<sup>60</sup>Fe–<sup>60</sup>Ni chronology of core formation in Mars

The timescales of accretion, core formation, and magmatic differentiation in planetary bodies can be constrained using extinct radionuclide systems. Experiments have shown that Ni becomes more siderophile with decreasing pressure, which is reflected in the progressively higher Fe/Ni ratios in the mantles of Earth, Mars and Vesta. Mars formed rapidly and its mantle has a high Fe/Ni ratio, so the 60Fe–60Ni decay system ( t 1 / 2 = 2.62   Myr ) is well suited to establish the timescale of core formation in this object. We report new measurements of 60Ni/58Ni ratios in bulk SNC/martian (Shergotty–Nakhla–Chassigny) meteorites and chondrites. The difference in ε Ni 60 values between SNC meteorites and the building blocks of Mars assumed to be chondritic (55% ordinary chondrites + 45% enstatite chondrites) is + 0.028 ± 0.023 (95% confidence interval). Using a model of growth of planetary embryo, this translates into a time for Mars to have reached ∼44% of its present size of 1.9 − 0.8 + 1.7   Myr with a strict lower limit of 1.2 Myr after solar system formation, which agrees with a previous estimate based on 182Hf–182W systematics. The presence of Mars when planetesimals were still being formed may have influenced the formation of chondrules through bow shocks or by inducing collisions between dynamically excited planetesimals.

利用灭绝放射性核素系统可以限制行星体中吸积、核形成和岩浆分异的时间尺度。实验表明，镍的亲铁性随压力降低而增强，这反映在地球、火星和灶神星地幔中逐渐升高的铁/镍比值上。火星形成迅速，其地幔具有较高的铁/镍比值，因此$^{60}$Fe - $^{60}$Ni衰变系统（半衰期$t_{1/2} = 2.62$百万年）非常适合确定火星核形成的时间尺度。我们报道了对SNC/火星（辉玻无球粒陨石 - 纳克勒陨石 - 夏塞尼陨石）陨石和球粒陨石中$^{60}$Ni/$^{58}$Ni比值的新测量结果。SNC陨石与假定为球粒陨石质（55%普通球粒陨石 + 45%顽火辉石球粒陨石）的火星组成部分之间的$\varepsilon_{Ni}^{60}$值差异为$+0.028 \pm 0.023$（95%置信区间）。利用行星胚胎生长模型，这意味着火星在太阳系形成后达到其当前大小约44%的时间为$1.9_{ - 0.8}^{ + 1.7}$百万年，严格下限为1.2百万年，这与先前基于$^{182}$Hf - $^{182}$W体系的估计相符。在小行星仍在形成时火星的存在可能通过弓形激波或诱导动态激发的小行星之间的碰撞影响了球粒的形成。