Glossary Mm

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

MACH NUMBER - Ratio of the speed of an object in a given medium to the speed of sound in that medium.

MACHO - Massive Astrophysical Compact Halo Objects (q.v.)

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

MAGELLANIC CLOUDS - Two irregular galaxies, the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) that orbit the Milky Way once every 1,500 million years and each other once every 900 million years. The Magellanic Clouds lie ~200,000 light years away. Their metallicity is much lower than that of the Milky Way. These lower metallicities reflect conditions similar to those found in the early Universe (before the evolution and deaths of stars could enrich the interstellar medium).

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MAGELLANIC STREAM - Long trail of gas extending from the Magellanic Clouds visible at radio wavelengths. The Stream extends more than 180 degrees across the sky at an average distance of 180,000 light years and wraps almost half way around the Milky Way. It passes through the Magellanic Clouds and beneath the south galactic pole, and shows both a leading and a trailing 'arm'. The Stream is thought to be an example of a tidal tail that was torn out of the Magellanic Clouds hundreds of millions of years ago in an interaction involving the Milky Way. No stars associated with the Stream have ever been found.

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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.

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MAGNETAR - Also spelled "magnestar." Neutron star with an ultra-strong magnetic field (~1015 gauss). The magnetic field is between 100 and 1,000 times stronger than that of a radio pulsar, making them the most magnetic objects known. Magnetars form by core-collapse of a massive star in a supernova explosion. It is unclear why a magnetar is created rather than an ordinary neutron star or pulsar, but in order to achieve such strong magnetic fields, the neutron star must initially rotate between 100 and 1,000 times per second. Both soft g-ray repeaters (SGRs) and anomalous x-ray pulsars may be explained by a magnetar model, with the decay of the magnetic field powering the emission of x-rays and g-rays. However, it appears that magnetars are only x-ray bright for a short period of time since their pulse periods cluster between 6 and 12 seconds. If they remained active for an extended period of time, we should also see magnetars with longer pulse periods.

MAGNETIC MONOPOLE - Hypothetical particle that constitutes sources and sinks of the magnetic field. Magnetic monopoles have never been observed, but if observed would require fairly minor modifications to Maxwell's equations. They also seem to be predicted by some grand-unified theories. If magnetic monopoles do exist, they do not seem to be very common in our Universe.

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.

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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). Star color magnitudes are measured through these filters can be used to determine the star's color temperature.

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MAIN ASTEROID BELT - Belt located between 2.12 and 3.3 AU (between the orbits of Mars and Jupiter) with the greatest concentration of asteroids. The main asteroid belt is very sparsely populated and the distribution of asteroids within it is not uniform. The destruction of larger bodies yields families of asteroids which share very similar orbital properties and cluster together. There are zones in which almost no asteroids occur. These are the Kirkwood Gaps and result from orbital resonances with Jupiter. The main asteroid belt is a leftover from the early Solar System when the strong gravitational influence of Jupiter prevented the planetesimals in this region from coalescing to form a planetary core.

MAIN COMPONENT ELEMENT - Elements that are found in early forming solar-system minerals. The main component lithophile elements are: Mg, Si, Cr, and Li. The main component siderophile and chalcophile elements are: Fe, Ni, Co, and Pd.

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.

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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.

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MAIN-SEQUENCE LIFETIME - Overall lifespan of a star burning hydrogen into helium on the main sequence (MS). A star’s main sequence lifetime is determined by its mass. Massive stars need higher central temperatures and pressures to support themselves against gravitational collapse, and fusion reactions in these stars proceed at a faster rate than in lower mass stars. The result is that massive stars use up their core hydrogen fuel rapidly and spend less time on the main sequence before evolving into a red giant stars. An expression for the main sequence lifetime can be obtained as a function of stellar mass and is usually written in relation to solar units (for a derivation of this expression, see below):

where, tsun = Sun’s MS lifetime (1010 y), M = mass of star, Msun = Sun’s mass.

The lifetimes of main sequence stars therefore range from a million years for a 40 Msun 0-type star, to 560 billion years for a 0.2 Msun M-type star. Given that the Universe is only 13.7 billion years old, these long main sequence lifetimes for M-type stars mean that every M star that has ever been created is still on the main sequence! The Sun, a G-type star with a main sequence lifetime of ~ 10 billion years, is currently 5 billion years old - about half way through its main sequence lifetime.

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.

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MAJOR AXIS - Longest diameter of an ellipse; the line from one side of an ellipse to the other that passes through the foci.

MAJORITY CARRIER - Type of charged particle that is primarily responsible for current flow in a material. In conductors, current is produced by the motion of electrons through the material, so electrons are the majority carrier in conductors. But in some solutions and in p-typesemiconductors, the current is due to the motion of positive ions or holes. In these cases, the positive ion or the hole would be the majority carrier.

MANTLE - Intermediate zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume and is composed of silicate minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes convection currents in the semi-liquid mantle; rock rises and then slowly sinks again as it cools, driving the tectonic plates. The crust-mantle boundary, which occurs at an average depth of ~32 km, is called the Mohorovicic discontinuity The core-mantle boundary (CMB), the Gutenburg discontinuity, lies at an average depth of 2,900 km.The mantle is subdivided into upper and lower mantle, based upon changes in seismic wave velocity. The uppermost non-convecting mantle comprises the lower part of the lithosphere. Beneath the lithosphere at ~72–250 km depth, is the low-velocity zone; also called the asthenosphere. A rapid increase of seismic velocities occurs at ~670; this corresponds to a mantle phase change. Seismic velocities in the upper mantle are overall lower than those of the lower mantle, indicating that the lower mantle is much denser than the upper mantle. Other smaller changes in velocity correspond to other phase changes.

MANTLE PHASE CHANGES - Solid-state mineralogical changes that occur with increasing depth in a planet’s mantle. These are best understood for Earth’s mantle. The change at ~400 km corresponds to a transition from the α- to β-structures of multi-component olivine with ~6% increase in density. The small Fe concentration in natural mantle olivine results in a thin region where both phases coexist in equilibrium and where macroscopically observed density changes occur. Depending actual temperature-depth relationships, the thickness of the two-phase region varies. The ~670-km transformation is fundamentally different from the a-b transition, because it involves a change in chemical composition. The olivine and pyroxene-garnet components transform into magnesiowüstite, (Mg,Fe)O, and perovskite with ~10% density increase across the 1-2 km thick phase change region. An addition transformation has been proposed for lowermost 100s of km of the mantle in which perovskite transforms in to a “post-perovskite” phase. Mantle mineralogy and phase changes (after Ringwood) are shown in the diagram below.

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MARE - Broad low plains surrounded by basin-forming mountains, originally thought to be a sea (pl. maria). This term is applied to the basalt-filled impact basins common on the face of the Moon visible from Earth.

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MARTIAN METEORITES - Over 30 of the meteorites found on Earth almost certainly came from Mars (see and 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. The one oddball is an orthopyroxenite. Trapped gases within several SNC meteorites exactly match the composition of the Martian atmosphere as measured by the Viking landers, confirming its place of origin.

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Oddly, the orthopyroxenite, ALH 84001, became the center of controversy in 1996 when a group of NASA scientists claimed it contained fossil bacteria. It represents a deeper part of Mars, and has a much older crystallization age of 4 Ga. The 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 × 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.

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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.

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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.

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MAXWELL’S EQUATIONS - Four elegant equations which describe classical electromagnetism often found on nerd’s T-shirts. In addition to describing electromagnetism, his equations also predict that waves can propagate through the electromagnetic field, and always propagate at the speed of light.

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

MEGAPARSEC (Mpc) - Distance measurement equal to one million parsecs or 3.26 million light years.

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 × 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.

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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.

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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 - Element that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn from B to Po separates the metals from the nonmetals. Elements on this line are metalloids, sometimes called semi-metals; elements to the lower left are metals; elements to the upper right are nonmetals. Nonmetals are more abundant in nature than metals, but metals make up most of the periodic table. Astronomers refer to all elements heavier than He 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).

METALLIC HYDROGEN - State that results when hydrogen is sufficiently compressed and undergoes a phase change – an example of degenerate matter. Metallic H consists of a lattice of protons with a spacing that is significantly smaller than a Bohr radius (comparable to an electron wavelength). The electrons are unbound and behave like the conduction electrons in a metal. As pressures rises to 1.4 million atmospheres, the electronic energy band gap (a measure of electrical resistivity) fell to almost zero. The electronic energy band gap of hydrogen in its uncompressed state is about 15 eV, making it an insulator, but as pressure rises, the band gap gradually falls to 0.3 eV. Because 0.3 eV are provided by the thermal energy of the fluid, hydrogen at this point may be considered fully metallic.

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Metallic H is present in tremendous amounts in the gravitationally compressed interiors of Jupiter and Saturn. The helium rainout region shown above may explain why Saturn's surface composition is He depleted relative to the Sun.

METALLICITY - Fraction of a star's mass composed of metals relative to hydrogen and helium; approximately 2% in the case of the Sun. However, metallicity fails to completely specify the chemical composition of a star because the same metallicity can be achieved in an infinite number of ways. For example, a star with any given metallicity could have its metals as all Fe or as all O. Instead, abundance ratios are used to specific a star’s chemical make-up. Stars with low metallicities occur in the galactic halo; high metal stars are in the disk as shown by the survey results below.

Galactic metallicity

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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 ablates and vaporizes in Earth’s atmosphere. Most incoming meteoroids are only a few mm across, but they enter the atmosphere at extremely high speeds (11–72 km/s) compressing the air in front of them. This compressed air raises the surface temperature of the meteoroid to >2,000 K and the outer layers begin to vaporize in a process called ablation. Vaporized atoms collide with air molecules to create an ionized 'trail', which produces the bright steak of light that is only ~1 m across but it can be many km (typically 20-30 km) long depending on the speed of the meteoroid. It may also display different colors, the result of the de-excitation of different atmospheric molecules. Most meteors appear at altitudes of 80-110 km.

METEOR SHOWER - Phenomenon that occurs when the orbit of the Earth passes through a meteor stream. For example, the Earth's orbit intersects the orbit of the comet Tempel-Tuttle every November, producing the Leonid meteor shower. All the meteoroids in the stream basically travel parallel to each other and when they hit Earth's atmosphere appear to originate from a single point (the radiant). The meteors appear to come from a specific constellation (the constellation Leo in the case of Tempel-Tuttle).

The duration of a shower depends on how long it takes for Earth to pass through the meteor stream. It is measured by calculating the number of meteors that a single visual observer would see in an hour assuming a limiting magnitude of 6.5 and the radiant at the zenith. This is known as the zenithal hourly rate (ZHR) and is a measure of the concentration of material in the stream. The ZHR changes from year to year as the meteor stream constantly evolves. For example, if the comet has recently visited the inner Solar System, there generally will be more meteoroids in the stream and a higher ZHR results. Extremely active showers with ZHR >1000 are usually called 'meteor storms'.

METEOR STREAM - Relatively narrow band of meteoroids stretched out along the orbital path of a comet. It consists of dust released from the nucleus of a comet during its perihelion passage. The dust grains escape the weak gravity of the nucleus and travel on their own independent, heliocentric orbits. Although these orbits remain similar to that of the parent comet, the different velocities at which they were ejected from the nucleus give them slightly different semi-major axes and orbital periods. Slight differences in the resultant orbital periods cause the individual dust grains to spread out along the orbital path of the parent comet. If the Earth's orbit intersects the meteor stream, a meteor shower is observed. Meteor streams are relatively short-lived phenomena with most only lasting of order 10,000 years. They are dispersed by radiation pressure, collisions between the meteoroids, and gravitational perturbations and collisions with other planets.

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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.

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 MeteoritesChondritesenstatite chondrite, carbonaceous chondrite, ordinary chondrite
Metachondritescarbonaceous-affinity, ordinary-affinity, lodranite, acapulcoite, winonaite
AchondritesHED group (howardite, eucrite, diogenite), lunar, Martian, aubrite, angrite, ureilite, brachinite
Stony-iron Meteoritesmesosiderite, pallasite
Iron Meteoritesaxatite, 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.

MICA - Phyllosilicate mineral with TOT layers alternating with filled interlayer cation layers. Micas have no exchangeable water. The cation Al3+ substitutes for every fourth Si4+ in tetrahedral layers, producing a negative charge on the surfaces of TOT layers. This negative charge is balanced by positively charged interlayer cations (X+) in 12-fold coordination, which bind the structure tightly.

The octahedral layers may be occupied by two 3+ cations (e.g., Al3+), making the mica “dioctahedral”, or can contain three 2+ cations (e.g., Mg2+, Fe2+) making the mica “trioctahedral.” Micas can have mixed octahedral and interlayer occupancies.

Micas can be divided into true and brittle based upon their interlayer occupancy; true micas have monovalent cations, whereas brittle micas have 2+ cations (compensated by two Al3+ in the tetrahedral layer). The higher interlayer charge makes the mineral harder and the layers less flexible, hence the term "brittle."

Dioctahedral micas include: muscovite and paragonite (true micas) and margarite (brittle mica). They exhibit a partial solid solution series (Na+ ↔ K+) and minor substitution of Ca2+ into structure (more at the paragonite end). Significant amounts of Na+ fit into margarite.

Trioctahedral micas include: biotite and phlogopite (true micas) and clintonite (brittle mica). Biotite consist of the solid solution series (Mg2+ ↔ Fe2+) with end-members of phlogopite, KMg3(AlSi3)O10(OH)2, and annite, KFe3(AlSi3)O10(OH)2; however, pure annite never observed in nature. The cation Ti4+ substitutes in the octahedral sites by complex coupled substitution whose nature is not entirely clear. One important process is loss of protons from the hydroxyl groups, to yield the Ti-biotite end-member KMg2Ti(AlSi3)O12. The addition of F to biotite increases its stability to higher temperatures and pressures.

The general mica formula is XY2-3Z4O10(OH,F,Cl)2. The interlayer cation (X) can be K+, Na+, Ca2+ (also Ba2+, Rb+, Cs+). The octahedral cations (Y) can be Al3+, Mg2+, Fe2+, Fe3+ (also Mn2+, Cr3+, Ti4+) and the tetrahedral cations (Z) are Si4+ and Al3+. All micas, except those with high F contents, contain 4-5 wt. % H2O.

MICROMETEORITE - Meteorite so small that it falls to Earth essentially unchanged from how it existed in space. If a meteoroid entering the Earth's atmosphere is sufficiently small (generally less than 10-6 m), it will be slowed by collisions with molecules in the upper atmosphere to a degree where ablation does not occur. Micrometeorites constitute almost all of the interplanetary debris entering the Earth's atmosphere every day. They are commonly found in places where the terrestrial dust content is low, such as ocean floor sediments, Antarctic ice and in the stratosphere.

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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. The Milky Way is a fairly typical spiral galaxy with distinct structural components. It contains a thin disk (diameter ~100,000 light years), a thick disk, a bulge (diameter ~25,000 light years), and a stellar halo which is home to about 150 globular clusters. Originally thought to be of Hubble type Sb or Sc (Sbc), astronomers now believe that the Milky Way is has a central bar and is therefore a loosely wound, barred spiral galaxy - Hubble type SBbc.

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The Sun, one of the hundreds of billions of stars comprising the Milky Way, is situated in the thin disk, about 25,000 light years from the centre of the Galactic bulge. In addition to its visible structure, and similarly to other spiral galaxies, the Milky Way contains a dark halo of presumably non-stellar (perhaps even non-baryonic) matter. This spherical halo extends well beyond the edge of the thin disk, and the motions of galaxies around the Milky Way suggest it has a total mass of 1,000 billion solar masses. This is about ten times the mass contained within the visible material! The rotational curve for the Milky Way demonstrates the existence of dark matter.

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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.

MIRROR PLANE – Symmetry element for which the corresponding operation resembles reflection in a mirror coincident with the plane. It converts an object or a structure into one of opposite “handedness,” related to the first as is any object and its mirror image.

MOLALITY - Number of moles of solute dissolved in one kilogram of solvent. Note: be careful not to confuse molality and molarity. Molality is represented by a small "m," whereas molarity is represented by an upper case "M."

MOLARITY - Number of moles of a solute dissolved in a liter of solvent.

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

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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 (below).

MOLECULAR HYDROGEN - Molecule, H2, formed by two H sharing electrons. H2 is notoriously difficult to detect astronomically. However, it has been shown that for every 10,000 H2 in the interstellar medium there is a carbon monoxide molecule, CO, and one can map out the H2 distribution in the Galaxy by searching for CO. This has revealed that molecular hydrogen is found primarily in the molecular clouds that trace the spiral arms in spiral galaxies. Molecular hydrogen is a vital ingredient in active star formation.

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. The example is a terrestrial granite breccia.

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MONS – Large area of high relief; mountain (pl. montes).

MOTIF - Identical repeating unit, also called the "basis," which is associated with each point of the crystal lattice. In the simplest cases, the motif of a crystal is a single atom. In more complex structures the basis may be several atoms or ions or a molecule. For example, in the case of sodium chloride NaCl, the motif consists of one sodium ion and one chloride ion.

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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