Terms and Definitions
Terms and Definitions
A, B, C,
D, E, F,
G, H, I,
J, K, L,
M, N, O,
P, Q, R,
S, T, U,
V, W, X,
- Accumulation of dust and gas into larger bodies.
- 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 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
- Alpha Centauri
- The closest bright star to our solar system.
- A unit of length = 1.0E-08cm.
- antipodal point
- 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.
- 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;
1 AU is 149,597,870 kilometers (92,960,116 miles).
- One atmosphere is 14.7 pounds per square inch;
standard 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.
- 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 australis.
- A unit of pressure, equal to the sea-level pressure of Earth's
atmosphere; 1 bar is equivalent to 0.987 atmosphere or 10,000 newtons
per square meter.
- 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.
- black hole
- An object whose gravity is so strong that the escape velocity exceeds
the speed of light.
- An exploding meteorite.
- bow shock
- 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.
- calcium K
- 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 volcano.
- A compound containing carbon and oxygen; an example is calcium carbonate (limestone).
- 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.
- 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.
- A distinctive area of broken terrain.
- A canyon.
- The lower level of the solar atmosphere between the photosphere and the corona.
- Loose, vesicular volcanic ejecta 4 to 32 millimeters (.16 to 1.28 inches) in diameter.
- cinder cone
- 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.
- composite volcano
- A volcano composed of interbedded lava and pyroclastic material
commonly with steep slopes.
- Fluid circulation driven by large temperature gradients;
the transfer of heat by this automatic circulation (see also
Educator's Guide to Convection).
- 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.
- 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.
- 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 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 boundary
- 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 melt.
- 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.
- 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.
- dielectric constant
- The ratio of electric flux density to electric field.
- 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.
- A ridge.
- 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
- effusive eruption
- 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.
- An oval. 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.
- explosive eruption
- 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 1980.
- A bright region of the photosphere seen
in white light, seldom visible except near the solar
- 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
- 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.
- Gaia Hypothesis
- 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.
- Galilean moons
- Jupiter's four largest moons:
Callisto; discovered independently by
- 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
- A pattern of small cells seen on the surface of
the Sun caused by the convective motions of the hot solar gas.
- greenhouse effect
- 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,
- 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
- 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 mineral.
- hot spot
- 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.
- Rrock or mineral that solidified from molten or partly molten material.
- impact melt
- 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
The inclination of a moon's orbit is
the angle between the plane of its orbit and the plane of its
- 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.
- 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) or more.
- Jovian planet
- Any of the four outer, gaseous
- kelvin (K)
- Zero K is absolute zero; water melts at 273 K (0° C,
32° F); water boils at 373 K ( 100° C, 212° F).
- kilogram (kg)
- One kilogram is equivalent to 1,000 grams or 2.2 pounds;
the mass of a liter of water.
- kilometer (km)
- One kilometer is equivalent to 1,000 meters or 0.62 miles.
- A landslide.
- An intersecting valley complex.
- 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.
- A general term for molten rock that is extruded onto the surface.
- lava tube
- A 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.
- 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.
- light year
- 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.
- magnetic field
- 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
- 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 1,013 millibars.
- minor planets
- Another term used for asteroids.
- A mountain.
- 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 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
equivalence E=mc2. This is the source of the Sun's
energy and, ultimately, of (almost) all energy on Earth.
- An ocean.
- 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 point.
- 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 fountaining.
- A circular feature on the surface of dark icy moons such as
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.
- A swamp.
- 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 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;
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
- 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.
- 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.
- pressure ridge
- 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 cape.
- 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
- red giant
- 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."
- 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
- A fissure.
- Roche limit
- 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 planetary object.
- 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.
- 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 that constrains the extent of a planetary ring
through gravitational forces.
- 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.
- 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
- 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).
- A bay.
- solar cycle
- The approximately 11-year, quasi-periodic variation in the 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 almost 350 kilometers (217 miles)
- spatter cone
- 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).
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.
- sulfuric acid
- 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 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. These orbits are commonly used for communications
- 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
- SAR is a side-looking imaging system that uses the
Doppler effect to sharpen the effective
resolution in the cross-track direction.
- target rocks
- 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.
- tidal forces
- 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 forces.
- tidal heating
- The frictional heating of a satellite's interior due to flexure
caused by the gravitational pull of its parent planet and possibly
- trailing hemisphere
- The hemisphere that faces backwards, away from the direction of
motion of a satellite that keeps the same face toward the planet.
- Trojan satellites
- 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
- 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
- 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.
- Widespread lowlands.
- 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
- 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.
- 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.