Mars in the aftermath of a colossal impact
Mars in the aftermath of a colossal impact
复制标题
遭受巨大撞击后的火星
DOI：
10.1016/j.icarus.2019.05.015
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发表时间：
2019
期刊：
影响因子：
3.2
通讯作者：
S. J.
中科院分区：
文献类型：
--
作者：
Woo;J. M. Y.;Genda;H.;Brasser;R.;Mojzsis;S. J.
从火星陨石中推断出的火星地幔中高亲铁元素（HSEs）的丰度表明，晚饰面（LV）质量添加量为~0.8 wt%，具有广泛的球粒陨石组成。一个谷神星大小（~1000 千米直径）的天体在后期对火星的吸积可以解释所需LV质量的一部分，地质年代学约束表明，这一定发生在大约4480 Ma之前。本文采用光滑粒子流体力学模拟和解析理论相结合的方法，对假设的LV巨大撞击火星的结果进行分析。结果表明，一般情况下，约50%的撞击体金属核心破碎成~10 m的碎片，这些碎片随后在再吸积时破碎成低于mm的金属冰雹。与正面碰撞和肇事逃逸的碰撞相比，这将返回一个有希望的HSEs进入火星地幔；在这两种情况下，小于10%的撞击器核心物质破碎并最终嵌入火星地幔。来自火星陨石的同位素证据和来自大气测绘数据的解释表明，在诺亚纪早期（约4100 Ma之前）可能存在一个全球地表水水库。因此，毫米大小的金属冰雹可以与火星水圈反应，产生约3巴的氢气，这足以作为温室，保持火星早期的温暖。然而，我们也发现这种大气是短暂的。根据太阳早期预期的极紫外线（EUV）通量，它通常存活时间短于3 Myr；如果太阳是一个缓慢的旋转体，那么相应地较弱的极紫外通量可以将其寿命延长到10兆尔。稠密的前诺亚宙co2大气通过红外发射降低了氢的逃逸效率。一个更详细的早期氢大气的流体动力学大气模型是有必要的，以检验它对诺亚亚前火星的影响。
The abundance of highly siderophile elements (HSEs) inferred for Mars' mantle from martian meteorites implies a Late Veneer (LV) mass addition of ~0.8 wt% with broadly chondritic composition. Late accretion to Mars by a differentiated Ceres-sized (~1000 km diameter) object can account for part of the requisite LV mass, and geochronological constraints suggests that this must have occurred no later than ca. 4480 Ma. Here, we analyze the outcome of the hypothetical LV giant impact to Mars with smoothed particle hydrodynamics simulations together with analytical theory. Results show that, in general about 50% of the impactor's metallic core shatters into ~10 m fragments that subsequently fragment into sub-mm metallic hail at re-accretion. This returns a promising delivery of HSEs into martian mantle compared to either a head-on and hit-and-run collision; in both cases, <10% of impactor's core materials are fragmented and finally embedded in the martian mantle. Isotopic evidence from martian meteorites, and interpretations from atmospheric mapping data show that a global surface water reservoir could be present during the early Noachian (before ca. 4100 Ma). The millimeter-sized metal hail could thus react with a martian hydrosphere to generate ~3 bars of H2, which is adequate to act as a greenhouse and keep early Mars warm. Yet, we also find that this atmosphere is transient. It typically survives shorter than 3 Myr based on the expected extreme ultraviolet (EUV) flux of the early Sun; if the Sun was a slow rotator an accordingly weaker EUV flux could extend this lifetime to >10 Myr. A dense pre-Noachian CO2atmosphere should lower the escape efficiency of hydrogen by IR emission. A more detailed hydrodynamic atmospheric model of this early hydrogen atmosphere is warranted to examine its effect on pre-Noachian Mars.