L GROUP METEORITES - Ordinary chondrites low in free Ni-Fe metal (4 to 10 vol. %), containing olivine (Fa22-26) and the orthopyroxene hypersthene (Fs19-22). Average chondrule diameters (0.7 mm) are larger than those in H chondrites. The asteroid 433 Eros is suspected as a parent body, based on reflectance spectra, but most L chondrites show signs of severe shock metamorphism suggesting a violent history of the parent. Possibly the L chondrites came from a relative or a former part of Eros that was entirely broken up when it collided with another asteroid.
LL GROUP METEORITES - Ordinary chondrites ("low Fe"/"low metal") with only 1 to 3% free metal. Their olivine is more Fe-rich than in the other ordinary chondrites (Fa27-32), implying that the LL types must have formed under more oxidizing conditions than their H or L cousins. Orthopyroxene compositions are also Fe-the rich (Fs23-26). LLs have the largest chondrules found in ordinary chondrites, averaging ~1 mm. Scientists are still searching for a probable parent body for this group. One small main belt asteroid, 3628 Boznemcová, shows a similar reflectance spectrum, but with a diameter of just 7 km it seems too small to be regarded as the progenitor of the LL members.
LAFAYETTE METEORITE - A Mars meteorite - a nakhlite. It is named after Lafayette, Indiana, where it was identified as a meteorite in 1931 by O. Farrington having sat for years unrecognized in a Purdue University geological collection. The exact location and date of its fall are not known. However, the Lafayette meteorite is very similar to Nakhla, which fell in Egypt in 1911, and it is possible that it may have been mislabeled and be part of the Nakhla fall. It weighs 800g and is shaped like a truncated cone 4-5 cm across. Most Mars meteorites show clear signs of having been exposed to moisture and salty water before they were ejected from the Martian surface. Enough weathered minerals are present in the Lafayette specimen to allow determination of a fairly precise date on when the water exposure took place - 670 Ma ago. Lafayette contains the most water of any Martian meteorite (0.387 wt. %) as hydrated salts. Altered olivine indicates the meteorite was originally a Fe-rich, volcanic rock that was exposed to water around 700 Ma ago. About 11 Ma ago, the fragment was ejected from Mars and it landed on Earth ~2,900 years ago. The salts identified in the Lafayette alteration formed by fractional evaporation of acid brine on Mars.
LAGRANGE POINTS - Five points, labeled L1 through L5, in the vicinity of two massive bodies where each others' respective gravities balance. L1, L2, and L3 lie along the centerline between the centers of mass between the two masses: L1 is on the inward side of the secondary, L2 on the outward side of the secondary; and L3 on the outward side of the primary. L4 and L5, the so-called Trojan points, lie along the orbit of the secondary around the primary, sixty degrees ahead and behind of the secondary.
L1 through L3 are points of unstable equilibrium; any disturbance will move a test particle there out of the Lagrange point. L4 and L5 are points of stable equilibrium, provided that the mass of the secondary is less than ~1/24.96 the mass of the primary. These points are stable because centrifugal pseudoforces work against gravity to cancel it out.
LAMBERT’S LAW - Relationship describing how the intensity of electromagnetic radiation decreases exponentially (from I0 to I) with distance, t, as it travels through an absorbing medium:
The extinction coefficient, K, is in units of cm-1. In materials science, extinction is often described by a “mass absorption coefficient,” m, which has units of cm2/g. In this case, the material density, r, must also be specified:
Mass absorption coefficients vary depending upon the wavelength (energy) of the radiation; they generally decrease with decreasing wavelength.
LANTHANIDES - Rare earth elements.
LARGE-SCALE STRUCTURE - Structures composed of galaxy superclusters, galactic sheets, filaments and voids that span significant fractions of the observable Universe. These structures are observed in galaxy redshift surveys such as the Australian-led 2 degree Field (2dF) project (below). Knowledge of large-scale structure is extremely important because it places severe constraints on cosmological models, and provides a probe of conditions before the epoch of recombination in the very early Universe, indicating the quantum mechanical effects that were important at that time.
LATE HEAVY BOMBARDMENT (LHB) - Period between ~4.0 to 3.8 Ga ago when the Moon and other objects in the Solar System were pounded heavily by wayward asteroids. The evidence for LHB includes the lunar maria basins and similar structures elsewhere, such as the Caloris Basin on Mercury and the great craters in the southern hemisphere of Mars. On Earth, LHB would have produced 22,000 or more impact craters with diameters >20 km, ~40 impact basins with diameters ~1000 km, and several impact basins with diameters ~5,000 km with a serious environmental damage event occuring about every 100 years. However, plate tectonics and erosion have erased the evidence from Earth's surface. Conclusive evidence that Earth experienced LHB was not discovered until 2002, when British and Australian researchers announced they had found W isotopes in 3.7 Ga rocks from Greenland and Canada that can only be extraterrestrial. Recent computer models also suggest that resonances and perturbations caused by the four large outer planets settling into their current orbital configurations would cascade large volumes of asteroidal material into the inner solar system.
LATE-TYPE GALAIES - Late-type galaxies include all classes of spiral and irregular galaxies. The term 'late-type' derives from the original interpretation of the Hubble tuning fork diagram, in which is was assumed that galaxies evolved from the left (early-type ellipticals) to the right (late-type spirals) in a sequence.
LATENT HEAT - Heat absorbed or released as the result of a phase change. There are three basic types of latent heat each associated with a different pair of phases: fusion (solid-liquid), vaporization (liquid-gas), and sublimation (solid-gas). No temperature change occurs during a phase change, thus there is no change in the kinetic energy of the particles in the material. The energy released comes from the potential energy stored in the bonds between the particles. Processes may either release or absorb latent heat. Endothermic phase changes absorb heat from the environment and are cooling processes. Exothermic phase changes release heat to the environment and are warming processes. Transformations between pairs of phases are given different names depending upon whether the process is endothermic or exothermic. For example, the solid-liquid phase change may be called melting or fusion (endothermic) or crystallization, freezing or solidification (exothermic).
LATTICE - Regularly spaced array of points that represents the structure of a crystal. Crystals are composed of groups of atoms repeated at regular interval in three dimensions with the same orientation. The smallest division of the lattice which can still be used to represent the entire structure is called the unit cell. Each such group of atoms is replaced by a representative point; the collection of points so formed is the space lattice or lattice of the crystal. Each crystal lattice is a Bravais lattice.
LATTICE VIBRATION - Regular vibration of atoms within a crystal lattice about their equilibrium positions.
LAWS OF THERMODYNAMICS - Statements of basic thermodynamic relationships in a system.
The 1st Law states that the internal energy, E, of an isolated system is constant. In a closed system, there cannot be a loss or gain of mass, but there can be a change in energy, dE, as a result of exchanging heat or doing work with the system. The net change in energy is the difference between the heat, Q, gained or lost, and the work, W, done by the system:
We define work, W, as force, F, acting over a distance, D:
Since pressure, P, is F/A:
where, V = volume (A x D). If work is done at constant pressure, then W = PdV. Substitution of this relationship into the equation (1) yields a restatement of the first law of thermodynamics:
The 2nd Law states that the change in heat energy, dQ, of the system is related to the amount of disorder in the system. Entropy, S, is a measure of a system’s disorder; at constant T and P:
Substituting into equation (2) yields:
LE CHATELIER'S PRINCIPLE - Principle that any change imposed on a system in equilibrium, tends to shift the equilibrium to reduce the effect of that applied change.
LENTICULAR GALAXY - Galaxy that is shaped like a lens. They are of an intermediate type between an ellipticalgalaxy and a spiral galaxy. They exhibit a bulge and disk similar to spiral galaxies, but lack spiral arms or significant quantities of interstellar material. They consist primarily of old, population II stars and for this reason, are often misclassified as elliptical galaxies when viewed face-on. The origins of S0 galaxies are still unknown, but one possibility is that represent spiral galaxies that lost or used up their interstellar material through interactions with another galaxy.
LEPTON - Particles that are not acted on by the strong nuclear force and are not built of quarks. There are six leptons, three of which have electrical charge and three of which do not. They appear to be point-like particles without internal structure. The best known lepton is the electron (e-). The other two leptons are the muon (μ) and tau (τ), negatively charged particles that appear to be heavier analogs of the electron. The mass of a μ particle is 105.6 MeV and that of a τ is 1.78 GeV. The remaining leptons are types of neutrinos (ν), which have no electrical charge, zero or very little mass, and consequently very hard to detect. For each lepton there is a corresponding antimatter antilepton.
LIGHT-EMITTING DIODE (LED) - Diode that has been constructed to optimize the emission of light as conductionelectrons and holes recombine at a p-n junction and maximize the fraction of the produced light that escapes the device and is seen. LEDs are energy-efficient sources of light in widespread use as indicator lights and in traffic signals, among other applications.
LIGHT YEAR - Distance that light travels in a year through a vacuum. This unit is derived from the fact that light (electromagnetic radiation) takes a finite length of time to travel through space. Defined as 299,792,458 m/s.
LIMB - Extreme edge of visible disk of the sun, moon or planet.
LITHOPHILE ELEMENT - Element that tends to be concentrated in the silicate phase, e.g., B, O, halogens, alkali earths, alkali metals, Al, Si, Sc, Ti, V, Cr, Mn, Y, Zr, Nb, REE, Hf, Ta, W, Th, and U.
LITHOSPHERE - Rigid outer layer of a planet. The base of the lithopshere is defined by the temperature at which the brittle/ductile transition occurs in the mantle.
LOBATE SCARP - Landform on Mercury consisting of curving cliffs produced by compressional forces. These cliffs vary from 10s to 100s of km in length and from 100-3000 m in height.
LOCAL BUBBLE (LB) - Cavity of ~100 pc radius that may have been formed by a supernova explosion and a density of ~0.005-0.05 atoms/cm3 - at least 10 times lower than the average interstellar medium (ISM) in the Galaxy. The Bubble was formed ~105-106 years ago by several relatively nearby supernova explosions that pushed aside gas and dust in the ISM leaving the current depleted expanse of hot, low density material. "Bubble" may be a misnomer since it appears to have an hourglass shape that is narrowest in the galactic plane and that widens above and below the plane like a chimney.
Inside the Bubble are numerous shells of gas. The Sun, along with several neighboring stars, ilies very close to the edge of a cloudlet named the Local Interstellar Cloud and is moving roughly perpendicular to it.
LOCAL FLOW - The motions of groups or clusters of galaxies not attributable to Hubble flow (due to the expansion of the Universe) or orbits within a groups or cluster. Local flows result from large-scale gravitational effects. For example, the Local Group of galaxies (including the Milky Way) has a local flow of about 600 km/s in the direction of the superclusters of galaxies that make up the Great Attractor in Centaurus.
LOCAL INTERSTELLAR CLOUD (LIC) - Group of sheet-like cloudlets in the interstellar medium near the Sun with density of ~0.5 atoms/cm3, a temperature of ~7000 K, and a size of several parsecs. These patches of neutral hydrogen atoms were produced during expansion of a larger bubble created by supernovae and stellar winds in the Scorpius-Centaurus Association, which lies ~500 light-years away. The LIC is flowing away from the Scorpius-Centaurus Association of young stars and resides in a low-density hole in the interstellar medium called the Local Bubble. The Sun will leave the LIC in ~10,000 years.
LODRANITES (LOD) - Rare type of primitive achondrite named after the Lodran meteorite that fell in Pakistan in 1868. Initially, lodranites were grouped with the stony-iron meteorites because they contain silicates (olivine, orthopyroxene, and minor plagioclase) and Fe-Ni metal in nearly equal proportions. However, since discovery of the closely related acapulcoite group, lodranites have been classified as primitive achondrites. Both groups have similar mineralogical and oxygen isotopic compositions, and probably come from the same parent body - most likely an unidentified S-class asteroid. Lodranites have coarser-grained olivines and pyroxenes and experienced higher temperatures than acapulcoites, suggesting that they originated within the deeper layers of the acapulcoite/lodranite parent body where they were subjected to a more intense and prolonged thermal processing. Fragment of Lodran shown below.
LONG-LIVED RADIONUCLIDES - Radioactive isotopes with half lives (t½) exceeding ~500 Ma. They include: 238U (t½ = 4.47 Ga), 232Th (t½ = 14 Ga), 235U (t½ = 0.704 Ga), 40K (t½ = 1.25 Ga), and 87Rb (t½ = 48.8 Ga) and 147Sm (t½ = 106 Ga). Measuring the amount of the parent nuclide and the amount of the daughter isotope, and knowing the decay rate, allows calculation of the time since the daughter isotope began to accumulate. Long-lived radionuclides have reveled the absolute age of the solar system (4.567 Ga) and the timing of major events on the Earth, Moon, and Mars. However, for the parent radionuclide to still be around to measure, it must decay very slowly. This means that the chronometers based on long-lived radionuclides inherently have low precision. The Pb-Pb system, based upon decay of U and Th, provides the most precise dates of early solar system events.
LONG-PERIOD COMET – Comets with orbital periods longer than 200 years, often with highly elliptical orbits highly inclined to the ecliptic. These comets probably were perturbed from orbits in the Oort cloud by a passing star or giant molecular cloud, or by tidal forces generated by the bulge and disk of our Galaxy. The high inclinations of the orbits arise since they can enter the inner Solar System from any angle. Long-period comets tend to be the most spectacular comets; because they have not made many (if any) passes through the inner Solar System, and still retain a large percentage of their initial volatiles. Sublimation of these volatiles from the comet nucleus as it nears the Sun produces the coma and highly-visible tail. Comets Hale-Bopp (1997), Hyakutake (1996), and McNaught (2007), shown below, were long-period comets.
LONG-PERIOD VARIABLE - A highly evolved, very luminous red giant star whose atmosphere expands and contracts in repeating cycles (i.e., it pulsates) with periods from several months to several years.
LONSDALEITE - Hexagonal polymorph of carbon that forms from meteoric graphic during impact. The great heat and stress of the impact transforms the graphite into diamond, but retains graphite's hexagonal crystal lattice (below). Lonsdaleite was first identified from the Canyon Diablo meteorite at Barringer Crater (also known as Meteor Crater) in Arizona in 1967.
LUMINOSITY - Basic property used to characterize stars, luminosity is defined as the total energy radiated by a star each second. An object’s luminosity is often compared to that of the Sun (Lsun = 4 × 1033 ergs/s = 3.9 × 1026 Watts). Luminosity has the same units as power (energy per second). Luminosity can be related to the absolute magnitude, MV, by the equation:
Where, L* is the luminosity of the object in question and Lstd is a reference luminosity (often the luminosity of a 'standard' star such as Vega). Luminosity can be quoted for the energy emitted within a finite waveband (e.g. optical luminosity), or for the energy emitted across the whole electromagnetic spectrum (“bolometric” luminosity). Note that measurement of the luminosity requires an object’s apparent magnitude and the distance to the object. Thus, estimates of luminosity rely on accurate distance measurements.
LUMINOSITY CLASS - Classification scheme which groups stars according to the width of their spectral lines. For a group of stars with the same temperature, luminosity class differentiates between supergiants (I), giants (II, III, IV), main-sequence stars (V), and subdwarfs (right).
LUNAR METEORITES - Achondrites from the surface of the Moon. Over 40 lunar meteorites have been identified since the 1970s with a total mass of ~8.5 kg. Most were found in Antarctica and in the deserts of northern Africa and Oman, although one, a 19-gram specimen, was recovered in 1990 from Calcalong Creek, Australia. These stones are of great importance because, in many cases, they provide specimens of the Moon from regions not visited by the manned Apollo or unmanned Russian sample-return missions. Most were blasted out of the lunar highlands rather than the low-lying maria, which served as the Apollo landing sites.
The lunar meteorites found so far, represent four distinct types of Moon rock and are categorized into groups LUN A (anorthositic highland breccias), LUN B (mare basalts), LUN G (gabbro), and LUN N (norite). One interesting specimen is a LUN B meteorite, found in Morocco in 2000, that crystallized from lava just 2.8 Ga ago and provides evidence for surprisingly recent lunar volcanism.
The only known LUN N meteorite, found in three pieces near Dchira in the Western Sahara and named NWA 773, is especially important because it represents a type of rock never sampled by the Luna or Apollo landing missions, but detected from orbit at several sites on the surface. The Aitken basin, a large impact structure near the lunar South Pole that is famous for its noritic composition and secondary impact craters, is a possible source of NWA 773. The large impact that excavated the Aitken basin removed the upper crust, exposing lower crustal layers that contain olivine-rich norites and gabbronorites.
All lunar meteorites can be considered mixtures of mare basalts and highlands rocks as shown by their bulk chemistries when plotted on a FeO vs. Al2O3 diagram.
LYMAN SERIES - Hydrogen emission series in which electrons jump to the ground state. All of the lines are in the ultraviolet.