Meteorite fractures and the behavior of meteoroids in the atmosphere

Arguably the major difficulty faced to model the atmospheric behavior of objects entering the atmosphere is that we know very little about the internal structure of these objects and their methods of fragmentation during fall. In a study of over a thousand meteorite fragments (mostly hand-sized, some 40 or 50 cm across) in the collections of the Natural History Museums in Vienna and London, we identified six kinds of fracturing behavior. (1) Chondrites usually showed random fractures with no particular sensitivity to meteorite texture. (2) Coarse irons fractured along kamacite grain boundaries, while (3) fine irons fragmented randomly, c.f. chondrites. (4) Fine irons with large crystal boundaries (e.g. Arispe) fragmented along the crystal boundaries. (5) A few chondrites, three in the present study, have a distinct and strong network of fractures making a brickwork or chicken-wire structure. The Chelyabinsk meteorite has the chicken-wire structure of fractures, which explains the very large number of centimeter-sized fragments that showered the Earth. Finally, (6) previous work on Sutter's Mill showed that water-rich meteorites fracture around clasts. To scale the meteorite fractures to the fragmentation behavior of near-Earth asteroids, it has been suggested that the fracturing behavior follows a statistical prediction made in the 1930s, the Weibull distribution, where fractures are assumed to be randomly distributed through the target and the likelihood of encountering a fracture increases with distance. This results in a relationship: σl = σs(ns/nl)α, where σs and σl refers to stress in the small and large object and ns and nl refer to the number of cracks per unit volume of the small and large object. The value for α, the Weibull coefficient, is unclear. Ames meteorite laboratory is working to measure the density and length of fractures observed in these six types of fracture to determine values for the Weibull coefficient for each type of object.