Glossary Gg

Ga (GIGA ANNUM) - One billion, 109, years (1 Ga = 1000 Ma).

GABBRO - Course-grained igneous rock rich in olivine, pyroxene, and plagioclase.

GAIA HYPOTHESIS - Idea that the Earth as a whole should be regarded as a living organism and that biological processes stabilize the environment. This hypothesis was first proposed by British biologist James Lovelock in 1969.

GALACTIC COORDINATES (l, b) - Coordinate system based on the plane of the Galaxy, it is centered on the Sun with the zero point of longitude and latitude pointing directly at the galactic centre. The symbols used for galactic coordinates are l (longitude) and b (latitude).

GALAXY - Concentration of 106 to 1012 stars. There are four main types of galaxies: elliptical Galaxies, lenticular galaxies, spiral Galaxies, and irregular galaxies. Our galaxy contains ~2 × 1011 stars.

GALACTIC HALO - Spherical region around a spiral galaxy that contains dim stars and globular clusters. The radius of the Milky Way's halo extends ~50 kpc from the galactic center (Earth is at ~28 kpc in the galactic disk).

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GALAXY CLUSTER - Group of galaxies which are mutually gravitationally bound. There are two important types of galaxy clusters: Regular and Irregular Galaxy Clusters. Regular clusters are huge spherical groupings of galaxies with large numbers of galaxies concentrated in their centers. They tend to contain thousands of galaxies and to have many bright elliptical and S0 type galaxies. Irregular clusters are not too centrally condensed with a somewhat nonspherical overall shape. They contain a few to hundreds of galaxies. Our Local Group (below) is an example of an irregular cluster of galaxies.

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GALILEAN MOONS - Four largest of Jupiter's moons: Io, Europa, Ganymede and Callisto. Dscovered independently by Galileo and Marius, Galileo proposed that they be named the “Medicean stars,” in honor of his patron Cosimo II de Medici. However, the present names are due to Marius.

GALILEO MISSION - Mission to study Jupiter and its system of satellites (see The spacecraft launched in 1989, consisted of two main components: an atmospheric entry probe and planetary orbiter. Galileo encountered and imaged asteroids: Gaspra (in 1991) and Ida (in 1993) on its way to Jupiter. Additionally, it had a direct view of the impact sites when fragments of comet Shoemaker-Levy 9 collided with Jupiter (the spacecraft was ~1 AU from Jupiter at this time). The entry probe made measurements of atmospheric composition between levels in the atmosphere corresponding to pressures from 0.4 bar to 22 bar. It made abundance measurements of He, H2, NH3, CH4, H2S, and H2O. The orbiter made close approaches to each of the major satellites and measured their gravitational and magnetic fields. It also made spectroscope composition measurements and provided close-up images.

GAMMA RAYS (γ-RAYS) - Most energetic form of radiation, similar to x-rays and light, except with shorter wavelengths (<0.01 nm) and higher energies (>105 eV). Despite their high energies and penetrating power, γ-rays from cosmic sources are absorbed by the atmosphere. In nuclear reactions, decay by γ-ray emission permits an excited nucleus drop lower (more stable) energy state.

GAMMA RAY BURST (GRB) - Long duration g-ray bursts that coincide with powerful supernovae (hypernovae) that occur when a massive star collapses to a black hole.

GAMOV PEAK (or WINDOW) - Narrow range of energies where nuclear reactions involving charged nuclei occur in stars. The probability for penetrating the Coulomb barrier (green) decreases rapidly with decreasing energy but at a given temperature the possibility of having a particle of high energy (and therefore high velocity) decreases rapidly with increasing energy (pink). The sum of these opposing effects produces an energy window for the nuclear reaction: only if the particles have energies approximately in this window can the reaction take place.

Image source: Figure 10.6, Introduction to Modern Astrophysics, 2nd ed., Carroll & Ostlie, 2006.

GANGUE - Material of no value that accompanies the desired material in raw ores.

GARNET - Mineral generally found in terrestrial metamorphic rocks, although igneous examples are not uncommon. Garnet is a significant reservoir of Al in the Earth's upper mantle.

The garnet structure consists of isolated SiO4 tetrahedra bound to two cation sites. The A site holds relatively large divalent cations (Ca2+, Mg2+, Fe2+, Mn2+); the 6-fold B site holds smaller trivalent cations (Al3+, Cr3+, Fe3+). The BO6 octahedra share corners with tetrahedra forming a continuous network and the A sites are surrounded by 8 oxygens that form an irregular polyhedron.

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The general formula of garnet is A3B2(SiO4)3. However, Ca2+ cation is appreciably larger than the Mg2+, Fe2+, and Mn2+ cations (100 pm vs. 72 to 83 pm), resulting in separate mineralogical series of Ca-rich and Ca-poor garnets.

Ca-rich garnet comprise the ugrandite series; Ca-poor garnet form the pyralspite series. The ugrandite series gets is name from the main minerals in it: uvarovite, Ca3Cr2(SiO4)3, grossular, Ca3Al2(SiO4)3, and andradite, Ca3Fe3+2(SiO4)3. Site occupancies are: A = Ca2+, B = Al3+, Cr3+, Fe3+. The names based upon the B site occupancy. Grossular-andradite forms a complete solid solid solution. The pyralspite series name is also based upon its minerals: pyrope, Mg3Al2(SiO4)3, almandine, Fe3Al2(SiO4)3, and spessartine, Mn3Al2(SiO4)3. Site occupancies are: A = Mg2+, Fe2+, and Mn2+, B = Al3+. The names based upon the A site occupancy. Pyrope-almandine and almandine-spessartine form complete solid solutions.

Pyralspite compositions

There is limited solid solution between the pyralspite and ugrandite series as shown below.

There are many types of synthetic garnets. The gadolinium gallium garnet (GGG), Gd3Ga2(GaO4)3 is used in magnetic bubble memories, and the yttrium aluminum garnet (YAG), Y3Al2(AlO4)3 isd used for synthetic gemstones and as lasing medium in lasers (doped with Nd3+).

GATE - Part of a transistor that controls the flow of electric current through the device. Applying a voltage to the (red) gate terminal in the transistor shown below allows conduction electrons to flow from one (green) n-type semiconductor region, through the (blue) p-type semiconductor region, to the other n-type region.

GENESIS MISSION - Mission to collect samples of charged particles in the solar wind (see and Genesis carried four instruments: (1) solar wind collector arrays designed to entrap solar wind particles; (2) an ion monitor, to record the speed, density, temperature and approximate composition of the solar wind ions; (3) an electron monitor, to make similar measurements of electrons in the solar wind; and (4) an ion concentrator, to focus elements like O and N onto a special collector tile. Genesis collected solar wind for 884 days capturing ~1020 ions (4 x 10-4 g). Due to a human error, the return capsule's parachutes failed to deploy and the capsule crashed into the Utah desert. Fortunately, preliminary results indicate that surface contamination of the collection wafers will be manageable for many analytical techniques. Results should be forthcoming.

GEOCHEMISTRY - Study of the chemical composition of Earth and other planets, chemical processes and reactions that govern the composition of rocks and soils, and the cycles of matter and energy that transport Earth's chemical components in time and space.

GEODESIC - Shortest distance between two points on a sphere.

GEOSYNCHRONOUS ORBIT - Orbit in which a satellite's orbital velocity matches the rotational velocity of a planet. As a result the satellite appears to hang motionless above one position on the planet's surface.

GIANT IMPACT THEORY - 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.

GIBBOUS - Phase of Moon between first quarter and full or between full and last quarter.

GIOTTO MISSION - Mission that studied the coma and nucleus of Comet P/Halley at close quarters during its 1986 perihelion passage (see Despite the damage sustained during Halley flyby, the mission was extended to visit a second comet - P/Grigg-Skjellerup.

GLIDE PLANE - Symmetry element for which the symmetry operation is reflection across a plane combined with translation in a direction parallel to the plane.

GLOBULAR CLUSTER - Spheroidal collections of 105-106 stars found orbiting in the halos of all large galaxies. Some globular clusters are almost as old as the Universe itself, making them among the oldest stellar populations known. With diameters of only several 10s to ~200 light years, they are some of the most densely packed stellar systems in the Universe.

All the stars in a globular cluster were formed at the same time and from the same cloud of gas; thus, they constitute a single stellar population. However, one aspect of this stellar population is unique: the extremely high packing density of stars in the centers means that interactions, and even collisions, between stars can occur. Consequently, globular clusters include some exotic classes of stars, including blue stragglers, low-mass x-ray binaries and millisecond pulsars. The stars in these clusters are so close that their outer layers are contaminated by stellar winds from nearby stars and supernova debris, giving them unique chemical properties.

Most globular clusters, in both the Milky Way and other galaxies, contain primarily old stars. Metallicities range from extremely metal-poor (< 1/100th of the solar value) to values close to the Sun’s. This variation in metallicity gives rise to the two distinct types of globular clusters in galaxies. In the Milky Way (at least), redder clusters are more metal-rich and are associated with the galactic bulge, whereas bluer clusters are more metal-poor and tend to be associated with the halo. Some scenarios suggested to explain the two types include:  multi-phase primordial collapse in which globular clusters form in a primordial gas cloud that collapses to form stars in two distinct phases; violent, gas-rich galaxy mergers in which globular clusters form from the disturbed cold gas of the merging galaxies; accretion of dwarf galaxies which bring their own clusters with them when they merge with a host galaxy; and secular evolution in disks where the cold gas in the spiral arms of galaxies can form large star clusters over an extended time.

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GLUON - Hypothetical particle which binds quarks together into hadrons.

GOLDSCHMIDT CLASSIFICATION - Geochemical classification system developed by Victor Goldschmidt, the founder of modern geochemistry and crystal chemistry. His system groups the chemical elements according to their preferred host phases: siderophile (iron-loving), lithophile (rock-loving = silicate-loving), chalcophile (sulfur-loving), and atmophile (gas-loving).

Image source: Figure 7.2, Chapter 7. Geochemistry, W. M. White, 2007.

GPa (GIGAPASCAL) - Conventional unit of pressure used when discussing the deep Earth and meteoritic shock: 1 GPa = 10 kilobars = 10,000 times air pressure at sea level.

GRAM FORMULA WEIGHT - Amount of a substance equal in grams to the sum of the atomic weights; a mole.

GRANULATION - Mottled appearance of the solar surface, caused by rising (hot) and falling (cool) material in convective cells just below the photosphere.

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GRAPHITE - Opaque form of carbon found in some iron and ordinary chondrites and in ureilite meteorites. Each C atom is bonded to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons. The two known forms of graphite, α (hexagonal) and β (rhombohedral), have identical physical properties, except for their crystal structure. Naturally occurring graphite contains up to 30% of the β form; synthetically produced graphite only contains the α form. The α form can be converted to the β form through mechanical treatment; the β form reverts to the α form when heated above 1000 °C.

Egg-shaped graphite nodules are found in a number of meteorites. These nodules are often referred to as "primary structures" in iron meteorites. Most nodules have rims of taenite and schreibersite that has precipitated on the nodules during cooling and crystallization. The enveloping taenite is highly resistant to weathering which eats away at a meteorite.

GRAVITATIONAL CONSTANT (G) - Fundamental constant of nature, which determines the strength of the gravitational interaction. G = 6.674 x 10-8 dyne cm2 g-2 = 6.67 x 10-11 N m2 kg-2 = 6.67 x 10-11 m3 s-2 kg-1.

GRAVITATIONAL FORCE - One of the four fundamental forces, gravitation is the force of mutual attraction that is exerted between massive bodies and between particles that have mass. Although gravitation is far weaker than the other three fundamentalforces over short ranges, it is the dominant force on large scales because its range of influence is far greater than that of the nuclear forces and because, unlike electrical charges, which can be positive or negative, all mass is mutually attractive. Gravitation alone determines motions of, and mutual interactions between, planets, stars and galaxies, and dominates the overall dynamics of the universe.

Newton’s law of universal gravitation works in most situations, but is inadequate where gravity is very strong (e.g., in the vicinity of a black hole, or when dealing with the structure and dynamics of the universe as a whole). The best present theory of gravitation is Einstein’s general theory of relativity, where mass distorts the geometry of 4-dimensional space-time. In this theory, paths followed by material particles, or rays of light, near massive bodies are determined by local distortions of space.

For two bodies, with masses m1 and m2, separated by distance, r, the mutual force of attraction, F, predicted by Newtonian theory is:

where, G = gravitational constant. An equal and opposite force acts on each mass, but the force acting on m1 is in the opposite direction of that on m2.

GRAVITATIONAL POTENTIAL ENERGY - Energy inherent in the potential for gravitational collapse. The gravitational potential energy of a gas of atoms and molecules is:

where R = radius of the ball of gas, and M = total mass.

GRAVITATIONAL RADIATION - Very weak wave-like disturbances in the geometry of space and time produced by an accelerating, oscillating or violently disturbed mass, or system of masses. The resulting gravitational radiation (or gravitational waves) waves ripple outwards through space, traveling at the speed of light. Gravitational waves vibrate in a plane perpendicular to the direction in which it is propagating. As they pass through a solid body, they will stretch it first along one direction (while compressing it at right angles to this direction) and then, similarly, in the plane perpendicular to this direction. There is strong indirect observational evidence that gravitational waves follow the predictions of General Relativity. Their weakness has made them undetectable by present instruments. The electromagnetic force between a hydrogen atom's electron and proton is 2 x 1039 times stronger than their mutual gravitational attraction!

GRAVITATIONAL REDSHIFT - Shift in the frequency of a photon to lower energy as it climbs out of a gravitational field.

GRAVITON - Hypothetical force-carrying particle (gauge boson) for gravitation. Although there is, as yet, no generally accepted quantum theory of gravity, it is widely believed that it should be possible to formulate gravitation in quantum terms with the gravitational interaction between particles of matter conveyed by gravitons. The hypothesized graviton has zero mass, zero charge and a spin value of 2 (in units of h/2π).

GRAVITY - Attractive force between all matter; one of the four fundamental forces.

GREAT RED SPOT See Jupiter’s Great Red Spot.

GREISEN-ZATSEPIN-KUSMIN CUTOFF - Computed limit to cosmic ray energies above which interactions with the microwave background should prevent transmission over cosmological distances.

GROSSITE - Calcium aluminate, CaAl4O7, first found in metamorphosed Israeli limestone and recently in CAIs in CV3 and CR-CH-CB carbonaceous chondrites.

GROUND STATE - Energy state of an atom, or of a solid, when its total energy equals the minimum possible energy for that atom or solid. For solids, this only occurs at a temperature of absolute zero. Individual atoms may be in their ground state at non-zero temperatures, but they will not stay there indefinitely. Instead, they can occasionally absorb thermal energy and move to an excited state for a while before they decay back to the ground state. For the energy levels of electrons in atoms and solids, the “ground state” refers to the situation when all electrons have their lowest possible energies.

GROUP - Vertical columns (major classes or divisions) into which elements are arranged in the periodic table of elements. Elements in a group have similar configurations of their valence shell electrons, which gives them similar properties.  There are three common numbering systems for these groups. (1) The IUPAC system, which numbers each column from 1–18. (2) The old IUPAC system, which labeled columns with Roman numerals followed by either 'A' or 'B'. Columns were numbed such that columns 1–7 were numbered 'IA' through 'VIIA', columns 8–10 were labeled 'VIIIA', columns 11–17 were numbered 'IB' through 'VIIB' and column 18 was numbered 'VIII'. (3) The CAS system, which also used Roman numerals followed by 'A' or 'B'. This method, however, labeled columns 1 and 2 as 'IA' and 'IIA', columns 3–7 as 'IIIB' through 'VIB', column 8–10 as 'VIII', columns 11 and 12 as 'IB' and 'IIB' and columns 13–18 as 'IIIA' through 'VIIIA'. Because of the confusion the old IUPAC and the CAS system created, the IUPAC adopted their new system; this is the preferred system.

GUANINE - One of the five nitrogen-containing bases occurring in nucleotides.