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.  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 produces so that, unless Mars accreted in less than a million years, the mantle would be more heterogeneous than we infer today. As melting cannot do the job, a giant impact appears necessary to homogenize Mars. As a bonus, a giant impact at about 15 million years would help erase evidence of crust expected to have formed earlier by melting inside the proto-Mars.
Citation: Zhang, Z., Bercovici, D., & Jordan, J. S. . A two‐phase model for the evolution of planetary embryos with implications for the formation of Mars. Journal of Geophysical Research: Planets, 126, e2020JE006754. https://doi.org/10.1029/2020JE006754
―Laurent G. J. Montési, Editor-in-Chief, JGR: Planets