METEORITICS& PLANETARY SCIENCE
Volume 39, Issue 3    
March 2004

ABSTRACTS
Please, contact the corresponding author for reprints of all published articles. © Meteoritical Society, 2004

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Dynamical and compositional assessment of near-Earth object mission targets
Richard P. BINZEL,* Ettore PEROZZI, Andrew S. RIVKIN, Alessandro ROSSI, Alan W. HARRIS, Schelte J. BUS, Giovanni B. VALSECCHI, Stephen M. SLIVAN
*Corresponding author. Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. E-mail: rpb@mit.edu

Using an H-plot analysis, we identify 234 currently known near-Earth objects that are accessible for rendezvous with a “best case” delta-V of less than 7 km/s. We provide a preliminary compositional interpretation and assessment of these potential targets by summarizing the taxonomic properties for 44 objects. Results for one-half (22) of this sample are based on new spectroscopic measurements presented here. Our approach provides an easy-to-update method for giving guidelines to both observers and mission analysts for focusing on objects for which actual mission opportunities are most likely to be found. Observing prospects are presented for categorizing the taxonomic properties of the most accessible targets that are not yet measured.

Simulation of the interaction of galactic cosmic ray protons with meteoroids: On the production of ³H and light noble gas isotopes in isotropically irradiated thick gabbro and iron targets
I. LEYA,* F. BEGEMANN, H. W. WEBER, R. WIELER, and R. MICHEL
*Corresponding author. Center for Radiation Protection and Radioecology, University of Hannover, Hannover, Germany and Isotope Geochemistry and Mineral Resources, NO C61, Zürich, Switzerland. E-mail: Leya@erdw.ethz.ch

Thick spherical targets, one made of gabbro (R = 25 cm) and one made of iron (R = 10 cm), were irradiated isotropically with 1.6 GeV protons at Laboratoire National Saturne (LNS)/Saclay to simulate the interactions of galactic cosmic ray protons with meteoroids in space. At various depths,
both artificial meteoroids contained a large number of high-purity, single-element target foils and chemical compounds of up to 28 target elements. In these individual target foils, the elemental production rates of radionuclides and noble gas isotopes were measured. Here, we report the results
for the light noble gas isotopes ^{3, 4}He, ^{20, 21, 22}Ne, and ^{36, 38, 39}Ar for the most cosmochemically relevant target elements as well as for some meteoritic material from Jilin, Farmington, and Cape York. From ³He analyses done several years apart, ³H diffusive losses during sample storage have been obtained, and direct as well as cumulative ³He production rates for O, Mg, Al, Si, Fe, Ni, and the meteoritic material are given. Losses by diffusion of tritium from metallic Mg and Fe are found to occur on time scales of months, while metallic Al, Si, and stone meteorites are much more retentive. The production rate ratios P(³H)/P(³He)d obtained in the simulation experiments are 0.73, 1.28, and 1.16 for O, Al, and Si, respectively. These rates are based on our best knowledge about the ³H and ³He production rates and should, therefore, replace data published earlier (Leya et al. 2000a). The earlier calculations for ⁴He, ^{20, 21, 22}Ne, and ^{36, 38, 39}Ar remain valid. The new modeled correlation ³Hecum/²¹Ne versus ²²Ne/²¹Ne for chondrites exposed to cosmic rays with an energy spectrum characterized by a modulation parameter of Φ= 650 MeV is in fair agreement with the empirical relationship (“Berne plot”). However, for small meteorites and little shielding in larger ones, there are systematic differences that most likely are due to an underestimation of the spallogenic ²²Ne/²¹Ne ratio by ~2%.

Devgaon (H3) chondrite: Classification and complex cosmic ray exposure history
S. V. S. MURTY,* V. K. RAI, A. D. SHUKLA, G. SRINIVASAN, P. N. SHUKLA, K. M. SUTHAR, N. BHANDARI, and A. BISCHOFF
*Corresponding author. Planetary and Geosciences Division, Physical Research Laboratory, Ahmedabad 380009, India. E-mail: murty@prl.ernet.in

The Devgaon meteorite fell in India on February 12, 2001 and was immediately collected. It is an ordinary chondrite having a number of SiO₂-rich objects and some Ca, Al-rich inclusions. Olivines (Fa_17–19) are fairly equilibrated, while pyroxenes (Fs_4–20) are unequilibrated. Occasionally,
shock veins are visible, but the bulk rock sample is very weakly shocked (S2). Chondrules and chondrule fragments are abundant. Based on chemical and petrological features, Devgaon is classified as an H3.8 group chondrite. Several cosmogenic radionuclides ranging in half-lives from
5.6 d (⁵²Mn) to 7.3 × 10⁵ yr (²⁶Al), noble gases (He, Ne, Ar, Kr, and Xe), and particle track density have been measured. The track density in olivines from five spot samples varies between (4.6 to 9) × 10⁶ cm^–2 showing a small gradient within the meteorite. The light noble gases are dominated by cosmogenic and radiogenic components. Large amounts of trapped gases (Ar, Kr, and Xe) are present. In addition, (n, γ) products from Br and I are found in Kr and Xe, respectively. The average cosmic ray exposure age of 101 ± 8 Ma is derived based on cosmogenic ³⁸Ar, 83Kr, and 126Xe. The track production rates correspond to shielding depths of about 4.9 to 7.8 cm, indicating that the stone suffered type IV ablation. Low ⁶⁰Co, high (²²Ne/²¹Ne)_c, and large neutron produced excesses at ⁸⁰Kr, ⁸²Kr, and ¹²⁸Xe indicate a complex exposure history of the meteoroid. In the first stage, a meter-sized body was exposed for nearly 10⁸ yr in the interplanetary space that broke up in ~50 cm-sized fragments about a million years ago (stage 2), before it was captured by the Earth.

A chemical sequence of macromolecular organic matter in the CM chondrites
Hiroshi NARAOKA,* Hajime MITA, Masatoshi KOMIYA, Shigekazu YONEDA, Hideyasu KOJIMA, and Akira SHIMOYAMA
*Corresponding author. Department of Chemistry, Tokyo Metropolitan University, 1–1 Minami-Ohsawa, Hachioji, Tokyo 192–0397, and Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3–1–1 Yoshinodai, Sagamihara, Kanagawa 229–8510, Japan. E-mail: naraoka-hiroshi@c.metro-u.ac.jp

A new organic parameter is proposed to show a chemical sequence of organic matter in carbonaceous chondrites, using carbon, hydrogen, and nitrogen concentrations of solvent-insoluble and high-molecular weight organic matter (macromolecules) and the molecular abundance of solvent-extractable organic compounds. The H/C atomic ratio of the macromolecule purified from nine CM chondrites including the Murchison, Sayama, and seven Antarctic meteorites varies widely from 0.11 to 0.72. During the H/C change of ~0.7 to ~0.3, the N/C atomic ratio remains at ~0.04,
followed by a sharp decline from ~0.040 to ~0.017 between H/C ratios from ~0.3 to ~0.1. The H/CN/C sequence shows different degrees of organic matter thermal alteration among these chondrites, in which the smaller H/C-N/C value implies higher alteration levels on the meteorite parent body. In addition, solvent-extractable organic compounds such as amino acids, carboxylic acids, and polycyclic aromatic hydrocarbons are abundant only in chondrites with macromolecular H/C values >~0.5. These organic compounds were extremely depleted in the chondrites with a macromolecular H/C value of <~0.5. Possibly, most solvent-extractable organic compounds could have been lost during the thermal alteration event that caused the H/C ratio of the macromolecule to fall below 0.4.

Radar observations of asteroid 25143 Itokawa (1998 SF36)
Steven J. OSTRO,* Lance A. M. BENNER, Michael C. NOLAN, Christopher MAGRI, Jon D. GIORGINI, Daniel J. SCHEERES, Stephen B. BROSCHART, Mikko KAASALAINEN, David VOKROUHLICKÝ, Steven R. CHESLEY, Jean-Luc MARGOT, Raymond F. JURGENS,
Randy ROSE, Donald K. YEOMANS, Shigeru SUZUKI, and Eric M. DE JONG
*Corresponding author. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109–8099, USA. E-mail: ostro@reason.jpl.nasa.gov

We observed 25143 Itokawa, the target of Japan’s Hayabusa (MUSES-C) sample-return mission, during its 2001 close approach at Arecibo on twelve dates during March 18–April 9 and at Goldstone on nine dates during March 20–April 2. We obtained delay-Doppler images with range
resolutions of 100 ns (15 m) at Arecibo and 125 ns (19 m) at Goldstone. Itokawa’s average circular polarization ratio at 13 cm, 0.26 ± 0.04, is comparable to that of Eros, so its cm-to-m surface roughness probably is comparable to that on Eros. Itokawa’s radar reflectivity and polarization
properties indicate a near-surface bulk density within 20% of 2.5 g cm^–3. We present a preliminary estimate of Itokawa’s shape, reconstructed from images with rather limited rotation-phase coverage, using the method of Hudson (1993) and assuming the lightcurve-derived spin period (12.132 hr) and pole direction (ecliptic long., lat. = 355°, –84°) of Kaasalainen et al. (2003). The model can be described as a slightly asymmetrical, slightly flattened ellipsoid with extents along its principal axes of 548 × 312 × 276 m ± 10%. Itokawa’s topography is very subdued compared to that of other asteroids, for which spacecraft images or radar reconstructions are available. Similarly, gravitational slopes on our Itokawa model average only 9° and everywhere are less than 27°. The radar-refined orbit allows accurate identification of Itokawa’s close planetary approaches through 2170. If radar ranging planned for Itokawa’s 2004 apparition succeeds, then tracking of Hayabusa during its 2005 rendezvous should reveal Yarkovsky perturbation of the asteroid’s orbit.

Geology, petrography, shock petrography, and geochemistry of impactites and target rocks from the Kärdla crater, Estonia
Puura V.,* Huber H., Kirs J., Kärki A., Suuroja K., Kirsimäe K., Kivisilla J., Kleesment A., Konsa M., Preeden U., Suuroja S., and Koeberl C.
*Corresponding author. Institute of Geology, University of Tartu, Vanemuise str. 46, 51014 Tartu, Estonia. E-mail: puura@math.ut.ee

The Kärdla crater is a 4-km-wide impact structure of Late Ordovician age located on Hiiumaa Island, Estonia. The 455-Ma-old buried crater was formed in shallow seawater in Precambrian crystalline target rocks that were covered with sedimentary rocks. Basement and breccia samples from thirteen drill cores were studied mineralogically, petrographically, and geochemically. Geochemical analyses of major and trace elements were performed on 90 samples from allochthonous breccias, sub-crater and surrounding basement rocks. The breccia units do not include any melt rocks or suevites. The remarkably poorly mixed sedimentary and crystalline rocks were deposited separately within the allochthonous breccia suites of the crater. The most intensely shock-metamorphosed allochthonous granitoid crystalline-derived breccia layers contain planar deformation features (PDFs) in quartz, indicating shock pressures of 20-35 GPa. An apparent K-enrichment and Ca-Na-depletion of feldspar- and hornblende-bearing rocks in the allochthonous breccia units and sub-crater basement is interpreted to be the result of early stage alteration in an impact-induced hydrothermal system. The chemical composition of the breccias shows no definite sign of an extraterrestrial contamination. By modeling of the different breccia units with HMX-mixing, the indigenous component was determined. From the abundances of the siderophile elements (Cr, Co, Ni, Ir, and Au) in the breccia samples no unambiguous evidence for the incorporation of a meteoritic component above about 0.1 wt% chondrite-equivalent was found.

Production rates and proton-induced production cross sections of ¹²⁹I from Te and Ba: An attempt to model the ¹²⁹I production in stony meteoroids and ¹²⁹I in a Knyahinya sample
C. SCHNABEL,* I. LEYA, M. GLORIS, R. MICHEL, J. M. LOPEZ-GUTIÉRREZ, U. KRÄHENBÜHL, U. HERPERS, J. KUHNHENN, and H. A. SYNAL
*Corresponding author. Zentrum für Strahlenschutz und Radioökologie (ZSR), Universität Hannover, Am Kleinen Felde 30, D-30167 Hannover, Germany; Institut für Teilchenphysik, ETH Hönggerberg, CH-8093 Zürich, Switzerland; Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland, Scottish Universities Environmental Research Center, Scottish Enterprise Technology Park, East Kilbride G75 0QF, UK. E-mail: c.schnabel@suerc.gla.ac.uk

Proton-induced production cross sections of ¹²⁹I from Te and Ba are presented. Earlier assumptions that Te is the most important target element in meteoroids are confirmed. Based on this data set and the experimental production rates of ¹²⁹I from thick-target experiments, the production of
¹²⁹I in stony meteoroids is modeled using a GCR flux density of 4.06 cm^–2 s^–1. The results of this modeling must be considered preliminary because the contribution from neutron capture on ¹²⁸Te needs further investigation. We obtained modeled production rates that agree with experimental results for samples from two medium-sized meteorites (Abee and Knyahinya). However, we find that this model does not describe ⁴¹Ca in lunar rocks well and seems to overestimate ¹²⁹I production in larger bodies, such as Allende. We present elemental production rates from Te and Ba based on our modeling as well as for a model that describes neutron capture products. For ¹²⁹I analysis of Knyahinya, a novel method to separate Te and analysis using ICP-MS was used, enabling the use of experimental elemental concentrations obtained from the same meteorite to calculate ¹²⁹I production.

Ejecta formation and crater development of the Mjølnir impact
Valery SHUVALOV* and Henning DYPVIK
*Corresponding author. Institute of Geosphere Dynamics, Russian Academy of Sciences, 38 Leninsky Prospect, Building 1, 119334, Moscow, Russia. E-mail: shuvalov@idg.chph.ras.ru

Crater-ejecta correlation is an important element in the analysis of crater formation and its influence on the geological evolution. In this study, both the ejecta distribution and the internal crater development of the Jurassic/Cretaceous Mjølnir crater (40 km in diameter; located in the Barents Sea)
are investigated through numerical simulations. The simulations show a highly asymmetrical ejecta distribution, and underscore the importance of a layer of surface water in ejecta distribution. As expected, the ejecta asymmetry increases as the angle of impact decreases. The simulation also
displays an uneven aerial distribution of ejecta. The generation of the central high is a crucial part of crater formation. In this study, peak generation is shown to have a skewed development, from approximately 50–90 sec after impact, when the peak reaches its maximum height of 1–1.5 km. During this stage, the peak crest is moved about 5 km from an uprange to a downrange position, ending with a final central position which has
a symmetrical appearance that contrasts with its asymmetrical development.

Exposure history and terrestrial ages of ordinary chondrites from the Dar Al Gani region, Libya
K. C. WELTEN*, K. NISHIIZUMI, R. C. FINKEL, D. J. HILLEGONDS, A. J. T. JULL, L. FRANKE, and L. SCHULTZ
*Corresponding author. Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA. E-mail: kcwelten@uclink4.berkeley.edu

We measured the concentrations of noble gases in 32 ordinary chondrites from the Dar al Gani (DaG) region, Libya, as well as concentrations of the cosmogenic radionuclides ¹⁴C, ¹⁰Be, ²⁶Al, ³⁶Cl and ⁴¹Ca in 18 of these samples. Although the trapped noble gases in five DaG sam-ples show ratios typical of solar or planetary gases, in all other DaG samples they are dominated by atmospheric contamination, which increases with the degree of weathering. Cosmic-ray ex-posure (CRE) ages of DaG chondrites range from ~1 Myr to 53 Myr. The CRE age distribution of 10 DaG L chondrites shows a cluster around 40 Myr due to four members of a large L6 chon-drite shower. The CRE age distribution of 19 DaG H-chondrites shows only three ages coincid-ing with the main H-chondrite peak at ~7 Myr, whereas seven ages are <5 Myr. Two of these H-chondrites with short CRE ages (DaG 904 & 908) show evidence of a complex exposure history. Five of the H-chondrites show evidence of high shielding conditions, including low ²²Ne/²¹Ne ratios and large contributions of neutron-capture ³⁶Cl and ⁴¹Ca. These samples represent frag-ments of two or more large pre-atmospheric objects, which supports the hypothesis that the high H/L-chondrite ratio at DaG is due to one or more large unrecognized showers.
The ¹⁴C concentrations correspond to terrestrial ages <35 kyr, similar to terrestrial ages of chondrites from other regions in the Sahara, but younger than two DaG achondrites. Despite the loss of cosmogenic ³⁶Cl and ⁴¹Ca during oxidation of metal and troilite, concentrations of ³⁶Cl and ⁴¹Ca in the silicates are also consistent with ¹⁴C ages <35 kyr. The only exception is DaG 343 (H4) which has a ⁴¹Ca terrestrial age of 150 ± 40 kyr. This old age shows that not only iron meteorites and achondrites but also chondrites can survive the hot desert environment for more than 50 kyr. A possible explanation is that older meteorites were covered by soils during wetter periods, and were recently exhumed by removal of these soils due to deflation during more arid periods, such as the current one, which started ~3000 years ago.
Finally, based on the ²⁶Al/²¹Ne and ¹⁰Be/²¹Ne systematics in 16 DaG meteorites, we de-rived more reliable estimates of the ¹⁰Be/²¹Ne production rate ratio, which seems more sensitive to shielding than predicted by the semi-empirical model of Graf et al. (1990), but less sensitive than predicted by the purely physical model of Leya et al. (2000).

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