(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 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.
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 = 0.987 atmosphere = 101,300 pascals = 14.5 lbs/square inch = 100,000 Newtons
per square meter.
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.
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 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; an example is calcium carbonate (limestone).
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.
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.
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
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.
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.
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.
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
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.
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 1980.
A crack or break in the crust of a planet along which slippage or
movement can take place.
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.
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.
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.
An increase in temperature caused when the atmosphere absorbs incoming solar radiation but blocks outgoing thermal radiation; carbon dioxide is the major factor.
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.
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
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.
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.
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.
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.
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.
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
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 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.
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.
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
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.
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 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.
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.
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.
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.
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.
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.
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."
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 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 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 second.
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.
spicules
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.
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.
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
satellites.
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.
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 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.
The frictional heating of a satellite's interior due to flexure
caused by the gravitational pull of its parent planet and possibly
neighboring 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
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.
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.
(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.
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.
