• Europa's dynamic icy crust

      Greenberg, R.; Geissler, P. (The Meteoritical Society, 2002-01-01)
      Europa's icy crust records active resurfacing by tectonic and thermal processes over tens of millions of years, as 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.
    • Cosmogenic nuclides in the Brenham pallasite

      Honda, M.; Caffee, M. W.; Miura, Y. N.; Nagai, H.; Nagao, K.; Nishiizumi, K. (The Meteoritical Society, 2002-01-01)
      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 iron meteorites. 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.
    • The geology of 433 Eros

      Robinson, M. S.; Thomas, P. C.; Veverka, J.; Murchie, S. L.; Wilcox, B. B. (The Meteoritical Society, 2002-01-01)
      The global high-resolution imaging of asteroid 433 Eros by the Near-Earth Asteroid Rendezvous (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 ~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 ~200 m 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.
    • 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.
    • 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
    • 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.
    • Coeval argon-40/argon-39 ages of moldavites from the Bohemian and Lusatian strewn fields

      Schwarz, W. H.; Lippolt, H. J. (The Meteoritical Society, 2002-01-01)
      40Ar/39Ar ages of four tektites (moldavites) from southern Bohemia (near Cesk Budejovice, Czech Republic) and a tektite from Lusatia (near Dresden, Germany) have been determined by 11 step-degassing experiments. The purpose of the study was to enlarge the 40Ar/39Ar data base 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 Nӧrdlinger 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.
    • 2002 Appreciations

      The Meteoritical Society, 2002-01-01
    • Reevaluating the impact cratering kill curve

      Kring, D. A. (The Meteoritical Society, 2002-01-01)
    • From the Editors

      Sears, D. (The Meteoritical Society, 2002-01-01)
      Learned socities, leadership, journals and the progress of planetary science.
    • Workshop summary on physical and chemical properties of potential Earth impactors

      Huebner, W. F.; Greenberg, J. M. (The Meteoritical Society, 2002-01-01)
      From 2001 June 17 to 25, we held the first international workshop in Erice, Italy, dedicated to the determination of geological and geophysical properties of near-Earth objects (NEOs). The goal was to develop a roadmap for determining the physical chemical properties of NEOs in the coming decades to meet the scientific requirements for development of Earth collision avoidance technology. We identified many properties that are desired, but four measurements are needed most critically for any potentially hazardous NEO: (1) its mass, (2) its mass distribution, (3) its material strengths, and (4) its internal structure. Global (whole-body) properties, such as material strengths and internal structure, can be determined best from the analyses of permeating waves: artificially initiated seismology and multifrequency reflection and transmission radio tomography. Seismology provides the best geophysical (material strengths) data of NEOs composed of consolidated materials while radio tomography provides the best geological data (e.g., the state of fracture) of electrically nonconducting media. Thus, teh two methods are complimentary: seismology is most suitable for stony and metallic asteroids, while radio tomography is most appropriate for comet nuclei and carbonaceous asteroids. The three main conclusions are (1) remote sensing for physical characterization should be increased, (2) several dedicated NEO missions should be prepared for geophysical and geological investigations, and (3) that it is prudent to develop and prove the technology to make geophysical measurements on NEOs now.
    • Calcium-aluminum-rich inclusions and amoeboid olivine aggregates from the CR carbonaceous chondrites

      Aléon, J.; Krot, A. N.; McKeegan, K. D. (The Meteoritical Society, 2002-01-01)
      Calcium-aluminum-rich refractory inclusions (CAIs) in CR chondrites are rare (<1 vol%), fairly small (<500 micrometers) 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-isotopic compositions of 27 CAIs and AOAs from seven CR chondrites demonstrate that most of the CAIs are 16O-rich (Delta-17O of hibonite, melilite, spinel, pyroxene, and anorthite <-22 ppm) and isotopically homogeneous within 3-4 ppm. Likewise, forsterite, spinel, anorthite, and pyroxene in AOAs have nearly identical, 16O-rich compositions (-24 ppm < Delta-17O < -20 ppm). In contrast, objects which show petrographic evidence for extensive melting are not as 16O-rich (Delta-17O less than -18 ppm). 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-isotopic compositions of their CAIs.
    • 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.
    • 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.
    • 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.
    • 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.
    • 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.