RAPID ACCRETION AND EARLY DIFFERENTIATION OF MARS INDICATED BY ND-142ND-144 IN SNC METEORITES

Small differences in the ratio of neodymium-142 to neodymium-144 in ea rly formed mantle reservoirs in planetary bodies are the result of in situ decay of the extinct radionuclide samarium-146 and can be used to constrain early planetary differentiation and therefore the time scal e of planetary accretion. The martian meteorite Nakhla (similar to 1.3 billion years old), the type sample of the nakhlite subgroup of the S hergottite-Nakhlite-Chassigny (SNC) meteorites, exhibits a 59 +/- 13 p arts per million excess in the ratio of neodymium-142 to neodymium-144 relative to normal neodymium. This anomaly records differentiation in the martian mantle before 4539 million years ago and implies that Mar s experienced no giant impacts at any time later than 27 million years after the origin of the solar system.