Q.-Z. Yin

Radar-Enabled Recovery of the Sutter’s Mill Meteorite, a Carbonaceous Chondrite Regolith Breccia

The Meteor That Fell to Earth In April 2012, a meteor was witnessed over the Sierra Nevada Mountains in California. Jenniskens et al. (p. 1583) used a combination of photographic and video images of the fireball coupled with Doppler weather radar images to facilitate the rapid recovery of meteorite fragments. A comprehensive analysis of some of these fragments shows that the Sutters Mill meteorite represents a new type of carbonaceous chondrite, a rare and primitive class of meteorites that contain clues to the origin and evolution of primitive materials in the solar system. The unexpected and complex nature of the fragments suggests that the surfaces of C-class asteroids, the presumed parent bodies of carbonaceous chondrites, are more complex than previously assumed. Analysis of this rare meteorite implies that the surfaces of C-class asteroids can be more complex than previously assumed. Doppler weather radar imaging enabled the rapid recovery of the Sutter’s Mill meteorite after a rare 4-kiloton of TNT–equivalent asteroid impact over the foothills of the Sierra Nevada in northern California. The recovered meteorites survived a record high-speed entry of 28.6 kilometers per second from an orbit close to that of Jupiter-family comets (Tisserand’s parameter = 2.8 ± 0.3). Sutter’s Mill is a regolith breccia composed of CM (Mighei)–type carbonaceous chondrite and highly reduced xenolithic materials. It exhibits considerable diversity of mineralogy, petrography, and isotope and organic chemistry, resulting from a complex formation history of the parent body surface. That diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to C-class asteroids are interpreted.

Isotopic Evidence of Cr Partitioning into Earth's Core

Chromium isotopes in meteorites reveal Earth’s accretion history. The distribution of chemical elements in primitive meteorites (chondrites), as building blocks of terrestrial planets, provides insight into the formation and early differentiation of Earth. The processes that resulted in the depletion of some elements [such as chromium (Cr)] in the bulk silicate Earth relative to chondrites, however, remain debated between leading candidate causes: volatility versus core partitioning. We show through high-precision measurements of Cr stable isotopes in a range of meteorites, which deviate by up to ~0.4 per mil from those of the bulk silicate Earth, that Cr depletion resulted from its partitioning into Earth’s core, with a preferential enrichment in light isotopes. Ab initio calculations suggest that the isotopic signature was established at mid-mantle magma ocean depth as Earth accreted planetary embryos and progressively became more oxidized.