Glossary of Cosmochemistry

accretion: The build-up of planets and planetesimals by the low-relative-velocity collisions of smaller bodies in the solar nebula.

achondrite: Igneous stony meteorite; because of high-temperature processing, they lack chondrules.

agglomeration: Early accretion of chondritic constituents (chondrules, refractory inclusions, silicate matrix material) in the solar nebula to form small aggregations of chondritic matter.

albedo: The fraction of incident light reflected from a planetary surface.

alkali element: The elements from group 1 on the periodic table, i.e., lithium, sodium, potassium, rubidium and cesium.

amino acid: A small molecule containing –NH2 at one end and –COOH at the other that can link up with other amino acids to form a protein.

angular momentum: The tendency of spinning or orbiting bodies to continue their motion due to inertia.

aphelion: The point in a body’s orbit that is farthest from the Sun.

asteroid: A rocky, metallic or icy body ranging in diameter from about 10 meters to 1000 km that is in heliocentric orbit.

Asteroid Belt: The region between 2.1 and 3.3 AU from the Sun located between Mars and Jupiter that contains the vast majority of asteroids.

asteroid family: A group of asteroids with similar orbital parameters formed by the collisional disruption of a single larger asteroid.

astronomical unit (au): The mean distance between the Earth and the Sun, about 150 million kilometers.

atomic number: A number equivalent to the number of protons in the nucleus of an atom.

AU see astronomical unit.

aurora: High-altitude, multi-colored, variable luminosity visible from the surface at night most often at high latitudes on Earth caused by the acceleration of charged particles (typically of solar origin) in the Earth’s magnetic field.

au (sometimes AU): see astronomical unit.

Australasian tektite field:   A region extending from Hainan Island China to Tasmania Australia that was populated by tektites 0.77 Ma ago.

baddeleyite: The mineral zirconium oxide (ZrO2), often formed as a shock-induced breakdown product of zircon.

basalt: A dark, fine-grained igneous rock consisting mainly of the minerals plagioclase and pyroxene.

Bode’s Law: An empirical mathematical relationship approximating the heliocentric distances of the planets Mercury through Uranus.

body-centered cubic: A crystal structure that has one atom at each corner of the unit cell and one atom in the center of the cell.

bolide: A meteoric fireball that creates sonic booms and breaks apart in the atmosphere.

breccia: A rock that is a mechanical mixture of mineral and rock fragments; the proportions of these fragments and unbrecciated material can vary significantly.

Calcium-aluminum-rich inclusion (CAN): A chondritic inclusion rich in refractory elements, particularly calcium, aluminum and titanium. Also known as a refractory inclusion.

brecciation: The process by which a coherent rock is broken into fragments and recemented.

carbonaceous chondrite: A class of chondritic meteorites containing a lower fraction of chondrules and a higher abundance of refractory oxides than the ordinary and enstatite chondrites.  Some groups contain several percent organic matter including amino acids.  The CI carbonaceous chondrites has volatile abundances very similar to those in the Sun’s photosphere.

celestial mechanics:   The science of the motion of celestial bodies governed by gravitational forces.

chondrite: The most abundant class of stony meteorite.  Most groups contain chondrules and all are similar in bulk composition to the Sun’s photosphere.  They are among the first solids to have formed in the solar nebula.

chondrite group:   A collection of individual chondritic meteorites that have a narrow range in texture and bulk composition and are believed to have been derived from the same parent asteroid. 

chondrules: A typically submillimeter-size crystalline and/or glassy quasi-spheroidal or ellipsoidal inclusion found in chondritic meteorites.

clast: A mineral or rock fragment embedded in another rock.

comet: A sub-planet-size icy and dusty body that formed in the outer solar system and orbits the Sun, typically in highly elliptical or parabolic orbits.

condensate: In the solar nebula, a mineral phase that precipitates directly from a cooling vapor.

core:   The central region of a body.  In the Earth and differentiated asteroids, the cores are rich in metallic iron-nickel.

cosmic ray:   Highly energetic atomic nuclei in outer space that bombard the Earth from all directions.

cosmic-ray exposure (CRE) age:   The period of time in which an object existed as a meter-size body in interplanetary space and was thus subject to irradiation by cosmic rays.

cosmochemistry: The study of the chemistry of planetary materials.

crater: A circular depression in the ground formed by a volcanic explosion or meteorite impact.

Cretaceous-Tertiary boundary: The 65-million-year-old boundary between the Cretaceous Period and the succeeding Tertiary Period. The extinctions were caused by the impact of a large (>10-km diameter) comet or asteroid with the Earth.

crust: The outermost layer of the Earth. Continental crust consists predominantly of granite and related rocks; oceanic crust consists mostly of basalt.

crystal settling:   A process by which crystals forming in a magma sink toward the bottom of the magma chamber under the influence of gravity and are unavailable for further reaction with the liquid.

differentiation: A process in which a homogeneous chondritic body containing metal and silicate melts and forms distinct layers of different densities.  In most cases, a metallic core and a silicate-rich mantle and crust are formed.

ecliptic: The plane of the Earth’s orbit around the Sun.

Edgeworth-Kuiper belt: A group of comets residing mainly near the ecliptic at distances of 34-45 AU from the Sun.

ejecta:   Material ejected from a crater due either to volcanism or meteorite impact.

electron microprobe: Instrument in which a beam of electrons is focused on a sample to produce x-rays.  The amount and energy of the x-rays indicate the chemical composition of the sample.

ellipse: A plane curve drawn so that the sum of the distances from any point on the curve to two fixed points within the curve (the foci) is constant.  The shape resembles an oval and is the characteristic orbital path of planets and asteroids around the Sun.

equilibrated chondrite: A chondrite with minerals of uniform composition (e.g., all of the olivine grains have the same composition) due to diffusion during thermal metamorphism.  Such chondrites would be petrologic type 4 to 6.

escape velocity: The velocity that an object must have to escape the primary gravitational influence of a larger body.

exothermic reaction: A chemical reaction that liberates heat energy.

extinction:   The permanent disappearance of a species after all individuals have died off.

face-centered cubic: A crystal structure that has one atom at each corner of the unite cell and one atom in the center of each face.

fall: A meteorite that was seen to fall.

feldspar: A group of alumino-silicate minerals containing a solid solution of calcium, sodium and potassium.

find:   A recovered meteorite that was not seen to fall.

fireball:   A very bright meteor.

fossil meteorite:   The textural, mineralogical or compositional remnant within a sedimentary rock of a meteorite that fell millions of years ago.

fractional crystallization: A crystallization process in which minerals crystallizing from a magma are isolated from contact with the liquid.

fulgurite: A tubular, glassy object produced by melting materials at the Earth’s surface during a lightning strike.

gabbro: A coarse-grained igneous rock of basaltic composition that formed at depth and consists mainly of plagioclase and pyroxene.

galaxy: A huge, gravitationally bound aggregation of stars, dust and gas, typically with masses ranging from that of a few million to a few trillion times the mass of the Sun.

gamma ray: A photon of very high energy.

geocentric:   Centered around the Earth.

geomagnetism: The Earth’s magnetic field.

glacier: A large concentration of ice that does not completely melt in the summer and moves downhill under its own weight.

granite: An igneous rock consisting mainly of the minerals quartz and alkali feldspar.

greenhouse effect: A warming of the surface of a planet caused by the absorption in the atmosphere of infrared radiation emitted by the planet after receiving a dose of solar radiation.

half-life: The period of time it takes half of a given number of atoms of a radioactive element to decay.

heliocentric: Centered around the Sun.

Hertzsprung-Russell (H-R) diagram:   A diagram of stellar luminosity against temperature, color or spectral class that separates stars at different evolutionary stages.

highlands:   The highly cratered, topographically high, ancient crust of the Moon; it is made mostly of a plagioclase-rich rock called anorthosite.

hydrocarbon: An organic compound composed of hydrogen and carbon arranged in rings or chains.

ice age: A period of time characterized by extensive glaciation.

ice sheet: Large continental glaciers, typically several kilometers thick.

igneous rock: A rock formed by crystallization of a melt.

immiscible:   The property of liquids that are not mutually soluble such as oil and water or metallic and silicate melts.

impact basin: A large impact crater, typically many tens to hundreds of kilometers in diameter.

impact crater: A circular depression caused by compression and excavation of target material during the explosive impact of a projectile.

inertia: The tendency of an object to maintain its motion.

interplanetary dust particle (IDP): Small dust particle in interplanetary space, a micrometeoroid.

ion: A positively or negatively charged atom.

ion microprobe: An instrument in which a focused beam of ions ionizes atoms in a sample and ejects them for analysis with a mass spectrometer. 

iron meteorite: A meteorite consisting mainly of metallic iron-nickel minerals.

isotope: Two or more atoms of the same chemical element that differ in the number of neutrons in their nucleus.

Ivory Coast tektite:   A tektite recovered in or near the Ivory Coast in Africa.

kamacite: An iron-nickel mineral with less than about 6% nickel by weight that has a body-centered cubic structure.

Kirkwood gap:   Paucity of main-belt asteroids with periods that are simple fractions of Jupiter’s period due to resonance with Jupiter’s gravitational influence.

Kuiper belt: See Edgeworth-Kuiper belt.

lava: Molten rock derived from a volcano.

layered tektite: A relatively large tektite made of multiple layers of glass; also called Muong Nong tektite.

lechatelierite:  A form of silica glass.

Libyan Desert Glass: Massive, light-colored chunks of high-silica glass found in western Egypt near the Libyan border.  The glass is most likely an impact-melt product similar to layered tektites.

limestone: A sedimentary rock composed of calcium carbonate.

loess: An unconsolidated, clay-rich sediment deposited by the wind.

magma: Molten volcanic rock beneath a planet’s surface.

magnetite: A magnetic iron oxide mineral (Fe3O4).

main sequence stars: Stable stars that are converting hydrogen into helium in their cores.  They form a diagonal swath on the H-R diagram.

mantle: The main, silicate-rich, layer of the Earth between the crust and the core.

maria:   Dark lunar plains covered with flows of basalt.

mass spectrometer: An instrument that determines the chemical or isotopic composition of a substance by separating gaseous ions of the substance by their mass.

matrix: Fine-grained silicate-rich material in chondrites that surrounds chondrules, refractory inclusions and other constituents.

mesosiderite: A stony-iron meteorite consisting of approximately 50% metallic iron-nickel and sulfide and 50% basaltic, gabbroic and orthopyroxenitic silicates.

metamorphism: Change in the mineralogy and texture of a rock due to heat and pressure.

meteor: The light phenomenon produced by a solid object (a meteoroid) plunging through a planetary atmosphere and frictionally heating the surrounding air.  

meteorite: A solid natural object reaching a planet’s surface from interplanetary space.

meteorite shower:   The simultaneous fall of numerous individual meteorites after the breakup of a single projectile in the atmosphere due to frictional stresses encountered during atmospheric passage.

meteoritics: The science involved in the study of meteorites and related materials.

meteoroid: A small solid object (larger than a dust particle and smaller than an asteroid) in interplanetary space. A meteoroid is transformed into a meteorite by surviving passage through the Earth’s atmosphere.

meteor shower: The light phenomena produced by particles (meteoroids) traveling through a planet’s atmosphere and heating the surrounding air.  Meteor showers occur at particular times of year when the Earth intersects debris traveling along the orbit of a comet.

micrometeorite: A small meteorite, typically of millimeter size.

microtektite: A tiny tektite, particularly those recovered in deep-sea drill cores.

microwave:   Electromagnetic radiation with a wavelength in the approximate range of 1 millimeter to 1 meter.

moldavite: A tektite recovered within or near the Czech Republic.

molecular cloud:   An interstellar gas cloud that is dense enough to allow the formation of molecules.

nebula: An immense, diffuse cloud of gas and dust from which a central star and surrounding planets and planetesimals condense and accrete.

North American tektite: A tektite recovered in North America, generally in Texas or Georgia.

nuclear fusion: The process in which atomic nuclei join together to make more massive nuclei and simultaneously release energy.

nucleotide: A compound consisting of a sugar linked with a phosphate group and an amino acid. Not needed.

nucleus:   In Biology, an organelle containing chromosomes that is surrounded by a membrane within eukaryotic cells. Not needed. In Physics, the central part of an atom that contains the protons and neutrons.

nuclide: A nuclear species characterized by Z protons and N neutrons.

obsidian: Silica-rich, dark volcanic glass.

olivine: A mineral consisting of magnesium and iron silicate (Mg,Fe)2SiO4.

Oort Cloud: Spherical cloud containing icy planetesimals more than about 20,000 AU from the Sun; this region is believed to be the source of many comets entering the inner solar system.

orbit: The elliptical path of one body around another, typically the path of a small body around a larger body such as the Moon’s path around the Earth or the Earth’s path around the Sun.

orbital eccentricity: The deviation of an orbit from circularity; circles have eccentricities of 0.

orbital period:   The length of time it takes an orbiting object to make one complete trip around its primary body.  The Earth’s orbital period around the Sun is 1 year.

ordinary chondrite: The class of meteorites most common among witnessed falls.  There are three principal groups: H chondrites (high total iron), L chondrites (low total iron) and LL chondrites (low total iron, low metallic iron).

ozone: A triatomic oxygen molecule (O3).

paleomagnetism:   The discipline of inferring the Earth’s ancient magnetic field and former continental positions through study of remanent magnetization in old rocks.

pallasite: A stony-iron meteorite consisting mainly of metallic iron-nickel and relatively coarse crystals of olivine.

parent body: The body from which a meteorite or meteoroid was derived prior to ejection.

partial melting: The process whereby incompletely melted rocks form liquids differing in composition from the original composition of the rock.

partitioning: The tendency of elements to prefer one mineral phase relative to another or to preferentially enter the solid or remain in the liquid during crystallization.

perihelion: The point in a body’s orbit that is closest to the Sun.

permafrost: Permanently frozen mix of ice and soil found in cold regions.

petrology:   The study of the composition, structure and origin of rocks.

photosphere: Visible “surface” of the Sun.

plagioclase: A feldspar mineral containing a solid solution of calcium and sodium.

planet: A sub-stellar, moderate-size (more than about 2000 km in diameter) body in orbit around a central star.

planetesimal: Bodies ranging in size from a few meters to a few hundred kilometers that accreted early in solar-system history.

polycyclic aromatic hydrocarbon (PAH): Organic compounds containing rings of six carbon atoms attached to hydrogen.

porosity: The volume percentage of a rock that consists of void space.

precession: The periodic shift in the rotational axis of a spinning body due to external gravitational influences.

presolar grain: A mineral grain that formed before the solar system, fell into the early solar system and was incorporated into chondritic material.  

pseudomorph: The occurrence of a mineral that has replaced a pre-existing mineral and has retained the shape of the original phase.

pyroxene: A class of silicate minerals, most of which contain calcium and form a solid solution between iron and magnesium. 

radiation pressure: The pressure exerted on tiny particles in interplanetary space by solar radiation due to absorption of light by the particles.

radioactivity: A property of some isotopes that causes their nuclei to undergo spontaneous disintegration resulting in the emission of subatomic particles.

radiometric age: The age of an object determined by the proportions of its original radioactive elements and their decay products.

radionuclide: A radioactive isotope.

rare-earth elements: Elements with atomic numbers 57 (lanthanum) to 71 (lutetium).These elements show closely related geochemical behaviors.

rare-earth pattern: The abundances of rare-earth elements relative to those in chondritic meteorites.

red giant: An old, highly luminous red star with a relatively cool surface that has burned most of the hydrogen in its core.  Red giants lose parts of their atmospheres and thus provide new elements into interstellar space.

refractory element: An element that would condense at high temperatures from a gas; such elements include calcium, aluminum, titanium, iridium and osmium.

refractory inclusion: An inclusion rich in refractory elements, particularly calcium, aluminum and titanium in a chondrite.  These are commonly known as calcium-aluminum inclusions or CAIs.

regolith: Unconsolidated and fragmented debris composed of rocks and  minerals located at the surface of a body lacking an atmosphere.

remanent magnetism:   Permanent magnetization acquired by rocks from the Earth’s magnetic field.

orbital resonance:   A dynamical relationship among bodies in heliocentric orbits in which a small body has an orbital period that is a simple fraction of a nearby larger body; the periodic gravitational tug of the large body induces changes in the orbit of the smaller body.

rubble pile asteroid: An asteroid consisting of jumbled rock fragments and void spaces.

semi-major axis:   Half the length of the long axis of an elliptical orbit, equivalent to the mean distance of an orbiting object from its primary body.

shergottite: Igneous stony meteorite consisting mainly of plagioclase (or a shocked glass of plagioclase composition) and pyroxene and believed to come from Mars.

siderophile element: A metallic element with a chemical affinity for metal.  Such elements, which include nickel, gold and iridium, mainly occur in chondrites as metals rather than oxides.

silica: The chemical compound of silicon dioxide (SiO2).

silicate: A large class of minerals containing silicon and oxygen.

silt:   Unconsolidated sediment consisting of small mineral particles intermediate in size between sand and clay.

solar flare:   An abrupt violent outburst of energy on the solar photosphere.

solar nebula: The primitive gas and dust cloud around the Sun from which planetary materials formed.

solar system: The Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids.

solar wind: The flow of ionized gas through the solar system originating in the Sun’s corona.

solid solution: A series of minerals of the same structure but containing a mixture of different elements in widely varying proportions.  A prime example is olivine which varies continuously from the magnesium end-member (forsterite; Mg2SiO4) to the iron end-member (fayalite; Fe2SiO4).

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

splash-form tektite:   Tektites that have the shape they acquired as spinning molten objects.  These include dumbbells, spheroids, teardrops, lenses and buttons.

stishovite: A very-high-pressure silica mineral formed from quartz in terrestrial craters during energetic impact events.

stony-iron meteorite: An igneous meteorite that consists mainly of metallic iron-nickel and silicates.  The two principal groups are pallasites and mesosiderites.

strata: Originally horizontal layers of rock.

strewn field: A typically elliptical distribution of recovered meteorites after a meteorite shower.

sunspot: A temporary, relatively cool region on the Sun’s photosphere associated with an intense magnetic field.

supernova: A highly energetic exploding star that produces heavy elements by neutron bombardment and ejects them into interstellar space.

taenite: An iron-nickel mineral with about 15-40% nickel by weight and having a face-centered cubic structure.

tektite: Glassy objects produced by the melting of silica-rich sediments on Earth by the impact of an asteroid or comet.

thermoluminescence (TL): Emission of light caused by the heating of certain minerals.

thermoremanent magnetization: Permanent magnetization acquired by igneous rocks in the presence of a magnetic field as the rocks cool through the Curie point.

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

till: An unconsolidated, poorly sorted sediment deposited by glaciers.  Till contains rock and mineral fragments of all sizes from clay-size particles to massive boulders.

T-Tauri star: A pre-main-sequence star of approximately solar mass characterized by erratic outbursts, flares and appreciable mass loss.

type-3 chondrite: An unequilibrated chondrite with a relatively unrecrystallized texture and containing mineral grains with heterogeneous compositions.

type-4-6 chondrite: Increasingly equilibrated, thermally metamorphosed chondrites.

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

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

volatile element: Elements that condense from a gas at relatively low temperatures.  Such elements include sodium, indium, gold, mercury, and the noble gases (e.g., helium, neon, argon, krypton, xenon).

Widmanstätten pattern: A texture in iron meteorites, enhanced by etching with a dilute solution of nitric acid, that shows the intergrowth of the two main iron-nickel minerals, low-Ni kamacite and high-Ni taenite.

x-rays:   High-energy, short-wavelength photons (intermediate in energy between gamma rays and ultraviolet photons).

zircon: The mineral zirconium silicate (ZrSiO4).

zodiacal light:   Sunlight scattered by interplanetary dust extending away from the Sun along the ecliptic and faintly visible in the eastern sky shortly before sunrise or in the western sky shortly after sunset.