Free Astronomy Magazine November-December 2014

SMALL BODIES second almost 8. In the two subse- quent fine-tooth comb searches, made on 21 April 2011 and 25 April 2012 (the latter after yet another plough- ing and sowing), two small sam- ples were recov- ered, one weigh- ing 2 grams and the other not even half a gram. The certainty that they are indeed meteor- ites produced by the Benešov bolide was soon established by the spectroscopic anal- ysis, which provided results similar to those collected during the fall. If a dozen grams of meteoritic material may seem little to jus- tify the commitment of several researchers over a rather long period of time, in reality the results achieved are more significant and rewarding than can be imagined. For example, it has never happened before to recover fragments scattered by a bolide so dated, and the new procedures adopted by the Czech team will in the future facilitate the discovery of meteorites associated to other bolides (as long as well documented). O utstanding microscope image revealing how in the same meteorite (M2) are present both the typical chon- drites materials (LL3.5), and a ba- saltic compound devoid of chon- drules, separated, moreover, by molten and recry- stallized material as a result of the powerful pres- sure. This is the confirmation that the meteoroid originated from the impact of asteroids. Bot- tom, the inner compound of the M1 sample, rich in metals (type H5) and therefore very different from that of the other fragments. [Spurný et al.] The real surprise, though, came from the mineralogical analysis of the samples re- covered, whose compound was found to belong to three classes very different from each other: LL chondrites (low metal con- tent), H chondrites (high metal content), primitive achondrites (basaltic material free of chondrules). This amazing variety is explained if we as- sume that the progenitor meteoroid was a mix of different materials produced by the collision between asteroids in the main belt. So far, such heterogeneity had been discovered only among the fragments of the Almahata Sitta meteorite. The only al- ternative way to explain the marked min- eralogical differ- ences among the samples of the Benešov meteorite would be to as- sume that they be- long to different events, a totally un- tenable hypothesis given that the re- currence of a sim- ilar phenomenon on the same area a few decades later (over longer peri- ods the fragments would be indistin- guishable) has a probability of 1 in 100,000. n