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Late Accretion on the Earliest Planetesimals Revealed by the Highly Siderophile Elements

Coming Late to the Planetesimal Highly siderophile (iron-loving) elements (Re, Os, Ir, Ru, Rh, Pt, Pd, and Au) must have been added to the mantles of Earth, the Moon, and Mars after their iron cores formed; otherwise the mantles would be devoid of these elements, which tend to be segregated to the core. Dale et al. (p. 72) report data on highly siderophile elements in rocks from different planetary bodies, including asteroid 4 Vesta and other differentiated asteroids, which are representative of the planetesimals from which the solar system planets formed. Like the larger planetary bodies, differentiated asteroids, which formed over the first few million years of the solar system, bear the evidence of the late addition of highly siderophile elements to their mantles. Thus, this process was not unique to Earth, the Moon, and Mars and happened over an extended period of time in the inner solar system. Analysis of meteorites shows that unprocessed material was accreted to both planets and asteroids 150 million years after the start of the solar system. Late accretion of primitive chondritic material to Earth, the Moon, and Mars, after core formation had ceased, can account for the absolute and relative abundances of highly siderophile elements (HSEs) in their silicate mantles. Here we show that smaller planetesimals also possess elevated HSE abundances in chondritic proportions. This demonstrates that late addition of chondritic material was a common feature of all differentiated planets and planetesimals, irrespective of when they accreted; occurring ≤5 to ≥150 million years after the formation of the solar system. Parent-body size played a role in producing variations in absolute HSE abundances among these bodies; however, the oxidation state of the body exerted the major control by influencing the extent to which late-accreted material was mixed into the silicate mantle rather than removed to the core.