Just how young is the average meteorite? One way to study the question is through the chondrules that make up stony meteorites. Chondrules are mineral deposits formed by rapid cooling; they give the appearance of tiny, spherical bits of glassy rock. Stony meteorites are generally called chondrites because they contain such chondrules. And it’s generally assumed that chondrites formed in the early Solar System in the condensation of the first solid materials.
But University of Toronto geologist Yuri Amelin and Alexander Krot (University of Hawaii) now have data that call that conclusion into question. Their paper in an August issue of Nature reports on chondrules that are the youngest ever found. The researchers used meteorites named Gujba and Hammadah al Hamra, studying their minerological structure and fixing an approximate isotopic age. “It soon became clear that these particular chondrules were not of a nebular origin,” says Amelin. “And the ages were quite different from what was expected. It was exciting.”
In fact, the chondrules Amelin, Krot and their colleagues found post-date the oldest asteroids. Amelin again: “We think these chondrules were formed by a giant plume of vapour produced when two planetary embryos, somewhere between moon-size and Mars-size, collided.”
Perhaps the formation of the Solar System wasn’t as cut and dried as some theories suggest. Indeed, this finding is the first suggestion that some chondrules and the meteorites that contain them formed at considerably different times than others, which implies that the planets — at least in early form — were already there when some chondrules were made. Another suggestion that our understanding of planetary formation is far from comprehensive.
The paper is Alexander N. Krot, Yuri Amelin et al., “Young chondrules in CB chondrites from a giant impact in the early Solar System,” Nature 436, (18 August 2005), pp. 989-992.