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.

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Chelyabinsk airburst, damage assessment, meteorite recovery, and characterization

Classification of meteorites is largely based on their mineralogical and petrographic characteristics and their whole-rock chemical and O-isotopic compositions According to the currently used classification scheme, meteorites are divided into chondrites, primitive achondrites, and achondrites There are 15 chondrite groups, including 8 carbonaceous (CI, CM, CO, CV, CK, CR, CH, CB), 3 ordinary (H, L, LL), 2 enstatite (EH, EL), and R and K chondrites Several chondrites cannot be assigned to the existing groups and may represent the first members of new groups Some groups are subdivided into subgroups, which may have resulted from asteroidal processing of a single group of meteorites Each chondrite group is considered to have sampled a separate parent body Some chondrite groups and ungrouped chondrites show chemical and mineralogical similarities and are grouped together into clans The significance of this higher order of classification remains poorly understood The primitive achondrites include ureilites, acapulcoites, lodranites, winonaites, and silicate inclusions in IAB and IIICD irons and probably represent recrystallization or residues from a low-degree partial melting of chondritic materials The genetic relationship between primitive achondrites and the existing groups of chondritic meteorites remains controversial Achondrites resulted from a high degree of melting of chondrites and include asteroidal (angrites, aubrites, howardites-diogenites-eucrites, mesosiderites, 3 groups of pallasites, 15 groups of irons plus many ungrouped irons) and planetary (martian, lunar) meteorites

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Systematics and Evaluation of Meteorite Classification

Oxygen isotope studies of ordinary chondrites

Presolar diamond, sic, and graphite in primitive chondrites : abundances as a function of meteorite class and petrologic type

Thermal metamorphism has affected most chondritic meteorites to some extent, and in most ordinary chondrites, some carbonaceous chondrites, and many enstatite chondrites it has significantly modified the primary characteristics of the meteorites. Metamorphic grade, as described by the petrologic type, is one axis of the current two-dimensional system for classifying chondrites. Many changes are produced during thermal metamorphism, including textural integration and recrystallization, mineral equilibration, destruction of primary minerals, and growth of secondary minerals. Understanding these changes is critical if one hopes to infer the conditions under which chondrites originally formed. In addition to summarizing metamorphic changes, we also discuss temperatures, oxidation states, and possible heat sources for metamorphism.

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Thermal Metamorphism in Chondrites

Measurements performed by a thermoluminescence sensitivity technique of the degree of metamorphism experienced by unequilibrated ordinary chondrites are reported. Samples of type 3 chondrites were ground and heated to 500 C to remove their natural thermoluminescence, then irradiated with either 50 krad from a Co-60 gamma ray source or 25 krad from a Sr-90 beta source. The resulting thermoluminescence measured as a function of temperature is found to differ as much among some type 3 chondrites as between type 3 and other types, leading to the proposal of scheme for subdividing type 3 ordinary chondrites based on their thermoluminescence sensitivity.

L. M. Ross

Papers

Measuring metamorphic history of unequilibrated ordinary chondrites

A New Measure of the Metamorphic History of Ordinary Chondrites