• Book Review: Planetary Rings, Larry Esposito

      Consolmagno, G.; Wood, C. A. (The Meteoritical Society, 2007-01-01)
    • Crumbs from the crust of Vesta: Achondritic cosmic spherules from the South Pole water well

      Taylor, Susan; Herzog, Gregory F.; Delaney, Jeremy S. (The Meteoritical Society, 2007-01-01)
      Ten glass cosmic spherules (CS) from the South Pole water well collection were analyzed by electron microprobe. Nine of them have Fe/Mn and Fe/Mg ratios in the range typical of chondrites. One of them (SP37-3), along with up to six other previously analyzed CS, have nonchondritic Fe/Mn and Fe/Mg ratios that agree well with values typical of either (basaltic) howardite, eucrite, and diogenite (HED) meteorites or Martian basalts, but not of lunar samples. SP37-3 also contains an anorthite relic grain. Anorthite has not previously been reported in cosmic spherules, but is well known in HED meteorites. The much greater frequency of HEDs among hand-sized meteorites suggests but does not prove that HED precursors are more likely for the nonchondritic spherules.We estimate that HED-like micrometeorites constitute ~0.5 +/- 0.4% of the total population of micrometeorites in the South Pole water well, a fraction that translates to a flux of 1.6 +/- 0.3 x 10^(-8) g HED micrometeorites/m^2-y. The ratio of HED-like objects to carbonaceous objects is about 100 times less in micrometeorites than among hand-size specimens. We infer that the comparative mechanical weakness of carbonaceous precursor materials tends to encourage spherule formation.
    • Fragmentation model analysis of the observed atmospheric trajectory of the Tagish Lake fireball

      Ceplecha, Zdeněk (The Meteoritical Society, 2007-01-01)
      A recently published meteoroid fragmentation model (FM) was applied to observational data on the Tagish Lake meteoric fireball. An initial mass of 56,000 kg, derived from seismic and infrasound data by Brown et al.(2002), proved to be consistent with a very low value of intrinsic ablation coefficient of 0.0009 s^2km^(-2). The average residual of the best fit to the observed light curve was +/- 0.10 stellar magnitude. The apparent ablation coefficient varied from 0.0009 to 1.52 s^2 km^(-2), with an average value of 0.054 s^2 km^(-2) (determined by the gross fragmentation [GF] model). The FM found 33 individual fragmentation events during the penetration of the 56,000 kg initial mass of the Tagish Lake meteoroid through the atmosphere, with five of the events fragmenting more than 10% of the instantaneous mass of the main body. The largest event fragmented 88% of the mass of the main body at a height of 34.4 km. The velocity of the main body mass of 2660 kg at a height of 29.2 km (the last observed light) was 13.1 km/s. Strong fragmentation at heights lower than 29.2 km is very probable. The extreme fragmentation process of the Tagish Lake meteoroid puts its classification well outside the IIIB type in the direction of less cohesive bodies. The light curve could not be explained at all by making use of only the apparent ablation coefficient and apparent luminous efficiency.
    • Geochemistry of 4 Vesta based on HED meteorites: Prospective study for interpretation of gamma ray and neutron spectra for the Dawn mission

      Usui, Tomohiro; McSween, Harry Y. (The Meteoritical Society, 2007-01-01)
      Asteroid 4 Vesta, believed to be the parent body of the howardite, eucrite, and diogenite (HED) meteorites, will be investigated by the Dawn orbiting spacecraft. Dawn carries a gamma ray and neutron detector (GRaND) that will measure and map some major- and trace-element abundances. Drawing on HED geochemistry, we propose a mixing model that uses element ratios appropriate for the interpretation of GRaND data.Because the spatial resolution of GRaND is relatively coarse, the analyzed chemical compositions on the surface of Vesta will likely reflect mixing of three endmember components: diogenite, cumulate eucrite, and basaltic eucrite. Reliability of the mixing model is statistically investigated based on published whole-rock data for HED meteorites. We demonstrate that the mixing model can accurately estimate the abundances of all the GRaND-analyzed major elements, as well as of minor elements (Na, Cr, and Mn) not analyzed by this instrument. We also show how a similar mixing model can determine the modal abundance of olivine, and we compare estimated and normative olivine data for olivine-bearing diogenites. By linking the compositions of well-analyzed HED meteorites with elemental mapping data from GRaND, this study may help constrain the geological context for HED meteorites and provide new insight into the magmatic evolution of Vesta.
    • Magnetometer survey of the proposed Sirente meteorite crater field, central Italy: Evidence for uplifted crater rims and buried meteorites

      Ormö, Jens; Gomez-Ortiz, David; McGuire, Patrick C.; Henkel, Herbert; Komatsu, Goro; Rossi, Angelo Pio (The Meteoritical Society, 2007-01-01)
      The Sirente crater field consists of a 120 m wide, rimmed main depression flanked to the northwest by about 30 smaller depressions. It has been dated to the first centuries A.D. An impact origin is suggested, but not confirmed. The small size combined with the properties of the target material (carbonate mud) would neither allow shock features diagnostic of impact, nor projectile vaporization. Consequently, a meteoritic component in the sediments would be very localized. At impacts of this size the projectile most likely is an iron meteorite. Any iron meteorites on the ground surface would, in Iron Age Europe, have been removed shortly after the event. However, if the depressions are of impact origin they should contain meteorites at great depth in analogy with known craters. The magnetic properties of iron meteorites differ distinctly from the very low magnetic sediments and sedimentary rocks of the Sirente area. We have used a proton precession magnetometer/gradiometer to produce magnetic anomaly maps over four of the smaller depressions (~8 m diameter), as well as two crossing profiles over a fifth depression (~22 m diameter). All show distinct magnetic anomalies of about 20 nT, the larger depression up to 100 nT. Magnetic modeling shows a best fit for structures with upturned strata below their rims, excluding a karstic origin but supporting an explosive formation. The 100 nT anomaly can only be explained by highly-magnetic objects at a few meters depth. All together, the magnetic data provides a strong indication for an impact origin of the crater field.
    • Olivine-dominated asteroids and meteorites: Distinguishing nebular and igneous histories

      Sunshine, Jessica M.; Bus, Schelte J.; Corrigan, Catherine M.; McCoy, Timothy J.; Burbine, Thomas H. (The Meteoritical Society, 2007-01-01)
      Melting models indicate that the composition and abundance of olivine systematically co-vary and are therefore excellent petrologic indicators. However, heliocentric distance, and thus surface temperature, has a significant effect on the spectra of olivine-rich asteroids. We show that composition and temperature complexly interact spectrally, and must be simultaneously taken into account in order to infer olivine composition accurately. We find that most (7/9) of the olivine-dominated asteroids are magnesian and thus likely sampled mantles differentiated from ordinary chondrite sources (e.g.,pallasites). However, two other olivine-rich asteroids (289 Nenetta and 246 Asporina) are found to be more ferroan. Melting models show that partial melting cannot produce olivine-rich residues that are more ferroan than the chondrite precursor from which they formed. Thus, even moderately ferroan olivine must have non-ordinary chondrite origins, and therefore likely originate from oxidized R chondrites or melts thereof, which reflect variations in nebular composition within the asteroid belt. This is consistent with the meteoritic record in which R chondrites and brachinites are rare relative to pallasites.
    • Petrology of the Miller Range 03346 nakhlite in comparison with the Yamato-000593 nakhlite

      Imae, N.; Ikeda, Y. (The Meteoritical Society, 2007-01-01)
      We petrologically examined the Miller Range (MIL) 03346 nakhlite. The main-phase modal abundances are 67.7 vol% augite, 0.8 vol% olivine, and 31.5 vol% mesostasis. Among all known nakhlites, MIL 03346's modal abundance of olivine is the smallest and of mesostasis is the largest. Augite occurs as cumulus phenocrysts having a homogeneous core composition (En36-38Fs24-22Wo40), which is identical with other nakhlites. They accompany thin ferroan rims divided into inner and outer rims with a compositional gap at the boundary between the two rims. Olivine grains have magnesian cores (Fa is greater than or equal to 55) and show normal zoning toward ferroan rims (Fa is less than or equal to 84). Mesostasis consists mostly of glass (26.0 vol%) with minor skeletal fayalites, skeletal titanomagnetites, acicular phosphate, massive cristobalite, and sulfides. We conclude that MIL 03346 is the most rapidly cooled nakhlite among all known nakhlites based on the petrography.We obtain the intercumulus melt composition for MIL 03346 from the mass balance calculation using the modal abundances and discuss the crystallization sequence of MIL 03346 in comparison with that of Yamato (Y-) 000593. Although magnesian olivines of Y-000593 are phenocrystic, magnesian olivine grains of MIL 03346 seem to have texturally crystallized from the intercumulus melt. After the MIL 03346 magma intruded upward to the Martian surficial zone, the magnesian olivine crystallized, and then the ferroan inner rim formed on phenocrystic core augite. The outer rim of phenocrystic augites formed after the crystallization of skeletal fayalites and skeletal titanomagnetites, resulting in a compositional gap between the inner and outer rims. Finally, glassy mesostasis formed from the residual melt. This crystallization sequence of MIL 03346 is different from those of other nakhlites, including Y-000593.
    • Spectroscopy of synthetic Mg-Fe pyroxenes I: Spin-allowed and spin-forbidden crystal field bands in the visible and near-infrared

      Klima, Rachel L.; Pieters, Carlé M.; Dyar, M. Darby (The Meteoritical Society, 2007-01-01)
      Understanding the fundamental crystal chemical controls on visible and near-infrared reflectance spectra of pyroxenes is critical to quantitatively assessing the mineral chemistry of pyroxenes viewed by remote sensing. This study focuses on the analysis ofspectroscopic measurements of a comprehensive set of synthetic Mg-Fe pyroxenes from the visible through the near-infrared (0.3-2.6 micrometers) to address the constraints of crystal structure and Fe^2+ content on spin-forbidden and spin-allowed crystal field absorptions in Ca-freeorthopyroxenes. The chemistry and oxidation state of the synthetic pyroxenes are characterized. Coordinated Mössbauer spectroscopy is used to determine site occupancy of Fe^2+ in the M1 and M2 crystallographic sites. Properties of visible and near-infrared absorption bands of the synthetic pyroxenes are quantified using the modified Gaussian model. The 1 and 2 m spin-allowed crystal field absorption bands move regularly with increasing iron content, defining a much tighter trend than observed previously. A spin-allowed crystal field absorption band at 1.2 micrometers is explicitly verified, even at low total iron contents, indicating that some portion of Fe^2+ resides in the M1 site. The 1.2 micrometers band intensifies and shifts to longer wavelengths with increasing iron content. At visible wavelengths, spin-forbidden crystal field absorptions are observed in all iron-bearing samples. The most prominent absorption near 506 nm, attributed to iron in the M2 site, shifts to slightly longer wavelengths with iron content. The purity and extent of this pyroxene series allows visible wavelength absorption bands to be directly assigned to specific transitions of Fe^2+ in the M1 and M2 sites.
    • Stardust—An artificial, low-velocity "meteor" fall and recovery: 15 January 2006

      Revelle, D. O.; Edwards, W. N. (The Meteoritical Society, 2007-01-01)
      On January 15, 2006, Stardust, a man-made space capsule, plummeted to Earth for a soft landing after spending seven years in space. Since the expected initial speed of the body was about 12.9 km/s, a four-element ground-based infrasound array was deployed to Wendover, Nevada, USA, to measure the hypersonic booms from the re-entry. At a distance of ~33 km from the nominal trajectory, we easily recorded the weak acoustic arrivals and their continued rumbling after the main hypersonic boom arrival. In this paper, we report on subsequent analyses of these data, including an assessment of the expected entry characteristics (dynamics, energetics, ablation and panchromatic luminosity, etc.) on the basis of a bolide/meteor/fireball entry model that was specifically adapted for modeling a re-entering man-made object.Throughout the infrasonic data analyses, we compared our results for Stardust to those previously obtained for Genesis. From the associated entry parameters, we were also able to compute the kinetic energy density conservation properties for the propagating line source blast wave and compared the inviscid theoretical predictions against observed ground-based infrasound amplitude and wave period data as a function of range. Finally, we made a top-down bottom-up assessment of the line source wave normals propagating downward into the complex temperature/sound speed and horizontal wind speed environment during January 15, 2006. This assessment proved to be generally consistent with the signal processing analysis and with the observed time delay between the known Stardust entry and the time of observations of infrasound signals, and so forth.