晚期吸积对水星组成和密度的影响
Modification of the composition and density of Mercury from late accretion
摘要：
晚期增生是一个强烈调制水星表面地貌和地球化学特征的过程。然而，撞击物的命运及其在撞击时期对水星表面的影响尚不清楚。利用Monte-Carlo方法和撞击坑分析方法，我们研究了水星晚期吸积的物理和化学结果。考虑到后期吸积的不确定性，我们开发了以下参数的比例律作为撞击速度和后期吸积总质量的函数：（1）地壳侵蚀深度，（2）表面重塑的程度，和（3）从撞击物物质中吸积的质量。现有的动力学模型表明，水星经历了强烈的撞击轰击（总质量为$\sim 8 \times 10^{18} - 8 \times 10^{20}$ kg，典型的撞击速度为$30-40$ km s $^{-1}$）后，$4.5$ Ga。据此，我们发现晚期的吸积作用可以使水星早期地壳的厚度减少50米到10公里，但其核幔比的变化可以忽略不计。另一种可能是，早期太阳系中假定的分化微行星的地幔可能更容易被移除，并且它们各自的核心部分增加，如果水星最终从这些物体中吸积。虽然陨石坑是值得注意的擦除旧的地质表面记录在水星上，我们表明，$\sim 40-50$重量%的撞击物的外生物质，包括挥发性轴承材料，可以异质植入水星的表面作为后期单板（$>3\times 10^{18}-1.6 \times 10^{19}$公斤）。大约一半的撞击物的物质被蒸发，其余的在撞击时完全熔化。我们希望我们的理论结果和即将到来的水星表面观测，包括BepiColombo任务之间的进一步相互作用，将使我们更好地了解水星的起源和演变。
英文摘要：
Late accretion is a process that strongly modulated surface geomorphic and geochemical features of Mercury. Yet, the fate of the impactors and their effects on Mercury's surface through the bombardment epoch are not clear. Using Monte-Carlo and analytical approaches of cratering impacts, we investigate the physical and thermodynamical outcomes of late accretion on Mercury. Considering the uncertainties in late accretion, we develop scaling laws for the following parameters as a function of impact velocity and total mass of late accretion: (1) depth of crustal erosion, (2) the degree of resurfacing, and (3) mass accreted from impactor material. Existing dynamical models indicate that Mercury experienced an intense impact bombardment (a total mass of $\sim 8 \times 10^{18} - 8 \times 10^{20}$ kg with a typical impact velocity of $30-40$ km s$^{-1}$) after $4.5$ Ga. With this, we find that late accretion could remove 50 m to 10 km of the early crust of Mercury, but the change to its core-to-mantle ratio is negligible. Alternatively, the mantles of putative differentiated planetesimals in the early solar system could be more easily removed and their respective core fraction increased, if Mercury ultimately accreted from such objects. Although the cratering is notable for erasing the older geological surface records on Mercury, we show that $\sim 40-50$ wt% of the impactor's exogenic materials, including the volatile-bearing materials, can be heterogeneously implanted on Mercury's surface as a late veneer ($>3\times 10^{18}-1.6 \times 10^{19}$ kg in total). About half of the accreted impactor's materials are vaporized, and the rest is completely melted upon the impact. We expect that the further interplay between our theoretical results and forthcoming surface observations of Mercury, including the BepiColombo mission, will lead us to a better understanding of Mercury's origin and evolution.
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