A planetary embryo origin for Mars

There is considerable uncertainty as to how and when Mars formed. In particular, its small mass compared to Earth and Venus is difficult to explain and some have suggested that Mars could be a stranded planetary embryo that escaped collision and merging with other embryos. A diagnostic parameter to assess this idea is its accretion time, which can be calculated using ¹⁸²Hf-¹⁸²W (t_1/2=9 Myr) systematics of martian meteorites. A major difficulty with this approach has been the large uncertainty that plagued Hf/W ratio estimates of the martian mantle, yielding core formation ages between 0 to 15 Myr after solar system formation. To better constrain the Hf/W ratio of the martian mantle, we have measured the concentrations of Lu, Hf, U, Th by isotope dilution, as well as ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratios of 43 chondrites from all major groups (see Pourmand and Dauphas, 2010 for details on the technique). Variations in the ratios of these elements reflect redistribution during parent-body metamorphism. We estimate the Hf/W atomic ratio of the martian mantle to be 3.51±0.45. Using this Hf/W ratio, the measured value of ɛ¹⁸²W_Mars mantle=+2.6 can only be reproduced with an accretion timescale of ~2 Myr. This timescale is consistent with a stranded planetary embryo origin for Mars (Kobayashi et al. 2010). Objects formed in the first few million years of the formation of the solar system would have incorporated enough ²⁶Al to melt. We thus demonstrate that a magma ocean powered by ²⁶Al-decay must have been present on early Mars. The depth and longevity of this magma ocean remain open questions. Dauphas N., Pourmand A. 2011. Nature 473, 489-492. Kobayashi H. et al. 2010. Icarus 209, 836-847. Pourmand A., Dauphas N. 2010. Talanta 81, 741-753.