Ga (GIGA ANNUM) - One billion, 109, years (1 Ga = 1000 Ma).
GABBRO - Course-grained igneous rock rich in olivine, pyroxene, and plagioclase.
GALILEO MISSION - Mission to study Jupiter and its system of satellites (see http://galileo.jpl.nasa.gov/). 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 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.
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).
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 is an example of an irregular cluster of galaxies.
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.
GAMOV 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.
GENESIS MISSION - Mission to collect samples of charged particles in the solar wind (see http://genesis.lanl.gov/ and http://genesismission.jpl.nasa.gov/). 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.
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.
GIOTTO MISSION - Mission that studied the coma and nucleus of Comet P/Halley at close quarters during its 1986 perihelion passage (see http://sci.esa.int/science-e/www/area/index.cfm?fareaid=15). Despite the damage sustained during Halley flyby, the mission was extended to visit a second comet - P/Grigg-Skjellerup.
GLOBULAR CLUSTER - Dense, rich, spherical cluster of stars, held together by mutual gravity, and containing up to hundreds of thousands of stars within a diameter ~100 pc. Globular clusters are generally found in the halo of galaxies, and contain old Population II stars.
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).
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.
GRANULATION - Mottled appearance of the solar surface, caused by rising (hot) and falling (cool) material in convective cells just below the photosphere.
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 fundamental forces 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.
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. Newton's gravitational theory works in most situations, but is inadequate where gravitation 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 spacetime. In this theory, paths followed by material particles, or rays of light, near massive bodies are determined by local distortions of space.
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.
GREISEN-ZATSEPIN-KUSMIN CUTOFF - Computed limit to cosmic ray energies above which interactions with the microwave background should prevent transmission over cosmological distances.