a'a: blocky, angular, and rough type of lava flow
achondrite: a stony meteorite, coarsely crystallized, with sizable fragments of various minerals visible to the naked eye
accretion: the growth of planets from smaller objects by impact, one impact at a time. After formation, planets are said to have "accreted" from small objects"
Adams, John Couch 1819-1892: English astronomer and mathematician who, at the age of 24, was the first person to predict the position of a planetary mass beyond Uranus. But, unfortunately, Adams did not publish his prediction. Galle confirmed the existence of Neptune based on independent calculations done by Le Verrier
aeolian (eolian): pertaining to the wind, especially said of wind-related processes or of rocks, soils and landforms produced or eroded by the wind
albedo: the ratio of the amount of light reflected by an object and the amount of incident light; a measure of the reflectivity or intrinsic brightness of an object (a white, perfectly reflecting surface would have an albedo of 1.0; a black perfectly absorbing surface would have an albedo of 0.0)
altimetry: the measurement of elevation or altitude
anomaly: a deviation from the common rule, type, or form; something abnormal or inconsistent
anorthsite: a type of igneous rock composed almost entirely of plagioclase feldspar, a group of minerals that make up about 60% of the Earth's crust
antenna: a conductor by which electromagnetic waves are transmitted or received
antipodal point: the point that is directly on the opposite side of the planet
aphelion: the point in its orbit where a planet is farthest from the Sun; when referring to objects orbiting the Earth the term apogee is used; the term apoapsis is used for orbits around bodies. (opposite of perihelion)
apoapsis: the farthest point in an orbit from the body being orbited
apogee: apoapsis in Earth orbit
Apollo: U.S. Space Program which included 6 piloted lunar landings between 1969 and 1972. Apollo astronauts collected and returned 382 kilograms of rock and sediment samples from the Moon
arcuate: curved or bent
asteroid: a small, mostly rocky body orbiting the Sun. Asteroids range in size from 1000 kilometers in diameter to tiny objects you could hold in your hand. Most asteroids orbit the Sun between Mars and Jupiter, and are the source of most meteorites
astrobiology: study of the origin, distribution, and destiny of life in the universe
Atlas: in greek mythology, brother of Promethius and grandfather of Hermes (Mercury). Condemned to stand forever supporting the heavens on his shoulders. The Atlantic Ocean is named for him
atmosphere: mixture of gases that surrounds a planet
AU: (astronomical unit) The average Earth-Sun distance, equal to 149.5 million kilometers or 93 million miles
BPS: Bits Per Second, same as baud rate
barycenter: the common center for mass about which two or more bodies rotate
basalt: fine-grained, dark-colored igneous rock composed primarily of plagioclase feldspar and pyroxene; other minerals such as olivine and ilmenite are usually present. Basalt is the most common volcanic rock on the terrestrial planets
basin: a depressed area with no surface outlet
bedrock: continuous solid rock that underlies regolith and is exposed at outcrops
Brahe, Tycho 1546-1601: (a.k.a Tyge Ottesen) Danish astronomer whose accurate astronomical observations formed the basis for Johannes Kepler's laws of planetary motion
breccia: rock consisting of angular, coarse fragments embedded in a fine-grained matrix
c: the speed of light, 299,792 km per second
CCD: Charged Coupled Device, a solid-state imaging detector
caldera: a large, basin-shaped volcanic depression, more or less circular in form, the diametar of which is many times greater than that of the included vent, or vents, produced by the collapse of overlying materials into an emptied magma chamber
carbon: an element with atomic number 6; symbol: C. Carbon is one of the four elements essential for life. (The others are hydrogen, oxygen, and nitrogen.)
carbon dioxide: a compound formed by combining one carbon atom with two oxygen atoms, making the molecule CO2. Carbon dioxide is an important part of the atmospheres of Venus and Mars. Carbon dioxide gas condenses to a solid below -78o C. This solid is commonly known as dry ice. The polar ice caps on Mars are made of frozen water and carbon dioxide
carbonate: common rock-forming mineral containing the carbonate ion, CO32. Common carbonate minerals are calcite, CaCO3, siderite, FeCaCO3, and magnesite, MgCO3
Cassini, Giovanni Domenico 1625-1712: (a.k.a. Jean Dominique) Italian-born French astronomer and first director of the Royal Observatory in Paris; discoverer of four of Saturn's moons (Tethys, Dione, Rhea and Iapetus) and the major division in its rings
catena: a chain of craters
celsius: a temperature scale that assigns the value 0o C to the freezing point of water and the value of 100o C to the boiling point of water at standard pressure
chaos: a suface or land area consisting of short, jumbled ridges or plateaus and valleys
chasma: a large canyon
chassigny: one of the meteorites thought to have come from Mars. Chassigny is a dunite, a rock composed almost entirely of the mineral olivine. The stone fell in France in 1815
chondrite: a stony meteorite, composed of finely crystallized material
comet: a small celestial body composed at least partially of ices. Comets either orbit the sun or pass through the solar system on hyperbolic orbital paths
conjunction: an inferior planet is said to be "in inferior conjunction" when it is directly between the Earth and the Sun. It is "in superior conjunction" when it is on the opposite side of the Sun from the Earth. A superior planet is "in conjunction" when it is on the opposite side of the Sun from the Earth. A superior planet obviously cannot have an inferior conjunction. When the Earth is at inferior conjunction with respect to an observer on a superior planet we say that planet is "in opposition" from Earth's perspective
convection: fluid circulation driven by large temperature gradients; the transfer of heat by this automatic circulation
Copernicus, Nicolaus 1473-1543: Polish astronomer who advanced the heliocentric theory that the Earth and other planets revolve around the Sun. This was highly controversial at the time as the Ptolemaic view of the universe, which was the prevailing theory for over 1000 years, was deeply ingrained in the prevailing philosophy and religion. (It should be noted, however, that the heliocentic idea was first put forth by Aristarcus of Samos in the 3rd century BC, a fact known to Copernicus but long ignored.)
core: the central region of a planet or moon frequently made of denser materials than the surrounding regions (mantle and crust). Earth and the Moon are thought to have cores of iron and nickel
crater (see Impact): a hole or depression. Most are roughly circular or oval in outline. On Earth most natural craters are of volcanic origin. On the Moon most are of impact origin
crust: the outermost layer of a planet or moon, above the mantle
DSCC: Deep Space Communications Complex, one of three DSN tracking sites at Goldstone, California; Madrid, Spain; and Canberra, Australia; spaced about equally around the Earth for continuous tracking of deep space vehicles
DSN: Deep Space Network, JPL’s worldwide spacecraft tracking facility
DSS: Deep Space Station, the antenna front-end equipment at DSCC
Deimos: in greek mythology, a son of Ares (Mars) who, with brother Phobos, was a constant companion to his father
density: measured in grams per cubic centimeter (or kilograms per liter); the density of water is 1.0; iron is 7.9; lead is 11.3
differentiation: chemical zonation caused by differences in the densities of minerals; heavy materials sink, less dense materials float
downlink: signal received from a spacecraft
drainage pattern: the configuration or arrangement in plan view of the natural stream valeys in an area. It is related to local geologic and geomorphologic features and history
dune: a low mound, ridge, bank, or hill of loose, windblown material (generally sand) capable of movement from place to place but always retaining it's characteristic shape
ESA: European Space Agency
Earth: third planet from the sun, a terrestrial planet
earthquake: sudden motion or trembling of Earth caused by the abrupt release of slowly accumulated elastic energy in rocks
eccentricity: the measure of the degree of elongation of an ellipse. For example, a circle has an eccentricity of 0, and a parabola (an open figure) an eccentricity of 1
ecliptic: the plane of the solar system
Einstein, Albert 1879-1955: German-American physicist; developed the Special and General Theories of Relativity which along with Quantum Mechanics is the foundation of modern physics. (See fusion, speed of light)
ejecta: material thrown out from and deposited around an impact crater
ellipse: oval. That the orbits of the planets are ellipses, not circles, was first discovered by Johannes Kepler based on the careful observations by Tycho Brahe
eolian: see aeolian
equator: an imaginary circle around a body which is everywhere equidistant from the poles, defining the boundary between the northern and southern hemispheres
erosion: removal of weathered rocks by moving water, wind, ice, or gravity
eruption: (see Source) A break out or brust of volcanic matter
escarpment: a long, more or less continuous cliff or relatively steep slope facing out in one general direction, produced by erosion or faulting
exponential notation: "1.23e4" means "1.23 times 10 to the fourth power" or 12,300; "5.67e-8" means "5.67 divided by 10 to the eighth power" or 0.0000000567"
extrusive: refers to ingeous rocks erupted on the surface
farenheit: a temperature scale with the freezing point of water assigned the value 32o F and the boiling point of water 212o F
fault: a fracture along which there has been movement or displacement
feldspar: a group of rock-forming minerals that make up about 60% of the Earth's crust
fluvial: of or pertaining to a river or rivers, especially said of processes, landforms and deposits resulting from the action of rivers
G: giga (billion —109)
g: gram, a thousandth of the metric standard unit of mass (see kg). The gram was originally based on the weight of a cubic centimeter of water
Galileo Galilei 1564-1642: Italian astronomer and physicist. The first to use a telescope to study the stars. Discoverer of the first moons of an extraterrestrial body (see above). Galileo was an outspoken supporter of Copernicus's heliocentric theory. In reaction to Galileo, the Church declared it heresy to teach that the Earth moved and imprisoned him. The Church clung to this position for 350 years; Galileo was formally exonerated in 1992.
Galileo: U. S. mission to study Jupiter's atmosphere, moons, and magnetosphere. The Galileo spacecraft was carried into space and released in 1989 by the shuttle Atlantis. It used its own rocket booster to leave Earth orbit. The Galileo spacecraft arrived at Jupiter in 1995 and completed its two-year primary mission before going on to explore Europa. Project Galileo web site.
Galle, Johann Gottfried 1812-1910: German astronomer who, with Heinrich Louis d'Arrest, made the first observation of Neptune based on calculations by Le Verrier. Though Galle was the first to observe Neptune, its discovery is usually credited to Adams (who made an earlier calculation) and Le Verrier
Ganymede: in greek mythology, a beautiful Trojan boy, son of Tros and Calirrhoe. Befriended by Zeus and made cupbearer to the Olympian gods. The third of the large moons orbiting Jupiter, commonly referred to as the Galilean Satellites.
geocentric distance: the distance from Earth
geologist: scientist who studies Earth, its materials, the physical and chemical changes that occur on the surface and in the interior, and the history of the planet and its life forms. Planetary geologists extend their studies to the Moon, planets, and other solid bodies in the Solar System
geomorphology: the shape, nature, origin, and development of present landforms on a planetary surface and their relationships to underlying structures, and of the history of geologic changes as recorded by these surface features
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
giant impact theory: an explanation for the origin of the Moon from Earth debris which collected in space after a projectile the size of planet Mars smashed into a growing Earth
graben: a long, relatively depressed crustal unit or block that is bounded by faults along its sides; a trough
gravitation: the mutual attraction of all masses in the universe
HGA: High-Gained Antenna onboard a spacecraft
half-life: the amount of time required for half of the mass of a radioactive isotope to decay
Hall, Asaph 1829-1907: American astronomer who discovered the two moons of Mars, Deimos and Phobos
Halley, Edmond 1656-1742: English astronomer who applied Newton's laws of motion to historical comet data and predicted correctly the reappearance of the comet which now bears his name
heliocentric: sun-centered; see Copernicus, Kepler, Galileo
heliocentric distance: the distance from the Sun
helium: an element with atomic number 2; symbol: He. It is the second most common element in the Sun and outer planets, but rare on the rocky planets
Hellas: the Greek name for Greece
hematite: a mineral, Fe3O4, composed of oxidized iron that may be the cause of the red color on Mars
Herschel, Sir William 1738-1822: British astronomer who discovered Uranus and cataloged more than 800 double stars and 2,500 nebulae
Hubble, Edwin Powell 1889-1953: American astronomer whose observations proved that galaxies are "island universes", not nebulae inside our own galaxy. His greatest discovery was the linear relationship between a galaxy's distance and the speed with which it is moving. The Hubble Space Telescope is named in his honor
hummocky: uneven; describing a terrain abounding in irregular knolls, mounds or other small elevations
Huygens, Christiaan 1629-1695: Dutch physicist and astronomer who first described the nature of Saturn's rings (1655) and discovered its moon Titan; also pioneered the use of the pendulum in clocks
hydrogen: an element with atomic number 1; symbol: H. It is the most abundant element in the solar system, making up 90 percent of the Sun. Hydrogen, carbon, nitrogen, and oxygen are essential for life
ice: used by planetary scientists to refer to water, methane, and ammonia which usually occur as solids in the outer solar system
igneous: rocks or processes involving the formation and solidification of hot, molten magma
ilmenite: opaque mineral found in basalt; nearly pure iron-titanium oxide (FeTiO3.)
impact: (see also crater) the forceful striking of one body, such as a meteorite, against another body such as a moon or planet
inclination: the inclination of a planet's orbit is the angle between the plane of its orbit and the ecliptic; the inclination of a moon's orbit is the angle between the plane of its orbit and the plane of its primary's equator
inferior planets: the planets Mercury and Venus are called inferior planets because their orbits are closer to the Sun than is Earth's orbit. (The other planets are called "superior planets")
intrusive: refers to igneous rocks formed underground
iron: an element with atomic number 26; symbol: Fe. Iron is one of the most abundant elements in the rocky planets. It is the most abundant element in the metallic cores of the inner planets
isotopes: two or more varieties of the same chemical element. Isotopes have different masses because they have different numbers of neutrons (for example 18O and 16O), thus they have different physical and chemical properties. Unstable isotopes undergo radioactive decay
JPL: Jet Propulsion Laboratory, operating division of the California Institute of Technology
Jovian: Jupiter-like planets, the gas giants: Jupiter, Saturn, Uranus, and Neptune
Jupiter: fifth planet from the sun, a gas giant or jovian planet
k: kilo (thousand)
KSC: Kennedy Space Center, Cape Canaveral, Florida
kelvin: the fundamental unit of temperature. It is not calibrated in terms of the freezing and boiling points of water, but in terms of energy itself. The number 0 K is assigned to the lowest possible temperature, called absolute zero. The sizes of the "degree" are the same as on the Celsius scale. On the Kelvin scale, the freezing point of water is 273 K and the boiling point is 373 K
Kepler, Johannes 1571-1630: German astronomer and mathematician. Considered a founder of modern astronomy. Using the positional data carefully amassed by Tycho Brahe, Kepler formulated the famous three laws of planetary motion. They comprise a quantitative formulation of Copernicus's theory that the planets revolve around the Sun
kilogram (kg) : = 1000 grams = 2.2 pounds, the mass of a liter of water. (see also)
kilometer (km) : = 1000 meters = 0.62 miles
Kuiper, Gerard 1905-1973: Dutch-born American astronomer best known for his study of the surface of the Moon; discovered Miranda and Nereid, found an atmosphere on Titan. (Dr. Kuiper was solidly Americanized; his name is pronounced to rhyme with "viper")
LGA: low-Gained Antenna onboard a spacecraft
lagrange points: Lagrange showed that three bodies can lie at the apexes of an equilateral triangle which rotates in its plane. If one of the bodies is sufficiently massive compared with the other two, then the triangular configuration is apparently stable. Bodies at such points are sometimes referred to as Trojans. The leading apex of the triangle is known as the leading Lagrange point or L4; the trailing apex is the trailing Lagrange point or L5. The colinear with the other two bodies are the L1, L2 and L3 unstable equilibrium points which can sometimes be useful places for spacecraft, eg SOHO
Lagrange, Joseph Louis 1736-1813: French (originally Italian, Giuseppe Luigi Lagrangia; born in Turin, moved to Paris and became a French citizen) mathematician and astronomer; made a number of contributions to the study of celestial mechanics. (5k gif)
landslide: general term to describe the process of mass movement (or avalanche) of material downslope by falling, sliding, or flowing, under the force of gravity. It also describes the landform (pile of debris) produced
laser: Light Amplification by Stimulated Emission of Radiation
latitude: the angular distance North or South from the Earth's equator measured in degrees on the meridian of a point; Equator being 0o and the poles 90o N and 90o S
lava: fluid magma that flows onto the surface of a planet or moon; erupted from a volcano or fissure. Also, the rock formed by solidification of this material
Le Verrier, Urbain Jean Joseph 1811-1877: French mathematician whose prediction of the position of an undiscovered planet (Neptune) that caused perturbations in the orbit of Uranus was the first to be confirmed (by Galle) though Adams had made a similar but unpublished prediction some months earlier
levee: zones in a lava flow where the lava between the zones is moving faster than the lava outside the zones
light year: distance that light can travel in 1 year. Light travels at a speed of 300,000 km / sec, so this distance is equal to 9.46 x 1012kilometers
limb: the outer edge of the apparent disk of a celestial body
lineament: a linear topographic feature, such as a fault line, aligned volcanoes, or a straight stream course
lithosphere: the brittle solid outer portion of a planet, generally overlying warmer plastic or partially molten layer below
longitude: the angular distance East or West, between the meridian of a particular place on Earth and that of Greenwich, England, expressed in degrees or time
Lowell, Percival 1855-1916. : American astronomer. He founded the Lowell Observatory in Arizona (1894), where his studies of Mars led him to believe that the linear markings (first noted by Schiaparelli) on the surface were "canals" and therefore that the planet was inhabited by intelligent beings. His successors later discovered Pluto
m: meter (U.S. spelling; elsewhere metre), the international standard of linear measurement
magma: term applied to molten rock in the interior of a planet or moon. When it reaches the surface, magma is called lava
magma chamber: a resevoir of magma in the shallow lithosphere (to several kilometers depth), from which volcanic materials are derived
magma ocean: term used to describe the layer of magma, hundreds of kilometers thick; thought to have covered the Moon 4.5 billion years ago
magnesium: an element with atomic number 12; symbol: Mg. Magnesium is contained in the minerals olivine and pyroxene, and is very abundant in the mantles of the inner planets
magnetic field: the region of "altered space" that will interact with the magnetic properties of a magnet. It is located mainly between the opposite poles of a magnet or in the energetic space about an electric charge in motion
magnetite: a black, strongly magnetic iron-oxide mineral; Fe3O4
mantle: a mostly solid layer of Earth lying beneath the crust and above the core; consisting mostly of iron, magnesium, silicon, and oxygen. On other planets and moons, the mantle is the layer between the crust and core. Mantles contain information about a planet's total composition, a key parameter in understanding planet formation and how the planets vary in composition with distance from the Sun
mare "soil": sediment on the surface of the lunar maria; fragments of basalt rocks, broken mineral grains, and glass produced by impact"
mare basalt: rocks making up the dark, smooth, mare areas of the Moon
maria (singlular mare): dark areas on the Moon covered by basalt lava flows
Marius, Simon 1573-1624: (a.k.a. Mayr) German astronomer who gave Jupiter's "Galilean" moons their names. He and Galileo both claimed to have discovered them in 1610 and likely did so independently. Marius was also the first to observe the Andromeda Nebula with a telescope and one of the first to observe sunspots"
Mars: planet fourth in order from the sun. In Roman mythology, the god of war and discord (Greek name, Ares)
Mars Global Surveyor: U. S. mission to Mars launched in 1996. The spacecraft circles the planet once every 118 minutes at an average altitude of 378kilometers (235 miles). Main instruments include a camera, laser altimeter, thermal emission spectrometer, and magnetometer. After mapping ends in late January 2000, the spacecraft will function as a communications satellite to relay data back to Earth from surface landers launched as part of future Mars missions
Mars Pathfinder: the second in NASA's Discovery program of low-cost spacecraft with highly focused science goals, Mars Pathfinder landed successfully on Mars on July 4, 1997. Images, science results, and engineering data were released on the World Wide Web throughout, and beyond, the primary mission dates (July 4 - August 3, 1997.) The Jet Propulsion Laboratory, Pasadena, CA, developed and managed the Mars Pathfinder mission for NASA's Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology (Caltech)
mass wasting: a general term for the dislodgment and downslope movement of soil and rock material under the force of gravity
massif: a masssive topographical feature, commonly formed of rocks more rigid that those of its surroundings
Mercury: the planet closes to the sun. In roman mythology, the fleet-footed messenger gos and escort of dead souls to the underworld (Greek name, Hermes)
metamorphic: rocks that have recrystallized in a solid state as a result of changes in temperature, pressure, and chemical environment
meteor: (also "shooting star" or "falling star") a bright streak of light in the sky caused by the entry into Earth's atmosphere of a meteoroid or a small icy particle. Very large, bright ones are called fireballs and bolides"
meteorite: a metallic or stony (silicate) body that has fallen on Earth (or other planetary body) from outer space. Most meteorites come from asteroids, but a small number come from the Moon or Mars (see SNC meteorites).Meteorites smaller than 1 mm are called "micrometeorites"
meteoritic bombardment: intensive and prolonged impacts of a surface by meteorites or other impactors
meteoroid: a small rocky object orbiting the Sun; smaller than an asteroid
mineral: naturally occurring inorganic solid with a definite chemical composition and crystal structure
nakhlites: one of the types of meteorites thought to be from Mars. Nakhlites probably formed as lava flows with unusual compositions. This type is named after the Nakhla meteorite, which fell in Egypt in 1911
NASA: United States federal agency; National Aeronautics and Space Administration
Neptune: planet eighth in order from the sun. In Roman mythology, god of the sea (Greek name, Poseidon)
Newton, Isaac 1642-1727: English cleric and scientist; discovered the classical laws of motion and gravity; the bit with the apple is probably apocryphal
nitrogen: an element with atomic number 7; symbol: N. It is common in Earth's atmosphere and along with hydrogen, carbon, and oxygen is essential for life
nucleus: the central body of a comet
olivine: mineral found in basalt; ranges from Mg2 SiO4 to Fe2 SiO4
opposition: a superior planet is said to be "in opposition" when it is directly on the opposite side of the Earth from the Sun. This is generally the closest it comes to the Earth and the time at which it is most easily visible. (nice diagram)
organic: pertaining to carbon-containing compounds. Organic compounds can be formed by both biological and non-biological processes
organism: any form of animal or plant life
oxygen: an element with atomic number 8; symbol: O. It is actually the most common element in the crusts and mantles of the inner planets and rocky moons, making up all silicate minerals. Along with hydrogen, carbon, and nitrogen, oxygen is essential to life
pahoehoe: basaltic lava with a smooth, billowy, or ropy surface
pascal: the standard unit of pressure is the pascal, abbreviated Pa, which is equivalent to 1 kg/m2. The pressure at the surface of the Earth is100,000 Pa. Pressures inside planets are very large numbers, usually expressed as GPa (giga means billion)
patera: shallow crater; scalloped, complex edge
Pathfinder: (see Mars Pathfinder)
perihelion: the point in its orbit where a planet is closest to the Sun. when referring to objects orbiting the Earth the term perigee is used; the term periapsis is used for orbits around other bodies. (opposite of aphelion)
Phobos: a moon of Mars. In Greek mythology, a son of Ares (Mars) who, with brother Deimos, was a constant companion to his father
Pickering, William Henry 1858-1938: American astronomer. His photographs of Mars, among the earliest obtained, provided a basis for his opposition to Lowell's observations of supposed canals on Mars. Discovered Phoebe
pixel: a term used in digital image processing to describe a unique, single picture element (or, a single spot on an image) which has a unique color (or brightness) value
plagioclase feldspar: common mineral; ranges from NaAlSi3 O8 to CaAl2 Si2O8
plateau: any comparatively flat area of great extent or elevation
plan view: a view of a surface from above, as from a spacecraft viewing the surface of a planet. Maps represent plan views of the areas they cover
planetary geology: a science that applies geologic principles and techniques to the study of planets and their natural satellites. [syn. planetology, astrogeology, planetary geoscience]
planitia: a low plain.
plate tectonics: theory formulated in the late 1960s that states the Earth' s crust and upper mantle (a layer called the lithosphere) is broken into moving pieces called plates. The formation of mountains and volcanoes, and the occurrence of earthquakes have been explained using this theory
plume: a buoyant mass of hot, partially molten mantle material that rises to the base of the lithosphere
Pluto: planet ninth in order, and farthes from, the sun. In Greek mythology, god of the dead and the underworld (Roman mythology, Hades)
Ptolemy 87-150: (a.k.a. Claudius Ptolemaeus) Alexandrian astronomer, mathematician, and geographer who based his astronomy on the belief that all heavenly bodies revolve around the Earth
pyroclastic eruption: explosive eruption of lava producing and ejecting hot fragments of rock and lava
pyroxene: a silicate mineral common in basalt and composed mostly of iron(Fe), magnesium (Mg), calcium ( Ca), and silicon (Si). Composition varies as a mixture among FeSiO3, MgSiO3, and CaSiO3
RTG: Radioisotope Thermo-electric Generator onboard a spacecraft
radioactivity: the spontaneous breaking apart, or decay, of unstable nuclei in isotopes. The unstable radioactive isotope is called the parent, and the products of the decay of the parent are called the daughter isotopes
radiocarbon, 14C: the unstable, radioactive isotope of carbon having atomic number 6 and atomic mass 14. It is continuously produced in Earth's upper atmosphere by cosmic ray bombardment. In space, 14C is produced when cosmic rays interact with oxygen in silicate minerals in rocks. The half-life of14C is 5730 years, which makes this radioactive isotope suitable for dating rocks and archaeological items as old as about 75,000 years
ray: streak of material blasted out and away from an impact crater
regolith (see sediment, soil): loose, unconsolidated rock, mineral, and glass fragments. On the Moon, this debris is produced by impacts and blankets the surface
relief: the maximum regional difference in elevation
resolution: the amount of small detail visible in an image; low resolution shows only large features, high resolution shows many small details
retrograde: rotation or orbital motion in a clockwise direction when viewed from above the north pole of the primary (i.e. in the opposite sense to most satellites); the opposite of direct. The north pole is the one on the same side of the ecliptic as the Earth's north pole
rift: a valley formed at a divergence zone or other area of extension
rille: long channel on the Moon crossing the surface of maria; probably formed either as an open channel in a lava flow, or as an under-ground tube carrying hot lava which collapsed as the lava flowed out
roche limit: the closest a fluid body can orbit to its primary without being pulled apart by tidal forces. A solid body may survive within the Roche limit if the tidal forces do not exceed its structural strength. The Roche limit is calculated with the equation}RL = 2.456*R*(p'/p)^(1/3)}where p' is the density of the planet, p is the density of the moon, and R is the radius of the planet. (more)
rock: a naturally formed solid that is an aggregate of one or more minerals, glass and/or organic materials
STS: Space Transportation System (Space Shuttle)
satellite: any smaller body (natural or artificial) that revolves about a larger (primary) body, as in the Moon revolving around the Earth
Saturn: planet sixth in order from the sun. In Roman mythoogy, god of agriculture, and father of Jupiter (Greek name, Cronos)
scale: the relationship of a distance on a map or model to the true distance in space; written as a ratio, such as 1:24,000
scarp: line of cliffs produced by faulting or erosion
Schiaparelli, Giovanni Virginio 1835-1910: Italian astronomer who in 1877 first observed the "canals" on Mars. He believed that the features he observed included straight lines that joined in a complicated pattern. He called these lines 'canali', which means 'channels'. However, the Italian word was mistranslated into the English word 'canals'. That, combined with the suspicious straightness of the lines, bespoke of artificial structures, and this created a furor. Speculations concerning the possibility of intelligent life on Mars sprang up in the popular press. Even astronomers felt the pull of that dramatic possibility. Foremost among these was Percival Lowell, who carried matters far beyond Schiaparelli"
sediment: (see regolith) solid rock or mineral fragments transported and deposited by wind, water, gravity, or ice; precipitated by chemical reactions; or secreted by organisms; accumulated as layers in loose, unconsolidated form
sedimentary: rock formed when sediment is compacted and lithified
semimajor axis: the semimajor axis of an ellipse (e.g. a planetary orbit) is 1/2 the length of the major axis which is a segment of a line passing through the foci of the ellipse with endpoints on the ellipse itself. The semimajor axis of a planetary orbit is also the average distance from the planet to its primary. The periapsis and apoapsis distances can be calculated from the semimajor axis and the eccentricity by rp = a(1-e) and ra = a(1+e)
shergottites: one of the three types of meteorites (the SNC meteorites)thought to have come from Mars. Shergottites are basalts. The type is named after the Shergotty meteorite, which fell in India in 1865
shield volcano: a volcano in the shape of a flattened dome, broad and low, built by flows of very fluid, basaltic lava
silicate: a variety of minerals that always contain silicon (Si) and oxygen(O). (E.G. olivine)
silicon: an element with atomic number 14; symbol: Si. Silicon is the most abundant element besides oxygen in planets, and forms the basis for silicate minerals such as olivine, pyroxene, and plagioclase
sill: a tabular, parallel-sided sheet of ingeous rock, formed underground, that is parallel to, and was intruded between, the layered rocks around it
slumping: a landslide that results from the downward sliding of rock debris as a single mass, usually with a backward rotation relative to the slope along which the movement takes place
SNC meteorites: (see meteorite) general name for three groups of meteorites with relatively young ages (slightly over 1 billion years old) that probably came from Mars. The groups are the Shergottites, Nakhlites, and Chassigny
soil: (see regolith) the upper layers of sediment on Earth that support plant growth
solar conjunction: a condition where the solar elongation is zero, or practically speaking, so small that the celestial object cannot be seen in the glare of the Sun
solar power: energy derived from the Sun or sunlight for use as a source of electricity
solar system: the Sun and all the objects (planets, moons, asteroids, and comets) that orbit the Sun.
source: (see eruption) location where igneous matter (lava and gases) erupts onto the surface; vent, fissure, volcano, etc
spacecraft: vehicle capable of traveling in outer space
spectra: the intensity of electromagnetic radiation (such as ultraviolet, visible light, and infrared) across a range of wavelengths. A prism separates white light into the range of wavelengths we call a rainbow of colors. "Spectrum" is the singular form of the word"
spectroscopy: the study of spectra. This is a means of looking at the intensity or brightness of an object as a function of wavelength or color
speed of light: = 299,792,458 meters/second (186,000 miles/second). Einstein's Theory of Relativity implies that nothing can go faster than the speed of light; Scotty and Geordi know better
stratigraphic column: a drawing showing the vertical sequence of rock. Geologists use this to interpret the relative sequence of past geological events
stratigraphy: study of layered rock to understand the sequence of geological events
stratosphere: an upper portion of a planetary atmosphere, above the troposphere and below the ionosphere, characterized by relatively uniform temperature and horizontal winds
stratovolcano: a volcano that is constructed of alternating layers of lava and pyroclastics. Viscous, acidic lava may flow from fissures radiating from a central vent, from which pyroclastics are ejected [syn. composite volcano]
sublimation: a direct phase transformation from solid to gas, without passing through the liquid state
sulfur: An element with atomic number16; symbol: S. Sulfur is common on the surface of Jupiter's moon Io
superior planets: the planets Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are called superior planets because their orbits are farther from the Sun than Earth's orbit. (Mercury and Venus are called "inferior" planets)
TRC: NASA’s Teacher Resource Center
tectonic: deformation forces acting on a planet's crust
tectonics: general term referring to the large-scale change of rock in response to forces causing faulting and folding. The forces acting upon a rock mass are generally termed compressional (squeezing together), tensional(pulling apart), or shear (parallel sliding). Common landforms resulting from tectonic processes are mountain ranges, rift zones, faults, fractured rock, and folded rock masses
terminator: the dividing line between the illuminated and the unilluminated part of the moon's or a planet's disk
terrain: area of the surface with a distinctive geological character
terrestrial: belonging to the class of planets that are similar to the Earth in density and composition. (i.e. mercury, Venus, Mars)
tharsis: a region of volcanoes on the surface of Mars. In the Bible, a land at the western extremity of the known world. Thought to be a region in modern-day Spain
thermodynamics: the study of heat and its transformation to mechanical and other forms of energy
Thomson, William 1824-1907: a.k.a. Lord Kelvin, British physicist who developed the Kelvin scale of temperature. Also supervised the laying of a trans-Atlantic cable.
thrust fault: low-angle fault in which rock above the fault plane moves up in relation to rock below
tidal heating: frictional heating of a satellite's interior due to flexure caused by the gravitational pull of its parent planet and possibly neighboring satellites
tidal stresses: force per unit area acting on a planetary body resulting in periodic bulging (of the crust and, in the case on Earth, oceans) caused by the gravitational attraction of another object such as the Sun, a moon, or a planet. The alternate growth and decay of a tide in the crust of a planetary body can lead to frictional heating
Tombaugh, Clyde 1906-1997: American astronomer; discovered Pluto
topography: the shape and form of the surface of a planet
trajectory: the curved path defined by a moving body (such as a spacecraft in space)
trough: a long linear depression
uplink: signal sent from Earth to a spacecraft
Uranus: planet seventh in order from the sun. In Roman mythology, god of the sky, mate of the goddess of the Earth, and father of the Titans
Van Allen, James A. : American physicist who discovered the Earth's radiation belts (that now bear his name) with an instrument aboard the first successful American satellite, Explorer 1
vastitas: widespread lowlands
vent: the opening in the Earth's surface (or other planet or moon) through which volcanic materials are erupted
Venus: planet second in order from the sun.In Roman mythology, the goddess of love (Greek name, Aphrodite)
vesicle: bubble-shaped cavity in a volcanic rock formed by expanding gases
Viking: U. S. mission to Mars, composed of two spacecraft, launched in 1975.Viking 1 and Viking 2 both consisted of an orbiter and a lander. Primary mission objectives were to obtain high resolution images of the Martian surface, characterize the structure and composition of the atmosphere and surface, and search for evidence of life. Viking Mission web site from the National Space Science Data Center
viscosity: an internal property of a fluid that offers resistance to flow. If a fluid has a high viscosity, then it strongly resists flow. An example of a low viscosity fluid is water
volatile elements: chemical elements that vaporize (that is, boil) at relatively low temperatures. Examples are water (H2O), carbon dioxide (CO2),carbon monoxide (CO), methane (CH4), ammonia (NH3), potassium (K), sodium(Na), and lead (Pb)
volcanism: the process by which magma and it's associated gases rise into the crust and are extruded onto planetary surfaces
volcano: mountain formed from the eruption of igneous matter through a source vent
Voyager: U. S. mission consisting of two spacecraft launched in 1977 to explore Jupiter, Saturn, Uranus, Neptune, and their moons, rings, and magnetic environments. It took 2 years for the crafts to reach Jupiter. The last image was taken in 1989 and now both crafts are headed for the outer solar system.
weathering: the mechanical breakdown and chemical alteration of rocks and minerals at Earth’s surface during exposure to air, moisture, and organic matter
zenith: the point on the celestial sphere directly above the observer. Opposite the nadi