Please contact the correspondence author for reprints of all published articles
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Kgagodi Basin: The First Impact Structure recognized in Botswana
D. Brandt, H. Holmes, W.U. Reimold*, B.K. Paya, C. Koeberl and. P.J. Hancox
*Correspondence author's address: Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa; e-mail address: 065wur@cosmos.wits.ac.za
Abstract–The 3.4-km-wide, so-called Kgagodi Basin structure, which is centered at longitude 27º34.4'E and latitude 22º28.6'S in eastern Botswana, has been confirmed as a meteorite impact structure. This crater structure was first recognized through geophysical analysis; now, we confirm its impact origin by the recognition of shock metamorphosed material in samples from a drill core obtained close to the crater rim. The structure formed in Archean granitoid basement overlain and intruded by Karoo dolerite The crater yielded a gravity model consistent with a simple bowl-shape crater form. The drill coreextends to a depth of 274 m and comprises crater fill sediments to a depth of 158 m. Impact breccia was recovered only between 158 and 165 m depth, below which locally brecciated basement granitoids grade into fractured and eventually undeformed crystalline basement, from about 250 m depth. Shock metamorphic effects were only found in granitoid clasts in the narrow breccia zone. This breccia is classified as suevitic impact breccia due to the presence of melt and glass fragments, at a very small abundance. The shocked grains are exclusively derived from granitoid target material. Shock effects include multiple sets of planar deformation features (PDFs) in quartz and feldspar; diaplectic quartz, and partially and completely isotropized felsic minerals, and rare melt fragments were encountered. Abundances of some siderophile elements and, especially, Ir in suevitic breccia samples are significantly elevated compared to the contents in the target rocks, which provides evidence for the presence of a small meteoritic component.
Kgagodi is the first impact structure recognized in the region of the Kalahari Desert in southern Africa. Based on lithological and first palynological evidence, the age of the Kgagodi structure is tentatively assigned to the upper Cretaceous to early Tertiary interval. Thus, the crater fill has the potential to provide a long record of paleoclimatic conditions.
Coeval Argon-40/Argon-39 ages of moldavites from the Bohemian and Lusatian strewn fields
Winfried H. Schwarz* and Hans J. Lippolt
*Correspondence author's address: Laboratorium für Geochronologie, Universität Heidelberg, INF 234, D-69120 Heidelberg, Germany; e-mail address: winfried.schwarz@urz.uni-hd.de
Abstract–40Ar/39Ar ages of four tektites (moldavites) from southern Bohemia (near Ceské Budejovice/Czech Republic) and a tektite from Lusatia (near Dresden/Germany) have been determined by eleven step degassing experiments. The purpose of the study was to enlarge the 40Ar/39Ar data basis of moldavites and to check the age relations of the Bohemian and Lusatian samples. The mean plateau-age of the Bohemian samples, which range from 14.42 to 14.70 Ma, is 14.50 ± 0.16 (0.42) [2 sigma] Ma (errors in parentheses include age error and uncertainty of standard monitor age). The plateau age of the Lusatian sample of 14.38 ± 0.26 (0.44) [2 sigma] Ma confirms the previously published 40Ar/39Ar age of 14.52 ± 0.08 (0.40) [2 sigma] Ma, and demonstrates that the fall of Lusatian and Bohemian tektites were contemporaneous. Because of their geochemistry and their ages there is no doubt that the Lusatian tektites are moldavites. Accepting that moldavites are ejecta from the Nördlinger Ries impact the new ages also date the impact event. This age is slightly younger (about 0.2 - 0.3 Ma) than the age suggested by earlier K-Ar determinations.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Cosmogenic Nuclides in the Brenham Pallasite
M. Honda, M. W. Caffee, Y. N. Miura, H. Nagai, K. Nagao, and K. Nishiizumi*
*Correspondence author's address: Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA; e-mail address: kuni@ssl.berkeley.edu
Abstract–Cosmic-ray-produced (cosmogenic) nuclides were studied in fragments of the Brenham pallasite, a large stony iron meteorite. The contents of light noble gases (He, Ne, and Ar) and long-lived radionuclides (10Be, 26Al, 36Cl, and 53Mn), produced by nuclear reactions with cosmic rays, were measured in the separated metal and olivine phases from numerous samples representing a wide range of shielding conditions in the meteoroid. The distribution of cosmogenic nuclide concentrations in the metal follows patterns similar to that observed in large irons. Shielding effects were estimated from the relative proportions of low- and high-energy reaction products. The production rates varied, from surface to interior, by a factor of more than several hundred. The 36Cl-36Ar cosmic-ray exposure age of Brenham is 156±8 Myr. This determination is based on a multiple nuclide approach that utilizes cosmogenic nuclide pairs. This approach not only yields a "shielding independent" exposure age but also demonstrates that the production of cosmogenic nuclides occurred in a single stage. The depth profiles of 10Be in the stone phase and 53Mn in the metal phase are shown superimposed on corresponding profiles from the Apollo 15 long drill core.
Surprisingly low abundances of lithophile elements, such as K, U, and Th, provided a unique opportunity to examine the production systematics of those nuclides whose inventories typically have significant contributions from non-cosmogenic sources, particularly radiogenic contributions. The U and Th contents of the olivine samples are extremely low, allowing detection of cosmogenic 4He production from oxygen, magnesium, silicon, and iron.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Calcium-Aluminum-rich inclusions and amoeboid olivine aggregates from the CR carbonaceous chondrites
Jérôme Aléon*, Alexander N. Krot, and Kevin D. McKeegan
*Correspondence author's address: Department of Earth & Space Sciences, University of California, Los Angeles, CA 90095-1567, USA; e-mail address: aleon@oro.ess.ucla.edu
Abstract–Ca-Al-rich refractory inclusions (CAIs) in CR chondrites are rare (< 1 vol%), fairly small (<500 µm) and irregularly-shaped, and most of them are fragmented. Based on the mineralogy and petrography, they can be divided into grossite±hibonite-rich, melilite-rich, and pyroxene-anorthite-rich CAIs. Other types of refractory objects include fine-grained spinel-melilite-pyroxene aggregates and amoeboid olivine aggregates (AOAs). Some of the pyroxene-anorthite-rich CAIs have igneous textures, and most melilite-rich CAIs share similarities to both the fluffy and compact Type A CAIs found in CV chondrites. One major difference between these CAIs and those in CV, CM, and CO chondrites is that secondary mineral phases are rare.
In situ ion microprobe analyses of oxygen isotope compositions of 27 CAIs and AOAs from 7 CR chondrites demonstrate that most of the CAIs are 16O-rich (Delta 17O of hibonite, melilite, spinel, pyroxene, and anorthite < -22%o) and isotopically homogeneous within 3-4%o. Likewise, forsterite, spinel, anorthite, and pyroxene in AOAs have nearly identical, 16O-rich compositions (-24%o < Delta 17O < -20%o). In contrast, objects which show petrographic evidence for extensive melting are not as 16O-rich (Delta 17O > -18%o). Secondary alteration minerals replacing 16O-rich melilite in melilite-rich CAIs plot along the terrestrial fractionation line.
Most CR CAIs and AOAs are mineralogically pristine objects that largely escaped thermal metamorphism and secondary alteration processes, which is reflected in their relatively homogeneous 16O-rich compositions. It is likely that these objects (or their precursors) condensed in an 16O-rich gaseous reservoir in the solar nebula. In contrast, several igneous CAIs are not very enriched in 16O, probably as a result of their having melted in the presence of a relatively 16O-poor nebular gas. If the precursors of these CAIs were as 16O-rich as other CR CAIs, this implies either temporal excursions in the isotopic composition of the gas in the CAI-forming regions and/or radial transport of some CAI precursors into an 16O-poor gas. The absence of oxygen isotope heterogeneity in the primary minerals of melilite-rich CAIs containing alteration products suggests that mineralogical alteration in CR chondrites did not affect oxygen isotope compositions of their CAIs.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Physical Properties of Near-Earth Asteroids from Thermal-Infrared Observations and Thermal Modeling
Marco Delbo* and Alan W. Harris
*Correspondence author's address: INAF - Osservatorio Astronomico di Torino, strada osservatorio 20, 10025 Pino Torinese (TO), Italy; e-mail address: delbo@to.astro.it
Abstract–We review the physical principles on which asteroid
thermal models are based and their application in the derivation
of asteroid sizes and albedos. In particular, the use of simple
thermal models to derive reliable diameters and albedos of
near-Earth asteroids (NEAs) is discussed.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Infrared Observations of Asteroids from Space The Past and Future
Stephan D. Price
Author's address: Air Force Research Laboratory, Space Vehicles Directorate, 29 Randoph Rd., Hanscom AFB, MA 01731-3010, USA; e-mail address: steve.price@hanscom.af.mil
Abstract–Infrared observations from space have large sensitivity and total instantaneous field of view advantages over ground-based measurements. The limits to telescope performance from thermal emission from the atmosphere and sky noise are eliminated in space and the instrument can be cooled to temperatures where the photon noise from the zodiacal background provides the fundamental limit to the sensitivity of the system. Furthermore, the entire thermal infrared spectral range is available; the atmospheric is virtually opaque at the wavelengths of molecular absorption bands from water vapor and CO2 to ground-based observations. Space-based infrared radiometry from the experiments described in this article supplied the basis for the largest, consistent set of derived diameters and albedos of asteroids. Radiometry over a large spectral range and a large span of phase angles provides essential information of the detailed thermal properties of a body. Infrared measurements resolve the ambiguity of whether a visual observation is of a small highly reflective object or a large dark one. Infrared spectroscopy obtained by the previous space-based experiments, and the spectral capability of two infrared missions to be flown within the next several years, is a powerful remote sensing tool to assay the mineralogy of a surface. A description is given of what knowledge has been and will be gained from past and future infrared missions on the physical characteristics of asteroids. Why the database derived from previous satellites remains the major source of new radiometric measurements is explained and the benefits to be had from a space-based infrared spectrometer/photometer dedicated to studying small bodies in the solar system presented.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Thermophysical Analysis of Infrared Asteroid Observations
Thomas Mueller
*Correspondence author's address: Max-Planck-Institut fur extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany; e-mail address: tmueller@mpe.mpg.de
Abstract–Visual photometry, which measures reflected solar radiation, can be combined with infrared radiometry, which measures absorbed and re-radiated solar energy, to determine key properties of small solar system objects. This method can be applied via thermophysical model concepts not only for albedo and diameter determination, but also for studies of thermal parameters like thermal inertia, surface roughness or emissivity. Hence, a detailed analysis of the asteroid surface is possible and topics like surface mineralogy, the density of the regolith or the presence of a rocky surface, lightcurve influences due to shape or albedo, porosity of the surface material, etc. can be addressed.
The "radiometric technique" based on a recently developed thermophysical model is presented. The model was extensively tested against observations from the Infrared Space Observatory, including spectroscopic and photometric measurements at infrared wavelengths between 2 and 200 um of more than 40 asteroids.
The possible model applications are discussed in terms of the different levels of knowledge for individual asteroids. The effects of the thermal parameters are illustrated and methods are presented as to how to separate different aspects. Possibilities and limitations are evaluated for the possible transfer of this model to near-Earth asteroids. In the long run, this kind of studies of near-Earth asteroids may provide answers to questions about their surface properties which are crucial to develop mitigation scenarios.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Mineralogy and petrology of amoeboid olivine inclusions in CO3 chondrites: Relationship to parent-body aqueous alteration
Lysa J. Chizmadia*, Alan E. Rubin and John T. Wasson
*Correspondence author's address: Department of Earth and Planetary Sciences, University of New Mexico Albuquerque, NM 87131, USA; e-mail address: lchiz@unm.edu
Abstract–Petrographic and mineralogic studies of amoeboid olivine inclusions (AOI) in CO3 carbonaceous chondrites reveal that they are sensitive indicators of parent-body aqueous and thermal alteration. As the petrologic subtype increases from 3.0 to 3.8, forsteritic olivine (Fa0-1) is systematically converted into ferroan olivine (Fa60-75). We infer that the Fe, Si and O entered the assemblage along grain boundaries, forming ferroan olivine that filled fractures and voids. As temperatures increased, Fe+2 from the new olivine exchanged with Mg+2 from the original AOI to form diffusive haloes around low-FeO cores. Cations of Mn+2, Ca+2 and Cr+3 were also mobilized.
The systematic changes in AOI textures and olivine compositional distributions can be used to refine the classification of CO3 chondrites into subtypes. In subtype 3.0, olivine occurs as small forsterite grains (Fa0-1), free of ferroan olivine. In petrologic subtype 3.2, narrow veins of FeO-rich olivine have formed at forsterite grain boundaries. With increasing alteration, these veins thicken to form zones of ferroan olivine at the outside AOI margin and within the AOI interior. By subtype 3.7, there is a fairly broad olivine compositional distribution in the range Fa63-70, and by subtype 3.8, no forsterite remains and the high-Fa peak has narrowed, Fa64-67. Even at this stage, there is incomplete equilibration in the chondrite as a whole, e.g., data for coarse olivine grains in Isna (CO3.8) chondrules and lithic clasts show a peak at Fa39. We infer that the mineral changes in AOI identified in the low petrologic types required aqueous or hydrothermal fluids whereas those in subtypes >= 3.3 largely reflect diffusive exchange within and between mineral without the aid of fluids.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Clay mineral-organic matter relationships in the early Solar System
Victoria K. Pearson*, Mark A. Sephton, Anton T. Kearsley, Philip A. Bland, Ian A. Franchi and Iain Gilmour
*Correspondence author's address: Planetary and Space Sciences Research Institute, Open University, Milton Keynes, Buckinghamshire, MK7 6AA, United Kingdom; e-mail address: v.k.pearson@open.ac.uk
Abstract–As the Solar System formed, it inherited and perpetuated a rich organic chemistry, the molecular products of which are preserved in ancient extraterrestrial objects such as carbonaceous chondrites. These organic-rich meteorites provide a valuable and tangible record of the chemical steps taken towards the origin of life in the early Solar System. Chondritic organic matter is present in the inorganic meteorite matrix which, in the CM and CI chondrites, contains evidence of alteration by liquid water on the parent asteroid. An unanswered and fundamental question is to what extent did the organic matter and inorganic products of aqueous alteration interact or display interdependence? We have used an organic labelling technique to reveal that the meteoritic organic matter is strongly associated with clay minerals. This association suggests that clay minerals may have had an important trapping and possibly catalytic role in chemical evolution in the early Solar System prior to the origin of life on the early Earth.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Northwest Africa 482: A crystalline impact melt breccia from the lunar highlands
Ingrid J. Daubar*, David A. Kring, Timothy D. Swindle, And A. J. Timothy jull
*Correspondence author's address: Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, 85721, USA; e-mail address: ingrid@lpl.arizona.edu
Abstract–Northwest Africa 482 (NWA 482) is a crystalline impact melt breccia from the Moon with highlands affinities. The recrystallized matrix and the clast population are both highly anorthositic. Clasts are all related to the ferroan anorthosite suite, and include isolated plagioclase crystals and lithic anorthosites, troctolites, and spinel troctolites. KREEP and mare lithologies are both absent, constraining the source area of this meteorite to a highland terrain with little to no KREEP component, most likely on the far side of the Moon. Glass is present in shock veins cutting through the sample and in several large melt pockets, indicating a second impact event.
There are two separate events recorded in the 40Ar-39Ar system: one at ~3750 Ma, which completely reset the K-Ar system, and one at <~2400 Ma, which caused only partial degassing. These events could represent, respectively, crystallization of the impact melt breccia and later formation of the glass, or the formation of the glass and a later thermal event.
The terrestrial age of the meteorite is 8.6 ± 1.3 ka. This age corresponds well with the modest amount of weathering in the rock, in the form of secondary phyllosilicates and carbonates. Based on terrestrial age and location, lithology, and chemistry, NWA 482 is unique among known lunar meteorites.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
High-Albedo Asteroid 434 Hungaria: Spectrum, Composition and Genetic Connections
Michael S. Kelley* and Michael J. Gaffey
*Correspondence author's address: Department of Geology and Geography, Georgia Southern University, Statesboro, Georgia 30460-8149, USA; e-mail address: mkelley@gasou.edu
Abstract–New data in the wavelength region of approximately 0.4-2.5 um have been obtained for asteroid 434 Hungaria. This is the most complete visible to near-infrared spectrum to date for this object. The near-infrared portion of the spectrum (~0.8-2.5 um) is smooth, featureless, and agrees well in the overlap region with new visible region data. However, visible region (~0.45-0.9 um) data appear to exhibit weak, broad spectral absorption features near 0.5, 0.6-0.7, and 1 um. If real, the presence of such features would strongly constrain the compositional determination of Hungaria since it has a relatively high albedo of 46 percent. Most minerals that exhibit similar absorption features, and are commonly found in meteorites, have a much lower albedo. Asteroid 434 Hungaria has been observed more than six times in these overlapping spectral regions, and it is now possible to assess its mineral composition with some confidence. The dominant phase on this asteroid is an iron-free mineral, probably enstatite. Hungaria may contain secondary phases causing subtle, visible-region absorption features. Alternatively, the surface layer(s) of the asteroid may be contaminated by an absorbing species from an external source.
Heterogeneous Condensation of Presolar TiC Core-Graphite Mantle Spherules
Takeshi Chigai, Tetsuo Yamamoto* and Takashi Kozasa
*Correspondence author's address: Department of Earth and Planetary Sciences, Nagoya University, Nagoya 464-8602, Japan; e-mail address: ty@eps.nagoya-u.ac.jp
Abstract–We investigate heterogeneous nucleation and growth of graphite on pre-condensed TiC grains in the gas outflows from carbon-rich asymptotic giant branch (AGB) stars employing a newly-derived heterogeneous nucleation rate taking into account of the chemical reactions at condensation. Competition between heterogeneous and homogeneous nucleations and growths of graphite is investigated to reveal the formation conditions of the TiC core-graphite mantle spherules found in the Murchison meteorite. It is shown that no homogeneous graphite grain condenses whenever TiC condenses prior to graphite in the plausible ranges of the stellar parameters. Heterogeneous condensation of graphite occurs on the surfaces of growing TiC grains, and prevents the TiC cores from reaching the sizes realized if all available Ti atoms were incorporated into TiC grains. The physical conditions at the formation sites of the TiC core-graphite mantle spherules observed in the Murchison meteorite are expressed by the relation 0.2 < upsilon0.1 (M5zeta)-1/2L41/4 < 0.7, where upsilon0.1 is the gas outflow velocity at the formation site in units of 0.1 km s-1, M5 the mass loss rate in 10-5 Msun yr-1, L4 the stellar luminosity in 104 Lsun, and M/zeta is the effective mass loss rate taking account of non-spherical symmetry of the gas outflows. The total gas pressures Pc at the formation sites for the effective mass loss rates M/zeta = 10-5 - 10-3 Msun yr-1 correspond to 0.01 < Pc < 0.9 dyn cm-2, implying that the observed TiC core-graphite mantle spherules are formed not only at the superwind stage but also at the earlier stage of low mass loss rates. The constraint on the C/O abundance ratio, 1< epsilon <= 1.03, is imposed to reproduce the observed sizes of the TiC cores. The derived upper limit of the C/O ratio is lower than the values estimated from the calculations without taking into account of heterogeneous condensation of graphite, and is close to the lower end of the C/O ratios inferred from the astronomical observations of carbon-rich AGB stars. Brief discussion is given on other types of graphite spherules.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Heterogeneous Agglutinitic Glass and the F3 Model
Abhijit Basu*, Susan J. Wentworth and David S. McKay
*Correspondence author's address: Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA; e-mail address: basu@indiana.edu
Abstract–Evidence in favor of the model fusion of the finest fraction (F3) for the origin of lunar agglutinitic glass has been accruing. They include (1) theoretical expectations that shock pulses should engulf and melt smaller grains more efficiently than larger grains, (2) experimental results of impact shock, albeit at lower than presumed hypervelocity impacts of micrometeorites on the lunar regolith, and (3) new analyses confirming previous results that average compositions of agglutinitic glass are biased towards that of the finest fraction of lunar soils from which they had formed. We add another reason in support of the F3 model. Finer grains of lunar soils, obviously much lighter than coarser grains, are also much more abundant; hence, electrostatic forces associated with the rotating terminator region bring the finest grains at the surface of the Moon. This further contributes to the preferential melting of the finest fraction upon micrometeoritic impacts. New backscattered electron imaging shows that agglutinitic glass is inhomogeneous at submicron scale. Composition ranges of agglutinitic glass are extreme and deviate from that of the finest fraction, even by more than an order of magnitude for some components. Additionally, we show how an ilmenite grain upon impact would produce TiO2-rich agglutinitic glass in complete disregard to the fusion of the finest fraction. We propose an addition to the F3 model to accommodate these observations, i.e., that micrometeorite impacts indiscriminately melt the immediate target regardless of grain size or grain composition. We, therefore, suggest that (1) agglutinitic glass is the sum of (a) the melt produced by the fusion of the finest fraction of lunar soils and (b) the microvolume of the indiscriminate target, which melts at high shock pressures from micrometeoritic impacts, and that (2) because of the small volume of the melt and incorporating cold soil grains, the melt quenched so rapidly that it did not mix and homogenize to represent any preferential composition, for example, that of the finest fraction.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Dark inclusions in the Mokoia CV3 chondrite: Evidence for aqueous alteration and subsequent thermal and shock metamorphism
Ichiro Ohnishi and Kazushige Tomeoka*
*Correspondence author's address: Department of Earth and Planetary Sciences, Faculty of Science, Kobe University, Nada, Kobe 657-8501, Japan; e-mail address: tomeoka@kobe-u.ac.jp
Abstract–Mokoia is a CV3 chondrite that contains abundant phyllosilicate mineralization. We present a detailed petrographic and scanning electron microscopic study of twenty four dark inclusions (DIs) that we found from Mokoia. The overall texture and constituent minerals of the DIs resemble those in the host meteorite. Fe-bearing saponite and Na-rich phlogopite, the same phyllosilicates as in the host meteorite, occur in the DIs, which strongly suggests that the DIs have a similar alteration history to the host meteorite. However, the DIs show several distinct differences from the host meteorite. Olivine grains in the DI matrices are more homogeneous in Fe/(Fe+Mg) ratio than those in the host meteorite matrix. Phyllosilicates in the DIs are less abundant than in the host meteorite, and they have been dehydrated to various extents. These characteristics suggest that the DIs have experienced higher degree of thermal metamorphism than the host meteorite. In addition, the matrices in the DIs are more compacted than those in the host meteorite. Most olivine grains in the DIs show undulatory extinction in transmitted crossed-polarized light and some show planar fractures, while such olivine grains are rare in the host meteorite. Two of the DIs contain Si-, Mg-, Fe- and O-rich melt veins. These characteristics indicate that most DIs have been shocked to shock stage S3-S4, while the host meteorite is shock stage S1 (virtually unshocked). Thermal metamorphism of the DIs was likely caused by shock heating. These results are consistent with the contention previously proposed for the DIs in CV3 chondrites, i.e., the DIs have experienced aqueous alteration and subsequent dehydration on the CV parent body. We suggest that thermal and shock metamorphism occurred locally to various extents after pervasive aqueous alteration in the Mokoia parent body.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Probing the Interior of Asteroids and Comets Using Radio Reflection Tomography
A. Safaeinili*, S. Gulkis, M.D. Hofstadter, R.L. Jordan
*Correspondence author's address: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA; e-mail address: ali.safaeinili@jpl.nasa.gov
Abstract–Asteroids and comets are of great scientific interest: their interior structure and composition, which are poorly known, provide information about conditions and processes that occurred during the early stages of solar system development. They are also of interest for social and economic reasons. Their proximity to Earth and abundance in the solar system make them potential sources of raw materials as well as a threat, as evidenced by past catastrophic impacts. Information on their composition and structure is therefore important to assess both the potential benefit of these objects and mitigate the potential risk they pose. This paper describes the use of Radio Reflection Tomography (RRT) for studying the interiors of asteroids and comets. We discuss technical issues regarding benefits and challenges of implementing a radio reflection tomography instrument and present potential solutions. This paper addresses a range of topics including 1) data collection scenarios, 2) data processing and inversion, and 3) instrument implementation. A "strawman" instrument capable of imaging the full interior of an asteroid or a comet with dimensions of a few kilometers is presented. Such an instrument can play a significant role in studying the near-Earth objects (NEOs), both for scientific and socio-economic purposes.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Domenico Troili (1766): "The true cause of the fall of a stone in Albereto is a subterranean explosion that hurled the stone skyward."
Ursula B. Marvin* and Mario L. Cosmo
*Correspondence author's address: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA; e-mail address: umarvin@cfa.harvard.edu
Abstract–In mid-July, 1766, a stone fell at Villa Albareto near Modena in northern Italy. A sudden explosion like a cannon shot followed by fierce whistling sounds in the air frightened people over a wide area. Some saw a fiery body falling from the sky; others said it war dark and smoky. The stone made the ground shake when it plunged into the Earth and made a hole nearly a meter deep. The Abbé Domenico Troili collected eye-witness reports, examined the stone, and reported the presence of marchesita, an old name for pyrite. A century later, this mineral, which proved to be iron sulfide (FeS), was named "troilite" in his honor. Troili's description is unquestionably that of a meteorite fall, and therefore some scientists have argued that it is Troili, rather than Ernst F. F. Chladni, to whom we should give credit as the first person to record the fall of a stone from space. However, Troili, himself, had no such an idea; he wrote that a subterranean explosion had hurled the stone high into the sky from a vent in the Earth. He stoutly defended this explanation against his opponents, including the Bishop of Modena, who believed that the stone had been hurled aloft by a bolt of lightning. Both hypotheses reflect a conviction, held well into the 19th century, that any rocky objects that fall from the sky must originate on the Earth or in the atmosphere. In 1794 Chladni calculated that meteors and meteoritic fireballs course down the sky at such extremely high velocities that the bodies forming them must originate in space. He listed all the falls that he found credible in historic records. Partly through his efforts, meteorites had gained widespread acceptance by 1803, but the idea of their origin in space had not. For the next half century many scientists continued to argue that meteorites either consolidate in the upper atmosphere or are ejected by volcanoes on the Moon. Recent efforts to transfer honors from Chladni to Troili for being the first to describe meteorites as bodies falling from space are unwarranted.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Role of sticky interstellar organic material in the formation of asteroids
A. Kouchi*, T. Kudo, M. Arakawa and H. Nakano
*Correspondence author's address: Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan; e-mail address: kouchi@lowtem.hokudai.ac.jp
Abstract–Collision experiments and measurements of viscoelastic properties were performed involving an interstellar organic material analogue to investigate the growth of organic grains in the protosolar nebula. The organic material was found to be stickiest at a radius of between 2.3 and 3.0 AU, with a maximum sticking velocity of 5 m s-1 for millimeter-size organic grains. This stickiness is considered to have resulted in the very rapid coagulation of organic grain aggregates and subsequent formation of planetesimals in the early stage of the turbulent accretion disk. The planetesimals formed in this region appear to be represent achondrite parent bodies. In contrast, the formation of planetesimals at <2.1 AU and >3.0 AU begins with the establishment of a passive disk because silicate and ice grains are not as sticky as organic grains.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Near-Earth Objects: Origins and Need of Physical Characterization
A. Cellino*, V. Zappala and E. F. Tedesco
*Correspondence author's address: Osservatorio Astronomico di Torino, 10025 Pino Torinese (TO), Italy; e-mail address: cellino@to.astro.it
Abstract–Important improvements have been made in recent years in understanding the likely origins of Near-Earth Objects, and extensive observational campaigns are ongoing in order to assess their current inventory. From these studies we can hope to obtain a much better understanding of the different populations of minor bodies, the relationship with meteorites, and the overall history of the Solar System. At the same time, NEOs are important also in terms of impact hazard. Both the purely scientific issues, and the more pragmatic point of view focused on the need of developing credible strategies of impact mitigation, require a major effort in order to improve the current knowledge of the physical properties of these objects.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
The Trapped Noble Gas Component in Achondrites
Henner Busemann* and Otto Eugster
*Correspondence author's address: University of Bern, Physics Institute, Sidlerstr. 5, 3012 Bern, Switzerland; e-mail address: busemann@phim.unibe.ch
Abstract–The trapped noble gases Ar, Kr and Xe in several achondrites were analysed. We chose separates of the lodranites Lodran and Graves Nunataks (GRA) 95209 and bulk samples of the Tatahouine diogenite, Pasamonte eucrite, five aubrites and two angrites. Among these, Lodran, Tatahouine, Pasamonte and the aubrite Norton County have been reported to contain U-Xe, a noble gas component assumed to be the most primitive component in the solar system. U-Xe might have been incorporated into the early Earth. We found large concentrations of Xe in several separates of the Lodran lodranite, however, none of the measurements revealed U-Xe composition. The Xe composition of all achondrites can straightforwardly be explained with mixtures of trapped common Xe-Q, absorbed air and various amounts of fissiogenic and cosmogenic Xe. Re-analysis of literature data for Pasamonte, Angra dos Reis and some aubrites is consistent with Xe-Q as the trapped endmember component and contributions of fissiogenic Xe. The presence of Xe-Q in many primitive achondrites is in agreement with the formation of their parent bodies from originally chondritic precursor material. The Ar-Xe elemental composition of Lodran and the aubrites indicate subsolar composition, which is commonly found in E chondrites. This result supports a model of formation of the aubrites from E-chondritic precursor material.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
M.S. Robinson*, P.C. Thomas, J. Veverka, S.L. Murchie, B.B. Wilcox
*Correspondence author's address: Northwestern University, 1847 Sheridan Road, Evanston, Illinois 60208, USA; e-mail address: robinson@earth.northwestern.edu
Abstract–The global high-resolution imaging of asteroid 433 Eros by the NEAR Shoemaker spacecraft has made it possible to develop the first comprehensive picture of the geology of a small S-type asteroid. Eros displays a variety of surface features, and evidence of a substantial regolith. Large scale facets, grooves, and ridges indicate the presence of at least one global planar structure. Directional and superposition relations of smaller structural features suggest that fracturing has occurred throughout the object. As with other small objects, impact craters dominate the overall shape as well as the small scale topography of Eros. Depth/diameter ratios of craters on Eros average about 0.13, but the freshest craters approach lunar values of ~0.2. Ejecta block production from craters is highly variable; the majority of large blocks appear to have originated from one 7.6 km crater (Shoemaker). The interior morphology of craters does not reveal the influence of discrete mechanical boundaries at depth in the manner of craters formed on lunar mare regolith and on some parts of Phobos. This lack of mechanical boundaries, and the abundant evidence of regolith in nearly every high resolution image, suggests a gradation in the porosity and fracturing with depth. The density of small craters is deficient at sizes below ~200m relative to predicted slopes of empirical saturation. This characteristic, which is also found on parts of Phobos and lunar highland areas, probably results from the efficient obliteration of small craters on a body with significant topographic slopes and a thick regolith. Eros displays a variety of regolith features, such as debris aprons, fine-grained "ponded" deposits, talus cones, and bright and dark streamers on steep slopes indicative of efficient downslope movement of regolith. These processes serve to mix materials in the upper loose fragmental portion of the asteroid (regolith). In the instance of "ponded" materials and crater wall deposits, there is evidence of processes that segregate finer materials into discrete deposits. The NEAR observations have shown us that surface processes on small asteroids can be very complex and result in a wide variety of morphologic features and landforms that today seem exotic. Future missions to comets and asteroids will surely reveal still as yet unseen processes as well as give context to those discovered by the NEAR Shoemaker spacecraft.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Richard Greenberg* and Paul Geissler
*Correspondence author's address: Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA; e-mail address: greenberg@lpl.arizona.edu
Abstract–Europa's icy crust records active resurfacing by tectonic and thermal processes over tens of millions of years, a rapidity demonstrated by a paucity of craters. Tidal working causes rotational torque, surface stress, internal heating, and orbital evolution, which can explain the formation of observed tectonic crack patterns, ridges, crustal displacement, and chaotic terrain by processes involving connections between the surface and the underlying ocean through cracks, melt sites, and occasional impacts. These processes were recent, and thus most likely continue today. The permeability of the crust allows exchange of materials, including oxidants and exogenic organics from the surface and endogenic substances from the ocean, such that a habitable biosphere might extend to within a few centimeters of the surface. Continual changes in environmental conditions in the ice crust, such as deactivation of individual cracks after thousands of years (due to non-synchronous rotation) and crustal thawing (releasing any trapped organisms), could provide drivers for biological adaptation, as well as opportunity for evolution.
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