Planetary Science Research Discoveries (PSRD) is an educational site sharing the latest research being made by NASA-sponsored scientists on meteorites, planets, and other bodies in our Solar System. The website is supported by the Cosmochemistry Program of NASA's Science Mission Directorate and by Hawai'i Space Grant Consortium and is a vital link for planetary and space sciences, and for learning how science works.

'A'a: Blocky, angular, and rough type of lava flow.

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.

Achondrite: A class of stony meteorites that crystallized from magmas. The term means without chondrules.

AGB stars: Cool, luminous, and pulsating red giant stars. Most stars in the Universe that have left the main sequence will reach their final evolutionary stage as stars on the asymptotic giant branch (AGB).

Agglutinates: Common particle type in lunar regolith; agglutinates consist of small rock, mineral, and glass fragments bonded together with glass.

Albedo: The brightness of an object or surface; it is the percentage of incoming radiation (light) that the object or surface reflects.

Amazonian Epoch: The geologic history of Mars has been divided into three broad time periods, or Epochs. From oldest to youngest, these are Noachian, Hesperian, and Amazonian (named after places on Mars). These Epochs are defined by the number of impact craters on the ground surface; older surfaces have more craters. The Amazonian Epoch extends from about 1.8 billion years ago to the present day. The actual timing of the Epochs is not known because we have different models of the rates of meteorite falls on Mars.

Ammonia: A gas molecule composed of one part nitrogen and three parts hydrogen; NH₃.

Andesite: dark-colored, fine-grained extrusive igneous rock with about 52 to 63 weight percent silica (SiO₂). Andesite consists mainly of plagioclase and one or more mafic minerals. The word andesite is derived from the Andes Mountains, located along the western edge of South America, where andesite rock is common.

Angrite: A type of achondrite meteorite consisting of pyroxene, olivine, and plagioclase.

Anomaly: A deviation from the common rule, type, or form; something abnormal or inconsistent.

Anorthosite: An igneous rock made up almost entirely of plagioclase feldspar. This rock is a major part of the lunar highlands.

Antenna: A conductor by which electromagnetic waves are transmitted or received.

Antipode: A point on the surface of a sphere that is located diametrically opposite another point. For example, on the Earth the antipode of Hawaii is Botswana. The term is used commonly in lunar geology to describe locations on the Moon in relation to impact basins. Researchers have hypothesized that accumulations of converged ejecta materials, production of impact shock stresses, anomalies in lunar crustal magnetic fields, and geochemical anomalies are associated with antipodes of basins.

Apollo: U.S. Space Program that included 6 piloted lunar landings between 1969 and 1972. Apollo astronauts collected and returned 382 kilograms of rock and sediment samples from the Moon.

Apparition: An appearance; in astronomy, the period of observation of a returning periodic comet.

Argument of perihelion: Angle along the orbit of a planet or other solar system object as measured from the ascending node (analogous to right ascension and longitude).

Ascending node: The plane of a planet's orbit (or other solar system object) is usually tilted with respect to the plane defined by the solar system (the ecliptic). These 2 great circles intersect at 2 points. As the planet travels around its orbit, at one of the intersection points it will pass from below the ecliptic plane to above it (i.e. northward). This point is called the ascending node.

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 (the Asteroid belt), and are the source of most meteorites. Principal asteroids are: 1 Ceres (a dwarf planet), 2 Pallas, 3 Juno, and 4 Vesta.

Asteroid belt: The region of space between Mars (1.5 AU) and Jupiter (5.2 AU) where most asteroids are found.

Asteroid family: Group of asteroids with similar orbital elements (distance from the Sun, inclination of the orbital plane to the Earth's orbital plane, and eccentricity of the orbit) thought to be remnants of one larger asteroid parent.

Astrobiology: Study of the origin, distribution, and destiny of life in the universe. These are interdisciplinary studies involving astronomers, astrophysicists, biologists, chemists, geneticists, geologists, paleontologists, planetary scientists, and specialists from other scientific fields. Three broad themes of astrobiology: Is there life elsewhere in the universe? How did life begin and evolve? What is the future for life on Earth and beyond?

Astronaut: Person engaged in or trained for spaceflight.

Astronomical twilight: The time when the Sun is 12-18 degrees below the horizon. When the Sun moves from 19 to 18 degrees below the horizon, astronomers can begin to detect an increase in the brightness of the sky, although to the naked eye the sky still looks dark.

Atmosphere: Mixture of gases that surround and are gravitationally attached to a planet.

AU (Astronomical Unit): The mean Earth-Sun distance, equal to 149.5 million kilometers or 93 million miles.

Bacteria: Single-celled microorganisms whose cells lack a nucleus. Bacteria comprise a major domain of life called prokaryotes. In contrast, the cells in eukaryotes have a well-defined nucleus; eukarya include molds, plants, and animals. Another major domain is called archaea, microorganisms with genetic features distinct from prokarya and eukarya. Most methane-producing bacteria are actually archaea, but for simplicity many biologists informally refer to all non-eukaryotes as "bacteria."

Basalt: Fine-grained, dark-colored extrusive igneous rock with less than about 52 weight percent silica (SiO₂). Because of its low silica content, basalt has a low viscosity (resistance to flow.) Basalt is 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.

Biosphere: The part of Earth's crust, water, and atmosphere where living organisms can survive.

Breccia: Rock consisting of angular, coarse fragments embedded in a fine-grained matrix.

CAI: Calcium-aluminum-rich inclusions (CAIs) are found in chondritic meteorites. CAIs are primitive objects that formed in the solar nebula before the planets formed. CAIs are light-colored objects rich in refractory elements (that condense at a high temperature). Besides calcium and aluminum, this includes magnesium, titanium, and rare earth elements. CAIs range in size from about a millimeter to a centimeter. Meteoriticists have identified several distinct varieties of CAIs, but all share a high temperature origin. Some might be condensates from the solar nebula. Other CAIs might be evaporation residues.

Caldera: A large, basin-shaped volcanic depression caused by collapse.

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 star: A rare class of red giant stars unusually rich in carbon or carbon compounds.

Carbonate: Common rock-forming mineral containing the carbonate ion, CO₃^2-. Common carbonate minerals are calcite, CaCO₃; siderite, FeCaCO₃; magnesite, MgCO₃; dolomite, CaMg(CO₃)₂; and ankerite, Ca(Fe,Mg,Mn)(CO₃)₂.

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 -78^o C. This solid is commonly known as dry ice. The polar ice caps on Mars are made of frozen water and carbon dioxide.

Celestial body: A solid object found in space.

Celestial equator: An imaginary great circle in the sky drawn concentric to the Earth's equator.

Celestial sphere:- An imaginary sphere centered on the Earth, or arbitrary large radius on the surface of which the stars are considered to be fixed.

Celsius: A temperature scale that assigns the value 0^o C to the freezing point of water and the value of 100^o C to the boiling point of water at standard pressure.

Ceres: The largest of the known asteroids; a dwarf planet. Also called 1 Ceres because it was the first asteroid to be discovered (by Piazzi in 1801).

Chassignites: One of three types of meteorites from Mars (the SNC meteorites). Chassignites are dunites, a rock composed almost entirely of the mineral olivine. The signature stone, Chassigny, fell in France in 1815.

Chondrite: An abundant class of stony meteorites with chemical compositions similar to that of the Sun and characterized by the presence of chondrules (see definition below). Chondrites come from asteroids that did not melt when formed and are designated as H, L, LL, E, or C depending on chemical compositions. The H, L, and LL types are called ordinary chondrites. The L chondrites are composed of silicate minerals (mostly olivine and pyroxene, but feldspar as well), metallic nickel-iron, and iron sulfide (called troilite). Most L chondrites are severely shocked-damaged, probably by a large impact on the asteroid in which they formed. The C (or carbonaceous) chondrites contain water-bearing minerals and carbon compounds including a variety of organic molecules such as amino acids. Carbonaceous chondrites are the most primitive meteorites--primitive in a chemical way. For example, the CI group of carbonaceous chondrites are closest in composition to the photosphere (visible surface) of the Sun.

Chondrule: Roughly spherical objects found in a type of meteorite called chondrites. Most chondrules are 0.5 to 2 millimeters in size and are composed of olivine and pyroxene, with smaller amounts of glass and iron-nickel metal. The shapes of the mineral grains in them indicate that chondrules were once molten droplets floating freely in space.

Clementine: A global mapping mission to the Moon launched in 1994 by the U. S. Department of Defense, with science support from NASA.

Coma: A large cloud of dust and gas which escapes from the nucleus of an active comet.

Comet: A small body (a "dirty iceball", typically 1 km across, with dust and plasma tails) that circles the Sun with a highly elliptical orbit.

Convection: Movements of mantle material, laterally or in upward-downward directions, due to heat variations.

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.

Cosmic rays: Extremely high-energy subatomic particles that continuously bombard Earth from all directions. Most cosmic rays hit and break up atomic nuclei in the upper atmosphere.

Cosmochemistry: Interdisciplinary science that overlaps with geochemistry, geology, astronomy, astrophysics, and geophysics to discover the materials and fundamental processes in the solar nebula and our solar system. These sciences give us complementary ways of looking at our origins by addressing questions like, How did the Sun and planets form? Where did we come from? Cosmochemistry, because it is a careful examination of the building blocks of the cosmos, also plays an important role in developing local resources on the Moon and Mars, essential to sustained human presence in space.

Crater (see Impact, Transient crater): 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.

Crater chain: Several craters along a general line.

Crust: The outermost layer of a planet or moon, above the mantle.

Cryptomaria (singular: cryptomare): Buried mare basalt lavas on the Moon. Many light plains in the lunar highlands have impact craters surrounded by dark piles of ejecta, nicknamed "dark-haloed craters." The dark-haloed craters formed when mare basalt lava flows were covered with ejecta from large impact craters and basins, and then small craters punctured through the ejecta to toss out mare basalt.

Dark mantle deposits: Deposits of dark glass on the Moon, possibly products of volcanic fire fountaining.

Declination: Celestial coordinate which is equivalent to the latitude of an object on Earth. This is an angular measure of how far above the celestial equator an object lies.

Density: Mass per unit volume; a measure of how much material is in a given space.

Density current: Water movement due to differences in the density of the water from place to place usually caused by changes in the amount of material held in suspension or changes in temperature or variations in salinity.

Descartes: Lunar highlands site of Apollo 16 landing on April 21, 1972.

Differentiation: Chemical zonation caused by differences in the densities of minerals; heavy materials sink, less dense materials float. Differentiation transforms an initially homogeneous body into a body with compositionally distinct core, mantle, and crust.

Diffusion: The random movement (spreading) of molecules from regions of highest concentrations to regions of lowest concentrations.

Diurnal: A single daily cycle.

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 eccentricy of 0, and a parabola (an open figure) an eccentricity of 1. The deviation of an orbit from a perfect circle.

Ecliptic: The plane of the solar system.

Ejecta: Material thrown out from and deposited around an impact crater.

Electromagnet spectrum: The full range of frequencies, from radio waves to gamma rays, that characterizes light. The electromagnetic spectrum can be expressed in terms of energy, wavelength, or frequency (where the wavelength equals the speed of light divided by the frequency and energy equals Planck's constant times the frequency.)

Electron microprobe: An instrument that analyzes the chemistry of very small spots by bombarding the sample with electrons and measuring the X-rays produced.

Electron Volt: The energy given to an electron by accelerating it through 1 volt of electric potential difference.

Ephemeris: A table of positions in right ascension (celestial coordinate corresponding to longitude on earth) and declination (celestial coordinate corresponding to latitude on Earth), as a function of time for a moving celestial object.

Erosion: Removal of weathered rocks by moving water, wind, ice, or gravity.

Eruption: (see Source) A break out or burst of volcanic matter.

Eucrite: A common class of achondrite meteorites composed of pigeonite and plagioclase. These meteorites formed as basaltic flows on a parent body, probably asteroid 4 Vesta.

Exsolution: The segregation, during cooling, of a homogeneous solid solution into two or more different minerals.

Extrusive: Refers to igneous rocks erupted on the surface.

Fall: A meteorite that was observed as it fell through Earth's atmosphere and was retrieved. Since most falls are collected soon after hitting the ground, these meteorites are not badly degraded or weathered.

Fahrenheit: A temperature scale with the freezing point of water assigned the value 32 ^oF and the boiling point of water 212 ^oF.

Farside: The side of the Moon that always faces away from Earth.

Fault: A fracture along which there has been movement or displacement.

Feldspar: see plagioclase feldspar.

Farside: The side of the Moon that never faces Earth.

Find: A meteorite that was not observed falling through the Earth's atmosphere, but was found at some later date.

Fireball: The streak of light and loud noise of a large meteor passing through the Earth's atmosphere.

Fischer-Tropsch reaction: Production of organic molecules by the addition of hydrogen to carbon monoxide aided by a suitable catalyst.

Fissure: Crack extending far into a planet or moon through which magma travels to and erupts onto the surface.

Fra Mauro: Landing site of Apollo 14 on the Moon on February 5, 1971.

Fusion crust: The glassy, melted rind on a meteorite that forms when the rock passes through the Earth's atmosphere.

Gabbro: A course-grained igneous rock rich in the minerals olivine, pyroxene, and plagioclase feldspar.

Galilean Moons: Jupiter's four largest moons: Io, Europa, Ganymede, and Callisto, discovered independently by Galileo Galilei and Simon Marius in 1609-1610.

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. The 14-year mission ended in 2003. Project Galileo website.

Gamma ray: Gamma rays are like the light we see with our eyes, the x-rays used to probe our bodies in search of problems, and the radio waves that our television sets translate into inane shows. The only difference among these types of electromagnetic radiation is the wavelengths of the waves and how fast they vibrate. Gamma rays are the most energetic (and vibrate the most); radio waves are the least energetic and vibrate the least.

Geocentric distance: The distance from Earth.

Geologist: Scientist who studies Earth, its structure, its materials, the physical and chemical processes and 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.

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.

Geomorphology: The study of the shape and form of the landscape, and how the nature of landforms relates to their origin, development, and change over time.

Gigapascal (GPa):Conventional unit of pressure when discussing the deep Earth: 1 GPa = 10 kilobars = 10,000 times air pressure at sea level.

Globular clusters: Groups of thousands to millions of old stars held together in orbits about their own center of mass and arranged in almost symmetrical (spherical) clusters.

Hadley-Appenine: Landing site of Apollo 15 on the Moon on July 31, 1971.

Half-life: The amount of time required for half of the mass of a radioactive isotope to decay.

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.

Hematite: An iron-oxide mineral (Fe₂O₃) that may be the cause of the red color on Mars. Courser-grained gray hematite has the same chemical formula as the red variety, but a different crystalline structure. The deposits of gray hematite found in the Terra Meridiani region of Mars may suggest that water once circulated through the region's rock layers.

Hesperian Epoch: The geologic history of Mars has been divided into three broad time periods, or Epochs. From oldest to youngest, these are Noachian, Hesperian, and Amazonian (named after places on Mars). These Epochs are defined by the number of impact craters on the ground surface; older surfaces have more craters. The Hesperian Epoch extends from about 3.5 to about 1.8 billion years ago. The actual timing of the Epochs is not known because we have different models of the rates of meteorite falls on Mars.

Highland "soil": Sediment on the surface of the lunar highlands; composed of broken rock and mineral fragments, and glass produced by impact.

Highlands: Oldest exposed areas on the surface of the Moon; extensively cratered, and chemically distinct from the maria.

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.

Hydrosphere: The portion of Earth, or other planet, that is water, including liquid water, ice, and water vapor on the surface, underground, or in the atmosphere.

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 Crater) The forceful striking of one body, such as a meteorite, against another body such as a moon or planet.

Inclination: Angle between the plane of the object's orbit and the ecliptic (defined by Earth's orbit).

Inclusions: Fragments of older material (e.g. minerals, rock) that have been enclosed within another rock.

Interplanetary dust particles (IDPs): Microscopic dust particles that can be collected from Earth's atmosphere using an ER-2 aircraft.

Interstellar medium: The material between the stars, consisting of gas, dust and cosmic rays (high energy charged particles, moving at nearly the speed of light).

Intrusive: Refers to igneous rocks formed underground.

Ion Microprobe: A Secondary Ion Mass Spectrometer (SIMS), a precision instrument used to quantitatively analyze the elements and isotopes of materials at microscopic scales.

Iron: An element with atomic number 26; sumbol: 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.

Iron Meteorite: Iron meteorites are made, almost completely, of iron and nickel metal. They are chemically distinguished and grouped according to the abundances of the trace elements such as gallium and germanium, as well as nickel. Initially, irons were classified into four groups and were given Roman numerals I, II, III, and IV. Today twelve groups are recognized and designated further by letters A through F according to concentrations of siderophile ("iron-loving") trace elements. Iron meteorites that do not fit into the groups are called ungrouped. The two iron-nickel alloys in iron meteorites are called kamacite (low-nickel content, usually up to 7.5 wt% nickel) and taenite (high-nickel content, ~20 to 50 wt% nickel). These alloys are rare in terrestrial rocks.

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.

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.

KREEP: Type of lunar igneous rock rich in potassium (K), rare-earth elements (REE), phosphorus (P), thorium, and other incompatible elements. These elements are not incorporated into common rock-forming minerals during magma crystallization, hence they become enriched in the residual magma and in the rocks finally formed from it.

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.

Latitude: The angular distance North or South from the Earth's equator measured in degrees on the meridian of a point; Equator being 0^o and the poles 90^o N and 90^o 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.

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 10¹² kilometers.

Lithology: The description of the physical characteristics of rocks including color, mineral compositions, grain sizes, grain shapes, texture or fabric (relationship between grains).

Lithosphere: Layer in a planetary body consisting of the crust and upper mantle.

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.

Long-period: In astronomy, a comet with an orbital period greater than 200 years.

Luna: One of two successful series of Soviet unpiloted lunar missions. There were 24 missions to the Moon in the Luna series launched from 1959 to 1976. (The other series, Zond, had 5 lunar missions launched in 1965 to 1970.) The first image of the farside of the Moon was taken by the Luna 3 spacecraft in October, 1959. Three Luna landers (Luna 16, 20, and 24) collected a total of approximately 300 grams of lunar samples and returned them to Earth.

Lunar: Of or pertaining to the Moon.

Lunar Cataclysm: Term referring to the idea that a sharp increase in impact bombardment rate between about 3.92 and 3.85 billion years ago affected the inner solar system--most notably recorded in the numerous craters and large basins on the Moon. Also described as the late heavy bombardment.

Lunar meteorites: Meteorites that have been identified through their chemistry and minerals as being from the Moon. (See Lunar Meteorites compiled by Randy Korotev from Washington University, St. Louis.)

Lunar Prospector: U. S. Discovery-class mission to the Moon launched on January 6, 1998. Its instruments were designed to provide global maps and data sets of the Moon's composition and magnetic and gravity fields from a low polar orbit. The mission lasted 18 months and ended July 31, 1999 with a controlled crash landing into a crater at the Moon's south pole.

Lunar Reconnaissance Orbiter: NASA's Lunar Reconnaissance Orbiter (LRO) is a Moon-orbiting mission scheduled to launch in the fall of 2008. Its low-polar orbit and instrument payload will be used to acquire new data in support of future human landing sites and resource exploration including polar volatiles (especially water ice). These data will include high-resolution imaging and topography, lunar radiation environment, and global surface composition. The Lunar Crater Observation and Sensing Satellite (LCROSS) will launch with LRO, then travel independently for a controlled crash into a permanently-shadowed crater at the lunar south pole to support our search for water ice.

Mafic: Term describing high contents of dark-colored, magnesium-rich and iron-rich minerals.

Magma: Term applied to molten rock in the interior of a planet or moon. When it reaches the surface, magma is called lava.

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.

Magnetofossils: Small grains of magnetite occurring in sedimentary rocks and thought to have been produced by bacteria. (Some bacteria, called magnetobacteria, make magnetite, presumably as an aid to determining directions.)

Magnetometer: Sensitive scientific instrument used to measure the intensity at various points of Earth's (or another planetary body's) magnetic field.

Magnetosphere: The region around Earth dominated by Earth's magnetic field. In this region, the Earth's magnetic field lines guide the motions of charged particles (ions and electrons).

Magnitude: Astronomical brightness measured on a logarithmic scale, based on the ancient practice of noting that the brightest stars in the sky were of "first importance" or "first magnitude", the next brightest being "second magnitude" etc. The human eye is a logarithmic detector, and in 1854 Pogson formalized this scale and defined a difference of 5 magnitudes to be exactly a factor of 100 in brightness. The scale is calibrated to the bright star Vega which is defined to have a magnitude of 0. For reference, the faintest naked-eye magnitude visible from a dark site is +6. Comet C/1996 B2 Hyakutake reached magnitude 0 in late March 1996.

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. A second definition of mantle is a smooth blanket of surface material (generally fine-grained, sometimes layered) that was deposited by some process such as wind or water and that now obscures the underlying land.

Mare basalt: Rocks making up the dark, smooth, mare areas of the Moon.

Mare "soil": Sediment on the surface of the lunar maria; fragments of basalt rocks, broken mineral grains, and glass produced by impact.

Maria (singular: mare): Dark areas on the Moon covered by basalt lava flows.

Mariner 9: U. S. mission to Mars, launched in 1971, achieved global imaging of the surface, including the first detailed views of the Martian volcanoes, Valles Marineris, the polar caps, and the satellites Phobos and Deimos. The spacecraft gathered data on the atmospheric composition, density, pressure, and temperature and also the surface composition, temperature, gravity, and topography of Mars. Mariner 9 website.

Mars Exploration Rovers: U. S. missions to Mars with twin rovers equipped with tools to study a diverse collection of rocks and soils that may hold clues to past water activity on Mars. MER-A, named "Spirit," launched on June 10, 2003 for a seven-month journey to a landing site in Gusev Crater and MER-B, named "Opportunity," launched on July 7, 2003 toward a landing site in Terra Meridiani.

Mars Global Surveyor: U. S. mission launched in 1996 to orbit Mars. Main instruments include cameras (MOC), a laser altimeter (MOLA), thermal emission spectrometer TES, and magnetometer. During the primary mission (through January 2001), the spacecraft circled the planet once every 118 minutes at an average altitude of 378 kilometers (235 miles). Four extended mission phases began February 1, 2001 and continued to November 2006 when the orbiter stopped communicating with Earth (due to solar panel problems). Mars Global Surveyor website.

Mars Mission, Phoenix: see Phoenix Mars Mission.

Mars Odyssey: U. S. orbital mission to Mars, part of NASA's Mars Exploration Program, launched in April, 2001 and arrived at Mars in October, 2001. The mission's goals are to map chemical elements and minerals, look for water, and analyze the low-orbit radiation environment using three primary instruments: Thermal Emission Imaging System (THEMIS), Gamma Ray Spectrometer (GRS), and Mars Radiation Environment Experiment (MARIE). During and after its science mission, the Odyssey orbiter will also support other missions in the Mars Exploration program. It is providing communications relay for U.S. and international landers, including the Mars Exploration Rovers launched in June and July, 2003. 2001 Mars Odyssey website.

Mars Orbiter Camera (MOC): Camera system onboard the Mars Global Surveyor spacecraft. MOC consists of three cameras: A narrow angle system that provides grayscale high resolution views of the planet's surface (typically, 1.5 to 12 meters/pixel), and red and blue wide angle cameras. Malin Space Science Systems.

Mars Orbiter Laser Altimeter (MOLA): Instrument onboard the Mars Global Surveyor spacecraft that has been obtaining topographic data since March 1999. MOLA transmits focused infrared laser pulses toward Mars at a rate of ten times per second and measures the round-trip reflection time to determine the range of the MGS spacecraft to the Martian surface. Scientists use the range measurements to construct precise topographic profiles and maps of Mars in unsurpassed detail.

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).

Mars Reconnaissance Orbiter (MRO): NASA orbital mission to Mars launched August, 2005. During its two-year primary science mission, which began in November, 2006, MRO will conduct science investigations with a suite of instruments tasked for three purposes: global mapping, regional surveying, and high-resolution targeting of specific spots on the surface. Goals include studying the history of water on Mars, analyzing minerals, tracing how much dust and water are distributed in the atmosphere, and monitoring daily global weather.

Mars Science Laboratory (MSL): NASA robotic lander designed to determine the habitability of Mars. MSL is scheduled to launch in 2009 as part of NASA's long-term Mars Exploration Program. MSL science goals are to determine whether microbial life ever arose on Mars, to characterize the geology and climate of Mars, and to prepare for human exploration. MSL science instruments include three cameras, four spectrometers, two radiation detectors, and an environmental sensor. MSL homepage.

Martian meteorites: (See SNC meteorites and Meteorite.) Of the 24,000+ meteorites collected on Earth, only 30+ have been identified as meteorites from Mars. They are generally divided into three groups (all with relatively young ages slightly over 1 billion years old). The groups are the Shergottites, Nakhlites, and Chassignites. A Martian meteorite that does not fall into one of these groups is ALH 84001, a cumulate rock composed mostly of orthopyroxene, which is much older than all the others with an age of 4.5 billion years. A NASA-sponsored research team reported in 1996 that ALH 84001 may contain evidence of past primitive life on Mars--an idea that remains a topic of great debate and continued investigation. (See Mars Meteorites compiled by Ron Baalke from the Jet Propulsion Laboratory.)

Mass Spectrometer: Instrument that produces and measures, usually by electrical means, a mass spectrum. It separates ions according to the ratio of their mass to charge, allowing scientists to determine the abundances of each isotope.

Mass Spectrum: The pattern of the relative abundances of ions of different atomic or molecular mass (mass-to-charge ratio) within a sample. It frequently refers to the measured relative abundances of isotopes of a given element.

Megaregolith: Term coined by William K. Hartmann in 1973 to describe the lunar-wide zone of fragmented material developed on the Moon by mixing of impact basin/crater ejecta (transported by ballistic trajectories) and local rock. Estimates of the thickness of this mixed zone are between 1.5 to about 2.5 kilometers.

Mesosiderite: A class of stony-iron meteorites consisting of metal and fragments of igneous rocks. These meteorites formed as breccias, but most have been recrystallized during metamorphism.

Metamorphic: Rocks that have recrystallized in a solid state as a result of changes in temperature, pressure, and chemical environment.

Meteor: A relatively small body of matter traveling through interplanetary space.

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 found on Earth come from the Moon (see Lunar meteorites) or Mars (see Martian meteorites). Meteorite types include: iron, stony iron, chondrite, carbonaceous chondrite, and achondrite. A primitive meteorite is defined as a chondrite that experienced minimal heating and aqueous alteration on its parent asteroid. Primitive meteorites have the highest concentrations of presolar grains. [See "Meteors, Meteorites, and Impacts" from The Eight Planets website.] Meteorites smaller than 1 mm are called "micrometeorites." Meteorites collected on Earth are named for the location where they are found.

Meteoritic bombardment: Intensive and prolonged impacts of a surface by meteorites or other impactors.

Meteoriticist: Person specializing in the scientific studies of meteorites. They work at universities, government laboratories (such as at NASA Centers), or associated with important meteorite collections (such as natural history museums in many different countries, and the Vatican.)

Meteoritics: The scientific study of meteorites to better understand the origin, geologic history and evolution of our Solar System.

Methane: A compound composed of carbon and hydrogen: CH₄.

Mineral: Naturally occurring inorganic solid with a definite chemical composition and crystal structure.

Mineralogy: The scientific study of minerals. This is the field within geology that focuses on the chemistry, structure, properties, and classification of minerals.

Molecular cloud: Large concentration of gas (roughly 75% hydrogen and 21-24% helium with trace amounts of other molecules), dust, and mineral grains. Our Solar System formed out of one approximately 4.55 billion years ago.

Moonquake (see Earthquake): Sudden motion or trembling of the Moon caused by the abrupt release of slowly accumulated elastic energy in rocks.

Mountain: A natural elevation of a planetary surface.

Nakhlites: One of the three types of meteorites from Mars (the SNC meteorites). Nakhlites are cumulate igneous rocks characterized by a high abundance of pyroxene and lesser amounts of olivine. This type is named after the Nakhla meteorite, which fell in Egypt in 1911.

Nanometer: Unit of measure that is one-billionth of a meter. A nanometer-sized particle is smaller than a living cell and can only be seen with the most powerful microscopes.

Nautical twilight: The time when the Sun is 6-12 degrees below the horizon. Sky begins to get noticeably bright.

Nd:YAG: An acronym for neodymium-doped yttrium aluminum garnet, a synthetic crystalline compound used as the active medium for certain solid-state lasers.

NEAR (Near Earth Asteroid Rendezvous): U. S. Discovery-class mission launched on February 17, 1996. Primary scientific objectives were to return data on the bulk properties, composition, mineralogy, morphology, internal mass distribution, and magnetic field of 433 Eros as the spacecraft orbited the asteroid from February 14, 2000 to February 12, 2001. On February 12, 2001, the NEAR-Shoemaker spacecraft touched down on 433 Eros. Earlier in the mission, on June 27, 1997, NEAR-Shoemaker flew within 1200 km of the C-class asteroid 253 Mathilde, and it flew by the Earth on January 23, 1998. NEAR website.

Nearside: The side of the Moon that always faces Earth.

NIMS: Near-Infrared Mapping Spectrometer: an instrument carried to Jupiter on the Galileo planetary spacecraft, imaging in regions of the energy spectrum that humans cannot see. NIMS website at the Jet Propulsion Laboratory.

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.

Noachian Epoch: The geologic history of Mars has been divided into three broad time periods, or Epochs. From oldest to youngest, these are Noachian, Hesperian, and Amazonian (named after places on Mars). These Epochs are defined by the number of impact craters on the ground surface; older surfaces have more craters. The Noachian Epoch extends from the birth of Mars to between 3.8 and 3.5 billion years ago. The actual timing of the Epochs is not known because we have different models of the rates of meteorite falls on Mars.

Non-gravitational forces:- Forces that alter the path of a comet, which are not caused by gravitational interaction with the Sun and planets. Typically this refers to uneven outgassing from vents on the comet, which serve to act like rocket nozzles.

Norite: Igneous rock found in the lunar highlands composed of plagioclase and pyroxene.

Nucleosynthesis: Element formation by reactions inside stars.

Nucleus: (see comet) Kilometer-sized "dirty snowball" composed of dust (refractory material) and primarily water-ice which gives rise to all of the features observers associate with comets. As the nucleus approaches the Sun, the temperatures rise sufficiently to cause the sublimation of the ices. As the gas leaves the nucleus, it is able to drag some of the dust with it in the low gravity. Sunlight reflected off the dust is what we see as the yellowish coma and tail of the comet, and interaction of the solar radiation with the gases gives us the characteristic blue appearance of the plasma tail.

Ocean of Storms: Landing site of Apollo 12 on the Moon on Nov. 19, 1969; Oceanus Procellarum.

Olivine: Mineral found in basalt; ranges from Mg₂ SiO₄ to Fe₂ SiO₄.

Oort cloud: Vast spherical swarm of comets orbiting our Solar System from roughly 2,000 to 100,000 AU. The Oort cloud, while roughly spherical at the largest radius, is wedge-shaped where it merges with the outer planet region in the vicinity of the Kuiper belt of comets. The cloud is divided into different regions of dynamical stability: the Kuiper belt (35-50 AU; affected by planetary perturbations), a dynamically inert region (50-2000 AU; not affected by gravity of planets or stars), the inner Oort cloud (2,000-15,000 AU; affected by galactic tidal forces), and the outer Oort cloud (15,000-100,000 AU; affected by stellar perturbations). The Oort cloud has never been observed, rather its existence is inferred from the careful analysis of the orbits of comets which come in from the cloud. Recent discoveries of objects in the Kuiper belt have confirmed its existence.

Orange "soil": On the Moon, a mixture of very small dark orange and black glass balls which formed from quickly cooled lava droplets during a pyroclastic eruption.

Ordinary chondrite: The most common class of meteorite to fall on Earth. They contain variable amounts of metal and chondrules in a matrix of mostly silicate minerals. See chondrite.

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.

Parallax: Apparent motion of a nearby object as projected against more distant background objects due to the motion of the observer.

Parent body: An object (e.g. asteroid) from which meteorites come.

Parsec [pc]: A unit of astronomical distance equal to 3.26 light years, or 206265 AU, which equals 3.08 x 10¹³ kilometers.

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 is 100,000 Pa. Pressures inside planets are very large numbers, usually expressed as GPa (giga means billion).

Pathfinder: (see Mars Pathfinder)

Peridotite: A course-grained igneous rock consisting largely of the mineral olivine.

Perihelion: Point in the elliptical orbit of a comet about the Sun in which it passes closest to the Sun.

Petrology: The scientific study of rocks. This is the field within geology that focuses on the origin, occurrence, formation, structure, composition, and classification of rocks.

Phoenix Mars Mission: NASA lander mission; it is the first Mars "Scout class" mission to study the planet's north-pole ice cap, and the geologic history of water and habitability potential in the polar ice-rich soil. Phoenix launched in August, 2007 and touched down on May 25, 2008. The lander's robotic arm will dig down into the subsurface to collect ice and soil samples for onboard analyses by a suite of instruments. Phoenix Mars Mission homepage.

Photosynthesis: The process by which plants convert water and carbon dioxide into carbohydrates, using sunlight as the source of energy and the aid of chlorophyll.

Pixel: A term used in digital image processing to describe a unique, single picture element (or, a single data point on an image) which has a unique color (or brightness) value.

Plagioclase feldspar: Common mineral; ranges from NaAlSi₃ O₈ (albite) to CaAl₂ Si₂ O₈ (anorthite).

Planck's constant: A constant value: 6.626x10^-27 erg-seconds.

Planetesimals: Bodies ranging in size from meters up to hundreds of kilometers in diameter that formed during the process that formed the planets by accretion. Most planetesimals accreted to form the planets.

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.

Pressure ridges: Long, narrow wavelike folds in the surface of lava flows; formed where lava may have buckled up against slower moving or stationary lava downstream.

Polycyclic aromatic hydrocarbons: (PAH) One group of hydrocarbons (benzene) produced by decomposition or decay of organic matter. These are certainly aromatic: they stink!

Porosity: The percentage of the total volume of rock or soil that consists of open spaces. (The space between particles is called pore space.)

Primitive meteorite: A primitive meteorite is a chondrite that experienced minimal heating and aqueous alteration on its parent asteroid. Primitive meteorites have the highest concentrations of presolar grains.

Presolar grain: Preserved in primitive meteorites, presolar grains are stardust that formed in stellar outflows or ejecta in the molecular cloud, out of which our solar system formed.

Protostar: A star in the process of formation which has not yet become hot enough in the core to initiate the process of nuclear fusion (10⁷ K) to halt its gravitational collapse.

Pulsar: Variable radio star having stable, very short (around one second) periods of pulsations. Electrons moving rapidly in a pulsar's magnetic field produce narrow beams of radiation which sweep around as the pulsar spins (analogous to sweeping search-light beams).

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.

Pyroxenite: An igneous rock composed largely of pyroxene.

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 of ¹⁴C is 5730 years, which makes this radioactive isotope suitable for dating rocks and archaeological items as old as about 75,000 years.

Rare Earth Elements (REE): Fifteen closely related elements, the lanthanoid series, from atomic numbers 57 through 71 on the Periodic Table (some people define the series from atomic numbers 58 through 71). These elements usually occur together and are useful in geochemical modeling.

Ray: Streak of material blasted out and away from an impact crater.

Recovery: In astronomy, the first time a periodic comet is observed on its inbound journey toward perihelion. In meteoritics, the collection of a meteorite after its fall.

Reflectance spectroscopy: The study of light as a function of wavelength that has been reflected or scattered from a solid, liquid, or gas. Also see spectroscopy.

Refractory elements: Any chemical element that vaporizes (that is, boils) at high temperatures or condenses from a gas at high temperatures. Examples are calcium (Ca), aluminum (Al), uranium (U), titanium (Ti), the rare earth elements such as cerium (Ce), europium (Eu), and gadolinium (Gd); tungsten (W), zirconium (Zr), and thorium (Th). The opposite of refractory is volatile.

Refractory inclusion: Inclusions, enriched in the rare earths and the other elements mentioned in the definition of refractory, which are identified in rocks petrographically because of their mineralogy, which is dominated by Ca-, Al-, and Ti-rich phases. This is why these inclusions are often referred to as Ca-, Al-rich inclusions, or "CAIs." Most refractory inclusions contain the minerals spinel and melilite and/or hibonite.

Regmaglypt: Shallow depression, resembling a thumb print in clay, that is commonly seen on meteorites. Regmaglypts are formed by ablation from the surface by vortices of hot gas as a meteor falls through a planetary atmosphere.

Regolith (see Sediment, Soil): General term referring to the layers of fragmental and loose, incoherent, or unconsolidated rock, mineral, and glass fragments of any origin (residual or transported) that accumulate on the surface. On the Moon, this blanket of debris is produced by impacts.

Remote sensing: The acquisition of information on some property of a target object or material without being in physical contact with it. In the broadest terms, this is done by using instruments or scanners to sense and record reflected or emitted energy from the target and then processing the data in order to better analyze and understand the property in question.

Resolution: The degree to which fine details in an image are separated or resolved.

Right ascension (RA): Celestial coordinate which is equivalent to the longitude of an object on Earth. The starting point for longitude on Earth is at Greenwich England, whereas in the celestial coordinate system it is at a point in the sky called the vernal equinox. The two great circles defined by the celestial equator and the ecliptic, which is defined as the plane of the solar system, cross each other at 2 points: at the vernal equinox (first day of spring) and the autumnal equinox (first day of autumn). The vernal equinox is the starting point for the RA coordinate system.

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 smallest distance at which a planetary object that has no internal strength can orbit another body without being torn apart by the larger body's gravitational force.

Rock: A naturally formed solid that is an aggregate of one or more minerals.

S process: The buildup of heavy isotopes inside massive stars by the slow (hence the use of "s") absorption of neutrons by atomic nuclei.

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.

Sea of Serenity: One of the maria on the Moon's nearside; Mare Serenitatis.

Sea of Tranquillity: Landing site of Apollo 11 on the Moon on July 20, 1969; Mare Tranquillitatis.

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.

Shergottites: One of the three types of meteorites from Mars (the SNC meteorites). The type is named after the Shergotty meteorite, which fell in India in 1865. Basaltic shergottites consist mainly of plagioclase and pyroxenes. They are fine-grained and probably formed as volcanic lava flows or shallow intrusions. Lherzolitic shergottites are related to the basaltic shergottites but are coarser grained, have larger amounts of olivine, and are considered to be cumulate rocks.

Shock wave: A powerful compressional wave that lasts for an extremely brief time. Shock waves are created by explosive events, such as the impact of a meteorite on a planetary surface.

Short-period: In astronomy, a comet with an orbital period less than 200 years.

Siderophile: An element that concentrates in iron or that forms alloys easily with iron. Siderophile means "iron loving."

Silicate: A variety of minerals that always contain silicon (Si) and oxygen (O).

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 igneous rock, formed underground, that is parallel to the layered rocks around it.

SNC meteorites: (see Meteorite and Martian Meteorite) General name for three groups of meteorites with relatively young ages (slightly over 1 billion years old) that came from Mars. The groups are the Shergottites, Nakhlites, and Chassignites. Another Martian meteorite that does not fall into one of these groups is ALH 84001, a cumulate rock composed mostly of orthopyroxene, which is much older than all the others with an age of 4.5 billion years. (See Mars Meteorites compiled by Ron Baalke from the Jet Propulsion Laboratory.)

Soil: (see Regolith) The upper layers of sediment on Earth that support plant growth.

Sol: A local day on a planet, defined by its period of rotation. On Mars, for example, a local day lasts 24 hours and 37 minutes. During the Mars Pathfinder mission, scientists began counting Sols with the successful landing of the spacecraft on July 4, 1997 (Sol 1). Images from the Mars Pathfinder mission always contain a reference to the Sol on which it was taken.

Solar elongation: The angle between the lines of sight to the Sun and to the celestial body in question.

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 flare: A region of exceptional brightness in an atmospheric layer of the Sun, often associated with sunspots of complex magnetic fields.

Solar power: Energy derived from the Sun or sunlight for use as a source of electricity.

Solar nebula: The immense cloud of gas and particles of floating cosmic dust from which our Solar System (Sun and planets) condensed 4.6 billion years ago.

Solar System: The Sun and all the objects (planets, moons, asteroids, and comets) that orbit the Sun.

Solar wind: The stream of charged particles and atoms (mainly ionized hydrogen but actually a mixture of all atoms in the Sun) moving outward all the time from the Sun with low velocities in the range 300-500 kilometers per second.

Solubility: The ability or tendency of one substance to dissolve into another at a given temperature and pressure.

Source: (see Eruption) Location where igneous matter (lava and gases) erupts onto the surface; vent, fissure, volcano, etc.

Space weathering: Processes that change the physical structure, optical properties, or chemical or mineralogical properties of the surface of an airless planetary body from their original state. These processes include meteorite, micrometeorite, and cosmic dust bombardment, solar wind ion implantation, and sputtering, in deep vacuum and temperatures approaching absolute zero (-273 ^oC).

Spacecraft: Vehicle capable of traveling in outer space.

Spectra: The intensity of electromagnetic radiation (such as ultraviolet, visible light, and infrared, etc.) 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. Every atom can only emit or absorb certain energies or wavelengths. Scientists use instruments called spectrometers to study the location and spacing of reflectance, emission, and absorption (spectral) lines in specific wavelengths to detect any atoms or molecules that may be present in a target material (such as the surface of Mars).

Speed of light: A constant value: 299,792,458 meters per second (186,212 miles per second).

Sputtering: In the context of space weathering on the surfaces of airless planetary bodies, sputtering is the process in which atoms on the surface are knocked free by high-speed atomic particles in the solar wind; much higher-energy cosmic rays can also sputter surface materials.

Stardust Mission: The fourth in NASA's Discovery program of low-cost spacecraft with highly focused science goals, Stardust was comprised of a spacecraft and capsule that completed a seven-year, 3-billion-mile journey to a comet in 2006. The capsule returned to Earth, carrying an aerogel collector that captured particles from comet Wild 2 as well as interplanetary dust. The Stardust capsule is now on display at the Smithsonian's National Air and Space Museum.

Stratigraphy: Study of layered rock to understand the sequence of geological events. Normally, older layers are on the bottom unless the sequence has been overturned or disrupted.

Stratigraphic column: A drawing showing the vertical sequence of rock. Geologists use this to interpret the relative sequence of past geological events.

Subduction: The process describing when one lithospheric plate collides with and is overridden by, or descends under, an adjacent plate.

Sublimation: Phase transformation from solid to gas.

Sulfur: An element with atomic number 16; symbol: S. Sulfur is common on the surface of Jupiter's moon Io.

Taurus-Littrow: Landing site of Apollo 17 on the Moon on Dec. 11, 1972.

Tektite: Tektites are small beads of silica glass that are usually black but can also be shades of brown or green. People used to think they came from the Moon, but now we know they are from the Earth. Tektites formed during impacts on Earth when plumes of vapor and melted rock pierced the atmosphere. The impact debris traveled briefly through space before reentering the atmosphere--spreading the tektites over huge areas called strewnfields. The largest tektites, which are found on land, are several centimeters across. The smallest, which are found in layers of deep sea sediments, are only micrometers across.

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.

Terrain: Area of the surface with a distinctive geological character.

Thermocouple: A sensor for measuring temperature. It consists of two dissimilar metals, joined together at one end, which produce a small voltage proportional to the temperature.

Thermodynamics: The study of heat and its transformation to mechanical and other forms of energy.

Thrust fault: Low-angle fault in which rock above the fault plane moves up in relation to rock below.

Thin section: A thin slice or rock, usually only 30 micrometers thick. Thin sections are used to study rocks with a microscope.

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.

Troctolite: Igneous rock, found in the lunar highlands, composed of plagioclase and olivine.

Topography: The configuration (shapes, positions, arrangements) and relief (elevations, slopes) of the land surface.

Torino Scale: The official scale (created by Professor Richard P. Binzel and formalized in Torino, Italy in 1999) for categorizing the Earth impact hazard for asteroids and comets. Values range from 0 (no chance of collision with Earth) to 10 (collision with global climatic disasters). [Torino Scale]

Transient crater: (see: Crater) The initial crater excavated in the target rock during the first stage of an impact-cratering event.

Troilite: Iron-sulfide mineral with the chemical formula FeS.

Ultramafic: Term describing the chemical composition of an igneous rock composed almost entirely of magnesium-rich and iron-rich minerals (e.g., olivine, pyroxenes) and minimal silica.

Ultraviolet: Invisible part of the light spectrum where wavelengths are shorter than the visible spectrum and longer than X rays.

Unconsolidated: Materials loosely packed and not cemented together.

Vent: An opening in the Earth's surface (or other planet or moon) through which lava, gases, and hot particles come out.

Vesicle: Bubble-shaped cavity in a volcanic rock formed by expanding gases.

Vesta: The brightest of the known asteroids. Also called 4 Vesta because it was the fourth asteroid to be discovered (by Olbers in 1807).

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 website 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). The opposite of volatile is refractory.

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.

Yarkovsky Effect: An explanation for the shift in an asteroid's orbit around the Sun due to a thermal thrust caused by the body's uneven heating by sunlight. The illuminated side of an asteroid absorbs the sunlight and heats up. After a short delay produced by thermal inertia, the asteroid re-radiates the thermal energy and this causes a recoil spin that pushes the asteroid away from its current orbital path. In 2003, scientists used radar data from the Arecibo Observatory of Asteroid 6489 "Golevka" to obtain the first-ever measurements of the Yarkovsky Effect. (Named for the Russian engineer who came up with the idea around the year 1900.)