FACULA - Term applied to a bright spot on a planetary surface (e.g., Memphis Facula on Ganymede).
FAINT YOUNG SUN PARADOX - Theories of stellar evolution indicate that as stars mature on the main sequence, they grow steadily hotter and brighter. About the time of the formation of Earth (~4.56 Ga), the Sun’s luminosity was roughly two-thirds its present value; the surface temperature of early Earth would have had a mean temperature of -15 °C (258 K). However, there is no geological evidence on Earth for a cooler Sun. For example, there is evidence of liquid water on Earth as early as ~4 Ga. To resolve this disparity, it is often proposed that Earth’s greenhouse effect was greater early in its history (probably enhanced by high concentrations of methane, CH4). Indeed the Gaia hypothesis argues that the Earth’s temperature has been regulated by life throughout its existence.
The paradox less easily resolved in the case of Mars, which has features like those produced by running water, but would have had a mean temperature of -77 °C (196 K). It is possible the fluid eroding martian channels was not water but instead fluidized avalanche debris (a mixture of dust, rocks, and ice) supported by CO2 vapor and no unusual past climate needs to be invoked.
FALL - Meteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering.
FAR ULTRAVIOLET SPECTROSCOPIC EXPLORER (FUSE) - Astrophysics mission, launched on June 24, 1999, to explore the Universe using the technique of high-resolution spectroscopy in the far-ultraviolet spectral region. See: http://fuse.pha.jhu.edu/.
FARADAY - Unit of electrical charge corresponding to the charge on one mole of electrons, or 96,485.309 coulombs.
FARADAY’S LAWS - Laws of electromagnetic induction first proposed by M. Faraday. The first law states that an electromotive force is induced in a conductor when the magnetic field surrounding it changes. The second law states that the magnitude of the electromotive force is proportional to the rate of change of the field. The third law states that the sense of the induced electromotive force depends on the direction of the rate of the change of the field.
FASSITE - Obsolete name for Ca- and Ti-rich clinopyroxene, properly termed augite.
FAULT - Fracture along which there has been movement or displacement.
FELDSPAR GROUP - Most abundant rock-forming silcate minerals in Earth's crust. Feldspars are common in almost all stony meteorite and lunar and martian rocks. The structure of feldspar can an be considered a "stuffed" version of silica group structures, consisting of an infinite network of (SiO4)4- and (AlO4)5- tetrahedra. Alkali metal and earth cations are housed in available voids to maintain charge balance. K+ or Na+ (or rarely Rb+) must fill the voids when a single Al tetrahedron substitutes for a Si tetrahedron; Ca2+ (or rarely Sr2+ or Ba2+) must be added when two Al tetrahedra substitute for Si tetrahedra. Its structure consists of crankshaft-like zigzag paired chains running parallel to the a axis. Each chain is linked to adjacent chains. The cations fit into cavities between crankshafts. The two prominent cleavages observed in feldspar occur along the vertical plane between chains (001) and along the mirror planes between layers (010).
There are several polymorphs of alkali (Na, K) feldspar, based upon order-disorder rather than structural differences. The high-temperature polymorph, sanidine, has Al3+ and Si4+ randomly distributed between the tetrahedral sites with K+ in large interstices bonded to nine nearest oyxgens. The lower temperature polymorph, microcline, has Al3+ and Si4+ completely ordered in the tetrahedral sites. The intermediate polymorph, orthoclase, has an intermediate degree of ordering. This is displayed in the figure below, which shows the concentration of Al3+ in the four tetrahedral sites.
Whether high-temperature K-feldspar retains disordered structure or transforms depends largely upon the cooling rate. Microcline is found in deep-seated plutonic rocks and pegmatites (slow cooling), orthoclase in intrusives formed at intermediate temperatures, and sanidine in extrusive high-temperature lavas (rapid cooling). The Na-alkali feldspar, albite, also shows disordered and ordered forms depending upon temperature and cooling; whereas, the Ca-feldspar, anorthite, is perfectly ordered at room temperature.
The general chemical formula for feldspar is A2+xB+1-xAl1+xSi3-xO8. A cations are Ca2+ and Ba2+; B cations are K+, Na+, Rb+, and Cs+. All these cations are large. Al and Si occur in variable amounts to maintain charge balance by the coupled substitution of B+ + Si4+ ↔ A2+ + Al3+. Feldspar compositions are often expressed in terms of their molecular percentages of anorthite (An), albite (Ab), and orthoclase (Or). The results may be easily plotted on a ternary diagram (below). Alkali (Na-K) feldspars contain <5-10 mol% An, whereas, plagioclase (Ca-Na) feldspars contain <5-10 mol% Or. Intermediate compositions are sometimes given specific names, although this is falling out of common usage. Ba-rich and Sr-rich feldspars are are rare except in alkaline terrestrial rocks. Ba-rich feldspars are subdivided based upon Ba abundances: celsian has >90 mol% of the BaAl2Si2O8 component; hyalophane feldspars have <30 mol%.
FELDSPATHOID GROUP - Minersals occuring in low-Si terrestrial rocks, feldspathoids are similar to feldspars in structure and properties, but contain about two-thirds as much silica as alkali feldspar. They do not coexist with quartz, because the excess of SiO2 converts feldspathoids into feldspar. The most common feldspathoids are: nepheline, NaAlSiO4, falsilite, KAlSiO4, and leucite, KAlSi2O6. Their compositions can be displayed using a tetrahedron with quartz at the apex (below). The ranges of observed substitutions are shown by the colored fields.
FERMAT’S PRICIPLE - Path taken by a ray of light between any two points in a system is always the path that takes the least time.
FERMI ENERGY - Energy of the highest occupied state in a solid in its ground state. It represents the kinetic energy of electrons responsible for conduction.
FERMI PARADOX - The question why, if the Galaxy is filled with intelligent and technological civilizations, they have not visited Earth? Fermi is supposed to have asked, “So? Where is everybody?” Fermi realized that any civilization with a modest amount of rocket technology and an immodest amount of imperial incentive could colonize the entire Galaxy within a few million years (a very short time on Galactic time scales). The fact that aliens don't seem to be walking our planet is taken to imply that there are no extraterrestrial anywhere among the vast tracts of the Galaxy. Many researchers consider this a radical conclusion to draw from such a simple observation. Many solutions to the paradox have been proposed, but in general, solutions to Fermi's paradox come down to either (1) life is difficult to start and evolve (either hard for the process or hard to find the right conditions); or (2) advanced civilizations destroy themselves on short timescales.
FERMION - Class of elementary particles that have an odd half-integer (1/2, 3/2, and so forth) spin. Quarks and leptons, as well as most composite particles, like protons and neutrons, are fermions. Fermions obey the Pauli Exclusion Principle.
FERRIMAGNETISM - Phenomenon in iron or iron alloys with a body centered cubic crystal structure where there can be incomplete cancellation of antiferromagnetic arranged spins giving a net magnetic moment. This happens when the sublattices consist of different materials or ions (such as Fe<2+ and Fe<3+).
FERROMAGNETISM - Phenomenon exhibited by materials like iron (nickel or cobalt) that become magnetized in a magnetic field and retain this magnetism when the field is removed. Ferromagnetism is one of the strongest forms of magnetism. It is responsible for most of the magnetic behavior encountered in everyday life and, along with ferrimagnetism, is the basis for all permanent magnets (as well as the metals that are noticeably attracted to them).
FILAMENT - Dark, thread-like feature seen on the Sun in the red light of hydrogen (H-alpha). Filaments are dense, omewhat cooler, clouds of material suspended above the solar surface by loops of magnetic field.
FIND - Meteorite not seen to fall, but found at some later date. For example, many finds from Antarctica fell 10,000 to 700,000 years ago.
FISSION - Breaking apart of a body into smaller fragments. In nuclear physics, fission refers to splitting of a heavy atomic nucleus into two or more lighter nuclei with an associated release of energy. The mass of the nucleus before fission is greater than the combined masses of the resulting fragments; the mass difference (Δm) is released as energy:
FLAMSTEED NUMBER - Combination of a number and the name of a constellation (61 Cygni, 36 Ophiuchi etc.) used to identify naked-eye stars. The numbers were applied to John Flamsteed's star catalogue published in 1725. Not all naked-eye stars have a Flamsteed number and most stars in the far southern hemisphere do not have one.
FLUCTUS - Geomorphologic term referring to a complex association of digitate (flow-like) features emanating from a common source location or feature (pl. flucti).
FLUENCE - Energy density from an optical source impingent on a sample. The higher the energy density, the higher the fluence. Fluence is measured in Joules per square centimeter, (J/cm<2).
FLUORESCENCE - Emission of electromagnetic radiation, especially of light, that results from bombardment of a substance with other forms of electromagnetic radiation. This spontaneously emitted radiation ceases immediately after excitation ceases.
FLUORESCENT YIELD - Quantification of the fraction of x-rays produced by inner shell ionization of an element. For K-shell ionization, fluorescent yield is:
where, ωK = fluorescent yield, nK = number of Kα X-ray photons emitted from sample, and NK = number of K shell ionizations. Yields for light elements are generally <0.2 for K-lines, but yield increases sharply with increasing Z and the fraction of Auger electrons (1-ω) decreases. Fluorescent yields vs. atomic number (Z) are shown for the K and L shells below.
FLUX - Rate at which something is transferred through a surface. In astronomy, this is the quantity of energy per second (watts) passing through unit surface area (1 m2) perpendicular to the direction of the source and is expressed as W/m2. For example, the flux of radiation from the Sun at the Earth is 1367 W/m2.
FLUX DENSITY - Measure of the flux of radiant energy within a unit interval of frequency (a frequency band with a width of 1 Hz) and is expressed as W/m2Hz. The flux densities for most cosmic radio sources are extremely low: the unit of flux density used by radio astronomers is the jansky (Jy), where 1 Jy = 10-26 W/m2Hz.
FORBIDDEN LINE - Spectral line seen in emission nebulae but not seen in laboratory experiments, because under laboratory conditions, collisions kick the electron in question into some other state before emission can occur.
FORBIDDEN ZONE - Relatively large energy separation between an insulator's highest filled electron energy band and the next higher energy vacant band.
FORCE - That which produces acceleration. Intuitively, a push or a pull, an action of one thing upon another. Forces produce palpable effects. There are only four fundamental forces (q.v.).
FORMULA - Combination of symbols that indicates the chemical composition of a substance.
FORMULA UNIT - The smallest repeating unit of a substance: the molecule for nonionic substances and unit cell for ionic solids.
FORMULA WEIGHT - Mass of one formula unit of a substance in atomic mass units.
FORSTERITE - Mg-rich olivine, Mg2SiO4, common in meteorites. Fe substitutes for Mg with complete substitution yielding the Fe-rich olivine, fayalite, Fe2SiO4.
FORSTERITE (F) CHONDRITES - Material only known as inclusions in other meteorites and described by certain lithologies of the Cumberland Falls aubrite. They are thought to have derived from a small and primitive asteroid of F chondritic composition that collided with the aubrite parent body shortly after their formation in the early Solar System. Mineralogically, they consist primarily of forsterite. Forsterite chondrites are intermediate between the H and E chondrites in terms of their chemical makeup. No complete meteorites of this group have been found on Earth, and therefore this class should be considered hypothetical.
FORWARD BIAS - Voltage applied to a diode in a direction that produces an electric current. When a p-n junction is forward biased, the positive terminal of a battery is connected to the p-side of the diode, and the negative terminal to the n-side.
FOSSA - Long, narrow, shallow depression or a narrow, linear trench (pl. fossae).
FRACTIONATION - Concentration or separation of one mineral, element, or isotope from an initially homogeneous system.
FREE ENERGY (G) - Thermodynamic state function of a system that indicates the amount of energy, E, available for the system to do useful work at constant T and P (also called “Gibbs free energy”). The free energy change (DG) in any reaction is related to the enthalpy and entropy. Formally, Gibbs Free Energy,G, is defined as the energy, E, in excess of the internal energy (as defined by the 2nd Law of Thermodynamics):
for dE in this equation, yields:
which, reduces to:
The Gibbs Free Energy is a function of P and T. For a system in equilibrium at constant P and T, ΔG = 0. If we differentiate this equation with respect to P at constant T, the result is:
This indicates that phases with small volume are favored at higher pressure. Differentiating with respect to T at constant P, yields:
This indicates that phases with high entropy (high disorder) are favored at higher temperature.
FREQUENCY (ν) - Rate at which specific events occur. For radiation and sound, this corresponds to the number of wave crests that pass a given point per second. Frequency is measured in cycles per second, called a hertz (Hz). Common multiples are kilohertz (1 kHz = 1000 Hz), megahertz (1 MHz = 106 Hz), and gigahertz (GHz = 109 Hz).
FUN CAIs - CAIs that show large mass fractionated and non-mass fractionated isotopic effects. FUN derives from Fractionated and Unidentified Nuclear effects.
FUNDAMENTAL PARTICLES - Particles considered basic, indivisible, building blocks of atoms and all forms of matter. These include stable leptons (electrons and neutrinos), and quarks and bosons. Also called "elementary particles."
FUNDAMENTAL FORCES - Forces that govern the various interactions between particles. The four fundamental forces, or interactions, are (in order of increasing strength): gravitational force, the weak nuclear force, the electromagnetic force, and the strong nuclear force. The strong and weak nuclear interactions are short-range forces that are effective only within atomic nuclei. The range of the strong force is ~10-15 m and that of the weak force ~10-17 m. In contrast, the electromagnetic and gravitational interactions are long-range forces, their strengths inversely proportional to the square of distance (1/r2). Although the effect of gravitation on particles is far weaker than the electromagnetic force, because matter tends to be electrically neutral, gravitation controls the overall dynamics of planets, stars, and galaxies. According to quantum theories, fundamental forces are conveyed between real particles by means of "virtual" particles called gauge bosons.
In the present-day universe, at common energy levels, the four forces are separate and have different strengths. However, at very high energies (more than ~1011 eV), the weak and electromagnetic forces merge into a single electroweak force. According to Grand Unified Theories (GUT), the strong nuclear and electroweak forces will behave as a single unified force at particle energies in excess of ~1024 eV (~1012 times higher than achievable experimentally). The fundamental forces may be summarized as follows:
FUSION - Process in which two lighter atomic nuclei combine to form a heavier atomic nucleus. Very high temperatures are normally required in order for atomic nuclei to collide with sufficient energy to overcome the Coulomb barrier (their mutual electrostatic repulsions). Fusion that occurs under high-temperature conditions is called thermonuclear fusion.Fusion reactions involving light elements release large amounts of energy. The mass of the resulting nucleus is less than the combined masses of the two original nuclei. The difference in mass, Δm (mass defect), is released as energy:
Fusion of elements up to 56Fe results in the release of energy. Thermonuclear fusion powers stars. Main sequence stars are dominated by hydrogen burning fusion reactions. In red giants, He is converted into C by the triple-alpha process. In highly evolved high-mass stars, fusion reactions synthesize a succession of elements up to Fe by helium capture.
FUSION CRUST - Melted glassy exterior of a meteorite that forms when it passes through Earth’s atmosphere. Friction with air can raise a meteorite’s surface temperature to 4800 K. Surface minerals melt at such temperatures and flow backwards over the surface as shown in the Lafayette meteorite photograph below. As the meteorite slows and the fireball is extinguished, the molten material cools and fuses to form a thin, glassy skin which envelopes the whole meteorite. Often the fusion crust is black or bluish-black and helps to make the meteorite stand out against the background of terrestrial rocks. Primary crust is the material that forms from the beginning of incandescent flight until dark flight. Secondary crust form if a piece breaks off the main mass during incandescent flight and a new crust forms on the broken surfaces.