's surface. The fragments may ... Read More

Absolute Magnitude

Magnitude an object would have if placed at a distance of exactly 10 parsecs (= 32.6 light years). A supergiant ... Read More

Absolute Zero

Lowest possible temperature, according to the kinetic theory of gases, attained when a system is at its minimum possible energy, ... Read More


Transfer of energy to a medium as a particle or electromagnetic radiation passes through it. Absorption of electromagnetic radiation is ... Read More

Absorption Line

Narrow range of wavelengths in a spectrum that are darker (have lower intensity) than neighboring wavelengths. Absorption lines in the ... Read More

Absorption Spectrum

Spectrum consisting of dark absorption lines superimposed over a continuum spectrum, created when a cooler gas absorbs photons from a ... Read More

Abundance Ratio

Logarithm of the ratio of two metallic elements in a star relative to their ratio in the Sun. This is ... Read More


Primitive achondrite that belongs to a small group named after the Acapulco meteorite that was observed to fall in Mexico ... Read More

Accessory Mineral

Constituent mineral present in small quantity and not taken into account in identifying or classifying a rock ... Read More


Accumulation of smaller objects into progressively larger bodies in the solar nebula leading to the eventual formation of asteroids, planetesimals ... Read More

Accretion Disk

Disk-shaped cloud of gas and solids in orbital motion around a central protostar or some other massive central body ... Read More

Accretionary Lapilli

Pellets that form by accretion of fine ash around condensing water droplets or solid particles; particularly common in steam-rich volcanic ... Read More


An achondrite is a type of stony meteorite whose precursor was of chondritic origin and experienced metamorphic and igneous processes ... Read More


Applied to mineral morphology: bristly, spiny, and needle-like ... Read More


Elements 89Ac (actinium) to 103Lr (lawrencium) found in the bottom row of the inner-transition elements of the periodic table. These ... Read More

Activation Complex

Unstable combination of reacting molecules that is intermediate between reactants and products ... Read More

Activation Energy

Amount of energy that must be placed into a system to form an activated complex in a reaction. On an ... Read More


Attractive force between unlike molecules ... Read More


Process in which no heat enters or leaves a system. This is the case, for example, when an interstellar gas ... Read More

Adiabatic Lapse Rate

Rate of decrease of temperature with increasing altitude in the atmosphere. If heat is neither gained nor lost from the ... Read More


Adhesion of a species onto the surfaces of particles ... Read More


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


Oxygen-dependent or requiring oxygen ... Read More


Silicon-based solid with a porous, sponge-like structure in which 99.8% of the volume is empty space. Aerogel is used to ... Read More


Archaic term for a stony meteorite. First known use in 1807 and borrowed from French ... Read More

Aerosol Particles

A component of atmospheric air, comprised of minute solids particles part of which is almost certainly water ... Read More


Process of collecting in a jumbled cluster. In relation to meteorites, agglomeration refers to the early accretion of chondrules, refractory ... Read More


Common particle type in lunar regolith (photograph below). Agglutinates are small glassy breccias formed when micrometeorites (< 1 mm in ... Read More


Magnesium sulfide found in aubrites and EL chondrites. Its formula is MnS ... Read More


Ratio of the amount of light reflected by an object and the amount of incident light. Albedo is used as ... Read More

Albedo Feature

Dark or light marking on the surface of an object that may or may not be a geological or topographical ... Read More

Alkali Earths

Large divalent (2+) elements in the second column (from the left) of the periodic table: Be, Mg, Ca, Sr, and ... Read More

Alkali Feldspar

Variety of feldspar containing alkali metals potassium and sodium in a solid solution. A more complete explanation can be found ... Read More

Alkali Metal

Large univalent (1+) metal elements from Group 1 (leftmost column) on the periodic table: lithium (Li), sodium (Na), potassium (K), ... Read More


Material formed or introduced from somewhere other than the place it is presently found. In impact cratering this may refer ... Read More


Different forms of the same element in the same physical state. Allotropy in crystalline materials is called polymorphism. Ozone (O3) ... Read More


Metal-like substance produced by mixing two or more metals or by the mixture of a metal and another substance. Ceramics ... Read More

Alpha Decay

Nuclear decay by emission of an alpha particle ... Read More

Alpha Particle

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

Alpha Process

Term used to describe the addition of a helium nucleus (4He) to existing nuclei with in a giant star to ... Read More

Amino Acid

A small molecule containing and amine group (–NH2) at one end and a carboxylic acid group (–COOH) at the other ... Read More

Amoeboid Olivine Aggregate

Millimeter sized, fine-grained inclusions present to a few volume-percent in most carbonaceous chondrites. They can be round but can also ... Read More


Material without the regular, ordered structure of crystalline solids. Amorphous substances, like glass, lack a definite repeating pattern in their ... Read More

Amphibole Group

Complex family of hydrous double-chain silicate minerals. Amphiboles are common in terrestrial metamorphic and igneous rocks, forming in the presence ... Read More


Obsolete name for LL (low-low Fe content) ordinary chondrites ... Read More


Type of evolved achondrite meteorite that represent some of the earliest stages of asteroidal differentiation and magmatism in our solar ... Read More


Unit of length convenient for measuring wavelengths of electromagnetic radiation. Named after the Swedish physicist Anders Jonas Ångström (1814-1874), a ... Read More


Meaning not isotropic and having different properties in different directions. In the case of minerals, optical anisotropy is particularly marked ... Read More


Rare compositional variety of plagioclase and the calcium end-member of the plagioclase feldspar mineral series with the formula CaAl2Si2O8. Anorthite ... Read More


A phaneritic, intrusive igneous rock made with a modal composition (i.e. volume%) > 90% plagioclase feldspar of undefined composition (anorthitic ... Read More

Anthropic Principle

The definition of the "anthropic principle" from Wikipedia states that "The principle was formulated as a response to a series of ... Read More


Point directly on the opposite side of a planet from a reference point ... Read More


Point farthest from the sun in a body's elliptical orbit ... Read More


Point in an object’s orbit farthest from a planetary body ... Read More


Point in an object’s orbit farthest from Earth ... Read More

Archimedes Principle

Principle that a body submerged in a fluid is buoyed up by a force equal in magnitude to the weight ... Read More


Measure for small angles ('). 1 arcminute = 1/60 degree ... Read More


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


Noble gas represented by the atomic symbol Ar, that has Z = 18, and an atomic weight of 39.948. It ... Read More


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

Armored Chondrule

Chondrule covered on its outer surface by a metal-sulfide shell. Commonly found in CR2 meteorites. When these chondrules are sliced ... Read More

Aromatic Hydrocarbon

Hydrocarbon compounds in which the carbon atoms are connected by a ring structure that is planar and joined by σ ... Read More

Asteroid Belt

Belt located between 2.12 and 3.3 AU from the Sun and located between the orbits of Mars and Jupiter containing ... Read More


Meaning "star wound", an astrobleme is a pit-like structure on the surface of Earth or other extra-terrestrial body caused by ... Read More


Branch of astronomy entailing measurements of the positions of stars and other celestial bodies. It is the oldest search method ... Read More

Astronomical Unit

The astronomical unit for length is described as the "mean" distance (average of aphelion and perihelion distances) between the Earth ... Read More

Astrophysical Jets

Rapid outflows, launched in antiparallel directions, roughly perpendicular to an accretionary disk found circulating around a central gravitating mass. The ... Read More

Asymptotic Giant Branch

Path on the Hertzsprung-Russell diagram corresponding to the changes that a star undergoes after He burning ceases in the core ... Read More

Asymptotic Giant Branch (AGB) Stars

Stars on the Asymptotic Giant Branch, which represents a late stage of stellar evolution that all stars with initial masses ... Read More


Originally discovered by Michael Warner and his son Michael while searching for meteorites in Chile's Atacama desert in 2012. Atacamaites ... Read More


Work in Progress Relatively rare variety of iron meteorite (designated type D) made almost entirely of taenite, a solid solution ... Read More

Atmophile Element

Elements in the Goldschmidt (geochemical) classification that tend to remain on the surface or atmosphere including Hydrogen (H), Carbon (C), ... Read More

Atomic Mass

Mass of a neutral atom of a nuclide - also called "atomic weight." The atomic weight of an element is ... Read More

Atomic Mass Unit

All the atomic masses of the elements are determined relative to 12C. The atomic mass unit (AMU) is a precise ... Read More

Atomic Number

A number equivalent to the number of protons in the nucleus of an atom, commonly abbreviated as Z ... Read More

Atomic Spectra

Collection of wavelengths can be emitted or absorbed by an element. Light is emitted by an atom when electrons change ... Read More


Process by which a compound is reduced in concentration over time, through adsorption, degradation, dilution, and/or transformation. Radiologically, it is ... Read More


Aubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that ... Read More


High-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, that occurs in many igneous rocks, particularly those of basaltic composition. In order to be considered augite, ... Read More


Meaning "dawn", an aurora is a visible glow of variable luminosity and color in a planet's ionosphere caused by  the ... Read More


One of the allotropes of iron, also known as γ-iron. It is formed when iron is between 912° and 1,394° ... Read More


Tektites found in Australia and are part of the Australasian Tektite Field, a region extending as far as Sri Lanka ... Read More


Formed or generated in place. Commonly applied to minerals formed subsequent to the formation of a rock like for example, ... Read More


Ni-rich Fe metal, Ni3Fe, similar to taenite found in minor amounts in some meteorites. Awaruite is also known as josephinite, ... Read More


Subclass of C-type asteroids with higher albedo than average C-type ... Read More

Backscattered Electron

High energy primary electron that suffers large angle (> 90°) scattering and re-emerges from the entry surface of a specimen ... Read More


Primary radiation or particles (electrons, protons, etc.) deflected or secondary radiation and particles emitted in the general direction of the ... Read More


A rare zirconium oxide (ZrO2) mineral, often formed as a shock-induced breakdown product of zircon. This mineral can be found ... Read More

Ballen Structure

Microscopic shock-deformation feature in quartz. Oval quartz with rims of tiny vugs filled with amorphous material ... Read More

Ballistic Sedimentation

Emplacement of ballistically transported impact ejecta, and ejecta-surface interaction ... Read More


Unit of pressure equal to 100 kPa ... Read More

Barringer Crater

Best known and best preserved impact crater on Earth. The crater is named after Daniel Barringer and still owned by ... Read More


Nickel-iron phosphide, (Fe,Ni)2P, found in iron meteorites ... Read More


Basalt is the most common extrusive igneous rock on the terrestrial planets. For example, more than 90% of all volcanic ... Read More

Beta Decay

Nuclear decays which proceed via the weak interaction are beta (β) decays. These include all nuclear decays in which the ... Read More

Big Bang

Beginning point of time and space for the universe. A state of extremely high (classically, infinite) density and temperature from ... Read More

Big Bang Nucleosynthesis

Formation of elements in the Big Bang. Calculated abundances of these isotopes (D (deuterium = 2H), 3H, 3He, 4He, 6Li, ... Read More

Binding Energy

  1. Energy required to remove a particular electron from an atom to an infinite distance.
  2. Amount of energy released at ... Read More


Complete dry weight of organic material found in the biosphere ... Read More


Volume including the lower part of the troposphere (as high as living organisms can fly or be lofted) and the ... Read More


Property of a mineral if a ray of light passing through it experiences two refractive indices. The effect is to ... Read More

Bl Lac Object

Active galaxy characterized by very rapid (day to day) variability in total luminosity, no emission lines, strong nonthermal radiation, and ... Read More

Black Hole

Maximally gravitationally collapsed object predicted to exist by the theory of general relativity, from which no material object, light or ... Read More

Black Hole Dynamic Laws

Laws describing the interaction of black holes and their surroundings.
  1. The first law of black hole dynamics states that ... Read More


Ideal object that is a perfect absorber of light and also a perfect emitter of light. A perfect black body ... Read More

Bode’s Law

Also called the Titus-Bode Law, this empirical mathematical relationship for planetary distances from the Sun was discovered by Titus in ... Read More

Body-Centered Cubic Packing

Way in which atoms or ions (considered as hard spheres) pack together to fill space. In BCC packing, the spheres ... Read More

Boiling Point

Temperature at which the vapor pressure of a liquid is equal to the ambient pressure - thus, the boiling point ... Read More


A very bright fireball that breaks apart in the atmosphere with a minimum brightness approximately equivalent to the full moon ... Read More

Bolometric Magnitude

Magnitude of a celestial object corrected to take account of the radiation in parts of the spectrum other than the ... Read More

Bottom-Up Structure Formation

Hypothesis that small structures, perhaps galaxies or even smaller substructures, form first in the universe, followed later by larger structures ... Read More


Work in Progress ... A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts). A breccia ... Read More


The formation of a breccia through a process by which rock fragments of of various types are recemented or fused ... Read More

Burgers Vector

Crystal vector which denotes the amount and direction of atomic displacement which will occur within a crystal when dislocation moves ... Read More

C Class

Common asteroid class in the outer part of the Main Belt. Compositions, based on reflectance spectra, are probably similar to ... Read More

Calcium Aluminum-rich Inclusion

Sub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and ... Read More

Calcium Monoaluminate

Mineral, CaAl2O4. The only known natural occurrence of this mineral is in a grossite-rich inclusion in the CH chondrite NWA ... Read More


Large (>1 km) approximately circular volcanic depression, which may form by explosion or collapse, or a combination of the two ... Read More


A hardened natural cement of calcium carbonate, CaCO3, binds other materials like gravels and sand. Caliche is generally light-colored, but ... Read More


Element commonly found in meteorites, it occurs in several structural forms (polymorphs). All polymorphs are shown to the left with ... Read More

Carbon Burning

Fusion reactions between any nuclei larger than C require such high temperatures that they are actually quite uncommon in stars ... Read More

Carbon Monoxide

Molecule (CO) used to map the distribution of matter, especially molecular hydrogen, H2, in interstellar space. Molecular hydrogen is by ... Read More

Carbon Star

Red giant (or occasionally red dwarf) star whose atmosphere contains more carbon than oxygen, which combine in the outer layers ... Read More

Carbonaceous Chondrite

Carbonaceous chondrites represent the most primitive rock samples of our solar system. This rare (less than 5% of all meteorite ... Read More


Mineral or compound containing carbon and oxygen (i.e. calcium carbonate, CaCO3, calcite) ... Read More


Texture found in metamorphic rocks in which brittle minerals have been broken, crushed, and flattened during shearing ... Read More


Chemicals that are not consumed in a reaction, but, which speed up the reaction rate. Catalysts aid to form a ... Read More


Emission of visible light in response to electron bombardment ... Read More


Positively charged ion ... Read More

CB Chondrite

Class also known as bencubbinites, are named after the Bencubbin meteorite found in 1930 in Australia. Only a handful of ... Read More

Central Peak

Exposed core of uplifted rocks in center of a complex impact crater. Central peak material typically shows evidence of intense ... Read More

CH Chondrite

Rare carbonaceous chondrite class that is chemically very close to the CRs and CBs as evidenced by the CH/CBb meteorite ... Read More

Chain Silicate

Minerals composed of chains of silicon tetrahedra. These minerals are divided into single chain and double chain silicates. The former ... Read More

Characteristic X-Ray

X-ray emitted from an excited atom when an outer-shell (e.g. L) electron jumps in to fill an inner-shell (e.g. K) ... Read More


Term applied to a deep, elongated, steep-sided depression (canyon) on a planetary surface (e.g., Candor Chasma on Mars) ... Read More


The group is part of the SNC martian trio and named after the meteorite seen to fall in Chassigny, France, ... Read More

Chemical Bond

Strong attractive force between atoms that holds the atoms close together in space ... Read More

Chemical Lifetime

Length of time a chemical species can survive without reacting, photolyzing, dissociating, or otherwise changing into another chemical species. Highly ... Read More

Chemical Vapor Deposition

Method for growing solids in which a gaseous precursor (containing fragments of the desired solid) is decomposed and deposited onto ... Read More


Emission of absorbed energy as visible light as the result of a chemical reaction. Examples include chemical glow sticks and ... Read More

Chicxulub Crater

Large (150-180 km wide and 20 km deep), partially submerged, multi-ringed impact crater straddling the northwest coastline of the Yucatan ... Read More


Chondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate ... Read More


Roughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at  ~1400 ° C ... Read More


Brownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups ... Read More


Region immediately above a star’s photosphere ... Read More

CI Chondrite

Rare meteorite class named after the Ivuna meteorite that fell in Tanzania in 1938. They are among the most primitive, ... Read More

CK Chondrite

Class of carbonaceous chondrite named for the Karoonda meteorite that fell in Australia in 1930. They are more oxidized than ... Read More


A mineral or rock fragment embedded in another rock ... Read More

Clay Mineral

Hydrous aluminium phyllosilicates, with variable amounts of Fe, Mg, Ca, Na, K, and other cations. The structure of clay minerals ... Read More

CM Chondrite

Class of carbonaceous chondrites named after the Mighei meteorite that fell in Ukraine in 1889. They represent samples of incompletely ... Read More

CNO Cycle

Fusion process in stars contaminated by "metals," specifically carbon (C), nitrogen (N), and oxygen (O). The two branches of the ... Read More

CO Chondrite

Meteorite class named after the Ornans meteorite that fell in France in 1868, are related in chemistry and composition to ... Read More


High-pressure polymorph of silicon dioxide (SiO2). Has the same chemical composition as cristobalite, stishovite, seifertite and tridymite but possesses a ... Read More


Fe-Ni-Co carbide, (Fe,Ni,Co)3C, that occurs as an accessory constituent in several iron meteorites, and coarse octahedrites with < 7 wt ... Read More

Color Index

Parameter defining how a star's brightness varies with color. It is defined by taking the difference in magnitude (which is ... Read More


Spherical envelope of gas and dust surrounding the nucleus of an active comet, created when the ambient heat causes the ... Read More


Conglomeration of frozen water and gases (methane, ammonia, CO2) and silicates that that formed in the outer solar system and ... Read More

Comet P/Halley

Comet with 76-year orbital period. Images returned by the Giotto Mission revealed its nucleus was a dark (albedo 0.04-0.05), peanut-shaped ... Read More

Complex Crater

Craters with stepped terraces on the interior wall, a flat floor, and a single peak or group of peaks in ... Read More

Compound Chondrule

Object composed of two or more chondrules that were fused together ... Read More


A hard, rounded mass that formed by the precipitation of mineral cement within the spaces between sedimentary particles and enclosing ... Read More


In the solar nebula, product of a chemical condensation reaction where a mineral phase precipitates (condenses) directly from a cooling ... Read More

Condensation Nuclei

Dust grains in the interstellar medium which act as nuclei (cores) around which other material can condense. The presence of ... Read More

Condensation Temperature

The condensation temperature is the temperature at which 50% of the element would be condensed (in the form of a ... Read More


Transfer of heat as a result of collisions between molecules; when one end of an object is heated or excited, ... Read More


Transfer of heat energy by moving material. Temperatures increases with depth in planetary objects. Deep hot less-dense material physically rises ... Read More

Convective Zone

Region within a star, where the change in temperature with increasing radius is so rapid that energy transfer occurs by ... Read More


In the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon ... Read More


Extended outer atmosphere of the Sun. The glow of the corona is a million times less bright than that of ... Read More


Crystalline form of aluminium oxide, Al2O3, found in Ca-Al-rich inclusions (CAIs). Corundum-bearing  CAI are a rare class of high-temperature condensates ... Read More

Cosmic Ray Exposure Age

Time interval that a meteoroid was an independent body in space. In other words, the time between when a meteoroid ... Read More

Cosmic Rays

High-energy subatomic particles mainly originating outside the Solar System that continuously bombard the Earth from all directions. They represent one ... Read More


Also known as chemical cosmology, cosmochemistry is the study of the chemical composition of matter in the universe and the ... Read More

Cosmological Principle

Principle incorporating the axioms that there is no center to the universe, that the universe is the same in all ... Read More

Cosmological Redshift

Redshift caused by the expansion of space. The wavelength of light increases as it traverses the expanding universe between its ... Read More

Coupled Substitution

Substitution in which the charges of substituting ions are not same and charge balance is achieved by a second substitution ... Read More

Covalent Bond

Interaction between atoms by which they "share" valence electrons in the outermost energy shells, thereby filling the outer shell of ... Read More

CR Chondrite

Class named for the Renazzo meteorite that fell in Italy in 1824, are similar to CMs in that they contain ... Read More


Bowl-like depression ("crater" means "cup" in Latin) on the surface of a planet, moon, or asteroid. Craters range in size ... Read More

Crater Chain

Several craters along a general line that may be overlapping, touching, or separate from one another. A crater chain, or ... Read More

Crater Density

Number of craters of a certain size and larger per unit area. Higher crater density reflects an older age for ... Read More

Cretaceous-Tertiary Boundary

The 66-million-year-old stratigraphic boundary between the end of the Cretaceous Period and the beginning of the Tertiary Period. This boundary ... Read More


High temperature polymorph of silicon dioxide (SiO2). Has the same chemical composition as coesite, stishovite, seifertite and tridymite but possesses ... Read More

Critical Point

Point along a phase boundary on a phase diagram where the liquid and gas states cease to be distinct. For ... Read More


Outermost layer of a differentiated planet, asteroid or moon, usually consisting of silicate rock and extending no more than 10s ... Read More


Water or other liquid or vapor-phase volatiles together with gas-driven solid fragments produced by internal heating of a planetary body ... Read More


Portion of Earth which consists of the ice masses and snow deposits (continental ice sheets, mountain glaciers, sea ice, surface ... Read More


Eruption of water or other liquid or vapor-phase volatiles (collectively referred to as "cryomagma"), together with gas-driven solid fragments, onto ... Read More


Crypto meaning "hidden" refers to a rock texture in which individual crystals are too small to be distinguished even using ... Read More

Crystal Defect

Variation in the regular arrangement of the atoms or molecules of a crystal. Such variations include point defects, line defects, ... Read More

Crystal Lattice

Atoms or groups of atoms repeated at regular intervals in three dimensions with the same orientation. One may regard each ... Read More

Crystal Lattice Energy

Energy change when one mole of formula units of a crystalline solid is formed from its ions, atoms, or molecules ... Read More

Crystal Settling

A process by which crystals forming in a magma will sink toward the bottom of the magma chamber due to ... Read More

Crystal Structure

Mutual arrangement of atoms, molecules or ions that are packed together in a crystal lattice to form a crystal ... Read More

Crystal System

Atomic arrangement of the atoms of an element when it is in its solid state. In mineralogy, these are  best ... Read More

Crystalline Material

Substance with a structure consisting of a systematically repeated pattern of atoms. This term is also applied to rock types ... Read More


Physical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as ... Read More


Science of determining the arrangement of atoms in solids; the scientific study of crystals. Crystallography relies on the analysis of ... Read More

Cubic Closest-Packed

Way in which atoms (considered as hard spheres) pack together to fill space. In cubic closest-packing, there are three alternating ... Read More


Igneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber ... Read More

CV Chondrites

Meteorite class named after the Vigarano meteorite that fell in Italy in 1910. They have abundant large, well-defined rimless (?) ... Read More


One of the five nitrogen-containing bases occurring in nucleotides ... Read More

Dark Energy

Form of energy that is gravitationally repulsive, due to a negative effective pressure. Dark energy is spread almost uniformly throughout ... Read More

Dark Matter

Form of matter that does not emit light, absorb light, or otherwise interact with electromagnetic radiation. Its only interactions are ... Read More

Darwin Glass

Impact glass found in the environs of the 1.2 km-diameter Darwin impact crater in western Tasmania, Australia. In rare cases, ... Read More


Fe-Cr sulfide (FeCr2S4) that crystallizes with cubic symmetry and found as an inclusion in meteoritic iron ... Read More

Decay Mode

Disappearance of a radioactive substance due to nuclear emission of an α or β particle, capture of an atomic electron, ... Read More

Degenerate Matter

Highly compressed matter in which the normal atomic structure has broken down and which, because of quantum-mechanical effects, exerts a ... Read More


Mass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.) ... Read More

Destructive Interference

Result when two waves overlap such that they are completely out of phase. When destructive interference occurs, the two waves ... Read More


Any device used to sense the passage of a particle or photon (x-ray, γ-ray, etc.). X-rays can be detected using ... Read More


Also called heavy hydrogen, deuterium is an isotope of hydrogen (D, or 2H) whose nucleus contains one proton and one neutron ... Read More


One of the naturally occurring forms of carbon found in meteorites. Each C atom is bonded through covalent sp3 hydrid ... Read More

Diaplectic Glass

Natural glass formed by shock transformation from any of several minerals (more commonly feldspar) without melting. It is found in ... Read More

Differential Gravitational Force

Gravitational force acting on an extended object, such that the portions of the object closer to the source of gravitation ... Read More


A process by which a generally homogeneous chondritic body containing mostly metal, silicates and sulfides will melt and form distinct ... Read More


Movement of particles from higher chemical potential to lower chemical potential (chemical potential can in most cases of diffusion be ... Read More


Planar, blade-like, intrusive igneous body that cuts across preexisting layers usually at a high-angle to near-vertical orientation. By definition, a ... Read More

Dimict Breccia

Breccia made up of only two lithologies. The term is usually applied to a common type of rock collected on ... Read More


Diogenites belong to the evolved achondrite HED group that also includes howardites and eucrites. They are named after the Greek ... Read More

Distal Ejecta

Impact ejecta found more than 5 crater radii away from the rim of the source crater. About 10% of all ... Read More


Potassium, iron, copper sulfide, K3CuFe12S14, found in some meteorites. Djerfisherite mainly occurs outside of chondrules in EH chondrites and can ... Read More


Rounded volcanic structure produced by eruption or intrusion of viscous high-silica magma. Domes are common on Earth, but rare on ... Read More

Doppler Shift

Result of the relative motion of the light emitting object and the observer. If the source of light is moving ... Read More


Term applied to a ridge on a planetary surface (e.g., Gordii Dorsum on Mars) ... Read More

Drake Equation

Method of estimating the number of intelligent, technologically advanced species (i.e., able to communicate with other species) in existence in ... Read More

Dust Grain

Grains of carbon and silicate ~0.1-1.0 μm in size. Dust grains are a major component of the interstellar medium. Dust ... Read More

Dwarf Galaxy

Relatively small galaxy, typically containing ~107 stars. There is no official size below which a galaxy is designated a dwarf ... Read More

Dynamo Theory

Branch of magnetohydrodynamics concerned with self-excitation of magnetic fields in any large rotating mass of electrically conducting fluid in motion ... Read More


Ratio of the distance between the foci and the major axis of an ellipse (planetary orbit). Eccentricity is: where ra ... Read More


Obscuring of one celestial body by another, either by direct superposition or by the casting of a shadow. Solar eclipses ... Read More


Plane of Earth's orbit around the sun. All the planets except Mercury and Pluto have orbits in nearly the same ... Read More

Edgeworth-Kuiper Belt

A group of comets residing mainly near the ecliptic at distances of 34-45 AU from the Sun ... Read More

Effective Mass

Value of mass one must use to model electron behavior in solids as the motion of independent particles. The motion ... Read More


Fractured and/or molten rocky debris thrown out of a crater during a meteorite impact event, or, alternatively, material, including ash, ... Read More

Ejecta Blanket

Generally symmetrical apron of ejecta surrounding a crater; it is thick at the crater's rim and thin to discontinuous at ... Read More

Elastic Deformation

Change in shape of a material at low stress that is recoverable after the stress is removed. This type of ... Read More

Electron Capture Decay

Nuclear decay by capture of an atomic electron. If the decay energy is greater than 1.022 MeV, β+ emission can ... Read More

Electron Degeneracy Pressure

The force that supports white dwarf stars against gravity. Quantum mechanics restricts the number of electrons that can have low ... Read More

Electron Microprobe

Instrument that analyzes the chemistry of very small spots by bombarding the sample with a focused electron beam and measuring ... Read More


Substance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of ... Read More

Elemental Group

Vertical columns (major classes or divisions) into which elements are arranged in the periodic table of elements. Elements in a ... Read More

Elliptical Orbit

Describes the characteristic oval orbital path of planets and asteroids around the Sun ... Read More

Emission Line

More or less narrow range of wavelengths in a spectrum that is brighter than neighboring wavelengths, corresponding to emission of ... Read More


A chemical reaction that is accompanied by or requires the absorption of energy from its environment. A classic endothermic reaction ... Read More

Energy Diagram

Convenient way of depicting the thermodynamics of reactions. Typically, the y-axis is given in units of energy, and the x-axis ... Read More

Energy-Wavelength Relationship

Inverse relationship between the wavelength of electromagnetic radiation, λ, (Å) and the photon energy, E, (Joules): Conversion to Ångstroms and ... Read More


A mineral that is composed of Mg-rich pyroxene, MgSiO3. It is the magnesium endmember of the pyroxene silicate mineral series ... Read More

Enstatite Achondrite

Used in past as synonym for Aubrites. Present definition from the Meteoritical Bulletin states that this rare class is an ... Read More

Enstatite Chondrite

Type of meteorite high in the mineral enstatite and also referred to as E-chondrites. Although they contain substantial amounts of ... Read More


Thermodynamic state function, generally measured in kJ/mole. In chemical reactions, the enthalpy change (DH) is related to changes in the ... Read More


Thermodynamic quantity which is a measure of the degree of disorder within any system. The lower the degree of order, ... Read More

Equilibrated Chondrite

Chondrite with minerals of uniform composition (e.g., all of the olivine grains have the same composition) due to diffusion during ... Read More


Term used to describe physical or chemical stasis. Physical equilibrium may be divided into two types: static and dynamic. Static ... Read More


Removal of weathered rocks by moving water, wind, or ice ... Read More

Escape Velocity

Velocity that an object needs to escape the primary gravitational influence of a more massive object: where, m = the ... Read More


Most common type of achondrite meteorite and a member of the HED group. Eucrites are basalts composed primarily of pigeonite ... Read More

Eutectic Point

Temperature at which the melting point of a mixture of chemical constituents is as low as possible. All the constituents ... Read More


Process in which atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy ... Read More

Event Horizon

The surface surround the region out of which light itself cannot escape a black hole. No signal or information from ... Read More

Exchange Vector

Notation for indicating substitution that occurs in minerals; also called “exchange component.” Probably the most common vector in minerals is ... Read More

Excited State

Any energy state of an atom, or of a solid, above its ground state. For atoms and solids, excited states ... Read More


A chemical or nuclear reaction that liberates heat energy so that heat flows from the system to the surroundings. Exothermic ... Read More


Segregation, during cooling, of a homogeneous solid solution into two or more different solids ... Read More


In astronomy, the dimming of starlight as it passes through the interstellar medium. Dust scatters some of the light, causing ... Read More


Refers to igneous rocks erupted on a planetary body's surface ... Read More

Face-Centered Cubic

A crystal structure that has one atom at each corner of the unit cell and one atom in the center ... Read More


Term applied to a bright spot on a planetary surface (e.g., Memphis Facula on Ganymede) ... Read More

Faint Young Sun Paradox

Theories of stellar evolution indicate that as stars mature on the main sequence, they grow steadily hotter and brighter. About ... Read More


Meteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering ... Read More

Faraday’s Laws

Laws of electromagnetic induction first proposed by M. Faraday. The first law states that an electromotive force is induced in ... Read More


Obsolete name for Ca- and Ti-rich clinopyroxene, properly termed augite ... Read More


Fracture along which there has been movement or displacement ... Read More


Pure* iron end-member (Fe2SiO4) of the olivine solid solution series and an important mineral in meteorites. When iron (Fe) is ... Read More


An alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of ... Read More


Minerals occurring in low-Si terrestrial rocks, feldspathoids are similar to feldspars in structure and properties, but contain about two-thirds as ... Read More


One of the two broad categories of silicate minerals, the other being mafic, based on the magnesium (Mg) and/or iron ... Read More

Fermi Paradox

The question why, if the Galaxy is filled with intelligent and technological civilizations, they have not visited Earth? Fermi is ... Read More


Phenomenon in iron or iron alloys with a body centered cubic crystal structure where there can be incomplete cancellation of ... Read More


Phenomenon exhibited by materials like iron (nickel or cobalt) that become magnetized in a magnetic field and retain this magnetism ... Read More


A mineral that is composed of Fe-rich pyroxene, FeSiO3. It is the iron endmember of the pyroxene silicate mineral series ... Read More


Dark, thread-like feature seen on the Sun in the red light of hydrogen (H-alpha). Filaments are dense, somewhat cooler, clouds ... Read More


Meteorite not seen to fall, but recovered at some later date. For example, many finds from Antarctica fell 10,000 to ... Read More


A fireball is another term for a very bright meteor, generally brighter than magnitude -4, which is about the same magnitude of the ... Read More


Breaking apart of a body into smaller fragments. In nuclear physics, fission refers to splitting of a heavy atomic nucleus ... Read More


Geomorphologic term referring to a complex association of flow-like features emanating from a common source location or feature (pl. flucti) ... Read More


Energy density from an optical source impinging on a sample. The higher the energy density, the higher the fluence. Fluence ... Read More


Emission of electromagnetic radiation, especially of light, that results from bombardment of a substance with other forms of electromagnetic radiation ... Read More

Forbidden Line

Spectral line seen in emission nebulae but not seen in laboratory experiments, because under laboratory conditions, collisions kick the electron ... Read More

Forbidden Zone

Relatively large energy separation between an insulator's highest filled electron energy band and the next higher energy vacant band ... Read More


Pure* magnesium end-member (Mg2SiO4) of the olivine solid solution series and an important mineral in meteorites. When magnesium (Mg) is ... Read More

Forsterite Chondrite

Forsterite Chondrites or F-Chondrites are presently only known as inclusions in other meteorites and described by certain lithologies of the ... Read More


Long, narrow, shallow depression or a narrow, linear trench (pl. fossae) ... Read More

Fossil Meteorite

The textural, mineralogical or compositional remnant within a sedimentary rock of a meteorite that fell millions of years ago and ... Read More

Fractional Crystallization

A crystallization process in which minerals crystallizing from a magma are isolated from contact with the liquid. It is a ... Read More


Concentration or separation of one mineral, element, or isotope from an initially homogeneous system. Fractionation can occur as a mass-dependent ... Read More


Glassy, oftentimes tubular, object produced by melting materials at the Earth’s surface such as sand during a lightning strike ... Read More


Fractionated and Unidentified Nuclear effect (FUN) CAIs are a type of CAI that shows large mass-dependent fractionation and mass-independent fractionation ... Read More

Fundamental Forces

Forces that govern the various interactions between particles. The four fundamental forces, or interactions, are (in order of increasing strength): ... Read More

Fundamental Particles

Particles considered basic, indivisible, building blocks of atoms and all forms of matter. These include stable leptons (electrons and neutrinos), ... Read More


Process in which two lighter atomic nuclei combine to form a heavier atomic nucleus. Very high temperatures are normally required ... Read More

Fusion Crust

Melted exterior of a meteorite that forms when it passes through Earth’s atmosphere. Friction with the air will raise a ... Read More


Work in progress Coarse-grained igneous rock of basaltic composition that formed at depth and is 90% plagioclase. clinopyroxene, Gabbro The ... Read More


Concentration of 106 to 1012 stars, dust and gas, that are gravitationally bound. Our galaxy contains ~2 × 1011 stars. There are ... Read More

Galaxy Cluster

Group of galaxies which are mutually gravitationally bound. There are two important types of galaxy clusters: Regular and Irregular Galaxy ... Read More

Galilean Moons

Four largest of Jupiter's moons: Io, Europa, Ganymede and Callisto. Discovered independently by Galileo and Marius, Galileo proposed that they ... Read More

Gamma Ray Burst

Long duration g-ray bursts that coincide with powerful supernovae (hypernovae) that occur when a massive star collapses to a black ... Read More

Gamma Rays

Most energetic form of radiation, similar to x-rays and light, except with shorter wavelengths (<0.01 nm) and higher energies (>105 ... Read More


Mineral generally found in terrestrial metamorphic rocks, although igneous examples are not uncommon. Garnet is a significant reservoir of Al ... Read More


Centered around the Earth ... Read More


Study of the chemical composition of Earth and other planets, chemical processes and reactions that govern the composition of rocks ... Read More

Giant Impact Hypothesis

Theia is a hypothesized planet about the size of Mars that, according to the Giant Impact Hypothesis, smashed into the proto-Earth ... Read More


Phase of the Moon between first quarter and full or between full and last quarter ... Read More


An igneous rock consisting mainly of the minerals quartz and alkali feldspar ... Read More


Opaque form of carbon (C) found in some iron and ordinary chondrites and in ureilite meteorites. Each C atom is ... Read More

Gravitational Force

One of the four fundamental forces, gravitation is the force of mutual attraction that is exerted between massive bodies and ... Read More

Gravitational Potential Energy

Energy inherent in the potential for gravitational collapse. The gravitational potential energy of a gas of atoms and molecules is: ... Read More

Gravitational Radiation

Very weak wave-like disturbances in the geometry of space and time produced by an accelerating, oscillating or violently disturbed mass, ... Read More

Gravitational Redshift

Shift in the frequency of a photon to lower energy as it climbs out of a gravitational field ... Read More


Hypothetical force-carrying particle (gauge boson) for gravitation. Although there is, as yet, no generally accepted quantum theory of gravity, it ... Read More


Attractive force between all matter - one of the four fundamental forces ... Read More


Calcium aluminate, CaAl4O7, first found in metamorphosed Israeli limestone and recently in CAIs in CV3 and CR-CH-CB carbonaceous chondrites ... Read More

Ground State

Energy state of an atom, or of a solid, when its total energy equals the minimum possible energy for that ... Read More


One of the five nitrogen-containing bases occurring in nucleotides ... Read More

H Chondrite

Ordinary chondrites with a high content of free Ni-Fe metal (15-19 vol. %) and attracted easily to a magnet. Their ... Read More

Habitable Zone

Zone around a star in which water is in the liquid form (273-373 K). This zone will be farther from ... Read More


Period of time required for 50% (½) of the atoms of a radioactive nuclide in a sample to decay. After ... Read More


Reactive nonmetal that is in Group 17 (VIIA) of the periodic table: F, Cl, Br, I and At. All of ... Read More


Iron-nickel carbide, (Fe,Ni)23C6, found in iron meteorites ... Read More

Heat Capacity

Ability of matter to store heat. The heat capacity is the quantity of heat (J) required to increase a material’s ... Read More

Heat Conduction

Flow of internal energy (heat) from a region of higher temperature to one of lower temperature by the interaction of ... Read More

Heat of Condensation

Equivalent to heat of vaporization, and latent heat of a substance. It is the amount of heat that must be ... Read More

Heat of Crystallization

Equivalent to heat of melting (fusion) and latent heat of a substance. It is the amount of heat that must ... Read More

Heat of Reaction

Enthalpy which is given off or absorbed during a reaction. If the pressure of the reaction remains constant, one may ... Read More

Heat of Vaporization

Heat required to vaporize one mole of a substance at its boiling point under standard pressure (101,325 Pa). The heat ... Read More

Heisenberg Uncertainty Principle

Conclusion that it is impossible to know simultaneously the absolute exact position and the absolute exact speed of a particle ... Read More


Centered around a sun. Our own Solar System is centered around the Sun so that all planets such as Earth ... Read More


Region of space containing plasma and magnetic fields of solar origin; a cavity carved in the interstellar medium by the ... Read More


Helium (He) Second lightest and second most abundant element (after Hydrogen) in the universe. The most abundant isotope is 4He ... Read More

Helium Burning

The triple alpha process. This is the main source of energy production in red giants and red supergiants ... Read More

Helium Capture

Formation of heavy elements by the capture of helium nuclei. For example, at ~6 x 108 K, carbon can form ... Read More

Helium Precipitation

Mechanism responsible for the low abundance of helium of Saturn's atmosphere. Helium condenses in the upper layers to form a ... Read More


Fe-oxide mineral (Fe2O3) that may be the major cause of the red color on Mars. Coarser-grained gray hematite has the ... Read More


Fe-Mg spinel, (Fe,Mg)Al2O4, found in many meteorite classes ... Read More

Heterogeneous Equilibrium

Equilibrium involving species in more than one phase. Compare homogeneous equilibrium ... Read More

Hexagonal Closest Packing

Way in which atoms (considered as hard spheres) pack together to fill space. In cubic closest-packing, there are three alternating ... Read More


One of the main types of iron meteorites composed almost entirely of kamacite and named for its cubic (hexahedral) cleavage ... Read More


Refractory mineral, Ca-aluminate (CaAl12O19) that occurs in terrestrial metamorphic rocks and in CAIs of many chondrites. Meteoritic hibonite tends to ... Read More

High-Pressure Mineral Phases

Mineral forms that are stable only at the extremely high pressures typical of Earth's deep interior, but not its surface ... Read More

Homogeneous Equilibrium

Equilibrium involving only one species in a single phase. For example, all gases, all liquids or all solids ... Read More


One type of meteorite in the HED (Howardite, Eucrite, Diogenite) achondrite group. Howardites are named after the English chemist Edward ... Read More

Hubble Space Telescope

A 2.4-meter reflecting telescope, which was deployed in low-Earth orbit (600 kilometers) by the crew of the space shuttle Discovery ... Read More


Reaction of a substance with water ... Read More


Organic compound containing only carbon and hydrogen ... Read More


Study of fluid in motion ... Read More


Lightest and most common element in the universe (~92% by atoms; ~75% by mass). Hydrogen's isotopes are:
  1. 1H (99.9885 ... Read More

Hydrogen Bond

Type of attractive intermolecular force that exists between two partial electric charges of opposite polarity. Although stronger than most other ... Read More

Hydrogen Burning

Processes by which hydrogen (1H) is fused into helium (4He) with in a star. The five possible fusion paths can ... Read More


Reaction of a substance with water or its ions ... Read More


Portion of a planet that is water, including liquid water, ice, and water vapor on the surface, underground, or in ... Read More

Hydrostatic Equilibrium

Balance between gravity and gas pressure. In the case of a star, gravity originates in mutual gravitational attraction of the ... Read More


Refers to the OH- ion ... Read More

Igneous Rock

One of the three basic types of rock that also include sedimentary and metamorphic. Igneous rocks are formed by the ... Read More


Ti-Fe oxide, TiFeO3, found in achondrites, lunar mare basalts, and shergottites. Ilmenite forms as a primary mineral in mafic igneous rocks ... Read More


The property of liquids that are mutually insoluble (won't mix together) such as oil and water or metallic and silicate ... Read More

Impact Basin

Very large impact structure, typically many tens to hundreds of kilometers in diameter, consisting of one or more concentric rings ... Read More

Impact Breccia

Rock consisting of broken fragments of rock (clasts) cemented by a fine-grained matrix formed from regolith during subsequent impacts ... Read More

Impact Crater

Crater formed by high-speed impact of a meteoroid, asteroid, or comet on a solid surface. Craters are a common feature ... Read More

Impact Melt Rock

Rock that has been made temporarily molten as a result of the energy released by the impact of a large ... Read More


Slag-like glassy object found on surface of the Earth formed from rock melted by the impact of a meteor. This ... Read More


Angle between the plane of an object's orbit and the ecliptic; the inclination of a moon's orbit is the angle ... Read More


Fragment of foreign (xeno-) material enclosed within the primary matrix of a rock or meteorite ... Read More

Inert Gases

Noble gases ... Read More

Intercrater Plains

Regions on the surface of Mercury that do not show extensive cratering, but are relatively smooth ... Read More

International Astronomical Union

The International Astronomical Union (IAU) was founded in 1919. Its mission is to promote and safeguard the science of astronomy ... Read More

Interplanetary Dust Particles

Extremely small (~10 μm in diameter) particles found on Earth (or collected during high altitude flights) that are probably from ... Read More

Interstellar Dust

Grains of carbon and silicate ~0.1-1.0 mm in size. Dust grains are a major component of the interstellar medium. Dust ... Read More

Interstellar Medium

Material between the stars, consisting of gas, dust and cosmic rays (high energy charged particles moving at nearly the speed ... Read More


Term applied to ions or atoms occupying sites between lattice points ... Read More

Interstitial Impurity

Atom or ions of a species not normally found in a crystal, located between normal atomic sites ... Read More

Interstitial Substitution

Substitution in which interstitial ions are added or removed. This is most common in cyclosilicate minerals, which have channel-like structures ... Read More


Refers to igneous rocks that crystallized underground ... Read More


Atom with a net electrical charge because it has lost, or gained, one or more electrons relative to the number ... Read More

Ion Microprobe

An instrument in which a focused beam of ions ionizes atoms in a sample and ejects them for analysis with ... Read More

Ionic Bonding

Chemical bond formed by electrostatic attraction resulting when one atom “donates” valence electron(s) to another atom, resulting in filled energy ... Read More


Process whereby atoms lose one or more electrons to become cations. Ionization occurs by ionizing radiation or if an atom ... Read More

Ionization Energy

Amount of energy required to remove the highest-energy electron from an isolated neutral atom in the gaseous state. There is ... Read More

Ionizing Radiation

Radiation with sufficient energy to eject electrons from electrically neutral atoms, leaving behind an ion. There are four basic types ... Read More


Region of charged particles in a planet's upper atmosphere. The Earth's ionosphere is at an altitude of ~40-400 km ... Read More

Iron Meteorite

Iron meteorites consist mostly of metallic iron alloyed with typically between ~5 to ~30 wt% nickel. The main metal phases ... Read More

Iron-Wustite Buffer

Reference buffer to which the oxygen fugacity of a system may be compared: ... Read More


Isotopes with equal atomic mass. For example, 10Be, 10B, and 10C are isobars. Only one isobar will be stable, the ... Read More


Process that takes place without any change in pressure ... Read More


Process that takes place without any change in volume ... Read More


Mechanism whereby areas of the crust rise or subside until the mass of their topography is buoyantly supported or compensated ... Read More


Process that takes place without any change in temperature. Isothermal processes are often described as "slow." ... Read More


Isotopes with equal numbers of neutrons. For example, 3H, 4He, 5Li, and 6Be are isotones, each having 2 neutrons. More ... Read More


One of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has ... Read More


Molecular entity that differs only in isotopic compositions, e.g., C16O vs. C18O ... Read More


Same in all directions ... Read More


More common than taenite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Kamacite, α-(Fe,Ni), contains 4-7.5 wt% ... Read More

Kirkwood Gaps

Lack of asteroids in regions within the asteroid belt (between Mars and Jupiter) coinciding with orbital periods that are simple ... Read More


Lunar igneous rock rich in potassium (K), rare-earth elements (REE), phosphorus (P), thorium, and other incompatible elements. These elements are ... Read More

Kuiper Belt

Region in the outer solar system beyond Neptune's orbit that contains billions of small, icy planetesimals from the original protoplanetary ... Read More

Kuiper Belt Objects

Generally, small bodies found in the Kuiper Belt. The largest Kuiper Belt Object (KBO), Eris, is <3000 km in diameter ... Read More

L Chondrite

Ordinary chondrites low in free Ni-Fe metal (4 to 10 vol. %), containing olivine (Fa22-26) and the orthopyroxene hypersthene (Fs19-22) ... Read More


Include the following elements elements (preceded by their atomic numbers): 57 lanthanum (La), 58 cerium (Ce), 59 praseodymium (Pr), 60 ... Read More

Late Heavy Bombardment

Period between ~4.0 to 3.8 Ga ago when the Moon and other objects in the Solar System were pounded heavily ... Read More

Latent Heat

Heat absorbed or released as the result of a phase change. There are three basic types of latent heat each ... Read More


Regularly spaced array of points that represents the structure of a crystal. Crystals are composed of groups of atoms repeated ... Read More

Lattice Vibration

Regular vibration of atoms within a crystal lattice about their equilibrium positions ... Read More


Hot molten or semifluid rock derived from a volcano or surface fissure from a differentiated and magmatically active parent body ... Read More

Layered Tektite

Tektite made of multiple layers of glass and also called a Muong Nong tektite ... Read More


An amorphous silica glass formed naturally by the heat of impact/detonation of a large meteorite on/over quartz sand. Lechatelierite is ... Read More

Libyan Desert Glass

High-silica glass (SiO2) found in western Egypt near the Libyan border. The glass is often worn smooth due to aeolian ... Read More

Light Year

Distance that light travels in a year through a vacuum. This unit is derived from the fact that light (electromagnetic ... Read More


A common form of calcium carbonate (CaCO3). Other common forms of CaCO3 include chalk and marble ... Read More

Lithophile Element

Element that tends to be concentrated in the silicate phase, e.g., B, O, halogens, alkali earths, alkali metals, Al, Si, ... Read More


Rigid outer layer of a planet. The base of the lithopshere is defined by the temperature at which the brittle/ductile ... Read More

LL Chondrite

Ordinary chondrites ("low Fe" / "low metal") with only 1 to 3% free metal. Their olivine is more Fe-rich than ... Read More

Lobate Scarp

Landform on Mercury consisting of curving cliffs produced by compressional forces. These cliffs vary from 10s to 100s of km ... Read More


Rare type of primitive achondrite named after the Lodran meteorite that fell in Pakistan in 1868. Initially, lodranites were grouped ... Read More


An unconsolidated, clay-rich sediment deposited by the wind ... Read More

Long-Lived Radionuclides

Radioactive isotopes with half lives (t½) exceeding ~500 Ma. They include: 238U (t½ = 4.47 Ga), 232Th (t½ = 14 ... Read More

Long-Period Comet

Comets with orbital periods longer than 200 years, often with highly elliptical orbits highly inclined to the ecliptic. These comets ... Read More


Hexagonal polymorph of carbon (C) that forms from meteoric graphite during impact. The immense heat and stress of the impact ... Read More


Basic property used to characterize stars, luminosity is defined as the total energy radiated by a star each second. An ... Read More

Lunar Highlands

The highly cratered, topographically high, ancient crust of the Moon. This region is composed mostly of anorthosite, a plagioclase-rich rock ... Read More

Lunar Meteorites

Achondrite meteorites from the surface of the Moon. Most were found in the hot deserts of northern Africa and Oman ... Read More

Lunar Phase

Amount of illumination by sunlight of a planetary body as observed from Earth. The Moon goes through a cycle of ... Read More


One of the two broad categories of silicate minerals, the other being felsic, based on its magnesium (Mg) and/or iron ... Read More


Molten silicate (rock) beneath the surface of a planetary body or moon. When it reaches the surface, magma is called ... Read More

Magma Ocean

Completely molten surfaces of terrestrial planets or moons that formed soon after accretion. Samples returned by the Apollo missions provide ... Read More


Fe oxide, Fe2+Fe3+2O4, containing oxidized iron (Fe3+) found in the matrix of carbonaceous chondrites and as diagnostic component in CK ... Read More


The study of the interaction between a magnetic field and plasma treated as a continuous medium. Most of the universe ... Read More


Region around an astronomical object in which phenomena are dominated or organized by its magnetic field. A schematic diagram of ... Read More

Main Component Element

Elements that are found in early forming solar-system minerals ... Read More

Main Mass

Largest fragment of a meteorite, typically at the time of recovery. Meteorites are commonly cut, sliced or sometimes broken thus ... Read More

Major Axis

Longest diameter of an ellipse; the line from one side of an ellipse to the other that passes through the ... Read More


Main silicate-rich zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume ... Read More

Mantle Phase Changes

Solid-state mineralogical changes that occur with increasing depth in a planet’s mantle. These are best understood for Earth’s mantle. The ... Read More


Broad low plains surrounded by basin-forming mountains, originally thought to be a sea (pl. maria). This term is applied to ... Read More

Martian Meteorite

Over 30 of the meteorites found on Earth almost certainly came from Mars (see http://www.imca.cc/mars/martian-meteorites.htm and http://www2.jpl.nasa.gov/snc/). All but one ... Read More


Natural glass composed of isotropic plagioclase produced during shock metamorphism (not melting) at pressures of ~30 GPa. Maskelynite is commonly ... Read More

Mass Fractionation

Fractionation of isotopes or elements that is dependent on their masses ... Read More

Mass Number

Sum of the number of neutrons and protons in a nucleus. Not to be confused with "atomic mass" ... Read More

Mass Spectrometer

Instrument used to measure the masses of molecules and atoms by volatilizing and then ionizing them. The ions are then ... Read More

Mass-to-Charge Ratio

Unit used by mass spectrometers to separate ions. The mass-to-charge ratio is simply a ratio of the mass to the ... Read More


Fine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents ... Read More

Maxwell Distribution

Maxwell (or Maxwell-Boltzmann) distribution gives the distribution of speeds of molecules in thermal equilibrium as given by statistical mechanics. Image ... Read More

Mean Free Path

Average distance traveled by a particle or photon before a collision occurs ... Read More

Megaparsec (mpc)

Distance measurement equal to one million parsecs or 3.26 million light years ... Read More


Group of minerals found in the CAIs of meteorites such as CV chondrites. Melilite consists almost exclusively of the binary ... Read More

Melting Point

Temperature at which a solid changes to a liquid. A completely pure crystalline substance has an exact temperature at which ... Read More


One of two main types of stony-iron meteorite, the other being pallasites. Mesosiderites are a mixture of approximately 50% basaltic, ... Read More


Last material to crystallize/solidify from a melt. Mesostasis can be found in both chondrules, in the matrix around chondrules, and ... Read More


Term used to describe a metamorphosed chondrite. Also referred to as a type 7 chondrite. Metachondrites are texturally evolved rocks ... Read More


Element that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud ... Read More


Rocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment ... Read More


How long Sonic booms Of the several 10s of tons of cosmic material entering Earth's atmosphere each day, only about ... Read More

Meteor Shower

The light phenomena produced by particles (meteoroids) traveling through a planet’s atmosphere and heating the surrounding air.   Meteor showers ... Read More

Meteor Stream

Relatively narrow band of meteoroids stretched out along the orbital path of a comet. It consists of dust released from ... Read More


Work in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that ... Read More

Meteorite Shower

The near simultaneous fall of numerous meteorites after the fragmentation of a single asteroid or large meteoroid in the atmosphere ... Read More


Science involved in the study of meteorites and related materials. Meteoritics are closely connected to cosmochemistry, mineralogy and geochemistry. A ... Read More


Small rocky or metallic object in orbit around the Sun (or another star) ... Read More


Meteorite so small that it falls to Earth essentially unchanged from how it existed in space. If a meteoroid entering ... Read More


Inorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical ... Read More


Tektite type whose name is derived from the Moldau (Vltava) river in the Czech Republic where the first pieces were ... Read More

Molecular Cloud

An interstellar gas cloud that is dense enough to allow the formation of molecules and comprised of a cold dense ... Read More

Molecular Exdended Lattice

Crystalline solids composed of individual molecules packed together in a regular repeating fashion. There are no bonds between the molecules, ... Read More

Molecular Spectra

Spectral lines resulting when molecules interact with electromagnetic radiation, typically at infrared wavelengths. Because molecules are usually fragile, molecular spectra ... Read More


Collection of atoms held together by chemical bonds into a discrete, finite structure. One way molecules are represented is by ... Read More

Monomict Breccia

Type of breccia whose clasts are composed of a single (mono-) rock type, possibly all from a single rock unit ... Read More


Large area of high relief; mountain (pl. montes) ... Read More

Murchison Meteorite

Carbonaceous Mighei-like chondrite which exploded into fragments over Murchison, Australia, located ~200 km north of Melbourne, on Sept. 28, 1969 ... Read More

Nakhla Meteorite

Achondrite that exploded over the Egyptian town of El-Nakhla, on June. 28, 1911. The Nakhla meteorite broke into ~40 fragments, ... Read More


One of the Martian SNC meteorites, nakhlites are basaltic cumulate clinopyroxenite rocks, and most all are comprised mainly of sub-calcic ... Read More

Near-Earth Asteroid

Asteroids with orbits that bring them within 1.3 AU (195 million km) of the Sun. NEAs are a dynamically young ... Read More

Near-earth Asteroid Rendezvous (near Shoemaker)

First mission to orbit an asteroid (see http://near.jhuapl.edu/). The spacecraft carried instruments, including a multi-Spectral Imager (MSI) used to determine ... Read More


An immense interstellar, diffuse cloud of gas and dust from which a central star and surrounding planets and planetesimals condense ... Read More

Neumann Lines

Fine twinning boundaries seen as parallel lines running along the plane of kamacite crystals of hexahedrite iron meteorites. They may ... Read More

Neutral Hydrogen

Electrically neutral hydrogen atom with one proton and one electron. It is commonly referred to as HI (pronounced H-one), and ... Read More


Charge-neutral hadron with a mass of 1.6748 x 10-27 kg, equivalent to 939.573 MeV, and an intrinsic angular momentum, or ... Read More

Neutron Star

Dense ball of neutrons that remains at the core of a star after a supernova explosion has destroyed the rest ... Read More


Principal constituent of the Earth’s atmosphere (78.08 vol. % at ground level). Nitrogen is the fifth most abundant element in ... Read More

Noble Gas

Element occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron ... Read More


Elements that do not exhibit the characteristics of metals. These elements differ markedly from metals in respect to electronegativity and ... Read More


Igneous rock composed of 90% plagioclase, 95% orthopyroxene (low-Ca pyroxene) and less than 10% olivine. Norite is most commonly found ... Read More

North American Tektite

A tektite recovered in North America including Texas (Bediasite) and Georgia (Georgiaite). There are reports of other tektites recovered in ... Read More


Star that, over a period of a few days, becomes 103 to 104 times brighter than it was previously. Novae ... Read More


Core of an atom, where nearly the entire mass and all positive charge is concentrated. It consists of protons and ... Read More


A nuclear species characterized by Z protons and N neutrons ... Read More


Most Common type of iron meteorite, composed mainly of taenite and kamacite and named for the octahedral (eight-sided) shape of ... Read More


Mn-Ca sulfide, (Mn,Ca)S, is a pale to dark brown accessory mineral found in minor amounts in highly reduced meteorites such ... Read More


Group of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in ... Read More


Obsolete name previously applied to H chondrites, because they are composed of olivine and bronzite (pyroxene with ~20 wt. % ... Read More


Obsolete name previously applied to L chondrites, because they are composed of olivine and hypersthene (pyroxene with 22-30 wt. % ... Read More

Oort Cloud

Vast spherical swarm ~1012-1013 long-period comets orbiting the Solar System with semimajor axes between 1,000 and 50,000 AU, typically with ... Read More


An object is at opposition when it is on the opposite side of the Earth from the Sun. The elongation ... Read More


The elliptical path of one body around another, typically the path of a small body around a much larger body ... Read More

Orbital Eccentricity

The deviation of an orbit from circularity. Circles have eccentricities of 0 ... Read More

Orbital Period

The length of time it takes an orbiting object to make one complete trip around its primary body. The Earth’s ... Read More

Orbital Resonance

Dynamic relationship between bodies in heliocentric orbits in which a small body has an orbital period that is a simple ... Read More

Ordinary Chondrite

Work in Progress Ordinary chondrites (OCs) are the largest meteorite clan, comprising approximately 87% of the global collection and 78% ... Read More


Pertaining to C-containing compounds. Organic compounds can be formed by both biological and non-biological (abiotic) processes ... Read More


A large carbonaceous Ivuna-like (CI1) chondrite that disintegrated and fell in fragments near the French town of Orgueil on May ... Read More

Oriented Meteorite

A meteorite develops orientation when it experiences a period of stable flight through Earth's atmosphere during which it is gradually ... Read More


Orthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These ... Read More


A rock composed primarily of orthopyroxene. Non-terrestrial orthopyoxenites include diogenites and a single martian meteorite, ALH 84001, that was found ... Read More


Mineral group also called "nesosilicate," containing isolated SiO4 tetrahedra that are bound by interstitial cations. Their structures are usually closely ... Read More

Oxidation & Reduction

Oxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical ... Read More


Element that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and ... Read More

Oxygen Fugacity

Used to express the idealized partial pressure of a gas, in this case oxygen, in a nonideal mixture. Oxygen fugacity ... Read More


An oxygen molecule (O3) composed of three oxygen atoms ... Read More


The discipline of inferring the Earth’s or any other differentiated parent body's ancient magnetic field and former continental positions through ... Read More


One of two main classes of stony-iron meteorite, the other being mesosiderites. Pallasites are igneous in nature and characterized by ... Read More


Small periodic shift of the apparent positions of nearby stars due to the changing position of the Earth as it ... Read More

Parent Body

The body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early ... Read More

Partial Melting

An igneous process whereby rocks melt and the resulting magma is comprised of the remaining partially melted rock (sometimes called ... Read More


The tendency of elements to prefer one mineral phase relative to another or to preferentially enter the solid or remain ... Read More


Irregular or complex crater with toothed edges ... Read More

Peak Ring

Central uplift characterized by a ring of peaks rather than a single peak. Peak rings are typical of larger terrestrial ... Read More


Fe-Ni sulfide, (Fe,Ni)9S8, that is often associated with troilite, and found in the matrix and chondrules of CO, CV, CK ... Read More


Term applied to A2+B4+O3 high-pressure minerals with a perovskite structure (general formula ABX3) where "A" is a metal that forms ... Read More

Petrologic Type

Measure of the degree of aqueous alteration (Types 1 and 2) and thermal metamorphism (Types 3-6) experienced by a chondritic ... Read More


Science dealing with the origin, history, occurrence, chemical composition, structure and classification of rocks ... Read More


Discrete bundle of light energy. Light of a given energy (frequency) cannot be broken up indefinitely. Rather for a given ... Read More


Point where a star's atmosphere appears to become completely opaque. Thus, the photosphere may be thought of as the imaginary ... Read More


Need info on phylo and METEORITE Class of hydroxyl-bearing silicate minerals with a sheet-like structure. They result from aqueous alteration ... Read More


Low-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, found as a major mineral in eucrites and shergottites. In order to be considered pigeonite, the clinopyroxene ... Read More


Also referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous ... Read More


The term "planet" originally comes from the Greek word for "wanderer" since these objects were seen to move in the ... Read More


Hypothetical solid celestial body that accumulated during the last stages of accretion. These bodies, from ~1-100 km in size, formed ... Read More


Fourth state of matter: a gas in which many or most of the atoms are ionized. In the plasma state ... Read More

Platinum Group Elements

Elements with geochemical properties similar to Pt (platinum) including Ru, Rh, Pd, Os, Ir, and sometimes Au. These occur in ... Read More


A fine-grained intergrowth of kamacite and taenite that fills in the wedges between wide kamacite and taenite bands in octahedrites ... Read More


Kuiper Belt object (KBO) that has an orbital periods very similar to that of Pluto. Found in the inner parts ... Read More


Geology: Igneous intrusive body that forms when magma is injected into host rocks and solidifies. Plutons occur in the crust ... Read More

Polycyclic Aromatic Hydrocarbon

Organic compounds containing rings of six carbon atoms attached to hydrogen ... Read More


Process of reacting molecules together to form three-dimensional networks or chains. Silicon tetrahedra are usually polymerized in silicate minerals and ... Read More

Polymict Breccia

General term for all breccias that are neither monomict nor dimict. Modified from image source: http://www.saharamet.com/meteorite/gallery/HED/index.html ... Read More


Situation in which a single chemical composition can exist with two or more different crystal structures. Transformations between crystal structures ... Read More


The volume percentage of a rock that consists of void space. Vesicular porosity is a type of porosity resulting from ... Read More


High-pressure form of MgSiO3 with a stacked SiO6-octahedral sheet structure that is a major component of the D" layer in Earth ... Read More


Parts per billion (109) ... Read More


Parts per million (106) ... Read More

Presolar Grains

Mineral grains that formed before our solar system. These tiny crystalline grains are typically found in the fine-grained matrix of ... Read More

Primitive Achondrite

Achondrite with an almost chondritic composition with age similar to the primordial chondrites. These should be better classified as "metachondrites" ... Read More

Primordial Elements

Elements and isotopes formed in the Big Bang including: hydrogen (1H), helium (3He, 4He), most deuterium (2H) and tritium (3H), ... Read More


Structure that occurs above the photosphere of the Sun. Prominences may reach high into the corona, often as graceful loops ... Read More

Protoplanetary Disk

Flattened and rotating disk of dense gas and dust/solids orbiting a young star from which planets can eventually form ... Read More


Star in the process of formation, which has not yet become hot enough in the core to initiate hydrogen burning ... Read More

Proximal Ejecta

Ejecta found up to 5 crater radii from the rim of the source crater. About 90% of all ejecta is ... Read More


The occurrence of a new mineral that has retained the shape of and replaced the original mineral. The term literally ... Read More


Rock formed by frictional melting of rocks during faulting and impact cratering as a result of high rates of deformation ... Read More


A class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to ... Read More


Single-chain silicates like pyroxene, but the tetrahedra composing chains are rotated and twisted. Octahedrally coordinated cations occur between chains as ... Read More


Iron sulfide group of minerals whose composition ranges widely between its end members pyrrhotite (Fe7S8) whose crystal structure is monoclinic, ... Read More

Q Gases

Major part of 'planetary' Ar, Kr, and Xe. These components (labeled Ar-Q, Kr-Q, and Xe-Q) usually dominate Ar, Kr, and ... Read More


Composed of SiO2, quartz is one of the silica group minerals most common in Earth's crust, but never found in ... Read More


Rapid (hence "r") absorption of neutrons by atoms when the neutron flux is very high (~1022 neutrons per cm2/s) and ... Read More


Point on the celestial sphere from which meteors of a given shower appear to radiate. The effect is due to ... Read More

Radiation Pressure

Minute pressure exerted by electromagnetic radiation (such as but not limited to light from the sun) on everything it encounters ... Read More

Radiative Zone

Portion of a star where the primary transport of energy is by photons (electromagnetic radiation) ... Read More

Radioactive Decay

Process in which an isotope's nucleus changes ('decays') to produce another isotope. The original atom is called the 'parent' and ... Read More

Radiometric Age

The age of an object determined by the proportions of its original radioactive elements at the time of object's formation ... Read More


Radioactive isotope - Atomic nuclide that decays radioactively ... Read More

Rare Earth Elements

Often abbreviated as “REE”, these 16 elements include (preceded by their atomic numbers): 21 scandium (Sc), 39 Yttrium (Y) and ... Read More

Red Giant

Giant and highly luminous red star in the later stages of stellar evolution after it has left the main sequence ... Read More

Refractory Elements

Using research by Wood (2019), any of the elements with a relatively high condensation temperature of 1291 K < TC,50 ... Read More

Refractory Inclusion

Inclusions found predominantly in carbonaceous chondrites and are rich in refractory elements particularly calcium, aluminum and titanium that in various ... Read More


Depressions resembling (and often called) thumbprints produced on the surface of some meteorites, regardless of type, during their atmospheric transit ... Read More


Mixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an ... Read More

Relict Grain

Coarse-grained precursor material typically found in chondrules, that survived the melting event that formed the host chondrule. In other words, ... Read More

Remanent Magnetism

Also called residual magnetism, refers to the permanent magnetization preserved within the ferromagnetic minerals inside rocks, like meteorites. The presence ... Read More


Type of simple sugar, or carbohydrate, with molecular formula C5H10O5. Ribose (specifically d-ribose) is a crucial component of ribonucleic acid ... Read More


Long narrow depression on the surface of the Moon; also called "sinuous rilles". Lunar rilles usually flow away from small ... Read More


High-pressure olivine polymorph with a spinel structure that is found in highly shocked meteorites (above ~50 GPa, shock level > ... Read More


Rapid proton capture (hence "rp") is a process that synthesizes elements by successive proton absorption and β+ decay; thus, it ... Read More

Rubble Pile Asteroid

A type of asteroid held together by friction and mutual gravitation consisting of loose jumbled rock fragments from potentially different ... Read More


Member of a rare group of chondrites, formerly named the Carlisle Lakes group, after a meteorite found in Australia in ... Read More


Term applied to scarps on planetary surfaces. Many scarps are thought to be the surface expression of faults within the ... Read More


Slow neutron capture by nuclei in massive stars. In the s-process, one starts with existing iron-group nuclei. Therefore, it would ... Read More


Body in orbit (such as a moon) around another larger body (such as a planet or star) ... Read More

Scattered Objects

Kuiper Belt Objects (KBOs) with large orbital eccentricities (perihelia between 30 and 48 AU, aphelia >60 AU). These objects were ... Read More


Ni-Fe phosphide mineral, (Fe,Ni)3P, yellowish in color and predominantly found in iron and stony-iron meteorites. Schreibersite can also be found ... Read More

Schwarzchild Radius

Radius of the event horizon surrounding a non-rotating black hole. Its size is given by: For a one solar mass ... Read More

Secondary Mineral

Mineral that forms through processes such as weathering, and in the case of meteorites can also include pre-terrestrial alteration. Secondary ... Read More


Name used for a large group of phyllosilicate minerals with the generalized formula X2-3 Y2 O5 (OH)4. Due to their ... Read More

Shatter Cone

Conical fragment of rock with regular thin grooves (striae) that radiate from the apex of the cone. Shatter cones range ... Read More

Shell Burning

Nuclear "burning" regions that occur in shells surrounding a star's core. For example, helium burning might take place in the ... Read More


Igneous stony meteorite with a Martian origin consisting mainly of plagioclase (or a shocked glass of plagioclase composition) and pyroxene ... Read More

Shield Volcano

Volcanic construction of a central type formed by repeated effusion of fluid (usually basaltic) lava. The shield is characterized by ... Read More

Shock Melting

Complete melting of target material produced by the shock of a meteoric impact. Different minerals will experience certain shock effects ... Read More

Shock Metamorphism

Metamorphism produced by hypervelocity impact between objects of substantial size moving at cosmic velocity (at least several kilometers per second) ... Read More

Shock Stage

A petrographic assessment, using features observed in minerals grains, of the degree to which a meteorite has undergone shock metamorphism ... Read More

Shock Wave

Abrupt perturbation in the temperature, pressure and density of a solid, liquid or gas, that propagates faster than the speed ... Read More

Short-lived Radionuclides (slrs)

the absolute age of which must be determined using long-lived radionuclide dating. Image source: http://www.psrd.hawaii.edu/Sept02/Al26clock.html ... Read More

Sidereal Time

Time measured in relation to the fixed stars: the length of a sidereal day is 23 hr, 56 min, 4.09 ... Read More


An obsolete term for an iron meteorite ... Read More


An obsolete term for a stony-iron meteorite ... Read More

Siderophile Element

Literally, "iron-loving" element that tends to be concentrated in Fe-Ni metal rather than in silicate; these are Fe, Co, Ni, ... Read More

Siegbahn Notation

Nomenclature for characteristic x-rays introduced by Manne Siegbahn in 1923. This system is based on relative intensity of lines from ... Read More


Silicon dioxide, SiO2 ... Read More

Silica Group

Mminerals formed exclusively from silica. There are ten known silica polymorphs, two of which are synthetic. Five of the naturall ... Read More


The most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, ... Read More

Silicon Carbide

Presolar interstellar dust grain found in CM and E chondrites; its formula is SiC ... Read More


Unconsolidated sediment consisting of small mineral particles intermediate in size between sand and clay ... Read More

Simple Crater

Smallest impact features, consisting of a bowl-shaped interior with smooth walls, an elevated rim, and a shallow inclination hummocky exterior ... Read More

Simple Cubic Packing

Way in which atoms (considered as hard spheres) pack together to fill space. In simple cubic packing, all layers are ... Read More

Simple Substitution

Substitution in which the cations or anions replacing one another have the same charge and similar radii (within ~15%). Simple ... Read More


Regions of space where the density of matter, or the curvature of space-time, becomes infinite and the concepts of space ... Read More

Small Magellanic Cloud

Smaller of the two irregular galaxies that make up the Magellanic Clouds. These two galaxies orbit the Milky Way once ... Read More

Snell’s Law

For a wavefront traveling through a boundary between two media, the first with a refractive index of n1, and the ... Read More

Solar Abundances

Amount of elements in the Sun as determined by spectral line intensities. Approximately 60 elements have been identified; the most ... Read More

Solar Apex

Imaginary point in the constellation Hercules, near the bright star Vega, towards which the Sun is moving. The Sun's velocity ... Read More

Solar Cosmic Rays

Cosmic rays with energies ~107 to 1010 eV, primarily ejected solar flares and coronal mass ejections (CME). Their composition is ... Read More

Solar Energetic Particle (sep) Noble Gases

Gases more deeply implanted than solar wind noble gases; probably of solar origin, but particles of interstellar origin may contribute ... Read More

Solar Flare

Sudden eruptions from the surface of the Sun. Flares typically last a few minutes and can release energies equivalent to ... Read More

Solar Luminosity (lsun)

Solar luminosity is 4 × 1033 ergs per second ... Read More

Solar Mass (msun)

Mass of the Sun, Msun; equal to 1.99 × 1030 kg ... Read More

Solar Nebula

The primitive gas and dust cloud around the Sun from which planetary materials formed ... Read More

Solar System

The Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids ... Read More

Solar Wind

Supersonic flow of high-speed charged particles continuously blowing off a star (mostly e- and p+). When originating from stars other ... Read More

Solar Wind (sw) Noble Gases

Noble gases implanted by the solar wind into mineral grain surfaces that have been part of an asteroidal regolith; present ... Read More

Solid Angle

Fraction of the surface of a sphere covered by a particular object, as seen by an observer at the sphere's ... Read More

Solid Solution

Compositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an ... Read More


Position of the sun when farthest north (summer solstice) or farthest south (winter solstice). The solstices occur because the rotation ... Read More

Space Group

Group or array of operations consistent with an infinitely extended regularly repeating pattern. It is the symmetry of a 3D ... Read More


The formation of new nuclides by interactions of high-energy cosmic ray protons with target nuclei that commonly produce several smaller ... Read More

Specific Gravity

Ratio of the density of a substance to the density of a standard substance. The standard is usually liquid water ... Read More

Spectral Class

Classification scheme, also called the "Harvard Spectral Sequence," based on the strength of stellar spectral lines, which indicate a star's ... Read More


Technique of splitting electromagnetic radiation (light) into its constituent wavelengths (a spectrum), in much the same way as a prism ... Read More

Speed Of Light

Speed at which electromagnetic radiation propagates in a vacuum. Although referred to as the speed of light, this should be ... Read More


Spikes of gas that rise through the chromosphere (right). Spicules are rising jets of gas that move upward at ~30 ... Read More

Spin-flip Transition

Origin of the 21-cm emission line that originates with a neutral 1H atom. The proton and the electron each have ... Read More

Spin-orbit Resonance

State that a body is said to be in if its rotation period and its orbital period are related in ... Read More


Mg-Al oxide, MgAl2O4, found in CAIs ... Read More

Spiral Arm

Characteristic feature of all spiral galaxies, a concentration of stars sweeping out from either the central bulge or the ends ... Read More

Spiral Galaxy

Galaxy with spiral arms. These are classified based on their appearance in optical light, into those in which the arms ... Read More

Splash-form Tektite

Tektites that have the shape they acquired as spinning molten objects.   These include dumbbells, spheroids, teardrops, lenses and buttons ... Read More

Standard Candle

Astronomical object with a known absolute magnitude. They are extremely important to astronomers since by measuring the apparent magnitude of ... Read More

Standard Pressure And Temperature (stp)

Standard conditions of 273.15 K and 1 atm (0°C at sea level) used as a baseline for calculations involving quantities ... Read More


Self-luminous object held together by its own self-gravity. Often refers to those objects which generate energy from nuclear reactions occurring ... Read More

Starburst Galaxy

Galaxies observed to be forming stars at an unusually fast rate (about 103 times greater than in a normal galaxy) ... Read More

Stardust Mission

Space mission to study Comet Wild 2 (see http://stardust.jpl.nasa.gov/home/index.html). During the encounter, Stardust performed a variety of tasks including making ... Read More

Stefan-boltzmann Law

Law of blackbody radiation that states that the amount of energy given off by a blackbody per second per unit ... Read More

Stellar Association

Loose group of stars moving in the same direction but not gravitationally bound to each other. The most highly visible ... Read More

Stellar Black Hole

One of the possible evolutionary endpoints of high mass stars. Stellar black holes have masses <100 Msun. Once the core ... Read More

Stellar Evolution

Changes in a stars luminosity and temperature over its lifetime; conventionally, plotted on an Hertzsprung-Russell (HR) diagram. All stars, irrespective ... Read More

Stellar Halo

Essentially spherical population of stars and globular clusters thought to surround most disk galaxies and the cD class of elliptical ... Read More

Stellar Jets

Linear streams of matter arising from a number of different sources (T Tauri stars, planetary nebulae, and compact objects). Stellar ... Read More

Stellar Wind

Fast continuous outflow of material (p+, e–, and atoms of heavier metals) ejected from stars. Stellar winds are characterized by ... Read More

Steradian (sr)

Unit used to express the dimensionless quantity of solid angle. A sphere subtends a solid angle of 4p (»12.5663706) for ... Read More


Dense, high-pressure phase of quartz; so far identified only in shock-metamorphosed, quartz-bearing rocks from meteorite impact craters. Stishovite was synthesized ... Read More


Quantitative relationships (proportions) among elements in compounds ... Read More


Pertaining to or involving substances that are in the exact proportions required for a given reaction ... Read More

Stony Meteorite

Meteorite composed of silicate minerals, but that may have up to 25% Ni-Fe metal by weight. Stony meteorites are extremely ... Read More

Stony-Iron Meteorite

Meteorite composed of roughly equal amounts by weight of silicate minerals and Ni-Fe metal. The stony-irons consist of two groups: ... Read More

Strömgren Sphere

Spherical region of ionized hydrogen (HII) around a single or tight cluser of young stars of spectral classes O or ... Read More


Originally horizontal layers of rock ... Read More

Strewn Field

Area on the surface containing meteorites and fragments from a single fall. Also applied to the area covered by tektites, ... Read More

Strong Lensing

Effect due to gravitational bending of light by mass concentrations which results in (1) the strong distortion of background galaxy ... Read More

Strong Nuclear Force

Fundamental force binding nucleons within an atomic nucleus and preventing like-charged protons from flying apart. Particles that are acted on ... Read More

Structure Formation

Development of organized structures in the universe, ranging from galaxies to clusters to superclusters, and possibly beyond to huge filaments ... Read More


Process in which a material changes from a frozen solid to a gas without passing through the liquid state. Whether ... Read More


Replacement of one ion or ionic group for another in the same structural site in a mineral yielding a solid ... Read More


Terrestrial breccia composed of angular fragments of different rock types and glass inclusions. Glass can make up more than half ... Read More


Our parent star. The structure of Sun's interior is the result of the hydrostatic equilibrium between gravity and the pressure ... Read More


Regions on the Sun’s surface that appear dark because they are cooler than the surrounding photosphere, typically by ~1500-1800 K ... Read More

Sunspot Cycle

Eleven-year periodicity in the number of sunspots observed on the Sun. Sunspot maxima are associated with times of high solar ... Read More


Cluster of galactic clusters ... Read More


Phenomena by which, at sufficiently low temperatures, a conductor can conduct charge with zero resistance. The current theory for explaining ... Read More


Phenomena by which, at sufficiently low temperatures, a fluid can flow with zero viscosity. Its causes are associated with superconductivity ... Read More

Supermassive Black Hole

Black holes that contain between a million and a billion times more mass than a typical stellar black hole. There ... Read More


Stellar explosion that expels much or all of the stellar material with great force, driving a blast wave into the ... Read More

Supernova Remnant (snr)

Diffuse, Expanding Nebula That Results From A Supernova Explosion. A Snr Consists Of Material Ejected In The Supernova Explosion And ... Read More


Property of an object if some spatial manipulation of it results in an indistinguishable object. A symmetric object can be ... Read More

Symmetry Axis

Element of rotational symmetry. Imaginary axis is placed through a perfect crystal so that during a single rotation about this ... Read More

Symmetry Element

A geometrical entity such as a point, line, or plane about which a symmetry operation is performed ... Read More

Symmetry Operators

Four operations which lead to superimposition of an object on itself. These are rotation, translation, reflection and inversion. The symmetry ... Read More

Synchronous Orbit

State of an object when its period of rotation is exactly equal to its average orbital period. The Moon is ... Read More

Synchrotron Emission

Type of non-thermal radiation generated by charged particles (usually electrons) spiraling around magnetic field lines at close to the speed ... Read More


Definable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy ... Read More

T Tauri Star

Protostar in the late stages of formation, often exhibiting both periodic and random fluctuations in brightness. T Tauri stars are ... Read More


Speculative particle presumed to travel faster than light. According to Einstein's equations of special relativity, a particle with an imaginary ... Read More


Less common than kamacite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Taenite, γ-(Fe,Ni), has 27-65 wt% ... Read More


A 1:2 Mg phyllosilicate, Mg3Si4O10(OH)2, best known for its extreme lark of hardness; crystals are rarely seen. The is little ... Read More


Natural, silica-rich glasses produced by melting of target rocks and dispersal as droplets during terrestrial impact events. In contrast to ... Read More


Dividing line between the illuminated and shadowed portions of the lunar or planetary disk ... Read More

Terrestrial Planets

Rocky planets: Mercury, Venus, Earth, and Mars. These planets have physical characteristics, chemical composition and internal structure similar to the ... Read More


Elevated area of complex ridged terrain found on Venus, crossed by lineations in two or more directions, having the appearance ... Read More

Thermal Doppler Broadening

Spreading of a spectral line due to the temperature of the emitting medium. In a gas, the individual atoms, elements ... Read More

Thermal Radiation

Emitted radiation, the characteristics of which depend on the temperature of the source. Blackbody radiation emitted by stars and heated ... Read More

Thermodynamic Phase

Part of a thermodynamic system having uniform properties (e.g., density, crystal structure, index of refraction). The most familiar examples of ... Read More

Thermodynamically Favored

Substance that of the possible products for a reaction is energetically the most stable. Sometimes, the thermodynamically favored product is ... Read More

Thermoluminescence (TL)

Emission of light caused by the heating of certain minerals ... Read More

Thermoremanent Magnetization

Permanent magnetization acquired by igneous rocks in the presence of a magnetic field as the rocks cool through the Curie ... Read More

Thick Disk

Features believed to exist in some, but not all disk galaxies; it clear whether the thick disk of the Milky ... Read More

Thin Disk

The defining morphological component of disk galaxies in general and spiral galaxies in particular. The thin disk contains stars, star ... Read More

Thin Section

Thin slice or rock, usually 30 µm thick. Thin sections are used to study rocks with a petrographic microscope ... Read More


One of the five nitrogen-containing bases occurring in nucleotides ... Read More

Tidal Heating

Frictional heating within a body caused by tidal stresses induced by differences in gravitational pull in different regions of the ... Read More

Tidal Tail

Long tails of material that are the result of gravitational interactions between galaxies. During the interaction, gas and stars are ... Read More

Tidal(tide-generating) Force

Differential gravitational force arising because the gravitational force exerted on one body by a second body is not constant across ... Read More


An unconsolidated, poorly sorted sediment deposited by glaciers.   Till contains rock and mineral fragments of all sizes from clay-size ... Read More

Top-down Structure Formation

Formation of large structures, such as galaxy superclusters or perhaps even the vast filaments and voids, prior to the formation ... Read More

Trans-neptunian Object (tno)

Any object with a mean orbital diameter larger than that of Neptune. Pluto and its moon Charon are both TNOs, ... Read More


When a small celestial body moves in front of a much larger one (as when Mercury or Venus appears in ... Read More

Transition Metals

Elements whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell ... Read More

Transition State

Maximas (peaks) on an energy diagram. A transition state represents the highest energy arrangement of atoms between two more stable ... Read More


Symmetry operation of shifting an object a given distance in a given direction. Translation may occur in 3 dimensions along ... Read More

Transport Phenomenon

Various mechanisms by which particles or quantities move from one place to another. Three common examples of transport phenomena are ... Read More


Silica group mineral in which the tetrahedra occur in sheets. Tetrahedra alternately point up or down to share oxygen with ... Read More

Triple-alpha Process

Production of 12C by fusion of 3 4He nuclei (α particles). Helium produced in hydrogen burning cannot undergo fusion reactions ... Read More


Troctolite is an intrusive igneous rock consisting of plagioclase feldspar and olivine. It is a member of gabbroic rocks family ... Read More


Brass colored non-magnetic Fe sulfide, FeS, found in a variety of meteorites ... Read More

Trojan Asteriod

Asteroid (or satellite) that orbits a body at one of the Lagrange points 60º ahead (L4) or behind (L5) a ... Read More


Lowermost region of a planetary atmosphere where convection keeps the gas mixed and maintains a steady decrease of temperature with ... Read More

Tully-fisher Relationship

Rotational velocity (vrotation) of a spiral galaxy as an indicator of its luminosity. The relation is approximately: ... Read More

Turn-off Point

Point on the main sequence where stars in a cluster have just started to move off toward the red giant ... Read More

Twenty-one Centimeter Emission

Radio wavelength emission that originates with a neutral 1H atom. The proton and the electron each have a quantum "spin," ... Read More

Type-3 Chondrite

An unequilibrated chondrite with a relatively unrecrystallized texture and containing mineral grains with heterogeneous compositions ... Read More

Type-4-6 Chondrite

Increasingly equilibrated, thermally metamorphosed chondrites ... Read More

U Geminorum Stars

Type of variable star displaying sudden and unpredictable increases in brightness, sometimes referred to as dwarf novae. Brightness may increase ... Read More

Ultra High Energy Cosmic Rays (uhecr)

Extremely energetic subatomic particles (mostly protons, but also some heavier atomic nuclei) with energies >109 MeV. The record holder so ... Read More


Term used for silicate minerals with cations predominantly Mg and/or Fe. Mafic minerals are dominated by plagioclase and pyroxene, and also contain smaller ... Read More

Uncertainty Principle

Principle central to quantum mechanics, which states that two complementary parameters (such as position and momentum, energy and time, or ... Read More

Unclassified Meteorite

Unclassified meteorites are meteorites that have not been analyzed by a meteoriticist, approved by the Nomenclature Committee (NomComm), published in ... Read More

Unequilibrated Chondrite

A chondrite with heterogeneous mineral compositions (e.g., olivine grains with differing FeO/(FeO+MgO) ratios ... Read More


Modifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group ... Read More

Unit Cell

Smallest repeating unit of a crystalline solid that can be used to describe the entire structure. Unit cells are like ... Read More

Universal Gravitational Constant (g)

Constant with value of 6.670 × 10-11 N m2/kg2 ... Read More


That which contains and subsumes all the laws of nature, and everything subject to those laws; the sum of all ... Read More


One of the five nitrogen-containing bases occurring in nucleotides ... Read More

Ureilite (ure)

perhaps the freshly fallen meteorite smelled good, or perhaps because it was shaped like a loaf of bread, which some ... Read More


Unoccupied sites within a crystal that are usually occupied by an atom. A vacancy may move when a neighboring atom ... Read More


Space entirely devoid of matter (called also, by way of distinction, absolute vacuum). In a more general sense, a space, ... Read More

Vacuum Enery Density

Amount of energy per unit volume associated with empty space itself ... Read More


Charge on an ion reflecting gained or lost electrons. For example, an iron atom that has lost two electron, Fe2+, ... Read More

Valence Electron

Electron in the valence shell of an atom, or the valence band of a solid. Valence electrons are more active ... Read More

Valence Shell

Outermost shell of electrons in an atom in its ground state ... Read More

Van Der Walls Bonds

van Allen Belts At least two doughnut-shaped regions of magnetically trapped charged particles high above Earth's atmosphere. Image source: http://commons.wikimedia.org/wiki/File:Van_Allen_radiation_belt.png ... Read More

Variable Star

Star whose luminosity varies over time. Broadly speaking, variable stars are of two types: (1) stars that are intrinsically variable, ... Read More

Velocity Dispersion

Random motions (orbits) of stars which support a self-gravitating body against collapse. Motions can be either ordered or random. The ... Read More

Vertical Circle

Arc of a great circle drawn from the zenith through a star and perpendicular to the horizon ... Read More


Vesicles appear in nature when they are produced within lava (extrusive aphanitic igneous rock) whose dissolved gases come out of ... Read More


Third largest and fourth brightest asteroid; it was discovered in 1807 by Heinrich Olbers and named for the ancient Roman ... Read More

Viral Theorem

Theorem that relates the total kinetic energy of a self-gravitating body due to the motions of its constituent parts (T) ... Read More

Virgo Cluster

Largest and nearest galaxy cluster to the Local Group. The Local Group is often considered to lie on the edge ... Read More


The degree to which a liquid resists flow.   High-viscosity liquids (e.g., "molasses in winter") resist flow to a great ... Read More


Huge region of space that is unusually empty of galaxies. Voids are not entirely empty, but are underdense and contain ... Read More


Substances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.) ... Read More

Volatile Elements

Chemical elements that condense (or volatilize) at relatively low temperatures. The opposite of volatile is refractory. Volatile elements can be ... Read More


Conversion of liquids into gases, usually by heating them ... Read More


Igneous rock that forms from cooling magma on the surface of a planet or asteroid ... Read More

Voltage (v)

Change in energy per unit of electric charge, measured in the SI unit of volt (V). For example, a 1.0 ... Read More

W Virginis Star

Another name for Type II Cepheid variable stars ... Read More


High pressure polymorph of olivine, β-Mg2SiO4, found on Earth and in some meteorites. It is thought to make up 50% ... Read More


Decrease of the illuminated portion of a object as observed from Earth. The waning moon refers to that portion of ... Read More


Propagating disturbance which transmits energy from one point to another without physically transporting the oscillating quantity. A wave is characterized ... Read More

Wave-particle Duality

Principle of quantum mechanics implying that light (and, indeed, all other subatomic particles) sometimes acts like a wave, and sometime ... Read More


Distance from one peak of a wave to the next. Wavelength is measured in units of distance. The wavelengths of ... Read More


Increase of the illuminated portion of a object as observed from Earth. The waxing moon refers to that portion of ... Read More

Weak Lensing

Small distortion and amplification of background galaxy images by foreground mass concentrations. In the weak lensing regime, distortion of an ... Read More

Weak Nuclear Force

Fundamental force which accounts for some particle interactions. The weak nuclear force causes radioactive decay, and, in particular, governs β ... Read More

Weakly Interacting Massive Particles (wimps)

Broad class of particles that were once in thermal equilibrium with the early universe but were "cold," i.e., moving non-relativistically ... Read More

Weird Terrain

Region on the surface of Mercury of oddly rippled features. This feature is probably the result of a strong impact ... Read More

White Dwarf

Remnant of a star with mass <8 Msun. White dwarfs have masses <1.4 Msun (the Chandrasekhar mass) and are supported ... Read More

Widmanstätten Pattern

Characteristic cross-hatched pattern visible on the surface of octahedrites, pallasites and even large metal blebs in chondrite melts and mesosiderite ... Read More

Wien’s Displacement Law

Relationship stating that wavelength at which a blackbody emits its maximum quantity of radiation is inversely proportional to its absolute ... Read More

Wilkinson Microwave Anisotropy Probe (wmap)

NASA Explorer mission launched in June 2001 to map the temperature fluctuations of the CMB radiation with much higher resolution, ... Read More


a partially differentiated asteroid that was disrupted just as it began to form an Fe core and a silicate-rich crust ... Read More

Wolf-Rayet Star

Massive stars at an advanced stage of stellar evolution, losing mass at a very high rate (right). They have masses ... Read More

X-ray Binary

Close binary system where a neutron star (or rarely a black hole) accretes matter from what is usually a main ... Read More

X-ray Bursts

Bursts of X-ray energy that occur in low-mass X-ray binary systems in which a neutron star and low-mass main sequence ... Read More

X-ray Diffraction

Analytical technique used to determine the structures of crystalline solids. A monochromatic beam of X-rays (usually Cu-Kα) is diffracted off ... Read More

X-ray Halo

Halo of high temperatures X-ray emitting gas (106 to 10 x 106 K) surrounding most large elliptical galaxies. Although found ... Read More

X-ray Pulsar

Neutron star with a powerful magnetic field in an X-ray binary. Gas accreted from the companion star is channeled to ... Read More


High-energy electromagnetic radiation, with short wavelength (~10-0.01 nm) and high frequency (greater than ~1016 Hertz). Although the boundaries are somewhat ... Read More


In the X-wind model the magnetic field of a young star interacts with the magnetic field of the circumstellar disk ... Read More


Fragment of a rock or meteorite that formed apart from the host material ... Read More

Zeeman Effect

Splitting of atomic energy levels into a larger number of levels when magnetic fields are present; the resulting spectral lines ... Read More


Point directly overhead, as determined by the indefinite upward extension of a plumb line ... Read More

Zero-age Main Sequence (zams)

Region on the Hertzsprung-Russell diagram, as predicted by theoretical models, where stars are located at the onset of nuclear hydrogen ... Read More


Orthosilicate mineral, Zr(SiO4), observed in all terrestrial rocks type and in ordinary chondrites, eucrites, mesosiderites, and lunar rocks ... Read More


Twelve constellations along the ecliptic through which the sun, the moon and the planets move: Ares (Ram), Taurus (Bull), Gemini ... Read More

Zodiacal Light

Light resulting from scattering of sunlight off small (1-300 µm) interplanetary dust particles lying in the plane of the ecliptic ... Read More

Zone Of Avoidance (zoa)

Region of sky that is obscured at optical wavelengths by our own galaxy, the Milky Way. It covers roughly 20% ... Read More

Zz Ceti Stars

Class of variable star characterized by modest luminosity variations (from 0.001 to 0.2 magnitudes or 0.1% to 20% of their ... Read More