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NASA NTRS2010

Cosmic Ray Positrons from Pulsars

Pulsars are potential Galactic sources of positrons through pair cascades in their magnetospheres. There are, however, many uncertainties in establishing their contribution to the local primary positron flux. Among these are the local density of pulsars, the cascade pair multiplicities that determine the injection rate of positrons from the pulsar, the acceleration of the injected particles by the pulsar wind termination shock, their rate of escape from the pulsar wind nebula, and their propagation through the interstellar medium. I will discuss these issues in the context of what we are learning from the new Fermi pulsar detections and discoveries.

Harding, Alice K.↗

DOE OSTI2018

Cooption of heat shock regulatory system for anhydrobiosis in the sleeping chironomid Polypedilum vanderplanki

Polypedilum vanderplanki is a striking and unique example of an insect that can survive almost complete desiccation. Its genome and a set of dehydration–rehydration transcriptomes, together with the genome of Polypedilum nubifer (a congeneric desiccation-sensitive midge), were recently released. Here, using published and newly generated datasets reflecting detailed transcriptome changes during anhydrobiosis, as well as a developmental series, we show that the TCTAGAA DNA motif, which closely resembles the binding motif of the Drosophila melanogaster heat shock transcription activator (Hsf), is significantly enriched in the promoter regions of desiccation-induced genes in P. vanderplanki, such as genes encoding late embryogenesis abundant (LEA) proteins, thioredoxins, or trehalose metabolism-related genes, but not in P. nubifer. Unlike P. nubifer, P. vanderplanki has double TCTAGAA sites upstream of the Hsf gene itself, which is probably responsible for the stronger activation of Hsf in P. vanderplanki during desiccation compared with P. nubifer. To confirm the role of Hsf in desiccation-induced gene activation, we used the Pv11 cell line, derived from P. vanderplanki embryo. After preincubation with trehalose, Pv11 cells can enter anhydrobiosis and survive desiccation. We showed that Hsf knockdown suppresses trehalose-induced activation of multiple predicted Hsf targets (including P. vanderplanki-specific LEA protein genes) and reduces the desiccation survival rate of Pv11 cells fivefold. Finally, cooption of the heat shock regulatory system has been an important evolutionary mechanism for adaptation to desiccation in P. vanderplanki.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2014

New insights into the physical nature of coronal mass ejections and associated shock waves within the framework of the three-dimensional structure

We present new insights into the physical nature of coronal mass ejections (CMEs) and associated shock waves within the framework of the three-dimensional (3D) structure. We have developed a compound model in order to determine the 3D structure of multiple fronts composing a CME, using data sets taken from STEREO, SDO, and SOHO. We applied the method to time series observations of a CME on 2012 March 7. From the analyses, we revealed that a CME could consist of two different fronts: one is represented well with the ellipsoid model, implying that CMEs are bubble-shaped structures, and the other is reproduced well with the graduated cylindrical shell model, indicating that CMEs are flux rope-shaped structures. The bubble-shaped structure is seen as the outermost front of the CME, and the flux rope-shaped structure is seen as the bright frontal loop or three-part morphology. From our results, we conclude that (1) a CME could consist of two distinct structures, a bubble-shaped structure and a flux rope-shaped structure, (2) the bubble-shaped structure is a fast magnetosonic shock wave, while the flux rope-shaped structure is the mass carried outward by the underlying magnetic structure, (3) the driven shock front could be either a piston-shock type or a bow-shock type, (4) the observed EUV wave in the low corona is the footprint of the bubble-shaped wave, and (5) the halo CME is primarily the projection of the bubble-shaped shock wave but not the underlying flux rope.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

Rail Shock and Vibration Pre-Test Modeling of a Used Nuclear Fuel Assembly

The U.S. Department of Energy Office of Nuclear Energy (DOE-NE), Office of Fuel Cycle Technology, has established the Used Fuel Disposition Campaign (UFDC) to conduct the research and development activities related to storage, transportation, and disposal of used nuclear fuel (UNF) and high-level radioactive waste (HLW). The mission of the UFDC is to identify alternatives and conduct scientific research and technology development to enable storage, transportation and disposal of used nuclear fuel and HLW generated by existing and future nuclear fuel cycles. The Storage and Transportation staff within the UFDC is responsible for addressing issues regarding the long-term or extended storage (ES) of UNF and its subsequent transportation. Available information is not sufficient to determine the ability of ES UNF, including high-burnup fuel, to withstand shock and vibration forces that could occur when the UNF is shipped by rail from nuclear power plant sites to a storage or disposal facility. There are three major gaps in the available information – 1) the forces that UNF assemblies would be subjected to when transported by rail, 2) the mechanical characteristics of fuel rod cladding, which is an essential structure for controlling the geometry of the UNF, a safety related feature, and 3) modeling methodologies to evaluate multiple possible degradation or damage mechanisms over the UNF lifetime. In order to address the first gap, options for tests to determine the physical response of surrogate UNF assemblies subjected to shock and vibration forces that are expected to be experienced during normal conditions of transportation (NCT) by rail must be identified and evaluated. The objective of the rail shock and vibration tests is to obtain data that will help researchers understand the mechanical loads that ES UNF assemblies would be subjected to under normal conditions of transportation and to fortify the computer modeling that will be necessary to evaluate the impact those loads may have on the integrity of the UNF assembly. The shock and vibration testing along with computer modeling is a vital part of research to achieve closure of a gap in information related to the ability of ES UNF to maintain its safety function when subjected to NCT. In support of this effort, preliminary structural dynamics modeling is presented herein. The modeling investigates the rigidity of a hypothetical cask and cradle structure by comparing it to a monolithic concrete mass. The concrete mass represents a practical option for achieving the necessary cask and cradle mass on a flatbed railcar, but this comparative modeling study investigates whether or not the dynamic loads transmitted through a monolithic concrete configuration are adequately representative of a realistic cask and cradle system. This modeling highlights the need for rail testing by reporting the phenomenon of structural transmissibility. As shown herein, this structural transmissibility can cause an amplification of shock and vibration loads through the structure, which could potentially lead to accelerated mechanical degradation of UNF under NCT.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

NASA NTRS1990

Unsteady transonic Navier-Stokes computations for an oscillating wing using single and multiple zones

Modern jet transports and maneuvering tactical fighters operating in the transonic regime often give rise to time-dependent fluid physics that interact with flexible structural components, e.g., vortical flow, shocks, and separation. Efficient computational fluid dynamic (CFD) methods are required to study such computationally intensive problems. In this work a numerical method is presented to address this problem. Time-dependent, compressible, Navier-Stokes equations are used to simulate unsteady transonic flow about a three-dimensional rigid wing undergoing a forced periodic motion in angle of attack. An efficient, implicit, diagonal algorithm is utilized because of its low operation count per time step compared to other methods that solve systems of block matrix equations. The formal time accuracy is addressed theoretically and demonstrated numerically by comparison of computational results with experimental data. A zonal grid approach, capable of treating complex geometries, is presented and its time accuracy is demonstrated by comparing a zonal computation with a single grid computation and experimental data.

Chaderjian, Neal M.↗

NASA NTRS2014

Detection of High Velocity Absorption Components in the He I Lines of Eta Carinae near the Time of Periastron

We have obtained a total of 58 high spectral resolution (R90,000) spectra of the massive binary star eta Carinae since 2012 in an effort to continue our orbital and long-term echelle monitoring of this extreme binary (Richardson et al. 2010, AJ, 139, 1534) with the CHIRON spectrograph on the CTIO 1.5 m telescope (Tokovinin et al. 2013, PASP, 125, 1336) in the 45507500A region. We have increased our monitoring efforts and observation frequency as the periastron event of 2014 has approached. We note that there were multiple epochs this year where we observe unusual absorption components in the P Cygni troughs of the He I triplet lines. In particular, we note high velocity absorption components related to the following epochs for the following lines: He I 4713: HJD 2456754- 2456795 (velocity -450 to -560 kms) He I 5876: HJD 2456791- 2456819 (velocity -690 to -800 kms) He I 7065: HJD 2456791- 2456810 (velocity -665 to -730 kms) Figures: Note that red indicates a high-velocity component noted above. He I 4713: http:www.astro.umontreal.carichardson4713.png He I 5876: http:www.astro.umontreal.carichardson5876.png He I 7065: http:www.astro.umontreal.carichardson7065.png These absorptions are likely related to the wind-wind collision region and bow shock, as suggested by the high-velocity absorption observed by Groh et al. (2010, AA, 519, 9) in the He I 10830 Atransition. In these cases, we suspect that we look along an arm of the shock cone and that we will see a fast absorption change from the other collision region shortly after periastron. We suspect that this is related to the multiple-components of the He II 4686 line that was noted by Walter (ATel6334), and is confirmed in our data. Further, high spectral resolution data are highly encouraged,especially for resolving powers greater than 50,000.These observations were obtained with the CTIO 1.5 m telescope, operated by the SMARTS Consortium, and were obtained through both SMARTS and NOAO programs 2012A-0216,2012B-0194, and 2013b-0328). We thank Emily MacPherson (Yale) for her efforts in scheduling the observations that we have and will obtain in the coming weeks.

binary star eta Carinae↗

DOE OSTI2015

Fast magnetic reconnection supported by sporadic small-scale Petschek-type shocks

Standard magnetohydrodynamic (MHD) theory predicts reconnection rate that is far too slow to account for a wide variety of reconnection events observed in space and laboratory plasmas. Therefore, it was commonly accepted that some non-MHD (kinetic) effects play a crucial role in fast reconnection. A recently renewed interest in simple MHD models is associated with the so-called plasmoid instability of reconnecting current sheets. Although it is now evident that this effect can significantly enhance the rate of reconnection, many details of the underlying multiple-plasmoid process still remain controversial. Here, we report results of a high-resolution computer simulation which demonstrate that fast albeit intermittent magnetic reconnection is sustained by numerous small-scale Petschek-type shocks spontaneously formed in the current sheet due to its plasmoid instability.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2016

Multi-wavelength study of the supernova remnant Kes 79 (G33.6+0.1): on its supernova properties and expansion into a molecular environment

Kes 79 (G33.6+0.1) is an aspherical thermal composite supernova remnant (SNR) observed across the electromagnetic spectrum and showing an unusual highly structured morphology, in addition to harboring a central compact object (CCO). Using the CO J = 1–0, J = 2–1, and J = 3–2 data, we provide the first direct evidence and new morphological evidence to support the physical interaction between the SNR and the molecular cloud in the local standard of rest velocity ∼105 km s{sup −1}. We revisit the 380 ks XMM-Newton observations and perform a dedicated spatially resolved X-ray spectroscopic study with careful background subtraction. The overall X-ray-emitting gas is characterized by an under-ionized (τ{sub c}∼6×10{sup 11} cm{sup −3}) cool (kT{sub c}≈0.20 keV) plasma with solar abundances, plus an under-ionized (τ{sub h}∼8×10{sup 10} cm{sup −3}) hot (kT{sub h}≈0.80 keV) plasma with elevated Ne, Mg, Si, S, and Ar abundances. The X-ray filaments, spatially correlated with the 24 μm IR filaments, are suggested to be due to the SNR shock interaction with dense gas, while the halo forms from SNR breaking out into a tenuous medium. Kes 79 appears to have a double-hemisphere morphology viewed along the symmetric axis. Projection effect can explain the multiple-shell structures and the thermal composite morphology. The high-velocity, hot (kT{sub h}∼1.4--1.6 keV) ejecta patch with high metal abundances, together with the non-uniform metal distribution across the SNR, indicate an asymmetric SN explosion of Kes 79. We refine the Sedov age to 4.4–6.7 kyr and the mean shock velocity to 730 km s{sup −1}. Our multi-wavelength study suggests a progenitor mass of ∼15–20 solar masses for the core-collapse explosion that formed Kes 79 and its CCO, PSR J1852+0040.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

c-KIT signaling is targeted by pathogenic Yersinia to suppress the host immune response

Background: The pathogenic Yersinia species exhibit a primarily extracellular lifestyle through manipulation of host signaling pathways that regulate pro-inflammatory gene expression and cytokine release. To identify host genes that are targeted by Yersinia during the infection process, we performed an RNA interference (RNAi) screen based on recovery of host NF-κB-mediated gene activation in response to TNF-α stimulation upon Y. enterocolitica infection. Results: We screened shRNAs against 782 genes in the human kinome and 26 heat shock genes, and identified 19 genes that exhibited ≥40% relative increase in NF-κB reporter gene activity. The identified genes function in multiple cellular processes including MAP and ERK signaling pathways, ion channel activity, and regulation of cell growth. Pre-treatment with small molecule inhibitors specific for the screen hits c-KIT and CKII recovered NF-κB gene activation and/or pro-inflammatory TNF-α cytokine release in multiple cell types, in response to either Y. enterocolitica or Y. pestis infection. Conclusions: We demonstrate that pathogenic Yersinia exploits c-KIT signaling in a T3SS-dependent manner to downregulate expression of transcription factors EGR1 and RelA/p65, and pro-inflammatory cytokines. This study is the first major functional genomics RNAi screen to elucidate virulence mechanisms of a pathogen that is primarily dependent on extracellular-directed immunomodulation of host signaling pathways for suppression of host immunity.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2025

Hydrogen Transportation and Distributed Energy Systems Seismic Risk Assessment for Cascadia Subduction Zone Airport Facilities

Portland International Airport in Oregon is exploring operating a fleet of 28 fuel cell electric buses to support airport operations and possibly provide backup power during outages. In this report, we evaluate the risk of hydrogen deployment at an airport considering the potential for seismic activity. We present a seismic risk assessment for pressurized piping for a generic hydrogen refueling station at the airport. Only pressurized piping was considered in the risk analysis because of its vulnerability and availability of fragility information unlike the other components. Seismic capacity of other components should be included as more information becomes available. To characterize the seismic hazard, we considered the initiating event frequency of a Cascadia Subduction Zone earthquake event based on data from the literature. Seismic stressors that translate to the site from an earthquake event are expressed as peak ground acceleration. Site disturbance is characterized as a function of soil conditions, represented by different shear wave velocity values, which affect how seismic waves propagate and impact structures. Pipe fragility curves as a function of seismic capacity were used to correlate ground acceleration and failure probability. These fragility characteristics were combined with site-specific soil conditions to calculate the probability of pipe rupture during a seismic event. It should be noted that multiple simultaneous failures due to an earthquake event as a common cause are not considered. Some epistemic uncertainties such as self-exciting shocks, aftershocks, and the time variant nature of the ground motion are not considered either.

08 HYDROGEN↗

NASA NTRS1974

Lightning damage to a general aviation aircraft: Description and analysis

The damage sustained by a Beechcraft King Air Model B90 aircraft by a single lightning discharge is presented and analyzed. The incident occurred during landing approach at Jackson, Michigan, on Feb. 19, 1971. In addition to the usual melted-metal damage at the lightning attachment points, there was severe implosion-type damage over a large area on the lower right side of the aircraft and impact- and crushing-type damage on the upper and lower surfaces on the left wingtip near the trailing edge. Analyses indicate that the implosion-type damage was probably caused by lightning-generated shock waves, that the impact-and crushing-type damage was caused by magnetic forces, and that the lightning discharge was a multiple strike with at least 11 strokes separated in time by about 4.5 milliseconds. The evidence indicates that the lightning discharge was rather different from the average in character severity.

Hacker, P. T.↗

NASA NTRS1976

Nonlinear periodic waves

Systematic perturbation procedures for the analysis of nonlinear problems are reviewed. The cases when the multiplicity of an eigenvalue is finite or infinite are treated for self-sustained and forced oscillations. The possibility of the formation of shock waves is discussed. Applications to acoustic problems are presented.

Ting, L.↗

NASA NTRS1992

Outer heliospheric radio emissions. II - Foreshock source models

Observations of LF radio emissions in the range 2-3 kHz by the Voyager spacecraft during the intervals 1983-1987 and 1989 to the present while at heliocentric distances greater than 11 AU are reported. New analyses of the wave data are presented, and the characteristics of the radiation are reviewed and discussed. Two classes of events are distinguished: transient events with varying starting frequencies that drift upward in frequency and a relatively continuous component that remains near 2 kHz. Evidence for multiple transient sources and for extension of the 2-kHz component above the 2.4-kHz interference signal is presented. The transient emissions are interpreted in terms of radiation generated at multiples of the plasma frequency when solar wind density enhancements enter one or more regions of a foreshock sunward of the inner heliospheric shock. Solar wind density enhancements by factors of 4-10 are observed. Propagation effects, the number of radiation sources, and the time variability, frequency drift, and varying starting frequencies of the transient events are discussed in terms of foreshock sources.

Cairns, Iver H.↗

NASA NTRS2019

The Orbital Variation of Non-Thermal X-Ray Emission from eta Carinae

The collision of strong stellar winds in massive binary systems creates powerful shocks,which accelerates a small amount of particles to relativistic energies at the shock interface.This process is important in fundamental astrophysics in two ways;i) some particles may contribute to cosmic-rays observed around the Earth, whose origin is not well known, andii) it provides a good laboratory for particle acceleration physics as the shock occurs steadily in a predictable environment.Some relativistic particles collide with stellar photons or ambient material near the binary systemand emit non-thermal X-rays and gamma-rays, which are good probes of particle acceleration.This emission had been searched for decades but not found convincingly without sensitive high-energy telescopes.The NuSTAR telescope observed the enigmatic supermassive binary system eta Carinae multiple times after 2014 and found conclusive evidence of non-thermal emission from the star in the extremely hard X-ray band.This emission is prominent between 20-50 keV, below which thermal emission from shock colliding plasma dominates.It is relatively stable throughout the binary orbit, but it disappears near periastron when the wind colliding activity shuts off.This variation indicates that the non-thermal X-ray emission originates from the head-on wind-wind collision.The flat spectrum is consistent with inverse-Compton of stellar UV photons by accelerated electrons.The spectrum smoothly connects to a gamma-ray spectrum of a Fermi source detected around eta Carinae,suggesting that acceleration occurs up to the GeV energy.This result provide the strongest evidence so far that particle acceleration occurs at the wind colliding shock of a massive colliding wind binary system.

Hamaguchi, Kenji↗

DOE OSTI2012

THE FERMI BUBBLE AS A SOURCE OF COSMIC RAYS IN THE ENERGY RANGE >10{sup 15} eV

The Fermi Large Area Telescope has recently discovered two giant gamma-ray bubbles that extend north and south of the Galactic center with diameters and heights of the order of H {approx} 10 kpc. We suggest that the periodic star capture processes by the Galactic supermassive black hole Sgr A*, with a capture rate of {tau}{sup -1}{sub cap} {approx} 3 Multiplication-Sign 10{sup -5} yr{sup -1} and an energy release of W {approx} 3 Multiplication-Sign 10{sup 52} erg per capture, can result in hot plasma injecting into the Galactic halo at a wind velocity of u {approx} 10{sup 8} cm s{sup -1}. The periodic injection of hot plasma can produce a series of shocks. Energetic protons in the bubble are re-accelerated when they interact with these shocks. We show that for energy larger than E > 10{sup 15} eV, the acceleration process can be better described by the stochastic second-order Fermi acceleration. We propose that hadronic cosmic rays (CRs) within the 'knee' of the observed CR spectrum are produced by Galactic supernova remnants distributed in the Galactic disk. Re-acceleration of these particles in the Fermi Bubble produces CRs beyond the knee. With a mean CR diffusion coefficient in this energy range in the bubble D{sub B} {approx} 3 Multiplication-Sign 10{sup 30} cm{sup 2} s{sup -1}, we can reproduce the spectral index of the spectrum beyond the knee and within it. The conversion efficiency from shock energy of the bubble into CR energy is about 10%. This model provides a natural explanation of the observed CR flux, spectral indices, and matching of spectra at the knee.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2001

Gaze Stabilization During Locomotion Requires Full Body Coordination

Maintaining gaze stabilization during locomotion places substantial demands on multiple sensorimotor subsystems for precise coordination. Gaze stabilization during locomotion requires eye-head-trunk coordination (Bloomberg, et al., 1997) as well as the regulation of energy flow or shock-wave transmission through the body at high impact phases with the support surface (McDonald, et al., 1997). Allowing these excessive transmissions of energy to reach the head may compromise gaze stability. Impairments in these mechanisms may lead to the oscillopsia and decreased dynamic visual acuity seen in crewmembers returning from short and long duration spaceflight, as well as in patients with vestibular disorders (Hillman, et al., 1999). Thus, we hypothesize that stabilized gaze during locomotion results from full-body coordination of the eye-head-trunk system combined with the lower limb apparatus. The goal of this study was to determine how multiple, interdependent full- body sensorimotor subsystems aiding gaze stabilization during locomotion are functionally coordinated, and how they adaptively respond to spaceffight.

Mulavara, A. P.↗

DOE OSTI2013

RADIO MONITORING OF THE TIDAL DISRUPTION EVENT SWIFT J164449.3+573451. II. THE RELATIVISTIC JET SHUTS OFF AND A TRANSITION TO FORWARD SHOCK X-RAY/RADIO EMISSION

We present continued multi-frequency radio observations of the relativistic tidal disruption event Swift J164449.3+573451 (Sw 1644+57) extending to t Almost-Equal-To 600 days. The data were obtained with the JVLA and AMI Large Array as part of our on-going study of the jet energetics and the density structure of the parsec-scale environment around the disrupting supermassive black hole. We combine these data with public Swift/XRT and Chandra X-ray observations over the same time-frame to show that the jet has undergone a dramatic transition starting at Almost-Equal-To 500 days, with a sharp decline in the X-ray flux by about a factor of 170 on a timescale of {delta}t/t {approx}< 0.2 (and by a factor of 15 in {delta}t/t Almost-Equal-To 0.05). The rapid decline rules out a forward shock origin (direct or reprocessing) for the X-ray emission at {approx}< 500 days, and instead points to internal dissipation in the inner jet. On the other hand, our radio data uniquely demonstrate that the low X-ray flux measured by Chandra at Almost-Equal-To 610 days is consistent with emission from the forward shock. Furthermore, the Chandra data are inconsistent with thermal emission from the accretion disk itself since the expected temperature of {approx}30-60 eV and inner radius of {approx}2-10 R{sub s} cannot accommodate the observed flux level or the detected emission at {approx}> 1 keV. We associate the rapid decline with a turn off of the relativistic jet when the mass accretion rate dropped below {approx} M-dot{sub Edd}{approx}0.006 M{sub Sun} yr{sup -1} (for a 3 Multiplication-Sign 10{sup 6} M{sub Sun} black hole and order unity efficiency) indicating that the peak accretion rate was about 330 M-dot{sub Edd}, and the total accreted mass by t Almost-Equal-To 500 days is about 0.15 M{sub Sun }. From the radio data we further find significant flattening in the integrated energy of the forward shock at t {approx}> 250 days with E{sub j,iso} Almost-Equal-To 2 Multiplication-Sign 10{sup 54} erg (E{sub j} Almost-Equal-To 10{sup 52} erg for a jet opening angle, {theta}{sub j} = 0.1) following a rise by about a factor of 15 at Almost-Equal-To 30-250 days. Projecting forward, we predict that the emission in the radio and X-ray bands will evolve in tandem with similar decline rates.

36 MATERIALS SCIENCE↗

NASA NTRS2014

Ar-Ar Analysis of Chelyabinsk: Evidence for a Recent Impact

The Chelyabinsk meteorite is an LL5 ordinary chondrite that fell as a spectacular fireball on February 15th, 2013, over the Ural region in Russia. The meteoroid exploded at an altitude of 25-30 km, producing shockwaves that broke windowpanes in Chelyabinsk and surrounding areas, injuring some 1500 people. Analyses of the samples show that the meteorite underwent moderate shock metamorphism (stage S4; 25-35 GPa) [1]. Most of the samples have a fusion crust ranging from ~0.1-1mm thick, and roughly a third of the samples were composed of a dark fine-grained impact melt with chondrule fragments which were targeted for chronometry. A Pb-Pb age obtained by [2] of a shock-darkened and potentially melted sample of Chelyabinsk is reported as 4538.3 +/- 2.1 Ma, while a U-Pb study [3] gave an upper concordia intercept of 4454 +/- 67 Ma and a lower intercept of 585 +/- 390. Galimov et al. 2013 [1] suggest the Sm-Nd system records a recent impact event [~290 Ma] that may represent separation from the parent body, while the Rb-Sr isotopic system is disturbed and does not give any definitive isochron. In order to better understand its history, we have performed 40Ar-39Ar analysis on multiple splits of two Chelyabinsk samples; clast- rich MB020f,2 and melt-rich MB020f,5. The term "clast-rich" lithology is meant to indicate a mechanical mixture of highly shock-darkened and less shocked components, both with some shock melt veining.

Beard, S. P.↗