How Not to Homogenize a Planet

It’s not so easy to build Mars without a giant impact. Isotope systems are commonly used to constrain the early evolution of planetary bodies. Mars shows enough isotopic homogeneity that is generally accepted its mantle must have been homogenized about 15 million years into its history. A giant impact could do that, but an alternative has been to consider Mars as a pristine planetary embryo whose mantle would have melted due to the heat produced by radioactive decay of aluminium-26 and by the differentiation of its core.

Zhang et al. [2021] show that this explanation is unlikely. They use a sophisticated numerical model to demonstrate that the heat in the mantle is effectively removed by the melt it…

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