Brennan Matthew C, Fischer Rebecca A, Nimmo Francis, O'Brien David P
Department of Earth and Planetary Sciences, Harvard University (20 Oxford Street, Cambridge, MA 02138, USA).
Department of Earth and Planetary Sciences, University of California Santa Cruz (1156 High Street, Santa Cruz, CA 95064, USA).
Geochim Cosmochim Acta. 2022 Jan 1;316:295-308. doi: 10.1016/j.gca.2021.09.022. Epub 2021 Sep 25.
Determining how and when Mars formed has been a long-standing challenge for planetary scientists. The size and orbit of Mars are difficult to reproduce in classical simulations of planetary accretion, and this has inspired models of inner solar system evolution that are tuned to produce Mars-like planets. However, such models are not always coupled to geochemical constraints. Analyses of Martian meteorites using the extinct hafnium-tungsten (Hf-W) radioisotopic system, which is sensitive to the timing of core formation, have indicated that the Martian core formed within a few million years of the start of the solar system itself. This has been interpreted to suggest that, unlike Earth's protracted accretion, Mars grew to its modern size very rapidly. These arguments, however, generally rely on simplified growth histories for Mars. Here, we combine likely accretionary histories from a large number of -body simulations with calculations of metal-silicate partitioning and Hf-W isotopic evolution during core formation to constrain the range of conditions that could have produced Mars. We find that there is no strong correlation between the final masses or orbits of simulated Martian analogs and their W anomalies, and that it is readily possible to produce Mars-like Hf-W isotopic compositions for a variety of accretionary conditions. The Hf-W signature of Mars is very sensitive to the oxygen fugacity (O) of accreted material because the metal-silicate partitioning behavior of W is strongly dependent on redox conditions. The average O of Martian building blocks must fall in the range of 1.3-1.6 log units below the iron-wüstite buffer to produce a Martian mantle with the observed Hf/W ratio. Other geochemical properties (such as sulfur content) also influence Martian W signatures, but the timing of accretion is a more important control. We find that while Mars must have accreted most of its mass within ~5 million years of solar system formation to reproduce the Hf-W isotopic constraints, it may have continued growing afterwards for over 50 million years. There is a high probability of simultaneously matching the orbit, mass, and Hf-W signature of Mars even in cases of prolonged accretion if giant impactor cores were poorly equilibrated and merged directly with the proto-Martian core.
确定火星如何形成以及何时形成，一直是行星科学家面临的长期挑战。在行星吸积的经典模拟中，火星的大小和轨道很难再现，这激发了一些调整后的内太阳系演化模型，旨在产生类似火星的行星。然而，这类模型并不总是与地球化学约束条件相关联。利用对核心形成时间敏感的灭绝铪 - 钨（Hf - W）放射性同位素体系对火星陨石进行分析，结果表明火星核心在太阳系形成之初的几百万年内就已形成。这被解释为意味着，与地球漫长的吸积过程不同，火星迅速增长到了其现代大小。然而，这些观点通常依赖于火星简化的增长历史。在这里，我们将大量多体模拟中可能的吸积历史与核心形成过程中金属 - 硅酸盐分配及Hf - W同位素演化的计算相结合，以限制可能产生火星的条件范围。我们发现，模拟的类火星天体的最终质量或轨道与其W异常之间没有很强的相关性，并且在各种吸积条件下都很容易产生类似火星的Hf - W同位素组成。火星的Hf - W特征对吸积物质的氧逸度（O）非常敏感，因为W的金属 - 硅酸盐分配行为强烈依赖于氧化还原条件。火星构建块的平均O必须落在低于铁 - 方铁矿缓冲剂1.3 - 1.6对数单位的范围内，才能产生具有观测到的Hf/W比值的火星地幔。其他地球化学性质（如硫含量）也会影响火星的W特征，但吸积时间是一个更重要的控制因素。我们发现，虽然火星必须在太阳系形成后的约500万年内在~500万年的时间内吸积了大部分质量，以再现Hf - W同位素约束，但它之后可能还持续增长了超过5000万年。即使在吸积过程延长的情况下，如果巨型撞击体核心平衡不佳并直接与原火星核心合并，同时匹配火星的轨道、质量和Hf - W特征的可能性也很高。
