Atmosphere Impact Losses
Atmosphere Impact Losses
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DOI：
10.1007/s11214-018-0471-z
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发表时间：
2018-01
影响因子：
10.3
通讯作者：
H. Schlichting;S. Mukhopadhyay
中科院分区：
文献类型：
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作者：
H. Schlichting;S. Mukhopadhyay
确定类地行星上挥发物的起源并量化行星形成过程中的大气损失对于了解行星大气的历史和演化至关重要。通过对稀有气体和主要挥发物的地球化学观测，我们可以确定当今挥发物的库存告诉我们有关地球的来源、吸积过程和早期分化的信息。我们进一步量化了地球形成过程中关键的挥发性损失机制和大气损失历史。挥发物在地球形成过程中不断吸积，但地球的早期吸积历史挥发性较差。尽管地幔深处的星云 Ne 和可能的 H 可能是这种早期增生的指纹，但大多数地幔并不记得这个暗示在增生过程中发生了挥发性损失的特征。目前挥发物的地球化学没有显示出流体动力逃逸的证据，因为大多数挥发物的同位素组成是球粒状的。这表明撞击产生的大气损失在地球形成过程中发挥了重要作用。虽然许多挥发物具有球粒状同位素比率，但它们的相对丰度肯定不是球粒状的，再次表明挥发物损失与影响有关。大气损失的地球化学证据来自卤素比率（例如 F/Cl）和低 H/N 比率。此外，地球化学比率表明，大部分水可能在月球形成撞击之前就已被输送，并且月球形成撞击并未将海洋赶走。鉴于影响在确定地球波动预算方面的重要性，我们研究了小型和大型影响对大气损失的贡献。我们发现，由于撞击造成的大气质量损失可以分为三种不同的情况：1）巨大的撞击，产生横贯整个地球的强烈冲击，并可能导致全球大气损失。 2）足够大的撞击器（对于当前的地球而言），能够喷射撞击地点切平面上方的所有大气，其中，and 是大气尺度高度、目标半径及其在地面的大气密度。 3）小型撞击器（对于当前的地球），只能将一小部分大气质量喷射到切平面上方。我们证明，每单位撞击器质量，小型撞击器是侵蚀大气的最有效撞击器。事实上，就地球目前的大气质量而言，它们的效率（每单位撞击体质量）比巨大撞击高出五个数量级以上，这意味着大气质量损失一定很常见。小型撞击器巨大的大气质量损失效率是由于它们的大部分撞击能量和动量可直接用于局部质量损失，而在巨大的撞击状态下，由于必须产生可以横贯整个地球的强烈冲击，从而实现全球大气损失，因此大量的能量和动量被“浪费”。在没有任何挥发物释放和逸出的情况下，我们表明，从月球陨石坑记录中推断出的含有 0.1% 的晚期撞击者群体能够侵蚀整个当前地球的大气层，这意味着侵蚀、逸气和挥发物释放的相互作用可能决定了早期地球的大气质量和成分。将地球化学观测与影响模型相结合表明，小型和大型影响之间存在有趣的协同作用，其中巨大的影响会产生巨大的岩浆海洋，而小型和较大的影响则推动……
Determining the origin of volatiles on terrestrial planets and quantifying atmospheric loss during planet formation is crucial for understanding the history and evolution of planetary atmospheres. Using geochemical observations of noble gases and major volatiles we determine what the present day inventory of volatiles tells us about the sources, the accretion process and the early differentiation of the Earth. We further quantify the key volatile loss mechanisms and the atmospheric loss history during Earth’s formation. Volatiles were accreted throughout the Earth’s formation, but Earth’s early accretion history was volatile poor. Although nebular Ne and possible H in the deep mantle might be a fingerprint of this early accretion, most of the mantle does not remember this signature implying that volatile loss occurred during accretion. Present day geochemistry of volatiles shows no evidence of hydrodynamic escape as the isotopic compositions of most volatiles are chondritic. This suggests that atmospheric loss generated by impacts played a major role during Earth’s formation. While many of the volatiles have chondritic isotopic ratios, their relative abundances are certainly not chondritic again suggesting volatile loss tied to impacts. Geochemical evidence of atmospheric loss comes from the, halogen ratios (e.g., F/Cl) and low H/N ratios. In addition, the geochemical ratios indicate that most of the water could have been delivered prior to the Moon forming impact and that the Moon forming impact did not drive off the ocean. Given the importance of impacts in determining the volatile budget of the Earth we examine the contributions to atmospheric loss from both small and large impacts. We find that atmospheric mass loss due to impacts can be characterized into three different regimes: 1) Giant Impacts, that create a strong shock transversing the whole planet and that can lead to atmospheric loss globally. 2) Large enough impactors (,for the current Earth), that are able to eject all the atmosphere above the tangent plane of the impact site, where,andare the atmospheric scale height, radius of the target, and its atmospheric density at the ground. 3) Small impactors (,for the current Earth), that are only able to eject a fraction of the atmospheric mass above the tangent plane. We demonstrate that per unit impactor mass, small impactors withare the most efficient impactors in eroding the atmosphere. In fact for the current atmospheric mass of the Earth, they are more than five orders of magnitude more efficient (per unit impactor mass) than giant impacts, implying that atmospheric mass loss must have been common. The enormous atmospheric mass loss efficiency of small impactors is due to the fact that most of their impact energy and momentum is directly available for local mass loss, where as in the giant impact regime a lot of energy and momentum is ’wasted’ by having to create a strong shock that can transverse the entirety of the planet such that global atmospheric loss can be achieved. In the absence of any volatile delivery and outgassing, we show that the population of late impactors inferred from the lunar cratering record containing 0.1%is able to erode the entire current Earth’s atmosphere implying that an interplay of erosion, outgassing and volatile delivery is likely responsible for determining the atmospheric mass and composition of the early Earth. Combining geochemical observations with impact models suggest an interesting synergy between small and big impacts, where giant impacts create large magma oceans and small and larger impacts drive the …