1] Shock waves produced by meteoroids are detectable by seismograph networks, but a lack of calibration has limited quantitative analysis of signal amplitudes. We report colocated seismic and infrasound observations from reentry of NASA's Stardust sample return capsule (SSRC) on 15 January 2006. The velocity of the SSRC (initially 12.5 km/s) was the highest ever for an artificial object, lying near the low end of the 11.2-72 km/s range typical of meteoroids. Our infrasonic/seismic recordings contain an initial N wave produced by the hypersonic shock front, followed $10 s later by an enigmatic series of weak, secondary pulses. The seismic signals also include an intervening dispersed wave train with the characteristics of an air-coupled Rayleigh wave. We determine an acoustic-seismic coupling coefficient of 7.3 ± 0.2 mm s À1 /Pa. This represents an energy admittance of 2.13 ± 0.15%, several orders of magnitude larger than previous estimates derived from earthquake or explosive analogs. Theoretical seismic response was computed using in situ V P and V S measurements, together with laboratory density measurements from samples of the clay-rich playa. Treatment of the air-ground interface as an idealized air-solid contact correctly predicts the initial pulse shape but underestimates its seismic amplitude by a factor of $2. Full-wave synthetic seismograms simulate the air-coupled Rayleigh wave and suggest that the secondary arrivals are higher-order Airy phases. Part of the infrasound signal appears to arise from coupling of ground motion into the air, much like earthquake-induced sounds.
Abstract-A new method for assessing the shock stage of enstatite chondrites has been developed, using in situ micro-X-ray diffraction (lXRD) to measure the full width at half maximum (FWHM v ) of peak intensity distributed along the direction of the Debye rings, or chi angle (v), corresponding to individual lattice reflections in two-dimensional XRD patterns. This lXRD technique differs from previous XRD shock characterization methods: it does not require single crystals or powders. In situ lXRD has been applied to polished thin sections and whole-rock meteorite samples. Three frequently observed orthoenstatite reflections were measured: (020), (610), and (131); these were selected as they did not overlap with diffraction lines from other phases. Enstatite chondrites are commonly fine grained, stained or darkened by weathering, shock-induced oxidation, and metal ⁄ sulfide inclusions; furthermore, most E chondrites have little olivine or plagioclase. These characteristics inhibit transmitted-light petrography, nevertheless, shock stages have been assigned MacAlpine Hills (MAC) 02837 (EL3) S3, Pecora Escarpment (PCA) 91020 (EL3) S5, MAC 02747 (EL4) S4, Thiel Mountains (TIL) 91714 (EL5) S2, Allan Hills (ALHA) 81021 (EL6) S2, Elephant Moraine (EET) 87746 (EH3) S3, Meteorite Hills (MET) 00783 (EH4) S4, EET 96135 (EH4-5) S2, Lewis Cliff (LEW) 88180 (EH5) S2, Queen Alexandra Range (QUE) 94204 (EH7) S2, LaPaz Icefield (LAP) 02225 (EH impact melt) S1; for the six with published shock stages, there is agreement with the published classification. FWHM v plotted against petrographic shock stage demonstrates positive linear correlation. FWHM v ranges corresponding to shock stages were assigned as follows: S1 < 0.7°, S2 = 0.7-1.2°, S3 = 1.2-2.3°, S4 = 2.3-3.5°, S5 > 3.5°, S6-not measured. Slabs of Abee (EH impactmelt breccia), and Northwest Africa (NWA) 2212 (EL6) were examined using lXRD alone; FWHM v values place both in the S2 range, consistent with literature values. Micro-XRD analysis may be applicable to other shocked orthopyroxene-bearing rocks.
Abstract-On November 20, 2008, the Buzzard Coulee H4 chondrite fell to Earth outside of Lloydminster, Alberta, Canada. Eighteen fresh samples obtained by the National Meteorite Collection of Canada, ranging from 8.80 to 109.14 g, were investigated in this study. Physical properties of the samples were first obtained using a suite of nondestructive techniques. The bulk density (Archimedean bead method: 3.48 AE 0.04 g cm À3 ; 3-D laser imaging: 3.46 AE 0.03 g cm À3 ; micro-computed tomography: 3.44 AE 0.03 g cm À3 ), porosity (6.2 AE 0.1%), bulk magnetic susceptibility (log v: 5.364 AE 0.056 9 10 À9 m 3 kg À1 at 825 Hz; 5.329 AE 0.052 9 10 À9 m 3 kg À1 at 19,000 Hz), and other derived magnetic properties (frequency dependence: 8.7 AE 6.2%; degree of anisotropy A%: 22.0 AE 2.0%; ellipsoid shape B%: À18.7 AE 8.7%) are typical of H chondrites. The coefficient of variation associated with the properties measured directly was low (0.10-1.15%), indicating that the samples are homogenous at the interfragment scale. The study then proceeded with detailed analyses at the intrafragment scale. Visual inspection of micro-computed tomographic images allowed the identification of an anomalous large clast with low metal content in a fragment. Another fragment exhibited macroscopic shock veins that warranted further examination. These fragments were cut and polished thin sections prepared for petrological analysis by optical and scanning electron microscopy. Based on mineralogical and textural similarities with several chondrules, the large clast was interpreted to be a macrochondrule. In a larger context, this study proposes a protocol for the systematic investigation of extraterrestrial material that can be exported to other new meteorite falls and finds, and specimens from sample return mission.
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