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Article
Glossary M (View 41934/Answer 0)

Ma (MEGA ANNUM) - One million, 106, years.

MAFIC - Term used for silicate minerals with cations predominantly Mg and/or Fe. Mafic minerals include olivine and pyroxenes.

MAGMA - Molten silicate (rock) in the interior of a planetary body or moon. When it reaches the surface, magma is called lava.

MAGMA OCEAN - Completely molten surfaces of terrestrial planets that formed soon accretion. Samples returned by the Apollo missions provide evidence of a lunar magma ocean, crystallization of which produced a stratified Moon with a low-density crust formed by accumulation of the mineral plagioclase overlying a higher density mantle of olivine and pyroxene.

MAGNETITE - Fe oxide, Fe2+Fe3+2O4, containing oxidized iron (Fe3+) found in the matrix of carbonaceous chondrites and as diagnostic component in CK chondrites. In Ck chondrites, magnetite is typically chromian, containing several wt. % Cr2O3.

MAGNETOHYDRODYNAMICS (MHD) - The study of the interaction between a magnetic field and plasma treated as a continuous medium. Most of the universe contains not normal gas, but instead plasma. Many dynamical astronomical processes are caused by the subtle nonlinear relationship between a magnetic field and plasma. There is virtually no interaction with a magnetic field in a neutral gas, but in a plasma extremely close coupling with the magnetic field means that whatever the plasma is doing intimately affects the magnetic field and vice versa.

Magnetohydrodynamics builds on the tools of both fluid dynamics and electromagnetism, but it possesses many new features that are present in neither. It does consider individual particles but, instead, treats plasma as a continuous medium. The assumption of a continuous medium is valid for length-scales much larger than the mean-free path for particle collisions, which is typically 3 cm in the solar chromosphere and 30 km in the solar corona.

MAGNETOSPHERE - Region around an astronomical object in which phenomena are dominated or organized by its magnetic field. A schematic diagram of the features of Earth's magnetosphere is shown.

MAGNITUDE - Logarithmic scale for the brightness of a star. Magnitudes may be apparent or absolute. Apparent magnitude is determined using the brightness as observed, with no consideration given to how distance is influencing the observation. Absolute magnitude is the apparent magnitude a star would have if it were at a standard (arbitrary) distance of 10 parsecs. A common convention is to use a lower-case "m" to denote an apparent magnitude and an upper-case "M" to denote an absolute magnitude. An additional complication is that the brightness of an object depends on the wavelength at which we observe it; thus, brightness measurements are made using standard color filters (U, B, and V).

MAIN MASS - Largest known fragment of a meteorite.

MAIN SEQUENCE - Stage of evolution in which stars spend the major part of their lifetimes. Their luminosity and temperatures are well correlated as shown by a well-defined curve on the Hertzsprung-Russell diagram. This line is so pronounced because both the spectral type and luminosity depend only on a star's mass as long as it is fusing H - and H-burning occupies most of a star's lifetime. Astronomers define a "zero age main sequence" (ZAMS) line based on computer models of where a star will be when it begins hydrogen fusion. A star's brightness and surface temperature typically increase gradually during its time on the main sequence. Stars enter and leave the main sequence when they are born and when they are starting to die, respectively. The properties of main sequence stars are shown in the diagram.

MAIN-SEQUENCE TURNOFF - Special point on the Hertzsprung-Russell diagram for a cluster, indicative of the cluster's age. If all the stars in the cluster are plotted, the lower mass stars will trace out the main sequence up to the point where stars begin to evolve off the main sequence toward the red giant branch. The point where stars are just beginning to evolve off is the main-sequence turnoff.

MARTIAN METEORITES - Over 30 meteorites have been found on Earth that almost certainly came from Mars (see http://www2.jpl.nasa.gov/snc/). All but one belongs to the group known as SNC meteorites, which includes the shergottites, nakhlites, and chassignites. SNC meteorites contain minerals that crystallized within the past 1.35 to 0.15 Ga, making them much younger than any other known achondrites. Trapped gases within EETA 79001 exactly match the composition of the Martian atmosphere as measured by the Viking landers, confirming its place of origin.

Oddly, the most famous Mars meteorite, ALH 84001, which became the center of controversy in 1996 when a group of NASA scientists claimed it contained fossil bacteria, is not a member of the SNC group, represents a deeper part of Mars, and has a much older crystallization age of 4 Ga. Lafayette meteorite, a nakhlite, contains the highest percentage of water of all Martian rocks.

MASKELYNITE - Diaplectic glass of plagioclase composition produced during shock metamorphism at pressures of ~30 GPa. It is commonly found in shergottites and ordinary chondrites.

MASS - Measure of how much "stuff" something has. Mass determines the inertia of an object (its resistance to being accelerated by a force) and how much gravitational force it exerts on another object. In classical physics, mass was conserved, but Einstein discovered that mass can be converted into energy and vice versa. Conservation of mass is still a good approximation since mass-energy conversions generally involve relatively small amounts of mass. The mass of astronomical objects is often measure in terms of the Sun's mass, which is 2 x 1033 grams.

MASS FRACTIONATION - Fractionation of isotopes or elements that is dependent on their masses.

MASS LOSS - Loss of material by a star during its evolution; causes of mass loss include stellar winds, bipolar outflows, the ejection of material to form a planetary nebula, or supernova.

MASS-LUMINOSITY RELATION - Dependence of the luminosity of a main-sequence star on its mass. The luminosity increases roughly as the mass raised raised to the power 3.5. For example, doubling the mass of a main sequence star increases luminosity by a factor 23.5 ~ 11.3. Thus, stars like Sirius that are about twice as massive as the Sun are more than 10 times as luminous. This particular relation between mass and luminosity holds only for stars on the main sequence. It does not hold, for example, for white dwarfs or for giant stars.

MASS NUMBER (A) - Sum of the number of neutrons and protons in a nucleus.

MASS SPECTROMETER - Instrument used to measure the masses of molecules and atoms by volatilizing and then ionizing them. The ions are then separated magnetically according to their mass-to-charge ratio.

MASS-RADIUS RELATION - Relationship between the radius, R, of a main-sequence star and its mass, M. If R and M are both in solar units, then R = M0.8.

MASS-TO-CHARGE RATIO - Unit used by mass spectrometers to separate ions. The mass-to-charge ratio is simply a ratio of the mass to the size of the electronic charge for any given molecular ion or fragment. Since the charge is often +1, the mass and the mass-to-charge ratio are often the same.

MASSIVE ASTROPHYSICAL COMPACT HALO OBJECT (MACHO) - Any dark object such as a brown dwarf, neutron star, or black hole that could account for some of the unobserved matter in the halos of galaxies. If it is made out neutrons and protons then it is baryonic dark matter, but primordial black holes are a non-baryonic dark matter version of MACHOs. Several experiments have attempted to detect MACHOs through gravitational microlensing (amplification) of the light from background stars, first using the Large Magellanic Cloud (LMC) as a background and more recently using Andromeda. A number of microlensing events toward the LMC were detected, but their interpretation is uncertain. As much as 30% of the dark matter in the Milky Way halo could be MACHOs.

MATRIX - Fine grained primary material in a meteorite that surrounds inclusions, chondrules and breccia clasts.

MAXWELL DISTRIBUTION - Maxwell (or Maxwell-Boltzmann) distribution gives the distribution of speeds of molecules in thermal equilibrium as given by statistical mechanics.

MEAN FREE PATH - Average distance traveled by a particle or photon before a collision occurs.

MELILITE - Group of minerals found in the CAIs of CV chondrites. Its composition varies between gehlenite (Ca2Al[Si,Al]2O7) and akermanite (Ca2MgSi2O7). The melilite in CAIs is closer to gehlenite in composition.

MELTING POINT - Temperature at which a solid changes to a liquid. A completely pure crystalline substance has an exact temperature at which it melts. Impure substances and amorphous substances will begin melting at one temperature and finish at another. For these substances the term "melting range" is more appropriate. Under some conditions a material will sublimate rather than melt, moving directly from a solid to gas state.

MESON - Type of hadron containing one quark and one antiquark; there are about 140 types of mesons. Mesons are short-lived and decay rapidly into leptons or photons either directly or by first decaying into other types of meson. One example of a meson is a pion (π+), which is made of an up quark and a down antiquark. Mesons are very unstable because they consist of a particle and an antiparticle. Their masses range from about 15% of the mass of a proton, in the case of the pion (π), to more than ten times the mass of a proton. Mean lifetimes range from 2.6 x 10-8 s, for π+ and π- (lifetimes for π0 are ~10-16 s), to <10-20 s for the heaviest mesons.

The first meson (π) was discovered experimentally 1947, but their existence in 1935 by Hideki Yukawa (1907-81) to explain how the strong nuclear force could be conveyed between protons and neutrons in an atomic nucleus. At the scale of an atomic nucleus, the π may be regarded as the force-carrying particle of the strong nuclear force. However, because it is, composed of quarks, it is not truly an elementary particle. At the fundamental level, the strong force is conveyed between quarks by particles called gluons.

MESOSIDERITE - One of two main types of stony-iron meteorite, the other being pallasites. Mesosiderites are a roughly 50:50 mixture of silicates and Ni-Fe metal. The name derives from the Greek "mesos" meaning "middle" or "half" and "sideros" for "iron;" hence "half-iron". The silicates are heavily brecciated igneous rocks, similar to eucrites, diogenites, and clearly came from the crust of an achondritic parent body. The metal in mesosiderites is similar to that of IIIAB irons, and came from the core of a differentiated asteroid, genetically unrelated to the precursor of the eucritic and diogenitic portion. Mesosiderites have no mantle-like material at all. This odd combination of crustal and core material can be explained by a collision of two differentiated asteroids in which the still-liquid core of one asteroid mixed with the solidified crust of the other. This model requires collisional disruption followed by gravitational reassembly of at least one of the asteroids.

MESOSTASIS - Last interstitial material to crystallize in an achondrite meteorite or a chondrule.

METACHONDRITE - Term used to describe a metamorphosed chondrite. Metachondrites are texturally evolved rocks derived from chondritic precursors and some have been classified as primitive achondrites.

METAL - Astronomers refer to all elements other than hydrogen and helium as "metals" even though these elements are not all metals as defined by cosmochemists.

METALLIC BOND - Bond between atoms with similar low electronegativities (ΔX ~0). Metallic bonding is essentially covalent bonding with delocalized electrons. Metallic nuclei sharre weakly bonded valence electrons, which form an "electron gas". Metals have many more empty orbits than electrons available to fill them. For example, Fe requires 5 electrons to fill its outermost 3d shell. Metallically bonded solids share the following properties: low to moderate hardness, malleability and ductility (weak delocalized bonds); very good thermal and electrical conductivity (free electrons in cloud); and form crystals with high symmetry (nuclei pack well).

METAMORPHIC - Rocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment.

METEOR - Flash and steak of light produced by a meteoroid as it vaporizes in Earth's atmosphere.

METEORITE - Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and textures. Meteorites range in size from microscopic to many meters across. Of the several 10s of tons of cosmic material entering Earth's atmosphere each day, only about one ton reaches the surface.

Frictional heating begins at an altitude of 100-120 km when a meteoroid encounters the Earth's atmosphere. An object's chance of survival depends on its initial mass, speed and angle of entry, and friability (tendency to break up). Micrometeoroids radiate heat so effectively that they are dramatically slowed without being vaporized and fall as a continuous, gentle, invisible rain. Meteoroids with masses between 10-6g and 1 kg tend to burn up completely as meteors. Friable meteoroids break up and are destroyed at altitudes of 80 to 90 km. Those which are tougher survive longer and produce fireballs as their surface undergo melting and ablation at temperatures of several thousand degrees. If meteoroids avoid destruction high up, they enter the lower, denser part of the atmosphere where they are rapidly decelerated. Finally, at subsonic speeds the fireball is extinguished and what remains falls to the ground as a meteorite. The last melted material on the surface of the object solidifies to form a thin, usually black, rind known as a fusion crust.

Stony Meteorites Chondrites enstatite chondrite, carbonaceous chondrite, ordinary chondrite
Metachondrites carbonaceous-affinity, ordinary-affinity, lodranite, acapulcoite, winonaite
Achondrites HED group (howardite, eucrite, diogenite), lunar, Martian, aubrite, angrite, ureilite, brachinite
Stony-iron Meteorites mesosiderite, pallasite
Iron Meteorites axatite, hexahedrite, octahedrite

METEOROID - Small rocky or metallic object in orbit around the Sun (or another star).

METRIC PREFIXES - Symbols used to specify orders of magnitude in the metric system of measurement, using factors of 103 between steps. Most common prefixes are shown below.

MICRON - More correctly a micrometer (μm). A unit of length convenient for measuring mineral grain sizes. 1 μm = 10-6 m.

MICROWAVES - Waves of electromagnetic radiation that oscillate from ~109 to 3 x 1011Hz (cycles per second).

MILKY WAY - Name of our galaxy. Also the name given to the band of diffuse light seen in the night sky.

MINERAL - Inorganic, naturally-occurring, homogeneous crystalline substance with a defined chemical composition. The chemical composition may vary between compositional end members in a mineral system. For example, the alkali feldspars comprise a continuous series between anorthite (KAlSi3O8 and NaAlSi3O8). Mineral-like substances that don't strictly meet the definition, such as opal, are classified as mineraloids.

MINORITY CARRIER - Type of charged particle that does NOT contribute significantly to current flow in a material. In n-type semiconductors, the number of conduction electrons is far larger than the number of holes, so holes are the minority carrier (and electrons are the majority carrier). In p-type semiconductors, the current is primarily due to the motion of hole, making conduction electrons the minority carrier.

MOLDAVITE - Tektite type found in the Czech Republic that is generally green in color . Moldavites may have formed during the Ries Impact event in Germany.

MOLECULAR CLOUD- Cold dense complex of interstellar gas and dust roughly 75% hydrogen and 21-24% helium. Clouds contain trace amounts of other molecules, of which well over 100 different types have now been discovered in space. Dust grains make up ~1 % of the mass of a cloud. The relatively high density of dust particles plays an important role in the formation and protection of the complex molecules. The emission of molecular lines often shows several distinct intensity peaks, each representing individual clumps or clouds of gas and dust in a region that characteristically extends for 50 light-years. Two distinct types of molecular cloud are known, both associated with star formation: giant molecular clouds (GMCs) and dwarf molecular clouds. GMCs are the coolest (10 to 20 K) and densest (106 to 1010 particles/cm3) portions of the interstellar medium. They typically stretch over 150 light-years and contain several 100,000s of Msun of material, making them are the largest gravitationally bound objects in the Galaxy. Molecular clouds are the only places where star formation is known to occur. Star formation occurs when deeply embedded clumps of interstellar gas and dust collapse under self-gravitation or due to impinging shock waves. Young stellar objects (YSOs) - newborn stars or stars in the process of forming - are obscured from direct optical view, and the only source of information from inside these clumps is provided by radio and infrared waves emitted by molecules and dust.

MOLECULAR EXDENDED LATTICE - Crystalline solids composed of individual molecules packed together in a regular repeating fashion. There are no bonds between the molecules, instead they are held to each other by weaker intermolecular forces. An example is the native element sulfur, where S is arranged in puckered rings forming tightly bound S8 units bonded together by van der Waals forces.

MOLECULAR SPECTRA - Spectral lines resulting when molecules interact with electromagnetic radiation, typically at infrared wavelengths. Because molecules are usually fragile, molecular spectra are important in relatively cool objects such as planetary atmospheres, the surfaces of very cool stars, and various interstellar regions.

MOLECULE - Collection of atoms held together by chemical bonds into a discrete, finite structure. One way molecules are represented is by a chemical formula where symbols for the elements are used to indicate the types of atoms present and subscripts are used to indicate the relative numbers of atoms. For example, forsterite can be written Mg2SiO4.

MOMENTUM - Linear momentum (ρ) is the mass of an object, m, times its velocity, v:

Since velocity has direction, so does momentum. It is conserved in the absence of forces.

MONOMICT BRECCIA - Breccia involving the mixing of clasts with the same textures and compositions. (e.g., L6 with L6). Monomict breccias are rare on the Moon because meteoroid impacts tend to mix different kinds of rocks.

MURCHISON METEORITE - Carbonaceous chondrite which exploded into fragments over Murchison, Australia, located ~200 km north of Melbourne, on Sept. 28, 1969. About 82 kg of the meteorite were recovered. Eyewitnesses reported smelling something like methanol or pyridine, an early indication that the object might contain organic material. Subsequent analysis by NASA scientists and a group led by Cyril Ponnamperuma revealed the presence of 6 amino acids commonly found in protein and 12 that did not occur in terrestrial life. All of these amino acids appeared in both dextrorotatory (right-handed) and laevorotatory (left-handed) forms, suggesting that they were not the result of Earthly contamination. The meteorite also contained abiogenic hydrocarbons enriched with a heavy isotope of carbon, confirming their extraterrestrial origin. Initial studies suggested that the amino acids in the Murchison meteorite showed no bias between left- and right-handed forms. However, in 1997, John R. Cronin and Sandra Pizzarello of Arizona State University reported finding 7-9% excesses of left-handed versions of four amino acids, a result confirmed independently by another group. More than 70 amino acids have been identified in Murchison altogether. To this organic mixture, in 2001, was added a range of polyols - organic substances closely related to sugars such as glucose.

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