Philip John Albert McCausland

Calibrating infrasonic to seismic coupling using the Stardust sample return capsule shockwave: Implications for seismic observations of meteors

[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 m ms � 1 /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 VP and VS 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.

Mineralogical and spectroscopic investigation of enstatite chondrites by X‐ray diffraction and infrared reflectance spectroscopy

[1] The mineralogy and infrared reflectance spectra of 13 Enstatite (E) chondrite meteorite finds spanning the full range of textural alteration grades in both EL and EH classes have been investigated. Rietveld refinement of high-resolution powder X-ray diffraction (XRD) data was used to determine quantitative major mineral abundances. Sample-correlated mid-infrared (2.0 to 25.0 μm; 4500 cm -1 to 400 cm -1 ) reflectance infrared spectra were collected for each meteorite. Spectral features due to the fundamental lattice vibrations of the silicates, primarily enstatite, dominate the spectra of these meteorites over most of the spectral range investigated. The spectral features related to primary (i.e., pre-terrestrial) mineralogy include fundamental stretching and bending lattice modes (-8.3-25.0 μm; 1200-400 cm -1 ), overtones and combinations of the fundamental modes (∼4.5-6.1 μm; 2200-1650 cm -1 ), and the principle Christensen feature (-8.3 μm; 1200 cm -1 ). Terrestrial weathering products including Fe-oxyhydroxides, gypsum, and carbonates occur in most of these meteorites and contribute to some spectral features: particularly an asymmetric feature near ∼2.6 to 3.8 μm (3800 to 2600 cm -1 ) attributed to adsorbed, hydrogen-bonded, and/or structural OH and H 2 O, and a feature near -6.2 μm (1625 cm - 1 ) attributed to adsorbed, hydrogen-bonded, and/or structural H 2 O. Modal mineral abundances determined by Rietveld refinement have been used to calculate model grain densities for each meteorite. Bulk magnetic susceptibility measurements combined with modal mineralogy and grain densities reveal a trend toward lower grain density and lower bulk susceptibility with increased terrestrial weathering.