Brennan Matthew C, Fischer Rebecca A, Irving Jessica C E
Harvard University, Department of Earth and Planetary Sciences.
Princeton University, Department of Geosciences.
Earth Planet Sci Lett. 2020 Jan 15;530. doi: 10.1016/j.epsl.2019.115923. Epub 2019 Nov 11.
The chemical and physical properties of the interiors of terrestrial planets are largely determined during their formation and differentiation. Modeling a planet's formation provides important insights into the properties of its core and mantle, and conversely, knowledge of those properties may constrain formational narratives. Here, we present a multi-stage model of Martian core formation in which we calculate core-mantle equilibration using parameterizations from high pressure-temperature metal-silicate partitioning experiments. We account for changing core-mantle boundary (CMB) conditions, composition-dependent partitioning, and partial equilibration of metal and silicate, and we evolve oxygen fugacity (O) self-consistently. The model successfully reproduces published meteorite-based estimates of most elemental abundances in the bulk silicate Mars, which can be used to estimate core formation conditions and core composition. This composition implies that the primordial material that formed Mars was significantly more oxidized (0.9-1.4 log units below the iron-wüstite buffer) than that of the Earth, and that core-mantle equilibration in Mars occurred at 42-60% of the evolving CMB pressure. On average, at least 84% of accreted metal and at least 40% of the mantle were equilibrated in each impact, a significantly higher degree of metal equilibration than previously reported for the Earth. In agreement with previous studies, the modeled Martian core is rich in sulfur (18-19 wt%), with less than one weight percent O and negligible Si. We have used these core and mantle compositions to produce physical models of the present-day Martian interior and evaluate the sensitivity of core radius to crustal thickness, mantle temperature, core composition, core temperature, and density of the core alloy. Trade-offs in how these properties affect observable physical parameters like planetary mass, radius, moment of inertia, and tidal Love number define a range of likely core radii: 1620-1870 km. Seismic velocity profiles for several combinations of model parameters have been used to predict seismic body-wave travel times and planetary normal mode frequencies. These results may be compared to forthcoming Martian seismic data to further constrain core formation conditions and geophysical properties.
类地行星内部的化学和物理性质在其形成和分化过程中很大程度上就已确定。对行星形成过程进行建模能为其地核和地幔的性质提供重要见解，反之，这些性质的相关知识也可能限制形成过程的描述。在此，我们提出了一个火星地核形成的多阶段模型，其中我们使用高压 - 高温金属 - 硅酸盐分配实验的参数化方法来计算地核 - 地幔平衡。我们考虑了不断变化的地核 - 地幔边界（CMB）条件、成分依赖的分配以及金属和硅酸盐的部分平衡，并且自洽地演化氧逸度（O）。该模型成功再现了基于陨石对火星整体硅酸盐中大多数元素丰度的已发表估计值，这些估计值可用于估算地核形成条件和地核成分。这种成分意味着形成火星的原始物质比地球的原始物质氧化程度显著更高（比铁 - 方铁矿缓冲剂低0.9 - 1.4对数单位），并且火星中的地核 - 地幔平衡发生在演化的CMB压力的42% - 60%。平均而言，每次撞击中至少84%的吸积金属和至少40%的地幔达到平衡，这一金属平衡程度比先前报道的地球情况要高得多。与先前的研究一致，模拟的火星地核富含硫（18 - 19 wt%），氧含量低于1 wt%且硅含量可忽略不计。我们利用这些地核和地幔成分建立了现今火星内部的物理模型，并评估了地核半径对地壳厚度、地幔温度、地核成分、地核温度以及地核合金密度的敏感性。这些性质如何影响诸如行星质量、半径、转动惯量和潮汐勒夫数等可观测物理参数之间的权衡确定了一系列可能的地核半径：1620 - 1870千米。已使用几种模型参数组合的地震速度剖面来预测地震体波传播时间和行星正常模式频率。这些结果可与即将到来的火星地震数据进行比较，以进一步限制地核形成条件和地球物理性质。
