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 - One of the most interesting of the known Mars meteorites - 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.
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.
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).
LEPTON - Particles that are not acted on by the strong nuclear force and are not buil 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.
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.
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 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.
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 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. 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, at all wavelengths (ergs per second). An objects luminosity is often compared to that of the Sun (Lsun = 4 x 1033 ergs/s). Luminosity has the same units as power (energy per second).
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.
