Planetary Science

Hammering Rims into Place

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Science  15 Apr 2011:
Vol. 332, Issue 6027, pp. 285
DOI: 10.1126/science.332.6027.285-b

Chondrites, the most primitive of meteorites, are fragments of parts of asteroids that never melted or suffered substantial alteration and thus sample the materials from which the solar system formed. They contain millimeter-sized grains called chondrules, which are surrounded by rims of finer grains. How these rims formed has been a matter of debate. Did they sample dust accreted onto the chondrules while these were still freely floating in the early protoplanetary nebula, or did they suffer alteration after the chondrules were incorporated into the meteorite's parent asteroid? To solve this question, Bland et al. used 3D electron backscatter diffraction to look at the preferred orientation of grains around the chondrules in a chondritic meteorite. Away from the chondrules, grains share a single preferred orientation that suggests they were compressed while in the asteroid. In the rims, in contrast, the grains have different orientations, which are spherically symmetric, centered on the chondrules. The analysis suggests that the rims were emplaced outside the parent asteroid and oriented themselves in response to gentle collisions in the protoplanetary nebula.

Nat. Geosci. 4, 244 (2011).

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