Attempts to constrain the accretion history of Mars using geochemical observations have focused on the chronology of core formation. However, given the possibility of incomplete metal‐silicate equilibration, most previous results cannot distinguish between the following two scenarios: (1) Mars was formed early through gradual accretion and remained isolated as a “planetary embryo” and (2) Mars underwent relatively protracted accretion involving collisions between planetary embryos. Here we use a two‐phase model to demonstrate that, despite strong heating from the short‐lived radionuclide ²⁶Al, the mantle of a Mars‐sized planetary embryo may stay effectively solid because the generation and migration of melts in relatively small fractions leads to efficient heat transport. In this scenario, the effectively solid mantle would have undergone substantial differentiation before the extinction of ²⁶Al, and the disparate planetary building blocks would not have been homogenized within the embryo. Hence, features of very early silicate differentiation (within the first few million years of the solar system) and significant nucleosynthetic isotope heterogeneity would be expected to exist in stranded planetary embryos. In contrast, meteorite observations suggest that the crust‐mantle system of Mars is homogeneous in nucleosynthetic isotope anomalies and the onset of silicate differentiation was no earlier than ∼15 Myr after solar system formation. This discrepancy implies that Mars is not a stranded planetary embryo. Instead, we suggest that the accretion of Mars involved at least one collision between planetary embryos with comparable sizes that caused complete melting and homogenization during the giant‐impact stage of planetary growth.

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行星胚胎演化的两阶段模型及其对火星形成的影响

利用地球化学观测来限制火星的积聚历史的尝试集中在岩心形成的年代上。但是，考虑到金属硅酸盐平衡不完全的可能性，大多数以前的结果无法区分以下两种情况：（1）火星是通过逐渐积聚而早期形成的，并且仍被孤立为“行星胚”，并且（2）火星经历了相对长期的发展涉及行星胚胎之间碰撞的吸积。在这里，我们使用两阶段模型来证明，尽管短期放射性核素会产生强烈的热量²⁶Al，火星大小的行星胚胎的地幔可能有效地保持固态，因为熔体的产生和迁移的比例相对较小，导致有效的热传递。在这种情况下，有效的地幔将在²⁶灭绝之前发生实质性的分化Al，以及不同的行星构造块都不会在胚胎中被均质化。因此，预计在绞合的行星胚胎中将存在硅酸盐早期分化（在太阳系的前几百万年内）和显着的核合成同位素异质性的特征。相反，陨石的观测结果表明，火星的地幔幔系统在核合成同位素异常中是均匀的，而硅酸盐的分化开始不早于太阳系形成后的〜15 Myr。这种差异意味着火星不是搁浅的行星胚胎。相反，我们建议火星的积聚涉及大小相近的行星胚胎之间的至少一次碰撞，从而在行星生长的巨大撞击阶段引起完全融化和均质化。