EPILOGUE

GLOSSARY - METEORITES AND ASTEROIDS

1. Meteorites

Meteorites are pieces of solar-system bodies falling to the ground when a Me­teor flashes through Earth's atmosphere at speeds of 15-70km/s. Most meteors originate from Asteroids shattered by impacts with other asteroids. Only a few me­teorites found come from the Moon and, presumably, from comets and the planet Mars. Meteorites are usually named for the location where they have been observed (Fall) or found (Find) such as Shergotty, Nakhla, and Chassigny (SNC) for the (presumably) Martian meteorites. In total, 287,115 kg of meteorites were found, excluding the Antarctic meteorites. There are three major classes of meteorites: Stony, Iron, and Stony-iron.

1.1. STONY METEORITES

Stony meteorites are the most numerous but only constitute 9.88% (28,367 kg) of the total meteorite mass found (excluding the Antarctic meteorites). Two groups of stony meteorites are distinguished: Chondrites and Achondrites. Most chondrites have remained unchanged since their formation 4.56 billion years ago, shortly after the formation of the Sun. Inside their primary Matrix, almost all chondrites contain the so-called Chondrules, which are small, spherical Inclusions that formed in the solar nebula. The class of chondrites is further subdivided into the most common Ordinary Chondrites (24,000 kg), Carbonaceous Chondrites (2,507 kg), Enstatite Chondrites (290 kg), and Rumuruti (R) Chondrites. For R Chondrites the reader is referred to, e. g., Schulze et al. (1994), whereas the others are characterized further below. Achondrites are much rarer and appear to have been chondritic before being altered by a heating or impact event.

1.1.1. Ordinary Chondrites Ordinary chondrites are further grouped by H, L, and LL classifications, indicating their Fe content. H denotes an Fe content of "-'27% by weight, L corresponds to "-'23% of Fe, and LL means "low iron" and "low metal" content, sometimes referred to as Amphoterites, with only 20% total Fe. H chondrites, also called Olivine-Bronzite, are composed of olivine, a group of Silicate minerals (containing Si, 0, and one or more metals) with a composition ranging from all magnesium­forsterite, Mg2Si04, to all iron-fayalite, Fe2Si04, and bronzite, a Pyroxene (major group of silicate minerals ranging from Enstatite, MgSi03, to Ferrosilite, FeSi03)

with about 20% FeSi03. L corresponds to Olivine-Hypersthene, composed of oli­vine and hypersthene, a pyroxene with 22-30% FeSi03.

The numbers 3-7 following the H, L, and LL classifications are petrologic (ac­cording to the scientific study of rocks) grades indicating the degree of chondrule alteration by heating. Well defined, unaltered chondrules have a petrologic grade of 3 or 4, whereas chondrules with a grade of 5 or 6 are less distinct.

.... Space Science Reviews 92: 415-418,2000. '" © 2000 Kluwer Academic Publishers.

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1.1.2. Carbonaceous (C) Chondrites C chondrites are rare and primitive - though complex - meteorites containing or­ganic compounds and water-bearing minerals that are evidence for the inclusion of water not long after formation. The sub-classes CI, CM, CV, CO, CK, and CR indicate chemical and mineralogical differences. They are named for the type spec­imens Ivuna, Mighei, Vigarano, Omans, Karoonda, and Renazzo, respectively. The petrologic grade designations of C chondrites include 1 and 2, indicating increasing metamorphism of the meteorite by water.

Allende CV3, Mexico, and Murchison CM2, Australia, are well-known C chon­drites. Allende formed 4.56 billion years ago and its high temperature Ca-Al-rich inclusions (CAls) represents some of the oldest condensed matter in the solar sys­tem. In the matrix of Allende and of other carbonaceous chondrites interstellar dust grains can be found. Short-lived, now extinct radionuclides, with half lives from 0.1 to a few million years, have also been detected. They may be remanents of a star that exploded before the birth of our Sun. Possibly, this event triggered the formation of our solar system. Murchison is believed to be of cometary origin because of its high water content, 12%. To date 92 amino acids have been found in Murchison. Only 19 of them are found on Earth.

1.1.3. Enstatite (E) Chondrites E chondrites are rare and have an unusually high content of enstatite, which is a mineral of the pyroxene group of silicates containing Mg and no Fe, i. e. MgSi03.

Most of the E chondrites' Fe is in the form of metal or sulfide, rather than oxides in silicates. This implies that E chondrites originally formed in a region of the solar nebula that was very poor in oxygen, possibly inside the present orbit of Mars.

E chondrites are further classified by Hand L, according to their Fe content, and by petrologic grades 3 to 6. EH chondrites contain approximately 30% Fe, while EL chondrites are about 25% Fe.

1.1.4. Achondrites Achondrites (1,570 kg) do not contain chondrules and have been found in more than a dozen different rare types. The more numerous types are the HED group from the asteroid Vesta (380 kg, Howardites, Eucrites, Diogenites), the SNC group presumably from the planet Mars (70 kg, Shergottites, Nakhlites, Chassignites, Or­thopyroxenite), the Aubrites (1100 kg), and the Ureilites (20 kg). Primitive Achon­drites (Acapulcoites, Brachinites, Lodranites, and Winonaites) are described in, e. g., Graham et al. (1985).

Achondrites are difficult to find because they resemble terrestrial igneous rocks, i. e. volcanic rocks formed from the solidification of magma, such as Basalt, which mainly consists of pyroxene and Plagioclase (a common rock-forming series of Feldspars, mixtures of silicates of Na, Ca, and AI).

They are believed to have formed on differentiated bodies in the solar sys­tem, i. e. bodies that are large enough (larger than a few 100 km in size) to melt

GLOSSARY - METEORITES AND ASTEROIDS 417

completely at one time, allowing heavier elements to sink towards the center of the layered mass to form chemically distinct Core, Mantle, and Crust areas. This complete melting removes all evidence of chondrules, which explains the name achondrites.

1.2. IRON METEORITES

The iron class represents only '"'-'5% of meteorites by number but most of their mass (248,900 kg). Iron meteorites are thought to be pieces of the shattered cores of dif­ferentiated asteroids. The variation in the structure of iron meteorites is a result of the ratio of the two Ni-Fe alloys, Kamacite ('"'-'27-65% Ni) and Taenite (;S7.5% Ni), that have crystallized forming the core of the parent asteroids. The cooling rate of the asteroid core and the taenite content in the metal affects the thickness of the kamacite bands in the Widmanstiitten pattern (Coarsest: >3.3 mm, 5-9% Ni; Coarse: 1.3-3.3 mm, 6.5-8.5% Ni; Medium: 0.5-1.3 mm, 7-13% Ni; Fine: 0.2-0.5 mm, 7.5-13% Ni; Finest: <0.2mm, 17-18% Ni; Plessitic: <0.2mm kamacite Spindles, 9-18% Ni). Many iron meteorites also contain inclusions of Graphite, Troilite (FeS), silicates and other minerals that give them a distinctive appearance.

In the chemical classification there are three basic groups: Hexahedrites (4.5-6.5% Ni), Octahedrites (6.5-13% Ni) and Ataxites (16-30% Ni). Octahedrites (235,000 kg) consist of a striking intergrowth of kama cite and taenite and are named for the octahedral shape of the kamacite. Hexahedrites have formed entirely from kamacite crystals and show no Widmanstiitten pattern but sometimes very fine par­allellines, the so-called Neumann lines, as the result of an impact event. Ataxites are almost entirely made of taenite. The heavy Hoba West (60,000 kg), found in 1920 in Namibia, is an ataxite; no fall of this type has ever been observed.

1.3. STONY-IRON METEORITES

There are two main groups of stony-iron meteorites (10,000 kg):

1.3.1. Mesosiderites Mesosiderites are the "dumping ground" of meteorites, as they appear to be a surface Regolith, a mixture of broken angular fragments (Breccia) of mantle rock and Ni-Fe alloy, stirred up and fused by repeated impacts. With respect to their silicate material they resemble the eucrite material in howardites, but chemical differences suggest additional mixing with other rock types. Unlike iron meteorites, mesosiderites have very uniform metal compositions, indicating a different origin. Some meso siderites have re-crystallized, suggesting they may have been deeply buried and somehow re-heated at some point in their history.

1.3.2. Pallasites Pallasites, some of the most attractive meteorites, are believed to have formed at the core-mantle boundary of asteroids. Embedded in the metallic matrix are large crystals of the green mineral olivine.

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2. Asteroids

Asteroids are rocky, metallic objects, considerably smaller than planets (diameters range from'" 1 cm to '" 1000 km), that orbit the Sun mostly in the so-called Asteroid Main Belt between the orbits of Mars and Jupiter. Some have hit the Earth in times past, as witnessed bye. g. the Barringer Meteor Crater near Winslow, Arizona. The estimated total mass of all asteroids would form a planet with a diameter ;S 1500 km. Asteroids are material left over from the formation of the solar sys­tem, most likely bodies that never coalesced into a planet. They are the origin of most meteorites (see above). Asteroids on a collision course with Earth are called Meteoroids. The parts that do not bum up when entering the atmosphere and cause the streak of light - meteors - strike Earth's surface as meteorites.

Asteroids are classified according to their composition and Albedo (reflective­ness). Some 75% of asteroids are C type (carbonaceous) with solar-like compo­sition and a low Albedo of 0.03-0.09 (very dark), rv 17% are S type (silicaceous) composed of metallic iron with iron- and magnesium-silicates and have an Albedo of 0.10-0.22 (bright), and "'8% are M type (metallic) with metallic composition and also a relatively high Albedo of 0.10-0.18. NEAs (Near-Earth Asteroids) have orbits to bring them within 1.3 AU of Earth; they are classified as Amors (crossing Mars' orbit but not reaching Earth's orbit), Apollos (crossing Earth's orbit with a period> 1 yr), and Atens (crossing Earth's orbit with a period of < 1 yr).

TABLE I

List of the largest Asteroids (from Hamilton, 1999)

# Name Radius (Ian) Distance (l06lan) Albedo Discoverer Date

Ceres 457 413.9 0.10 G. Piazzi 1801 511 Davida 168 475.4 0.05 R. Dugan 1903 15 Eunomia 136 395.5 0.19 De Gasparis 1851 52 Europa 156 463.3 0.06 Goldschmidt 1858

951 Gaspra 17xl0 205.0 0.20 Neujrnin 1916 10 Hygiea 215 470.3 0.08 De Gasparis 1849

243 Ida 58x23 270.0 ? J. Palisa 1884 704 Interamnia 167 458.1 0.06 V. Cerulli 1910

2 Pallas 261 414.5 0.14 H.Olbers 1802 16 Psyche 132 437.1 0.10 De Gasparis 1852 87 Sylvia 136 521.5 0.04 N. Pogson 1866 4 Vesta 262.5 353.4 0.38 H.Olbers 1807

References

Graham eta/.: 1985, 'Catalogue of Meteorites' , Univ. Arizona Press, Tucson, 460 p.

Schulze, H., Bischoff, A., Palme, H., Spettel, B., Dreibus, G., and Otto, J.: 1994, 'Mineralogy and Chemistry of Rumuruti: The First Meteorite Fall of the new R Chondrite Group', Meteoritics 29, 275-286.

New England Meteoritical Services: 2000, http://www.xensei.comluserslmeteor/metites.htm Planetary Studies Foundation: 2000, http://www.planets.org/met.htm Hamilton, C. J.: 1999, http://www.solarviews.comleng/asteroid.htm

AFM AGB AGU AMM AU BE BSE CAl CC CME EHD EPB FEB FWHM GSE HC HED HST IC ICE IDP 10M IR lRAS ISM ISO LREE LTE LWS HH KH MC MIR NEA NEO OIM OM PAH PB PC PDE PPM REE RT SCT

List of Acronyms

Atomic Force Microscope Asymptotic Giant Branch star American Geophysical Union Antarctic Micro Meteorites Astronomical Unit (distance Sun-Earth, 1.5x 1011 m) Bernoulli's Equation Bulk Silicate Earth Calcium-Aluminum-rich Inclusions in meteorites Carbonaceous Chondrites Coronal Mass Ejection Eucrite-Howardite-Diogenite meteorites (see HED) Eucrite Parent Body Falling Evaporating Bodies Full Width at Half Maximum Grad-Shafranov Equation Hot Cores Howardite-Eucrite-Diogenite meteorites (see EHD) Hubble Space Telescope Interstellar Cloud Inhomogeneous Chemical Evolution Interplanetary Dust Particles Insoluble Organic Matter Infrared light Infrared Astronomical Satellite Interstellar Medium Infrared Space Observatory Light Rare Earth Elements Local Thermal Equilibrium Long Wavelength Spectrometer on ISO Herbig-Haro objects Kelvin-Helmholtz instability Molecular Cloud Mid-infrared light Near-Earth Asteroids Near-Earth Objects Oxygen Isotope Mixing model Organic Matter Polycyclic Aromatic Hydrocarbons Parent Body Parsec, 1 AU = 5 xl 0-5 pc Partial Differential Equation Parabolic Piecewise Method of numerical simulations Rare Earth Elements Rayleigh-Taylor instability Simple Collision Theory

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420

SMOW SN SNC SWS STIS SPH TF TTS UAP WR

Standard Mean Ocean Water Supernova Shergotty, Nakhla, Chassigny (Martian meteorites) Short Wavelength Spectrometer on ISO Space Telescope Imaging Spectrograph on the HST Smoothed Particle Hydrodynamics Terrestrial Fractionation T Tauri Star University of Arizona Press Wolf-Rayet stars

Adams, F. c., 23 Artymowicz, P., 69

Balbus, S. A, 39 Benz, W., 1, 279 Blum, J., 265 Boss, A P., 13

Cellino, A, 397 Clayton, D. D., 133

Dubrulle, B., 201

Fegley, Jr., B., 177

Gautier, D., 201 Gilmour, J. D., 123 Glassgold, A E., 153

Halliday, A, 355 Hartmann, L., 55 Hawley, J. F., 39

Inaba, S., 311

Kallenbach, R, 1

Laughlin, G., 23 Lee, T., 153 Lodders, K., 341 Lugmair, G. W., 1, 225

Meyer, B. S., 133

Nelson, R P., 323 Nichols, Jr., R H., 113

Palme, H., 237 Papaloizou, J. C. B., 323 Pepin, R 0., 371

Robert, F., 201

Author Index

Shang, H., , 153 Shu, F.H., 153 Shukolyukov, A, 225

Terquem, C., 323

Vanhala, H. AT., 13

Weidenschilling, S. J., 295 Wetherill, G. W., 311 Wood,J.A,87,97

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List of Participants

F. Adams, University of Michigan, adams@pablo.physics.lsa.urnich.edu P. Artymowicz, Stockholm Observatory, pawel@astro.su.se S. Balbus, University of Virginia, sb@astsun.astro.virginia.edu F. Begemann, Max-Planck-Institut fUr Chemie, Kosmochemie W. Benz, Universitiit Bern, willy.benz@phim.unibe.ch J. Blum, UniversWit Jena, blum@astro.uni-jena.de A. Boss, Carnegie Institution of Washington, boss@dtm.ciw.edu A. Cellino, Osservatorio di Torino, cellino@fortuna.to.astro.it B. Fegley, Washington University in St. Louis, bfegley@levee.wustl.edu J. Geiss, International Space Science Institute, Bern, johannes.geiss@issi.unibe.ch J. Gilmour, Manchester University, james.gilmour@man.ac.uk M. Grenon, Observatoire de Geneve, rnichel.grenon@obs.unige.ch A. Halliday, Eidgenossische Technische Hochschule Zurich, halliday@erdw.ethz.ch L. Hartmann, Harvard Smithsonian Center for Astrophysics, hartrnann@fuor.harvard.edu R. Kallenbach, International Space Science Institute, Bern, kallenbach@issLunibe.ch G. Laughlin, NASA Ames Research Center, gpl@sun.berkeley.edu D. Lin, University of California, Santa Cruz, lin@ucolick.org K. Lodders, Washington University in St. Louis, lodders@levee.wustl.edu G. Lugmair, Max-Planck-Institut fur Chemie, Kosmochemie, lugmair@mpch-mainz.mpg.df B. Meyer, Clemson University, mbradle@clemson.edu R. Nichols, Washington University in St. Louis, rhn@levee.wustl.edu H. Palme, UniversiUit zu Koln, H.Palme@min.uni-koeln.de R. Pepin, University of Minnesota, Minneapolis, pepinOOl@maroon.tc.umn.edu F. Robert, CNRS Museum d'Histoire Naturelle, Paris, robert@cimrs1.mnhn.fr F. Podosek, Washington University in St. Louis, fap@levee.wustl.edu H. Shang, University of California, Berkeley, shang@rninerva.berkeley.edu C. Terquem, University of California, Santa Cruz, ct@helios.ucolick.org I. Villa, UniversiUit Bern, igor@mpLunibe.ch S. Weidenschilling, Planetary Science Institute, Tucson, sjw@psi.edu G. Wetherill, Carnegie Institution of Washington, wetherill@eros.ciw.edu J. Wood, Harvard Smithsonian Center for Astrophysics, jwood@cfa.harvard.edu

Space Science Series of ISSI

1. R. von Steiger, R. Lallement and M.A. Lee (eds.): The Heliosphere in the Local Interstellar Medium. 1996 ISBN 0-7923-4320-4

2. B. Hultqvist and M. 0ieroset (eds.): Transport Across the Boundaries of the Mag-netosphere. 1997 ISBN 0-7923-4788-9

3. L.A. Fisk, 1.R. Jokipii, G.M. Simnett, R. von Steiger and K.-P. Wenzel (eds.): Cosmic Rays in the Heliosphere. 1998 ISBN 0-7923-5069-3

4. N. Prantzos, M. Tosi and R. von Steiger (eds.): Primordial Nuclei and Their Galactic Evolution. 1998 ISBN 0-7923-5114-2

5. C. Frohlich, M.e.E. Huber, S.K. Solanki and R. von Steiger (eds.): Solar Composition and its Evolution - From Core to Corona. 1998 ISBN 0-7923-5496-6

6. B. Hu1tqvist, M. 0ieroset, Goetz Paschmann andR. Treumann (eds.): Magnetospheric Plasma Sources and Losses. 1999 ISBN 0-7923-5846-5

7. A. Balogh, J.T. Gosling, J.R. Jokipii, R. Kallenbach and H. Kunow (eds.): Co-rotating Interaction Regions. 1999 ISBN 0-7923-6080-X

8. K. Altwegg, P. Ehrenfreund,1. Geiss and W. Huebner (eds.): Composition and Origin of Cometary Materials. 1999 ISBN 0-7923-6154-7

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