• Annual Subject Index, Volume 37, 2002

      The Meteoritical Society, 2002-01-01
    • Volume 37 2002: The Year at a Glance

      The Meteoritical Society, 2002-01-01
    • Book Review: Storms in Space, John Freeman

      Cheng, A. F. (The Meteoritical Society, 2002-01-01)
    • Biological processes in impact craters, King's College, University of Cambridge, U. K., 2003 March 29 to April 1

      The Meteoritical Society, 2002-01-01
      Announcement: Biological processes in impact craters, King's College, University of Cambridge, U. K., 2003 March 29 to April 1
    • Annual Author Index, Volume 37, 2002

      The Meteoritical Society, 2002-01-01
    • Heterogeneous condensation of presolar titanium carbide core-graphite mantle spherules

      Chigai, T.; Yamamoto, T.; Kozasa, T. (The Meteoritical Society, 2002-01-01)
      We investigate heterogeneous nucleation and growth of graphite on precondensed 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 < v0.1 (M5 / zeta)^(-1/2) L4^(1/4) < 0.7, where v0.1 is the gas outflow velocity at the formation site in units of 0.1 km s^(-1), M^(-5), the mass loss rate in 10^(-5) M solar year^(-1), L4 the stellar luminosity in 10^4 L(solar), 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) M solar year-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 degrees C/O abundance ratio, 1 < eta which is less than or approximately equal to 1.03, is imposed to reproduce the observed sizes of the TiC cores. The derived upper limit of the degrees 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 degrees C/O ratios inferred from the astronomical observations of carbon-rich AGB stars. Brief discussion is given on other types of graphite spherules.
    • Near-Earth objects: Origins and need of physical characterization

      Cellino, A.; Zappala, V.; Tedesco, E. F. (The Meteoritical Society, 2002-01-01)
      Important improvements have been made in recent years in understanding the likely origins of near-Earth objects (NEOs), 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, their 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.
    • Probing the interior of asteroids and comets using radio reflection tomography

      Safaeinili, A.; Gulkis, S.; Hofstadter, M. D.; Jordan, R. L. (The Meteoritical Society, 2002-01-01)
      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 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, both for scientific and socio-economic purposes.
    • The role of sticky interstellar organic material in the formation of asteroids

      Kudo, T.; Kouchi, A.; Arakawa, M.; Nakano, H. (The Meteoritical Society, 2002-01-01)
      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 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 and >3.0 AU begins with the establishment of a passive disk because silicate and ice grains are not as sticky as organic grains.
    • Modeling the Ries-Steinheim impact event and the formation of the moldavite strewn field

      Stöffler, D.; Artemieva, N. A.; Pierazzo, E. (The Meteoritical Society, 2002-01-01)
      Using detailed geological, petrographic, geochemical, and geographical constraints we have performed numerical modeling studies that relate the Steinheim crater (apparent diameter Da = 3.8 km), the Ries crater (Da = 24 km) in southern Germany, and the moldavite (tektite) strewn field in Bohemia and Moravia (Czech Republic), Lusatia (East Germany), and Lower Austria. The moldavite strewn field extends from ~200 to 450 km from the center of the Ries to the east-northeast forming a fan with an angle of ~57 degrees. An oblique impact of a binary asteroid from a west-southwest direction appears to explain the locations of the craters and the formation and distribution of the moldavites. The impactor must have been a binary asteroid with two widely separated components (some 1.5 and 0.15 km in diameter, respectively). We carried out a series of three-dimensional hydrocode simulations of a Ries-type impact. The results confirm previous results suggesting that impacts around 30-50 degrees (from the horizontal) are the most favorable angles for near-surface melting, and, consequently for the formation of tektites. Finally, modeling of the motion of impact-produced tektite particles through the atmosphere produces, in the downrange direction, a narrow-angle distribution of the moldavites tektites in a fan like field with an angle of ~75 degrees. An additional result of modeling the motion of melt inside and outside the crater is the preferred flow of melt from the main melt zone of the crystalline basement downrange towards the east-northeast rim. This explains perfectly the occurrence of coherent impact melt bodies (some tens of meters in size) in a restricted zone of the downrange rim of the Ries crater. The origin of these melt bodies, which represent chemically a mixture of crystalline basement rocks similar to the main melt mass contained (as melt particles < 0.5 m in size) in the suevite, do not occur at any other portion of the Ries crater rim and remained enigmatic until now. Although the calculated distribution of moldavites still deviates to some degree from the known distribution, our results represent an important step toward a better understanding of the origin and distribution of the high-velocity surface melts and the low-velocity, deep-seated melt resulting from an oblique impact on a stratified target.
    • Domenico Troili (1766): "The true cause of the fall of a stone in Albereto is a subterranean explosion that hurled the stone skyward"

      Marvin, U. B.; Cosmo, M. L. (The Meteoritical Society, 2002-01-01)
      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 frightened people over a wide area. Some saw a fiery body falling from the sky; others said it was dark and smoky. The ground shook when the stone plunged into the soil making a hole nearly a meter deep. The Abb Domenico Troili collected eyewitness 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 nineteenth 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.
    • Thermophysical analysis of infrared observations of asteroids

      Müller, T. G. (The Meteoritical Society, 2002-01-01)
      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 micrometers 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 study of near-Earth asteroids may provide answers to questions about their surface properties which are crucial to develop mitigation scenarios.
    • Infrared observations of asteroids from space: The past and future

      Price, S. D. (The Meteoritical Society, 2002-01-01)
      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.
    • Physical properties of near-Earth asteroids from thermal infrared observations and thermal modeling

      Delbó, M.; Harris, A. W. (The Meteoritical Society, 2002-01-01)
      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 is discussed.
    • The trapped noble gas component in achondrites

      Busemann, H.; Eugster, O. (The Meteoritical Society, 2002-01-01)
      The trapped noble gases Ar, Kr and Xe in several achondrites were analysed. We chose separates of the lodranites Lodran and Graves Nunataks 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. Reanalysis 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.
    • Dark inclusions in the Mokoia CV3 chondrite: Evidence for aqueous alteration and subsequent thermal and shock metamorphism

      Ohniski, I.; Tomeoka, K. (The Meteoritical Society, 2002-01-01)
      Mokoia is a CV3 chondrite that contains abundant phyllosilicate mineralization. We present a detailed petrographic and scanning electron microscopic study of 24 dark inclusions (DIs) that we found in 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.
    • Heterogeneous agglutinitic glass and the fusion of the finest fraction (F3) model

      Basu, A.; Wentworth, S. J.; McKay, D. S. (The Meteoritical Society, 2002-01-01)
      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 are also much more abundant. Hence, electrostatic forces associated with the rotating terminator region bring the finest grains that are obviously much lighter than courser grains to 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 requirements of 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.