Reflectivity of an object; ratio of reflected light to incident
A dark or light marking on the surface of an object that might not
be a geological or topographical feature.
(1) Material that is formed or introduced from somewhere other than
the place it is presently found. (2) 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.
The closest bright star to our solar system.
A unit of length = 1.0E-08cm.
The point that is directly on the opposite side of the planet; e.g.,
the Earth's north pole is antipodal to its south pole.
The point in its orbit where a planet is farthest from the Sun.
The point in orbit farthest from the planet.
The point in orbit farthest from the Earth.
The fine-grained material produced by a pyroclastic eruption. An ash
particle is defined to have a diameter of less than 2 millimeters.
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; 1 AU is 149,597,870
kilometers (92,960,116 miles).
One atmosphere is 14.7 pounds per square inch (105 Newtons per
square meter); the average atmospheric pressure at sea level on Earth.
A glow in a planet's ionosphere caused by the interaction between
the planet's magnetic field and charged particles from the Sun.
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
A unit of pressure, equal to the sea-level pressure of Earth's
atmosphere; 1 bar = 0.987 atmosphere = 101,300 pascals = 14.5
lbs/square inch = 100,000 Newtons per square meter.
A general term for dark-colored, igneous rocks composed of minerals
that are relatively rich in iron and magnesium.
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.
An object whose gravity is so strong that the escape velocity
exceeds the speed of light.
An exploding meteorite.
The 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.
A course-grained rock, composed of angular, broken rock fragments
held together by a mineral cement or a fine-grained matrix.
A conspicuous, isolated, flattop hill with steep slopes.
A narrow wavelength of blue light which is emitted and absorbed by
ions of the element calcium.
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
A compound containing carbon and oxygen; an example is calcium
A texture found in metamorphic rocks in which brittle minerals have
been broken, crushed and flattened during shearing.
A chain of craters.
Hollows, irregular depressions.
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.
A distinctive area of broken terrain.
The lower level of the solar atmosphere between the photosphere and
Loose, vesicular volcanic ejecta 4 to 32 millimeters (.16 to 1.28
inches) in diameter.
A conical hill formed by the accumulation of pyroclastic fragments
that fall to the ground in an essentially solid condition.
A fragment of rock that has been transported, either by volcanic or
A small hill or knob.
The dust and gas surrounding an active comet's nucleus.
A volcano composed of interbedded lava and pyroclastic material
commonly with steep slopes.
Fluid circulation driven by temperature gradients; the transfer of
heat by this automatic circulation
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.
A special telescope which blocks light from the disk of the Sun in
order to study the faint solar atmosphere.
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
1) A depression formed by the impact of a meteorite. 2) A depression
around the orifice of a volcano.
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
A geological term denoting the interval of Earth history beginning
around 144 million years ago and ending 66 million years ago.
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.
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
Measured in grams per cubic centimeter (or kilograms per liter); the
density of water is 1.0, iron is 7.9, and lead is 11.3.
A natural glass formed by shock pressure from any of several
minerals without melting; it is found only in association with
meteorite impact craters.
The ratio of electric flux density to electric field.
The visible surface of the Sun (or any heavenly body) projected
against the sky.
The apparent change in wavelength of sound or light caused by the
motion of the source, observer or both.
Einstein's famous theory of relativity formula known as the
energy-mass relation. The energy e is equal to the mass m multiplied
by the speed of light squared c2. A small mass produces an enormous
amount of energy.
Noncircular; elliptical (applied to an orbit).
A value that defines the shape of an ellipse or planetary orbit; the
ratio of the distance between the foci and the major axis.
The cutting off of light from one celestial body by another.
The plane of Earth's orbit about the Sun
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
Material such as glass and fragmented rock thrown out of an impact
crater during its formation.
A closed curve that is formed from two foci or points in which the
sum of the distances from any point on the curve to the two foci is a
constant. Johannes Kepler first discovered that the orbits of the
planets are ellipses, not circles; he based his discovery 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.
Related to wind deposits and associated effects.
The ejection of volcanic materials (lavas, pyroclasts and volcanic
gases) onto the surface, either from a central vent, a fissure or a
group of fissures.
A dramatic volcanic eruption which throws debris high into the air
for hundreds of miles. The lava is low in silicate and can be very
dangerous for people near by. An example is Mount St. Helens in
A bright region of the photosphere seen in white light, seldom
visible except near the solar limb.
A crack or break in the crust of a planet along which slippage or
movement can take place.
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.
A narrow opening or crack of considerable length and depth.
A sudden eruption of energy on the solar disk lasting minutes to
hours, from which radiation and particles are emitted.
A cuspate linear feature
A flow terrain
A long, narrow, shallow depression.
Named for the Greek Earth goddess Gaea, this hypothesis holds that
the Earth should be regarded as a living organism. British biologist
James Lovelock first advanced this idea in 1969.
Jupiter's four largest moons: Io, Europa, Ganymede and Callisto;
discovered independently by Galileo and Marius.
An elongated, relatively depressed crustal unit or block that is
bounded by faults on its sides.
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
A pattern of small cells seen on the surface of the Sun caused by
the convective motions of the hot solar gas.
An increase in temperature caused when the atmosphere absorbs
incoming solar radiation but blocks outgoing thermal radiation; carbon
dioxide is the major factor.
A mutual physical force attracting two bodies.
A narrow wavelength of red light which is emitted and absorbed by
the element hydrogen; this wavelength is often used to study the Sun.
Sun centered; see Copernicus, Kepler, Galileo.
The point at which the solar wind meets the interstellar medium or
solar wind from other stars.
The space within the broundary of the heliopause containing the Sun
and solar system.
A half of the celestial sphere that is divided into two halves by
either the horizon, the celestial equator, or the ecliptic.
high-pressure mineral phase
In this 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
Center of persistent volcanism, thought to be the surface expression
of a rising hot plume in Earth's mantle.
Uneven, lumpy terrain.
Planetary scientists use this word to refer to water, methane, and
ammonia, which usually occur as solids in the outer solar system.
Rock or mineral that solidified from molten or partly molten
Rocks melted during impact, including small particles dispersed in
various 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 can contain a
small but measurable amount of the impactor.
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
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.
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.
A region of charged particles in a planet's upper atmosphere; the
part of the earth's atmosphere beginning at an altitude of about 400
kilometers (25 miles) and extending outward 400 kilometers (250 miles)
Any of the four outer, gaseous planets: Jupiter, Saturn, Uranus, and
Zero K is absolute zero; ice melts at 273 K (0° C, 32° F); water
boils at 373 K ( 100° C, 212° F).
One kilogram is equivalent to 1,000 grams or 2.2 pounds; the mass of
a liter of water.
One kilometer is equivalent to 1,000 meters or 0.62 miles.
An intersecting valley complex.
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.
A general term for molten rock that is extruded onto the surface.
A tunnel formed underneath the surface of a solidfying lava flow.
The hemisphere that faces forward, into the direction of motion of a
satellite that keeps the same face toward the planet.
The side of an object that is sheltered from the wind.
An embankment, continuous dike or ridge.
Electromagnetic radiation that is visible to the eye.
The distance light travels in a year, at the rate of 300,000
kilometers per second (671 million miles per hour); 1 light-year is
equivalent to 9.46053e12 km, 5,880,000,000,000 miles or 63,240 AU.
The outer edge of the apparent disk of a celestial body.
An elongate marking.
Linear topographic feature that may depict crustal structure.
Having lobes or resembling a lobe.
A dark spot.
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.
A region of space near a magnetized body where magnetic forces can
A special telescope which analyzes the color and polarization of
sunlight in order to measure the magnetic field of the Sun.
The boundary of the magnetosphere, lying inside the bow shock.
The region of space in which a planet's magnetic field dominates
that of the solar wind.
The portion of a planetary magnetosphere which is pushed in the
direction of the solar wind.
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.
Latin 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.
A mesa, flat-topped elevation.
A broad, flattop, erosional hill or mountain, commonly bounded by
The luminous phenomenon seen when a meteoroid enters the atmosphere,
commonly known as a shooting star.
A part of a meteoroid that survives through the Earth's atmosphere.
A small rock in space.
This is 1/1000 of a bar; the standard sea-level pressure is about
Another term used for asteroids.
A diffuse mass of interstellar dust and gas.
A fundamental particle supposedly produced in massive numbers by the
nuclear reactions in stars; they are very hard to detect because the
vast majority of them pass completely through the Earth without
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=mc2. This is the source of the Sun's energy and,
ultimately, of (almost) all energy on Earth.
The angle between a body's equatorial plane and orbital plane.
The blockage of light by the intervention of another object; a
planet can occult (block) the light from a distant star.
A planetary surface that has been modified little since its
formation typically featuring large numbers of impact craters;
(compare to young).
The path of an object that is moving around a second object or
Shaped like an egg.
A geological term denoting the time in Earth history between 570 and
245 million years ago.
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 of 1 to 10 meters (3 to 33 feet) hour and are
associated with low-effusion-rate eruptions with little to no
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.
Shallow crater; scalloped, complex edge.
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 kilometers (30 miles) in diameter.
The outer filamentary region of a sunspot.
The point in the orbit closest to the planet.
The point in the orbit closest to the Earth.
The point in its orbit where a planet is closest to the Sun.
To cause a planet or satellite to deviate from a theoretically
regular orbital motion.
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.
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.
Bright regions seen in the solar chromosphere.
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
Broad plains that occupy lowlands on planetary surfaces.
A plateau or high plain.
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.
A special property of light; light has three properties, brightness,
color and polarization.
A geological term denoting the time in Earth history prior to 570
million years ago.
A ridge formed by the uplift of a lava flow crust due to pressure of
the flowing lava.
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.
A generally circular crater produced by a phreatic eruption
resulting from emplacement of a lava flow over wet ground.
Pertaining to clastic (broken and fragmented) rock material formed
by volcanic explosion or aerial expulsion from a volcanic vent.
A light vesicular form of volcanic glass with a high silica content;
it is usually light in color and will float on water.
Energy radiated in the form of waves or particles; photons.
Regions of charged particles in a magnetosphere.
A star that has low surface temperature and a diameter that is large
relative to the Sun.
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.)
The amount of small detail visible in an image; low resolution shows
only large features, high resolution shows many small details.
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.
The rotation or orbital motion of an object in a clockwise direction
when viewed from the north pole of the ecliptic; moving in the
opposite sense from the great majority of solar system bodies.
Fine-grained extrusive igneous rock, commonly with phenocrysts of
quartz and feldspar in a glassy groundmass.
A fracture or crack in a planet's surface caused by extension. On
some volcanoes, subsurface intrusions are concentrated in certain
directions; this causes tension at the surface and also means that
there will be more eruptions in these "rift zones."
An elongated valley formed by the depression of a block of the
planet's crust between two faults or groups of faults of approximately
The closest a fluid body can orbit to its parent planet without
being pulled apart by tidal forces.
The term applied to scarps on planetary surfaces; many scarps are
thought to be the surface expression of faults within the crust of the
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.
A body that revolves around a larger body.
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.
A lobate or irregular scarp.
One-half of the longest dimension of an ellipse.
Striated conical fracture surfaces produced by meteorite impact into
fine-grained, brittle rocks such as limestone.
A satellite that constrains the extent of a planetary ring through
Any of several extensive regions where ancient Precambrian
crystalline rocks are exposed at the Earth's surface.
A volcano in the shape of a flattened dome, broad and low, built by
flows of very fluid lava.
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.
Of, relating to, or expressed in relation to stars or
Rotation time measured with respect to the fixed stars rather than
the Sun or body orbited.
This phrase literally means iron-loving elements. It includes
Iridium, Osmium, Platinum and Plladium, which are found in the
metal-rich interiors of chemically segregated asteroids and planets;
consequently, these elements are extremely rare on Earth's surface.
A rock or mineral whose structure is dominated by bonds of silicon
and oxygen atoms (ie. olivine).
The approximately 11-year, quasi-periodic variation in the frequency
or number of solar active events.
The large cloud of gas and dust from which the Sun and planets
condensed 4.6 billion years ago.
A tenuous flow of gas and energetic charged particles, mostly
protons and electrons -- plasma -- which stream from the Sun; typical
solar wind velocities are almost 350 kilometers (217 miles) per
A low, steep-sided cone built up from fluid pyroclasts coating the
surface around a vent.
A device that measures the amount of reflected or radiated energy
from a surface in two or more wavelengths.
The distribution of wavelengths and frequencies.
speed of light
Light speed equals 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.
The grass-like patterns of gas seen in the solar atmosphere.
The cold region of a planetary atmosphere above the convecting
regions (the troposphere), usually without vertical motions but
sometimes exhibiting strong horizontal jet streams.
A dense, high-pressure phase of quartz that has so far been
identified only in shock-metamorphosed, quartz-bearing rocks from
meteorite impact craters.
The process of one lithospheric plate descending beneath another.
Sublimation occurs when a substance changes directly from a solid to
a gas without becoming liquid.
Subparallel furrows and ridges.
A heavy, corrosive, oily, dibasic strong acid H2SO4 that is
colorless when pure; it is a vigorous oxidizing and dehydrating agent.
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
The planets Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are
superior planets because their orbits are farther from the Sun than
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. These orbits are commonly used for communications
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
SAR is a side-looking imaging system that uses the Doppler effect to
sharpen the effective resolution in the cross-track direction.
The surface rocks that an asteroid or comet impactor smashes into in
a meteorite impact event.
The deformation forces acting on a planet's crust.
Natural, silica-rich, homogeneous glasses produced by complete
melting, and dispersed as droplets during terrestrial impact events.
Tektites 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.
The dividing line between the illuminated and the unilluminated part
of the moon's or a planet's disk.
An extensive land mass.
A tile; polygonal ground.
A small domical mountain or hill.
The 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
The frictional heating of a satellite's interior due to flexure
caused by the gravitational pull of its parent planet and possibly
The hemisphere that faces backwards, away from the direction of
motion of a satellite that keeps the same face toward the planet.
Satellites which orbit at the Lagrangian points, 60° ahead of and
60° behind another satellite. For example, Telesto and Calypso are
trojans of Saturn's satellite Tethys.
The lower regions of a planetary atmosphere where convection keeps
the gas mixed and maintains a steady increase of temperature with
depth. Most clouds are in the troposphere.
The general term for consolidated pyroclastic debris.
Electromagnetic radiation at wavelengths shorter than the violet end
of visible light; the atmosphere of the Earth effectively blocks the
transmission of most ultraviolet light.
The dark central region of a sunspot.
A sinuous valley.
The opening in the crust through which volcanic material erupts.
Compounds with low melting temperatures, such as hydrogen, helium,
water, ammonia, carbon dioxide and methane.
(1) A vent in the planetary surface through which magma and
associated gases and ash erupt. (2) The form or structure produced by
the erupted materials.
The gravitational force exerted on a body.
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.
Electromagnetic radiation of very short wavelength and very high
energy; x-rays have shorter wavelengths than ultraviolet light but
longer wavelengths than cosmic rays.
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 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.