Terms and Definitions

A

accretion

accumulation of dust and gas into larger bodies

albedo

reflectivity of an object; ratio of reflected light to incident light

albedo feature

a dark or light marking on the surface of an object that may not be a geological or topographical feature

allocthonous

material that is formed or introduced from somewhere other than the place it is presently found; fragmented rock thrown out of the crater during its formation that either falls back to partly fill the crater or blankets its outer flanks after the impact event

angstrom

a unit of length = 1.0E-08cm

antipodal point

the point that is directly on the opposite side of the planet; the Earth's north pole is antipodal to its south pole

aphelion

the point in its orbit where a planet is farthest from the Sun

the point in orbit farthest from the planet

apogee

the point in orbit farthest from the Earth

ash

fine-grained material produced by a pyroclastic eruption; an ash particle is defined to have a diameter of less than 2mm

asteroid number

asteroids are assigned a serial number when they are discovered; it has no particular meaning except that asteroid N+1 was discovered after asteroid N

astronomical unit (AU)

the average distance from the Earth to the Sun; 1AU is 149,597,870 km

atmosphere

1 atmosphere is 14.7 pounds per square inch; standard atmospheric pressure at sea level on Earth

aurora

a glow in a planets ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun

aurora borealis

the Northern Lights caused by the interaction between the solar wind, the Earth's magnetic field and the upper atmosphere; a similar effect happens in the southern hemisphere where it is known as the aurora austrailis

B

bar

unit of pressure, equal to the sea-level pressure of Earth's atmosphere; 1 bar is equivalent to 0.987 atmosphere or 10000 newtons per square meter

basalt

a general term for dark-colored igneous rocks composed of minerals that are relatively rich in iron and magnesium

blackbody temperature

the temperature of an object if it is reradiating all the thermal energy that has been added to it; if an object is not a blackbody radiator, it will not reradiate all the excess heat and the leftover will go toward increasing its temperature

bolide

an exploding meteorite

bow shock

outermost part of a planetary magnetosphere; the place where the supersonic flow of the solar wind is slowed to subsonic speed by the planetary magnetic field

breccia

a course-grained rock, composed of angular, broken rock fragments held together by a mineral cement or a fine-grained matrix

butte

conspicuous isolated flattop hill with steep slopes

C

calcium K

a narrow wavelength of blue light which is emitted and absorbed by ions of the element calcium

caldera

a large, basin-shaped volcanic depression that is more or less circular in form; most volcanic calderas are produced by collapse of the roof of a magma chamber due to removal of magma by voluminous eruptions or subterranean withdrawal of the magma, although some calderas may be formed by explosive removal of the upper part of a volcano

carbonate

a compound containing carbon and oxygen (ie calcium carbonate aka limestone)

cataclastic

a texture found in metamorphic rocks in which brittle minerals have been broken, crushed and flattened during shearing

catena

a chain of craters

cavus

hollows, irregular depressions

central peak

the exposed core of uplifted rocks in complex meteorite impact craters; the central peak material typically shows evidence of intense fracturing, faulting and shock metamorphism

chaos

distinctive area of broken terrain

chasma

a canyon

chromosphere

the lower level of the solar atmosphere between the photosphere and the corona

cinders

loose, vesicular volcanic ejecta 4-32 mm in diameter

cinder cone

a conical hill formed by the accumulation of pyroclastic fragments that fall to the ground in an essentially solid condition

clast

a fragment of rock that has been transported, either by volcanic or sedimentary processes

colles

a small hill or knob

coma

the dust and gas surrounding an active comet's nucleus

composite volcano

volcano composed of interbedded lava and pyroclastic material commonly with steep slopes

convection

fluid circulation driven by large temperature gradients; the transfer of heat by this automatic circulation (see also Educators Guide to Convection)

corona

1) the upper level of the solar atmosphere, characterized by low densities and high temperatures (> 1.0E+06 K); it is not visible from the Earth except during a total eclipse of the sun or by use of special telescopes called coronagraphs; 2) an ovoid-shaped feature

coronagraph

a special telescope which blocks light from the disk of the Sun in order to study the faint solar atmosphere

cosmic ray

electromagnetic rays of extremely high frequency and energy; cosmic rays usually interact with the atoms of the atmosphere before reaching the surface of the Earth; some cosmic rays come from outside the solar system while others are emitted from the sun and pass through holes in the corona

crater

depression formed by the impact of a meteorite; depression around the orifice of a volcano

cratons

the relatively stable portions of continents composed of shield areas and platform sediments; typically cratons are bounded by tectonically active regions characterized by uplift, faulting and volcanic activity

Cretaceous period

a geological term denoting the interval of Earth history beginning around 144 million years ago and ending 66 million years ago

Cretaceous-Tertiary boundry

a major stratigraphic boundry on Earth marking the end of the Mesozoic Era, best known as the age of the dinosaurs; the boundary is defined by a global extinction event that caused the abrupt demise of the majority of all life on Earth

crystalline

rock types made up of crystals or crystal fragments, such as metamorphic rocks that recrystallized in high temperature or pressure environments or igneous rocks that formed from cooling of a melt

D

density

measured in grams per cubic centimeter (or kilograms per liter); the density of water is 1.0; iron is 7.9; lead is 11.3

diaplectic glass

a natural glass formed by shock pressure from any of several minerals without melting; it is found only in association with meteorite impact craters

disk

the visible surface of the Sun (or any heavenly body) projected against the sky

Doppler effect

the apparent change in wavelength of sound or light caused by the motion of the source, observer or both

dorsum

a ridge

E

eccentric

noncircular; elliptical (applied to an orbit)

eccentricity

a value that defines the shape of an ellipse or planetary orbit; the ratio of the distance between the foci and the major axis

ecliptic

the plane of Earth's orbit about the Sun

effusive erruption

a relative quiet volcanic eruption which puts out basaltic lava that moves at about the speed one walks; the lava is fluid in nature; the eruptions at the Kilauea volcano on the island of Hawaii are effusive

ejecta

material such as glass and fragmented rock thrown out of an impact crater during its formation

ellipse

oval; That the orbits of the planets are ellipses, not circles, was first discovered by Johannes Kepler based on the careful observations of Tycho Brahe.

en echelon fissures

fissures that are parallel in trend to each other, but offset to either the left or right

eolian

related to wind deposits and associated effects

eruption

the ejection of volcanic materials (lavas, pyroclasts and volcanic gases) onto the surface, either from a central vent or from a fissure or group of fissures

explosive erruption

a dramatic volcanic eruption which throws debris high into the air for hundreds of miles; lava is low in silicate; can be very dangerous for people near by; an example is Mount St. Helens in 1980

F

faculae

a bright region of the photosphere seen in white light, seldom visible except near the solar limb

filament

a strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun; a filament on the limb of the Sun seen in emission against the dark sky is called a prominence

fissure

a narrow opening or crack of considerable length and depth

flare

a sudden eruption of energy on the solar disk lasting minutes to hours, from which radiation and particles are emitted

flexus

cuspate linear feature

fluctus

flow terrain

fossa

a long, narrow, shallow depression

G

Gaia Hypothesis

named for the Greek Earth goddess Gaea, holds that the Earth should be regarded as a living organism. First advanced by British biologist James Lovelock in 1969.

Galilean Moons

Jupiter's four largest moons: Io, Europa, Ganymede and Callisto; discovered independently by Galileo and Marius.

garben

an elongated, relatively depressed crustal unit or block that is bounded by faults on its sides

geosynchronous orbit

a direct, circular, low inclination orbit in which the satellite's orbital velocity is matched to the rotational velocity of the planet; a spacecraft appears to hang motionless above one position of the planet's surface

granulation

a pattern of small cells seen on the surface of the Sun caused by the convective motions of the hot solar gas

greenhouse effect

increase in temperature caused when incoming solar radiation is passed but outgoing thermal radiation is blocked by the atmosphere (carbon dioxide is the major factor)

H

h-alpha

a narrow wavelength of red light which is emitted and absorbed by the element hydrogen; this wavelength is often used to study the Sun

heliocentric

Sun centered; see Copernicus, Kepler, Galileo

heliopause

the point at which the solar wind meets the interstellar medium or solar wind from other stars

heliosphere

the space within the broundary of the heliopause containing the Sun and solar system

hemisphere

a half of the celestial sphere divided into two halves by the horizon, the celestial equator, or the ecliptic

high pressure mineral phase

mineral forms that are stable only at the extremely high pressures typical of Earth's deep interior but not its surface; such pressures are generated instantaneously during meteorite impact; stishovite is the high-pressure polymorph of quartz, a common crustal mineral

hot spot

center of persistent volcanism, thought to be the surface expression of a rising hot plume in Earth's mantle

hummocky

uneven, lumpy terrain

I

ice

used by planetary scientists to refer to water, methane, and ammonia which usually occur as solids in the outer solar system

igneous

used to describe a rock or mineral that solidified from molten or partly molten material

impact melt

rocks melted during impact, including small particles dispersed in varous impact deposits and ejecta, and larger pools and sheets of melt that coalesce in low areas within the crater; impact melts are extremely uniform in their composition but highly variable in texture; they are composed predominantly of the target rocks but may contain a small but measurable amount of the impactor

inclination

the inclination of a planet's orbit is the angle between the plane of its orbit and the ecliptic; the inclination of a moon's orbit is the angle between the plane of its orbit and the plane of its primary's equator

inferior planets

the planets Mercury and Venus are inferior planets because their orbits are closer to the Sun than is Earth's orbit

interplanetary magnetic field (IMF)

the magnetic field carried with the solar wind

ion

an atom or molecular fragment that has a positive electrical charge due to the loss of one or more electrons; the simplest ion is the hydrogen nucleus, a single proton

ionosphere

a region of charged particales in a planet's upper atmosphere; the part of the earth's atmosphere beginning at an altitude of about 25 miles and extending outward 250 miles or more

J

K

Kelvin (K)

0 K is absolute zero; water melts at 273 K (0° C, 32° F); water boils at 373 K ( 100° C, 212° F)

kilogram (kg)

1 kilogram is equivalent to 1000 grams or 2.2 pounds; the mass of a liter of water

kilometer (km)

1 kilometer is equivalent to 1000 meters or 0.62 miles

L

labes

a Landslide

labyrinthus

an intersecting valley complex

lacus

a lake

Lagrangian point

one of the solutions to the three-body problem discovered by the eighteenth century French mathematician Lagrange; the two stable Lagrangian points, L-4 and L-5, lie in the orbit of the primary body, leading and trailing it by a 60-degree arc

lava

a general term for molten rock that is extruded onto the surface

lava tube

tunnel formed underneath the surface of a solidfying lava flow

leading hemisphere

the hemisphere that faces forward, into the direction of motion of a satellite that keeps the same face toward the planet

lee

the side of an object that is sheltered from the wind

light-year

the distance light travels in a year, at the rate of 300,000 km per second; 1 light-year is equivalent to 9.46053e12 km, 5,880,000,000,000 miles or 63,240 AU

limb

the outer edge of the apparent disk of a celestial body

linea

an elongate marking

lineament

linear topographic feature that may depict crustal structure

M

macula

a dark spot

magma

molten rock within the crust of a planet that is capable of intrusion into adjacent crustal rocks or extrusion onto the surface; Igneous rocks are derived from magma through solidification and related processes or through eruption of the magma at the surface

magnetograph

a special telescope which analyzes the color and polarization of sunlight in order to measure the magnetic field of the Sun

magnetopause

the boundary of the magnetosphere, lying inside the bow shock

magnetosphere

the region of space in which a planet's magnetic field dominates that of the solar wind

magnetotail

the portion of a planetary magnetosphere which is pushed in the direction of the solar wind

magnitude

the degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest visible star has magnitude 6, with the scale rule such that a decrease of one unit represents an increase in apparent brightness by a factor of 2.512; also called apparent magnitude

mare

Latan word for "sea"; Galileo thought the dark featureless areas on the Moon were bodies of water; even though the Moon is essentially devoid of liquid water, the term is still applied to the basalt-filled impact basins common on the face of the Moon visible from Earth

mensa

mesa, flat-topped elevation

mesa

broad flattop erosional hill or mountain, commonly bounded by steep slopes

millibar

1/1000 of a bar; standard sea-level pressure is about 1013 millibars

minor planets

another term used for asteroids

mons

a mountain

N

nebula

a diffuse mass of interstellar dust and gas

neutrino

a fundamental particle supposedly produced in massive numbers by the nuclear reactions in stars; they are very hard to detect since the vast majority of them pass completely through the Earth without interacting

nuclear fusion

a nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones; the difference in mass is converted to energy by Einstein's famous equivalence E=mc^2; this is the source of the Sun's energy therefore ultimately of (almost) all energy on Earth

O

oceanus

an ocean

obliquity

the angle between a body's equatorial plane and orbital plane

occulation

blockage of light by the intervention of another object; a planet can occult (block) the light from a distant star

old

a planetary surface that has been modified little since its formation typically featuring large numbers of impact craters; (compare young)

ovoid

shaped like an egg

P

Paleozoic

a geological term denoting the time in Earth history between 570 and 245 million years ago

pahoehoe

a type of basalt lava flow characterized by a smooth glassy skin, and constructed of innumerable "flow units" called "toes"; pahoehoe flows advance at rates between 1 and 10 meters/hour and are associated with low-effusion-rate eruptions with little to no fountaining

palimpsest

a circular feature on the surface of dark icy moons such as Ganymede and Callisto lacking the relief associated with craters; Pamlimpsests are thought to be impact craters where the topographic relief of the crater has been eliminated by slow adjustment of the icy surface

palus

a swamp

patera

shallow crater; scalloped, complex edge

peak ring

a central uplift characterized by a ring of peaks rather than a single peak; peak rings are typical of larger terrestrial craters above about 50 km in diameter

penumbra

the outer filamentary region of a sunspot

periapsis

the point in the orbit closest to the planet

periagee

the point in the orbit closest to the Earth

perihelion

the point in its orbit where a planet is closest to the Sun

perturb

to cause a planet or satellite to deviate from a theoretically regular orbital motion

photosphere

the visible surface of the Sun; the upper surface of a convecting layer of gases in the outer portion of the sun whose temperature causes it to radiate light at visible wavelengths; sunspots and faculae are observed in the photosphere

phreatic eruption

a volcanic eruption or explosion of steam, mud or other material that is not incandescent; this form of eruption is caused by the heating and consequent expansion of ground water due to an adjacent igneous heat source

plage

bright regions seen in the solar chromosphere

planar features

microscopic features in grains of quartz or feldspar consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain's crystal structure

planitia

broad plains that occupy lowlands on planetary surfaces

planum

plateau or high plain

plasma

a low-density gas in which the individual atoms are charged, even though the total number of positive and negative charges is equal, maintaining an overall electrical neutrality

polarization

a special property of light; light has three properties, brightness, color and polarization

precambrian

a geological term denoting the time in Earth history prior to 570 million years ago

prominence

an eruption of hot gases above the photosphere of the Sun; prominences are most easily visible close to the limb of the Sun but some are also visible as bright streamers on the photosphere

promontorium

a cape

pseudocrater

a generally circular crater produced by a phreatic eruption resulting from emplacement of a lava flow over wet ground

pyroclastic

pertaining to clastic (broken and fragmented) rock material formed by volcanic explosion or aerial expulsion from a volcanic vent

pumice

a light vesicular form of volcanic glass with a high silica content; it is usually light in color and will float on water

Q

R

red giant

a star that has low surface temperature and a diameter that is large relative to the Sun

regio

region

regolith

the layer of rocky debris and dust made by metoritic impact that forms the uppermost surface of planets, satellites and asteroids

Relativity, Theory of

more accurately describes the motions of bodies in strong gravitational fields or at near the speed of light than newtonian mechanics. All experiments done to date agree with relativity's predictions to a high degree of accuracy. (Curiously, Einstein received the Nobel prize in 1921 not for Relativity but rather for his 1905 work on the photoelectric effect.)

resolution

the amount of small detail visible in an image; low resolution shows only large features, high resolution shows many small details

resonance

a relationship in which the orbital period of one body is related to that of another by a simple integer fraction, such as 1/2, 2/3, 3/5

retrograde

rotation or orbital motion in a clockwise direction when viewed from the north pole of the ecliptic (or of the rotating object)

rhyolite

fine-grained extrusive igneous rock, commonly with phenocrysts of quartz and feldspar in a glassy groundmass

rift

a fracture or crack in a planet's surface caused by extension; on some volcanoes subsurface intrusions are concentrated in certain directions and this causes tension at the surface and also means that there will be more eruptions in these "rift zones"

rift valley

an elongated valley formed by the depression of a block of the planet's crust between two faults or groups of faults of approximately parallel strike

rima

fissure

Roche limit

the closest a fluid body can orbit to its parent planet without being pulled apart by tidal forces

rupes

term applied to scarps on planetary surfaces; many scarps are thought to be the surface expression of faults within the crust of the planetary object

S

sapping

a process of erosion where water leaks to the surface through the pores of rocks; as the water flows away it slowly removes material to form valleys and channel networks

scarp

a line of cliffs produced by faulting or erosion; a relatively straight, clifflike face or slope of considerable linear extent, breaking the general continuity of the land by separating surfaces lying at different levels

scopulus

lobate or irregular scarp

semimajor axis

one-half of the longest dimension of an ellipse

shatter cone

striated conical fracture surfaces produced by meteorite impact into fine-grained brittle rocks such as limestone

shepherd satellite

a satellite which constrains the extent of a planetary ring through gravitational forces

shield

any of several extensive regions where ancient Precambrian crystalline rocks are exposed at the Earth's surface

shield volcano

a volcano in the shape of a flattened dome, broad and low, built by flows of very fluid lava

shock metamorphism

the production of irreversible chemical or physical changes in rocks by a shock wave generated by impact, or detonation of high-explosive or nuclear devices

siderial

of, relating to, or expressed in relation to stars or constellations

siderial rotation

rotation time measured with respect to the fixed stars rather than the Sun or body orbited

siderophile elements

litterally, iron-loving elements, such as Iridium, Osmium, Platinum and Plladium, that in chemically segregated asteroids and planets, are found in the metal-rich interiors; consequently these elements are extremely rare on Earth's surface

silicate

a rock or mineral whose structure is dominated by bonds of silicon and oxygen atoms (ie. olivine)

sinus

a bay

solar cycle

the approximately 11-year quasi-periodic variation in frequency or number of solar active events

solar nebula

the large cloud of gas and dust from which the Sun and planets condensed 4.6 billion years ago

solar wind

a tenuous flow of gas and energetic charged particles, mostly protons and electrons -- plasma -- which stream from the Sun; typical solar wind velocities are near 350 kilometers per second

spatter cone

low steep-sided cone built up from fluid pyroclasts coating the surface around a vent

spectroradiometer [SPEC-tro-RAY-dee-om-it-er]

a device that measures the amount of reflected or radiated energy from a surface in two or more wavelengths

speed of light

= 299,792,458 meters/second (186,000 miles/second); Einstein's Theory of Relativity implies that nothing can go faster than the speed of light

spicules

grass-like patterns of gas seen in the solar atmosphere

stishovite

a dense, high-pressure phase of quartz that has so far been identified only in shock-metamorphosed quartz-bearing rocks from meteorite impact craters

subduction

process of one lithospheric plate descending beneath another

sublime

to change directly from a solid to a gas without becoming liquid

sulcus

subparallel furrows and ridges

sulfuric acid

a heavy corrosive oily dibasic strong acid H2SO4 that is colorless when pure and is a vigorous oxidizing and dehydrating agent

sunspot

an area seen as a dark spot on the photosphere of the Sun; sunspots are concentrations of magnetic flux, typically occurring in bipolar clusters or groups; they appear dark because they are cooler than the surrounding photosphere

superior planets

the planets Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are superior planets because their orbits are farther from the Sun than Earth's orbit

synchronous orbit radius

the orbital radius at which the satellite's orbital period is equal to the rotational period of the planet. A synchronous satellite with an orbital inclination of zero (same plane as the planet's equator) stays fixed in the sky from the perspective of an observer on the planet's surface (such orbits are commonly used for communications satellites).

synchronous rotation

a satellite's rotational period is equal to its orbital period; this causes the same side of a satellite to always face the planet; synchronous rotation occurs when a planet's gravity produces a tidal bulge in its satellite; the gravitational attraction and bulge acts like a torque which slows down the satellite until it reaches a synchronous rotation

synthetic-aperture radar

side-looking imaging system that uses the Doppler effect to sharpen the effective resolution in the cross-track direction

T

target rocks

the surface rocks that an asteroid or comet impactor smashes into in a meteorite impact event

tectonic

deformation forces acting on a planet's crust

tektites

natural, silica-rich, homogeneous glasses produced by complete melting and dispersed as droplets during terrestrial impact events; they range in color from black or dark brown to gray or green and most are spherical in shape; they have been found in four regional deposits or strewn fields on the Earth's surface: North America, Czechoslovakia ivory coast and australasia

terminator

the dividing line between the illuminated and the unilluminated part of the moon's or a planet's disk

terra

extensive land mass

tessera

tile; polygonal ground

tholus

small domical mountain or hill

tidal forces

gravitational pull on planetary objects from nearby planets and moons; when the tidal forces of a planet and several moons are focused on certain moons, particularly if the orbits of the various objects bring them into alignment on a repeated basis, the tidal forces can generate a tremendous amount of energy within the moon; the intense volcanic acivity of Io is the result of the interaction of such tidal forces

tidal heating

frictional heating of a satellite's interior due to flexure caused by the gravitational pull of its parent planet and possibly neighboring satellites

trailing hemisphere

the hemisphere that faces backwards, away from the direction of motion of a satellite that keeps the same face toward the planet

tuff

general term for consolidated pyroclastic debris

U

ultraviolet

electromagnetic radiation at wavelenghts shorter than the violet end of visible light; the atmosphere of the Earth effectively blocks the transmission of most ultraviolet light

umbra

the dark central region of a sunspot

undae

dunes

V

vallis

sinuous valley

vastitas

widespread lowlands

volatile

compounds with low melting temperatures, such as hydrogen, helium, water, ammonia, carbon dioxide and methane

volcano

a vent in the planetary surface through which magma and associated gases and ash erupt; the form or structure produced by the erupted materials

W

white dwarf

a whitish star of high surface temperature and low intrinsic brightness with a mass approximately equal to that of a Sun but with a density many times larger

X

x-ray

electromagnetic radiation of very short wavelength and very high energy; x-rays have shorter wavelengths than ultraviolet light but longer wavelengths than cosmic rays

Y

young

when used to describe a planetary surface "young" means that the visible features are of relatively recent origin, i.e. that older features have been destroyed (e.g. by erosion or lava flows); young surfaces exhibit few impact craters and are typically varied and complex; in contrast an "old" surface is one that has changed relatively little over geologic time; the surfaces of Earth and Io are young; the surfaces of Mercury and Callisto are old

Z

Copyright © 1995 by Calvin J. Hamilton. All rights reserved.
Last Modified: June 7, 1995