A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

The given definitions are arranged such that the simplest definition is first. We are not responsible for the links on this site.
 

Absolute age:  An age usually determined in thousands or millions of years, as from radioactive decay. ^1M

Accretion:  Gravitational accumulation of mass in a planet or protoplanet. ² The sticking together of solid particles to produce a larger particle¹. . Addition of matter to a body. Examples are gas falling into a star and asteroids colliding and sticking together. ³

Albedo:  The fraction of the light that hits an object that is reflected.¹ The reflectance or reflectivity of an object; specifically the ratio of reflected energy to incident energy (also called bond albedo).²
 

Angular momentum:  A measure of an object's tendency to keep rotating and to maintain its orientation. Mathematically it depends on the object's mass, m, radius, r, and rotational velocity, v, and is proportional to mvr.³ The tendency of a rotation body to continue rotating; mathematically, the product of mass, velocity and radius.¹  A measure of the momentum associated with rotational motion about an axis.²

Anorthosite:  Primary igneous rocks of the lunar highlands, a rock type that is very rare on Earth.²  Rocky aluminum and calcium silicates found in the lunar highlands.¹

Aperture: Diameter size of a telescope.
Aphelion: For an object orbiting the Sun, the point in the orbit that is farthest to the Sun. ²

Apoapsis: The orbital point, of an object orbiting a planet, of greatest distance from the planet.

Asteroid(s):  A small, generally rocky, solid body orbiting the Sun and ranging in diameter from a few meters to hundreds of kilometers.³  Small rocky worlds, most of which lie between Mars and Jupiter in the asteroid belt.¹  A small body (typically less than 1000 Km in diameter) orbiting the sun that does not display the atmosphere or tail associated with a comet; formerly called minor planets.²

Astronomical unit (AU):  Average distance from the Earth to the Sun; 1.5 x 10⁸ km, or 93 x 10⁶ miles.¹

Autumnal Equinox:  See Equinox.

Basalt:  Dark, igneous rock, characteristic of solidified lava.¹  Common igneous rock, composed primarily of silicon, oxygen, iron, aluminum, and magnesium, produced by the rapid cooling of lava. Basalts make up most of Earth's oceanic crust and are also found on other planets that have experienced volcanic activity.²

Basin:  A depressed area with no surface outlet. The term is widely applied, e.g. to a lake basin, to a ground-water basin, to a depression on the sea floor, to a circular depression on the Moon's surface, or to a tidal basin.  ⁴   The generic term "basin" describes the low topography and not the origin of that topography. However, the term is used loosely in planetary science and for the Moon and Mercury often refers to an impact feature larger than about 300 km in diameter. Impactors must have been 30-100 km in diameter.

Bathymetry:  The measurement of ocean depths and the charting of the topography of the ocean floor.⁴

Breccia:  A rock composed of fragments of pre-existing rocks bounded together. In planetary sciences the term refers to rocks usually  produced by meteoroid impact.^1M

Capture Hypothesis: The theory that the moon formed elsewhere in the solar system and was later captured by Earth.¹

Comet:  A small body in orbit around the Sun, with a core consisting of ice and rock, and a tail of gas and dust. The tail forms only when the comet is near the Sun.³ One of the small, icy bodies that orbit the Sun and produce tails of gas and dust when they near the Sun. ¹ The most primitive Sun-orbiting members of the solar system, consisting of a small nucleus composed of ices, silicates, and carbonaceous material, which when heated generates a tenuous temporary atmosphere as its volatiles evaporate. Generally these icy objects are called comets only when they are close enough to the Sun to outgas a visible coma or atmosphere.²

Composite volcano:  A common type of terrestrial volcano built up of layers of lava flows and ash falls. Steep sided and typically associated with subduction zones.¹

Conduction:  One of the basic ways of transferring energy, caused by the motion of atoms and electrons in a solid (there is no motion of the material as a whole). Although it is not as intrinsically efficient as convection or radiation, conduction is usually the dominant mechanism of energy transfer in a solid.^2M

Condensation Hypothesis:  The hypothesis that the Moon and Earth formed by condensing from the solar nebula as a dual planetary system.^1M
 

Convection:  One of the basic ways of transferring energy, caused by the large-scale (macroscopic) movement of material, such as the rising of pockets of hot gas and the sinking of cooler gas. When convection occurs, it is usually more effective at transferring energy than either radiation or conduction. ² Circulation in a fluid buoyancy driven by heat; hot or less dense material rises, and cool or more dense material sinks. ^1M  The rising and sinking motions on a liquid or gas that carry heat upward through the material. Convection is easily seen in a pan of heated soup on a stove.³

Convergent Plate Boundary:   A boundary between two plates that are moving toward each other. ^4M
 

Core:  The central zone or nucleus of a planet's interior.

Coronae: Large, circular tectonic features unique to Venus, often with associated volcanic activity. The fracturing (tectonism), volcanism and circular shape are thought to result from a rising plume or mantle material ^2M

Crater:  An abrupt circular depression formed by collapse following extrusion of volcanic material, or formed by impact of a meteorite.^5M A circular depression (from the Greek word for bowl or cup), generally of impact origin.² The size at which the transition from simple to complex craters occurs depends on the planet's gravity field.
    Complex: see class notes
    Simple: An impact crater of relatively small diameter, characterized by a uniform concave upward shape and a maximum depth in the center, and lacking a central uplift and marginal slumping.⁴

Crust:  The outer solid layer (based on chemical properties) of a planet; on Earth, roughly the upper 30 km.²

Differentiation: The gravitational separation of planetary material according to density.¹  The gravitational separation or segregation of different densities of material into different layers in the interior of a planet, as a result of heating.²  The separation of different previously mixed materials inside a planet or other object. This is the same separation that occurs when a heavy material, for example iron, settles to the planet's core, leaving lighter material on the surface. ³  Gravitational settling of more dense material to form layers of different composition- core (metal), mantle ( silicate rock) and crust (lighter silicate material).

Doppler Effect (Shift):  Apparent change in wavelength of the radiation from a source due to its relative motion toward, or away from, the observer.²  The change in the observed wavelength of radiation caused by the motion of the emitting body or the observer. The shift is an increase in the wavelength if the source and observer move apart and a decreased in the wavelength if the source and the observer approach.³

Dynamo:  The process by which a rotating, convecting body of conducting matter, such as Earth's liquid iron core, can generate a magnetic field. ^1M  A physical process for generating magnetic fields in astronomical bodies. In many cases, the process involves the generation of electric currents from an interaction between rotation and convection.³

Ejecta:  Pulverized rock scattered by meteorite impacts on a planetary surface. The characteristics (size and shape) of ejecta around impact craters depend mainly on properties of the target-rock (the surface that was cratered ).¹

Electromagnetic radiation:  Radiation consisting of electric and magnetic waves;  they include radio, infrared, visible light, ultraviolet, xrays, and gamma rays.¹

Ellipse:  a closed curve that describes the orbit of one object about another subject only to their mutual gravitational attraction.¹

Epicenter:  The point on the Earth's surface that is directly above the focus of an earthquake.⁴

Equinox:  The time when the sun crosses the plane of the Earth's equator, making night and day of approximately equal length all over the Earth and occurring about March 21 (vernal equinox or spring equinox) and September 22 (autumnal equinox).⁷  Visit The Seasons for more information.

Fault:  A tectonic crack or break in the crust of a planet along which slippage or movement can take place, sometimes accompanied by seismic activity.²
        Extension: Normal:  A steeply inclined fault in which the  hanging wall has moved downward in relation to the footwall.⁵
        Compression: Reverse: (or thrust)  A fault in which the hanging wall has moved upward in relation to the footwall.⁵
        Shear: Strike-Slip:  A fault in which movement has occurred parallel to the strike of the fault.⁵

Fission Hypothesis:  The theory that the Moon formed by breaking away from Earth.¹

Focus (hypocenter):  The initial rupture point of an earthquake. The point within the Earth, which is the center of an earthquake.⁴

Fold:  A significant bend, or flexure, in a rock.⁵ 

Half-life:  The time required for half the atoms in a radioactive sample to decay.¹

Hot spot:  See volcanic hot spot.

Heliocentric Model:  Sun-Centered Model.  Initiated by Copernicus, this model was based on the idea that the planets are in orbit around the Sun rather than the Earth.¹

Infrared Radiation:  Electromagnetic radiation with wavelengths (around 0.1 mm) intermediate between visible light and radio waves.¹
 

Isotope:  Any of two or more forms of the same element, whose atoms all have the same number of protons but different number of neutrons.²
    Daughter isotope:  An isotope produced by radioactive decay of  its parent isotope. The quantity of a daughter isotope continually  increases with time.⁵
    Parent Isotope:  The isotope of the element that decays.

Jovian Planet:  Jupiter-like planet with large diameter and low density.¹
 

Kuiper belt:  Disk-shaped region beyond the orbit of Neptune that contains many icy objects and is the main source of the short-period comets. ² The collection of icy planetesimals believed to orbit in a region from just beyond Neptune out to 100 AU or more. ¹ A region from which some comets come. The region appears to extend from  the orbit of Neptune, past Pluto, out to approximately 55 AU.³

Large-Impact Hypothesis:  The theory that  the moon formed from debris ejected during a collision between Earth and a large planetesimal.¹
 

Lithosphere:  The upper layer (based on physical properties) of the Earth, to a depth of 50 to 100 km, which is involved in plate tectonics. ²  The relative rigid outer zone of Earth, which includes the continental crust, the oceanic crust, and the part of the upper mantle lying above the weaker asthenosphere. ⁵

Lobate scarp:  A curved cliff of compressional origin such as those found on the surface of Mercury  .^1M

Lunar eclipse:  The darkening of the Moon when it moves through Earth's shadow.¹

Magnetic field: A region in which magnetic forces would be exerted on any magnetized bodies or electric currents present; the region of influence of a magnetized body or an electric current..⁴

Magnetosphere:  The volume of space around a planet within which the motion of charged particles is dominated by the planetary magnetic field rather than the solar wind. ¹ The region around the Earth (or other planet) occupied by its magnetic field, and within which the planetary field dominates over the interplanetary field associated with the solar wind.²

Mantle:  The layer (based on chemical properties) of dense rock and metal oxides that lies between the molten core and Earth's crust; also similar layers in other planets. ¹  The part of a planet or satellite between its crust and core; on Earth, the mantle is the largest part of the planet, with about 65% of the mass.²

Mare (pl. maria):  Latin for sea; name applied to the dark, relatively smooth features consisting of basaltic lava flows that cover 17% of the Moon.² One of the lunar lowlands filled by successive flows of dark lava.¹

Meteor:  The luminous phenomenon observed when a bit of material (cosmic dust) enters Earths atmosphere and burns up; popularly called a falling star or shooting star. ² A small bit of matter heated by friction to incandescent vapor as it falls into Earths atmosphere.¹

Meteorite: A rock that survives its fiery passage through the atmosphere (as a Meteor) and strikes the ground. ² A meteor that has survived its passage through the atmosphere and strikes the ground. ¹  The solid remains of a meteor that falls to the Earth.³   Lunar meteorites.

Meteoroid:  A meteor in space before it enters Earths atmosphere.¹

Occultation:  The passage of a larger body in front of a smaller body.¹

Oort cloud:  The cloud of icy bodies, extending from the outer part of our solar system out to roughly 100,000 AU from the Sun, which acts as the source of most comets.¹  The spherical region around the sun from which most long-period comets come, representing objects with aphelia at about 50,000 AU, or extending about a third of the way to the nearest other stars.²   A vast region in which a comet nuclei orbit. This cloud lies far beyond the orbit of Pluto.³

Orbital Period: The time it takes for a planetary body to make one complete orbit.

Periapsis: The orbital point, of an object orbiting a planet, of closest approach to the planet.

Perihelion: For an object orbiting the Sun, the point in the orbit that is the closest from the Sun.²

Permafrost:  Permanently frozen subsoil. Important on Earth and Mars. ⁵

Planet:  A non-luminous object, larger than a comet or asteroid, which orbits a star.¹

Planetesimal:  One of the small bodies that formed from the  solar nebula and eventually grew into protoplanets.¹

Plate:  A torsionally rigid thin segment of the Earth's lithosphere, which may be assumed to move horizontally and adjoins other plates along zones of seismic activity.⁴

Plate Boundary:  Zone along the edges of lithosphere plates, often with associated volcanic and/or seismic activity.^4M

Plate tectonics:  The constant destruction and renewal of Earth's surface by the motion of segments or plates of the outer layer of the Earth's mantle (the lithosphere) .^2M The hypothesis that the lithosphere of the Earth (or some other planet) is divided into large regions (plates) that move very slowly over the planet's surface. Interaction between plates at their boundaries creates mountains and activity such as volcanoes and earthquakes.³   A theory of global tectonics in which the lithosphere is divided into a number of plates whose pattern of horizontal movement is that of torsionally rigid bodies that interact with one another at their boundaries, causing seismic and tectonic activity along these boundaries.⁴

Plume (mantle plume):  A buoyant mass of hot mantle material that rises to the base of the lithosphere. Mantle plumes commonly produce volcanic activity and structural deformation in the central parts of lithospheric plates.⁵  Picture of an Io's volcanic plume. 

Protoplanet:  Massive object resulting from the coalescence of planetesimals in the solar nebula and destined to become a planet.¹

Radioactive decay:  The breakdown of an atomic nucleus by the emission of subatomic particles.³

Radiometric dating:  Calculating an age in years for geologic materials by measuring the presence of a short-life radioactive element, e.g., carbon-14, or by measuring the presence of a long-life radioactive element plus its decay product, e.g., potassium-40/argon-40. The term applies to all methods of age determination based on nuclear decay of naturally occurring radioactive isotopes.⁴

Regolith:  A soil made up of crushed rock fragments.¹

Relative age:   The age of a geological feature referred to other features. For example, relative ages tell us the lunar maria are younger than the lunar highlands.¹

Resolution:  The degree to which fine details in an image are separated or visible. Resolution can be specified in either angular or linear units, but is used here usually in the sense of the linear dimensions (in km) of the smallest features that can be studied on a planet or satellite.²

Resonance:  An orbital condition in which one object is subject to periodical gravitational perturbations by another; most commonly arising when two objects orbiting a third have periods of revolution that are simple multiples or fractions of each other. ² The coincidental agreement between two periodic phenomena: commonly applied to agreements between orbital periods, which can make orbits more or less stable.¹

Rift:  A place where the crust is being torn apart by tectonic forces; generally associated with the injection of magma from the mantle and with a slow separation  of lithospheric plates.²  A long, narrow continental trough that is bounded by normal faults. It marks a zone along which the entire thickness of the lithosphere has ruptured under extension.⁴

Rifting:  The breaking apart of a continental plate.³

Rift Fault:  A fault that bounds a rift valley. The term has been applied to normal faults and to large strike-slip faults.⁴

Rift valley:  A long, straight, deep valley produced by the separation of crustal plates.¹

Rille:  A narrow, winding valley on the Moon caused by ancient lava flows along narrow channels.¹

Roche limit:  The minimum distance between a planet and a satellite at which the satellite holds itself together by its own gravity. If a satellite's orbit brings it within its planet's Roche limit, tidal forces will pull the satellite apart.¹  The distance from an astronomical body at which its gravitational force breaks up another astronomical body.³  The smallest distance that a fluid satellite can orbit from the center of a planet without being torn apart by tidal forces. For a planet and moon with equal densities, the Roche limit is about 21 times the radius of the planet. In practice, since moons tend to be solid, the tensile force of the rock and ice of which they are composed helps prevent their breakup. Even so, the shattering of satellites in orbits well inside the Roche limit may explain the origin of some planetary ring systems.⁶

Satellite:  A body orbiting a planet. ³

Scientific method:  The process of observing a phenomenon, proposing a hypothesis on the basis of the observations, and then testing the hypothesis.³

Seismic wave:  Mechanical waves in solid Earth (or other planet) caused by earthquakes or impacts, which can be used to probe the structure of the interior.²   The two more important varieties are S and P waves. The former can travel only through solid material; the latter can travel through either solid or liquid material. ³
     P-waves:  Mechanical waves of compression and rarefaction that travel through Earth's interior.¹  This type of  seismic wave that involves particle motion (alternating  compression and expansion) in the direction of propagation. It is  the  fastest of seismic waves, traveling 5.5-7.2 km/s in the crust  and  7.8-8.5 km/s in  the
upper mantle. Sound waves are P- waves. The  P stands for primary; it is so named because it is the  first arrival from earthquakes.⁴  Primary seismic wave: a type of  seismic wave, propagated like a sound wave, in which the material involved in the wave motion is alternately compressed and  expanded.⁵
     S-waves:  Mechanical waves that travel through Earths interior  by the vibration of particles perpendicular to the direction of wave  travel.¹  A seismic wave propagated by shearing motion that  involves oscillation perpendicular to the direction of propagation. It  does not travel through liquids, or through the outer core of the  Earth. Its speed is often about half that of P-waves, about 3.0-4.0  km/s in the crust and 4.4-4.6 km/s in the upper mantle. The S  stands for secondary; it is so named because it arrives later than  the P-wave.⁴  Secondary seismic wave: a seismic wave in which  the particles vibrate at right angles from the direction in which the  wave travels.⁵

Shield volcano:  Wide, low profile volcanic cone produced by low viscosity lava. ¹  A broad volcano built up through the repeated non-explosive eruption of fluid basalts to form a low dome of shield-shape, typically with slopes of only 4-6 degrees, often with a large caldera at the summit. Examples include the Hawaiian volcanoes on Earth and the Tharsis volcanoes in Mars.²

Solar eclipse: The event that occurs when the Moon passes directly between Earth and the Sun blocking our view of the Sun.¹  The blocking of all or part the light of the Sun by the Moon.²

Solar wind:  Rapidly moving atoms and ions that escape from the solar corona and blow outward  through the solar system.¹

Stratigraphy:  The study of rock strata, (particularly the sequences of layers) and the information this provides on the geologic history and relative ages of a region.²

Subduction: The process of one lithospheric plate descending beneath the other.⁴

Subduction zone:  An elongated zone in which a lithospheric plate descends beneath another. A subduction zone is usually marked by an oceanic trench, lines of volcanoes, and crustal deformation associated with mountain building; e.g., along the Peru-Chile trench.⁵  See Plate Boundary  for more information.

Summer solstice: The point on the celestial sphere where the Sun is at its most northerly point; also, the time when the Sun passes this point, about June 22, and summer begins in the northern hemisphere.¹  See The Seasons  for more information.

Tectonic:  Associated with the stress in the crust of a planet, often leading to the formation of faults (cracks) and folded ridges; in the case or Earth, associated with even the larger scale motion of lithospheric plates (called Plate Tectonics).²

Terminator:  The dividing line between daylight and darkness on a planet or moon.¹

Terrestrial planet:  Earth-like planet:  small, dense, rocky.¹

Tidal Bulge:  A bulge on one body created by the gravitational attraction on it by another. Two tidal bulges form, one on the side near the attracting body and one on the opposite side.³

Tidal coupling:  The locking of the rotation of a body to its revolution around another body.¹

Tidal heating:  Heating of one object by tidal friction or repeated stressing resulting from its motion within the strong tidal field of its neighbor, as in the tidal heating of Io. ² The heating of a planet or satellite because of friction caused by tides.¹

Vernal equinox: See equinox.

Volatile:  A substance that has relatively low boiling temperature. Although usage of this term depends on the context, volatile in this text usually refers to substances that are gaseous at temperatures above 400K to 500K, such as water and carbon dioxide.²

Volcanic hot spot:  Location of a particularly large flow of heat from the interior of the planet. On Io, name given to the areas covering about 1% of the surface from which most of the internal heat escapes to space.²  Volcanic activity is typically associated with a hot spot.

Volcanic plain:  Surface formed by extensive lava or ash flows that cover topographic irregularities.⁴

Volcanism:  The process by which magma and gases are transferred from Earths interior to the surface.⁵

Wavelength:  The distance between two wave crests or wave troughs.¹

Winter solstice: The point on the  celestial sphere where the Sun is farthest south; also, the time of year when the Sun passes this point, about December 22, and winter begins in the northern hemisphere.¹  See The Seasons  for more information.

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References:
¹ The Solar System by Michael A. Seeds.
² The Planetary System by David Morrison and Tobias Owen.
³ Explorations, Volume I, Solar Systems by Thomas T. Arny.
⁴ Glossary of Geology, 4th edition, by Julia A. Jackson.
⁵ Earths Dynamic Systems, 8th edition, by W. Kenneth Hamblin and Eric H. Christiansen.
⁶ The Encyclopedia of Astrobiology, Astronomy, and Spaceflight http://www.angelfire.com/on2/daviddarling/ETEmain.htm
⁷ http://factmoster.com
^M Definition modified slightly.