Ancient meteorite is in a class of its own

More than four-fifths of the asteroids recovered on Earth as meteorites are ordinary chondrites—iron-poor rocks that contain small round grains, or chondrules. Over the past two decades, Birger Schmitz at Lund University in Sweden and his colleagues have assembled evidence that a subset of ordinary chondrites, known as L chondrites, are pieces of a single large asteroid that was shattered in a collision about 470 million years ago.

Now Schmitz and his team have conducted a detailed analysis of a peculiar meteorite that has the age of an L chondrite but not the composition. Österplana 065 (pictured here) was discovered in 2011 at the Thorsberg quarry in southern Sweden, where workers for a company that produces limestone floors have uncovered more than 100 L chondrites in ancient sediment. Compared with the L chondrites, Öst 65 contains far less oxygen-17 than it should based on its concentration of chromium-54; the ratio of the two isotopes is used to classify meteorites. The quirky space rock also lacks certain minerals and the chondrule texture that distinguish chondrites.

The researchers argue that the discovery of this one anomalous specimen out of the more than 52 000 meteorites classified worldwide has major implications. Schmitz and his colleagues propose that Öst 65 is the first evidence of an extinct meteorite, which no longer falls to Earth yet is representative of the kinds of rocks that exist elsewhere in the solar system. The researchers go further and suggest, based on the meteorite’s age, that Öst 65 is a fragment of the object that slammed into the parent body of the L chondrites. (B. Schmitz et al., Nat. Comm. 7, 11851, 2016, doi:10.1038/ncomms11851 .)