Wednesday 23 April 2014

The nature of the Košice Meteorites.

On 28 February 2010 a meteor shower fell over Slovakia, accompanied by a bright fireball and a series of sonic booms. Subsequently a number of meteorites were recovered from the area to the northwest of the city of Košice, in the east of the country, most within four weeks of the observed shower (meteors are ‘shooting stars’ observed in the sky, a meteorite is an actual piece of rock of presumed extra-terrestrial origin).

In a paper published in the April 2014 edition of the journal Planetary and Space Science and on the arXiv database at Cornell University Library on 4 April 2014 a team of scientists led by Tomáš Kohout of the Department of Physics at the University of Helsinki and the Institute of Geology of the Academy of Sciences of the Czech Republic, describe the results of an examination of 67 of these meteorites for bulk and grain density, porosity and magnetic susceptibility (a proxy for iron content). These are non-destructive methods, unlike conventional mineralogical and chemical methods, which enabled the examination of a greater number of samples.

A fusion encrusted meteorite from the Košice Meteorite Shower. Jon Taylor/Wikimedia Commons.

Kohout et al. found that the Košice Meteorites had an mean bulk density of 3.43 g/cm3, and a mean grain density of 3.79 g/cm3. This is typical for an H chondrite (High iron ordinary chondrite) type meteorite . This composition appeared to be true throughout all of the meteorites, with no grains of any material that did not conform, indicating that the meteorites were of a homogenous nature, and therefore presumably originated from a homogenous body.

The samples had a mean porosity of 9.88%, though this was more variable, ranging from 4.2% to 16.1%. This is again within the range of typical values for H chondrites, though the mean value is a little on the high side. There was a relationship between fragment sizes and porosity, with saller samples tending to have higher porosities. This is in line with prediction for such bodies, as an object breaking up in the atmosphere would tend to fracture more freely in areas with higher porosity, with the result that such areas will produce more numerous, smaller fragments.

The meteorites had a mean magnetic susceptibility of 5.35, again within the typical range of H chondrites. 

From this data Kohout et al. conclude that the parent body for the Košice Meteorites was a H chondrite with a homogenous composition (unhomogenous bodies are thought to be fragments of larger bodies, that had enough gravity for the separation of minerals to have occurred during their formation).

It had been suggested  that the Košice Meteorite shower was the product of two bodies breaking up in the atmosphere rather than one; Kohout et al.’s findings suggest that if this were to have been the case then the two bodies must have had a recent common origin; i.e. have been fragments of a parent body that broke up recently enough that these fragments had not had time to separate significantly.

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