Chloe+Y+Space+Dictionary

Space Dictionary

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**Accretion ** Accumulation of dust and gas into larger bodies. **Albedo ** Reflectivity of an object; ratio of reflected light to incident light.  A dark or light marking on the surface of an object that might not be a geological or topographical feature. **Allocthonous ** (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. **Angstrom ** 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. **Aphelion ** The point in its orbit where a planet is farthest from the Sun. **Apoapsis ** The point in orbit farthest from the planet. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Apogee ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The point in orbit farthest from the Earth. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Ash ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The fine-grained material produced by a [|pyroclastic] eruption. An ash particle is defined to have a diameter of less than 2 millimetres. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Asteroid number ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The average distance from the Earth to the Sun; 1 AU is 149,597,870 kilometres (92,960,116 miles). **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Atmosphere ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">One atmosphere is 14.7 pounds per square inch (105 Newtons per square meter); the average atmospheric pressure at sea level on Earth. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Aurora ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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 Australia//. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A general term for dark-colored, igneous rocks composed of minerals that are relatively rich in iron and magnesium.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">An object whose gravity is so strong that the escape velocity exceeds the speed of light.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">An exploding meteorite.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A course-grained rock, composed of angular, broken rock fragments held together by a mineral cement or a fine-grained matrix.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A conspicuous, isolated, flattop hill with steep slopes. 

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**<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">calcium K ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A narrow wavelength of blue light which is emitted and absorbed by ions of the element calcium.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">carbonate ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A compound containing carbon and oxygen; an example is calcium carbonate (limestone).  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A texture found in metamorphic rocks in which brittle minerals have been broken, crushed and flattened during shearing. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">catena ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A chain of craters. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">cavus ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Hollows, irregular depressions.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">chaos ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A distinctive area of broken terrain.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A canyon. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">chromosphere ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The lower level of the solar atmosphere between the photosphere and the corona.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Loose, vesicular volcanic ejecta 4 to 32 millimeters (.16 to 1.28 inches) in diameter.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">A conical hill formed by the accumulation of [|pyroclastic]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">fragments that fall to the ground in an essentially solid condition.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A fragment of rock that has been transported, either by volcanic or sedimentary processes. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">colles ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A small hill or knob. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">coma ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The dust and gas surrounding an active comet's nucleus.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A volcano composed of interbedded lava and pyroclastic material commonly with steep slopes.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Fluid circulation driven by temperature gradients; the transfer of heat by this automatic circulation (see also [|Educator's Guide to Convection]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">).  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">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]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">-shaped feature. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">coronagraph ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A special telescope which blocks light from the disk of the Sun in order to study the faint solar atmosphere.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">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]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">crater ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">1) A depression formed by the impact of a meteorite. 2) A depression around the orifice of a volcano.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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**] <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">A geological term denoting the interval of Earth history beginning around 144 million years ago and ending 66 million years ago. [ [|more]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">]  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">A major stratigraphic boundry on Earth marking the end of the [|Mesozoic Era]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">, best known as the age of the [|dinosaurs]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">. The boundary is defined by a global extinction event that caused the abrupt demise of the majority of all life on Earth.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A natural glass formed by shock pressure from any of several minerals without melting; it is found only in association with meteorite impact craters.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The ratio of electric flux density to electric field.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The visible surface of the Sun (or any heavenly body) projected against the sky.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The apparent change in wavelength of sound or light caused by the motion of the source, observer or both. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">dorsum ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A ridge. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Noncircular; elliptical (applied to an orbit).  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A value that defines the shape of an ellipse or planetary orbit; the ratio of the distance between the foci and the major axis.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The cutting off of light from one celestial body by another.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The plane of Earth's orbit about the Sun  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Material such as glass and fragmented rock thrown out of an impact crater during its formation.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">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]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">first discovered that the orbits of the planets are ellipses, not circles; he based his discovery on the careful observations of [|Tycho Brahe]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Fissures that are parallel in trend to each other, but offset to either the left or right.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Related to wind deposits and associated effects.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">A bright region of the [|photosphere]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">seen in white light, seldom visible except near the solar [|limb]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A crack or break in the crust of a planet along which slippage or movement can take place. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">filament ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">A strand of cool gas suspended over the [|photosphere]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">by magnetic fields, which appears dark as seen against the [|disk]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">of the Sun; a filament on the [|limb]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">of the Sun seen in emission against the dark sky is called a [|prominence]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A narrow opening or crack of considerable length and depth.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">A sudden eruption of energy on the solar [|disk]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">lasting minutes to hours, from which radiation and particles are emitted. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">flexus ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A cuspate linear feature **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">fluctus ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A flow terrain **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">fossa ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A long, narrow, shallow depression. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">'s four largest moons: [|Io]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">, [|Europa]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">, [|Ganymede]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">and [|Callisto]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">; discovered independently by [|Galileo]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">and [|Marius]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">An elongated, relatively depressed crustal unit or block that is bounded by faults on its sides. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">geosynchronous orbit ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">granulation ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A pattern of small cells seen on the surface of the Sun caused by the convective motions of the hot solar gas.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">An increase in temperature caused when the atmosphere absorbs incoming solar radiation but blocks outgoing thermal radiation; carbon dioxide is the major factor.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A mutual physical force attracting two bodies. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A narrow wavelength of red light which is emitted and absorbed by the element hydrogen; this wavelength is often used to study the Sun.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Sun centered; see [|Copernicus]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">, [|Kepler]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">, [|Galileo]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">heliopause ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">The point at which the [|solar wind]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">meets the interstellar medium or solar wind from other stars. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">heliosphere ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The space within the broundary of the heliopause containing the Sun and solar system.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">A half of the celestial sphere that is divided into two halves by either the horizon, the celestial equator, or the [|ecliptic]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">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]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">is the high-pressure polymorph of quartz, a common crustal mineral.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Center of persistent volcanism, thought to be the surface expression of a rising hot plume in Earth's mantle.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Uneven, lumpy terrain. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Planetary scientists use this word to refer to water, methane, and ammonia, which usually occur as solids in the outer solar system.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Rock or mineral that solidified from molten or partly molten material.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">The inclination of a planet's orbit is the angle between the plane of its orbit and the [|ecliptic]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">. The inclination of a moon's orbit is the angle between the plane of its orbit and the plane of its primary's equator. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">inferior planets ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The planets Mercury and Venus are inferior planets because their orbits are closer to the Sun than is Earth's orbit. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">interplanetary magnetic field (IMF) ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">The magnetic field carried with the [|solar wind]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Any of the four outer, gaseous planets: [|Jupiter]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">, [|Saturn]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">, [|Uranus]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">, and [|Neptune]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">. 

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**<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">kelvin (K) ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Zero K is absolute zero; ice melts at 273 K (0° C, 32° F); water boils at 373 K ( 100° C, 212° F). **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">kilogram (kg) ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">One kilogram is equivalent to 1,000 grams or 2.2 pounds; the mass of a liter of water. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">kilometer (km) ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">One kilometer is equivalent to 1,000 meters or 0.62 miles. 

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**<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">labes ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A landslide. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">labyrinthus ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">An intersecting valley complex. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">lacus ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A lake.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A general term for molten rock that is extruded onto the surface.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A tunnel formed underneath the surface of a solidfying lava flow.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The hemisphere that faces forward, into the direction of motion of a satellite that keeps the same face toward the planet.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The side of an object that is sheltered from the wind.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">An embankment, continuous dike or ridge.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Electromagnetic radiation that is visible to the eye.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">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]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The outer edge of the apparent disk of a celestial body. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">linea ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">An elongate marking.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Linear topographic feature that may depict crustal structure.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Having lobes or resembling a lobe. 

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**<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">macula ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A dark spot.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Molten rock within the crust of a planet that is capable of intrusion into adjacent crustal rocks or extrusion onto the surface. [|Igneous]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">rocks are derived from magma through solidification and related processes or through eruption of the magma at the surface.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A region of space near a magnetized body where magnetic forces can be detected. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">magnetograph ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A special telescope which analyzes the color and polarization of sunlight in order to measure the magnetic field of the Sun.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">The boundary of the magnetosphere, lying inside the [|bow shock]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">The region of space in which a planet's magnetic field dominates that of the [|solar wind]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">The portion of a planetary magnetosphere which is pushed in the direction of the [|solar wind]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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//. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">mare ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">mensa ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A mesa, flat-topped elevation.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A broad, flattop, erosional hill or mountain, commonly bounded by steep slopes.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The luminous phenomenon seen when a meteoroid enters the atmosphere, commonly known as a shooting star.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A part of a meteoroid that survives through the Earth's atmosphere.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A small rock in space.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">This is 1/1000 of a [|bar]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">; the standard sea-level pressure is about 1,013 millibars.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Another term used for asteroids. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">mons ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A mountain. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A diffuse mass of interstellar dust and gas.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">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]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">'s famous equivalence E=mc2. This is the source of the [|Sun]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">'s energy and, ultimately, of (almost) all energy on Earth. 

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**<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">oceanus ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">An ocean.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The angle between a body's equatorial plane and orbital plane.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The blockage of light by the intervention of another object; a planet can occult (block) the light from a distant star.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">A planetary surface that has been modified little since its formation typically featuring large numbers of impact craters; (compare to [|young]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">).  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Optical depth is a measure of the transparency of a ring system. When a ring is "optically thick" (i.e., the optical depth is large), the ring is nearly opaque and very little light passes through. When a ring is "optically thin" (i.e., the optical depth is small), very little material is present and most of the light passes through.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The path of an object that is moving around a second object or point.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Shaped like an egg. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A geological term denoting the time in Earth history between 570 and 245 million years ago.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">A circular feature on the surface of dark icy moons such as [|Ganymede]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">and [|Callisto]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">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. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">palus ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A swamp.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Shallow crater; scalloped, complex edge.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The outer filamentary region of a sunspot. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">periapsis ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The point in the orbit closest to the planet.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The point in the orbit closest to the Earth.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The point in its orbit where a planet is closest to the Sun.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">To cause a planet or satellite to deviate from a theoretically regular orbital motion.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">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]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">and [|faculae]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">are observed in the photosphere.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">plage ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Bright regions seen in the solar chromosphere.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">planitia ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Broad plains that occupy lowlands on planetary surfaces.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A plateau or high plain.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">polarization ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A special property of light; light has three properties, brightness, color and polarization.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A geological term denoting the time in Earth history prior to 570 million years ago.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A ridge formed by the uplift of a lava flow crust due to pressure of the flowing lava.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">An eruption of hot gases above the [|photosphere]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">of the [|Sun]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">. Prominences are most easily visible close to the limb of the Sun, but some are also visible as bright streamers on the photosphere. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">promontorium ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A cape.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A generally circular crater produced by a phreatic eruption resulting from emplacement of a lava flow over wet ground.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Pertaining to clastic (broken and fragmented) rock material formed by volcanic explosion or aerial expulsion from a volcanic vent.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A light vesicular form of volcanic glass with a high silica content; it is usually light in color and will float on water. 

 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Energy radiated in the form of waves or particles; photons.

 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Regions of charged particles in a [|magnetosphere]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A star that has low surface temperature and a diameter that is large relative to the Sun. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">regio ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Region.

 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The layer of rocky debris and dust made by metoritic impact that forms the uppermost surface of planets, satellites and asteroids. <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">, Theory of  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">More accurately describes the motions of bodies in strong gravitational fields or at near the [|speed of light]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">than [|newton]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">ian mechanics. All experiments done to date agree with relativity's predictions to a high degree of accuracy. (Curiously, [|Einstein]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">received the Nobel prize in 1921 not for Relativity but rather for his 1905 work on the photoelectric effect.) **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">resolution ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The amount of small detail visible in an image; low resolution shows only large features, high resolution shows many small details.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the [|ecliptic]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">; moving in the opposite sense from the great majority of solar system bodies.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Fine-grained extrusive igneous rock, commonly with phenocrysts of quartz and feldspar in a glassy groundmass.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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."  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">rima ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">A [|fissure]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Roche limit ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The Roche Limit was first described by Edouard Roche in 1848. It is the closest distance a body can come to a planet without being pulled apart by the planet's tidal (gravity) force. As a result, large moons cannot survive inside the Roche Limit. On July 7, 1992, Comet Shoemaker-Levy 9 broke apart into 21 pieces due to tidal forces when it passed within Jupiter's Roche Limit; on the subsequent pass, each of the comet's pieces collided with Jupiter. <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin-left: 36pt;">If a planet and a moon have identical densities, then the Roche Limit is 2.446 times the radius of the planet. The Roche Limits for the ringed planets are: · <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Jupiter - 175,000 km (108,000 miles) · <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Saturn - 147,000 km ( 92,000 miles) · <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Uranus - 62,000 km ( 39,000 miles) · <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Neptune - 59,000 km ( 37,000 miles) <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">This limit represents the rough boundary between each planet's ring system and its innermost moons.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">The term applied to [|scarps]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">on planetary surfaces; many scarps are thought to be the surface expression of faults within the crust of the planetary object. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A body that revolves around a larger body.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">scopulus ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A lobate or irregular scarp. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">semimajor axis ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">One-half of the longest dimension of an ellipse.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Striated conical fracture surfaces produced by meteorite impact into fine-grained, brittle rocks such as limestone.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A satellite that constrains the extent of a planetary ring through gravitational forces.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">Any of several extensive regions where ancient [|Precambrian]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">crystalline rocks are exposed at the Earth's surface.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A volcano in the shape of a flattened dome, broad and low, built by flows of very fluid lava.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Of, relating to, or expressed in relation to stars or constellations.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Rotation time measured with respect to the fixed stars rather than the Sun or body orbited.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A rock or mineral whose structure is dominated by bonds of silicon and oxygen atoms (ie. olivine). **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">sinus ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A bay. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">solar cycle ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The approximately 11-year, quasi-periodic variation in the frequency or number of solar active events.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">The large cloud of gas and dust from which the [|Sun]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">and planets condensed 4.6 billion years ago.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">A tenuous flow of gas and energetic charged particles, mostly protons and electrons -- [|plasma]<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">-- which stream from the Sun; typical solar wind velocities are almost 350 kilometers (217 miles) per second.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A low, steep-sided cone built up from fluid pyroclasts coating the surface around a vent.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A device that measures the amount of reflected or radiated energy from a surface in two or more wavelengths.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The distribution of wavelengths and frequencies. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">speed of light ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">spicules ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The grass-like patterns of gas seen in the solar atmosphere.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The cold region of a planetary atmosphere above the [|convecting] regions (the [|troposphere]), usually without vertical motions but sometimes exhibiting strong horizontal jet streams.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A dense, high-pressure phase of quartz that has so far been identified only in shock-metamorphosed, quartz-bearing rocks from meteorite impact craters.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The process of one lithospheric plate descending beneath another. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">sublime ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Sublimation occurs when a substance changes directly from a solid to a gas without becoming liquid. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">sulcus ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Subparallel furrows and ridges.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A heavy, corrosive, oily, dibasic strong acid H2SO4 that is colorless when pure; it is a vigorous oxidizing and dehydrating agent.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">superior planets ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The planets Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are superior planets because their orbits are farther from the Sun than Earth's orbit.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">SAR is a side-looking imaging system that uses the [|Doppler effect] to sharpen the effective resolution in the cross-track direction. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The surface rocks that an asteroid or comet impactor smashes into in a meteorite impact event.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The deformation forces acting on a planet's crust.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The dividing line between the illuminated and the unilluminated part of the moon's or a planet's disk. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">terra ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">An extensive land mass. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">tessera ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A tile; polygonal ground. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">tholus ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A small domical mountain or hill.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The frictional heating of a satellite's interior due to flexure caused by the gravitational pull of its parent planet and possibly neighboring satellites.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The hemisphere that faces backwards, away from the direction of motion of a satellite that keeps the same face toward the planet.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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].  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The general term for consolidated pyroclastic debris. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">umbra ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The dark central region of a sunspot. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">undae ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Dunes. 

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**<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">vallis ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">A sinuous valley. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">vastitas ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Widespread lowlands.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The opening in the crust through which volcanic material erupts. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt;">volatile ** <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Compounds with low melting temperatures, such as hydrogen, helium, water, ammonia, carbon dioxide and methane.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">(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. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">The gravitational force exerted on a body.  <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Electromagnetic radiation of very short wavelength and very high energy; x-rays have shorter wavelengths than [|ultraviolet] light but longer wavelengths than [|cosmic rays]. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">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. 

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 <span style="font-family: 'Tahoma','sans-serif'; font-size: 20pt; margin: 0cm 0cm 0pt 36pt;">Twelve constellations dividing the [|ecliptic] into approximately equal parts. Each month the [|Sun] is in a different constellation of the zodiac.