RESUMO
Within this work, we present the first true three-dimensional (3D) analysis of chondrule size. Knowledge about the physical properties of chondrules is important for validating astrophysical theories concerning chondrule formation and their aggregation into the chondritic meteorites (known as chondrites) that contain them. The classification of chondrites into chemical groups also relies on chondrule properties, including their dimensions. Within this work, we quantify the diameters of chondrules in five ordinary chondrites (OCs; comprised of the H, L, and LL chondrites) and one low-iron enstatite (EL) chondrite. To extract the chondrule size data, we use x-ray computed microtomography to image small (~1-2 cm3 ) chondrite samples followed by manual digital segmentation to isolate chondrules within the volumes or subvolumes. Our data yield true 3D results without stereographic corrections necessary for two-dimensional (2D) or petrographic thin section-based determinations of chondrule sizes. Our results are completely novel, but are consistent with previous surface analysis (2D) data for OCs. Within our OC chondrule diameter data, we find the trend of mean chondrule diameters increasing in the order H < L < LL. We also present the first detailed EL chondrite chondrule size-frequency distribution. Finally, we examine the shapes and collective orientations of the chondrules within the chondrites and show that chondrite petrofabrics can be explored with our methodology. Chondrule shape-preferred orientations are identical to the orientations of the metal and sulfide grains in the chondrites and this is likely due to impact-related compaction. HIGHLIGHTS: We present a first true three-dimensional analysis of chondrule size. Our ordinary chondrite chondrule diameter data demonstrate the trend of mean chondrule diameters increasing in the order H chondrites < L chondrites < LL chondrites. We also present the first detailed low-iron enstatite chondrite chondrule size-frequency distribution. We examine the shapes and collective orientations of the chondrules and show that chondrite petrofabrics can be explored with our methodology.
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X-ray microcomputed tomography is a useful means of characterizing cosmochemical samples such as meteorites or robotically returned samples. However, there are occasional concerns that the use of µCT may be detrimental to the organic components of a chondrite. Small organic compounds such as amino acids comprise up to ~10% of the total solvent extractable carbon in CM carbonaceous chondrites. We irradiated three samples of the Murchison CM carbonaceous chondrite under conditions akin to and harsher than those typically used during typical benchtop x-ray µCT imaging experiments to determine if detectable changes in the amino acid abundance and distribution relative to a non-exposed Murchison control sample occurred. After subjecting three meteorite samples to ionizing radiation dosages between ~300 Gray (Gy) and 3 kGy with bremstrahlung X-rays, we analyzed the amino acid content of each sample. Within sampling and analytical errors, we found no differences in the amino acid abundances and amino acid enantiomeric ratios when comparing the control samples (non-exposed Murchison) and the irradiated samples. We conclude that a polychromatic X-ray µCT experiment has no detectable effect on the amino acid content of a CM type carbonaceous chondrite.
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The asteroid impact near the Russian city of Chelyabinsk on 15 February 2013 was the largest airburst on Earth since the 1908 Tunguska event, causing a natural disaster in an area with a population exceeding one million. Because it occurred in an era with modern consumer electronics, field sensors, and laboratory techniques, unprecedented measurements were made of the impact event and the meteoroid that caused it. Here, we document the account of what happened, as understood now, using comprehensive data obtained from astronomy, planetary science, geophysics, meteorology, meteoritics, and cosmochemistry and from social science surveys. A good understanding of the Chelyabinsk incident provides an opportunity to calibrate the event, with implications for the study of near-Earth objects and developing hazard mitigation strategies for planetary protection.
Assuntos
Acidentes , Ar , Explosões , Meteoroides , Federação RussaRESUMO
Doppler weather radar imaging enabled the rapid recovery of the Sutter's Mill meteorite after a rare 4-kiloton of TNT-equivalent asteroid impact over the foothills of the Sierra Nevada in northern California. The recovered meteorites survived a record high-speed entry of 28.6 kilometers per second from an orbit close to that of Jupiter-family comets (Tisserand's parameter = 2.8 ± 0.3). Sutter's Mill is a regolith breccia composed of CM (Mighei)-type carbonaceous chondrite and highly reduced xenolithic materials. It exhibits considerable diversity of mineralogy, petrography, and isotope and organic chemistry, resulting from a complex formation history of the parent body surface. That diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to C-class asteroids are interpreted.
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New aspects of synchrotron Mössbauer microscopy are presented. A 5â µm spatial resolution is achieved, and sub-micrometer resolution is envisioned. Two distinct and unique methods, synchrotron Mössbauer imaging and nuclear resonant incoherent X-ray imaging, are used to resolve spatial distribution of species that are chemically and magnetically distinct from one another. Proof-of-principle experiments were performed on enriched (57)Fe phantoms, and on samples with natural isotopic abundance, such as meteorites.
