Lunar tungsten isotopic evidence for the late veneer

Precise measurements of the tungsten isotopic composition of lunar rocks show that the Moon exhibits a well-resolved excess of 182 W of about 27 parts per million over the present-day Earth’s mantle: this excess is consistent with the expected 182 W difference resulting from a late veneer with a total mass and composition inferred from previously measured highly siderophile elements. Late accretion to Earth and Moon Two papers published in this issue of Nature present precise measurements of tungsten isotope composition in lunar rocks that are best explained by the Earth and Moon having had similar composition immediately following formation of the Moon, and then having diverged as a result of disproportional late accretion of material to the two bodies. Mathieu Touboul et al . found small 182 W excess of about 21 parts per million relative to the present-day Earth's mantle in metals extracted from two KREEP-rich Apollo 16 impact-melt rocks, while Thomas Kruijer et al . measured tungsten isotopes in seven KREEP-rich whole rock samples that span a wide range of cosmic ray exposure ages, and found a 182 W excess of about 27 parts per million over the present-day Earth's mantle. According to the most widely accepted theory of lunar origin, a giant impact on the Earth led to the formation of the Moon, and also initiated the final stage of the formation of the Earth’s core 1 . Core formation should have removed the highly siderophile elements (HSE) from Earth’s primitive mantle (that is, the bulk silicate Earth), yet HSE abundances are higher than expected 2 . One explanation for this overabundance is that a ‘late veneer’ of primitive material was added to the bulk silicate Earth after the core formed 2 . To test this hypothesis, tungsten isotopes are useful for two reasons: first, because the late veneer material had a different 182 W/ 184 W ratio to that of the bulk silicate Earth, and second, proportionally more material was added to the Earth than to the Moon 3 . Thus, if a late veneer did occur, the bulk silicate Earth and the Moon must have different 182 W/ 184 W ratios. Moreover, the Moon-forming impact would also have created 182 W differences because the mantle and core material of the impactor with distinct 182 W/ 184 W would have mixed with the proto-Earth during the giant impact. However the 182 W/ 184 W of the Moon has not been determined precisely enough to identify signatures of a late veneer or the giant impact. Here, using more-precise measurem