Bekaert David V, Broadley Michael W, Marty Bernard
Centre de Recherches Pétrographiques et Géochimiques, UMR 7358 CNRS - Université de Lorraine, 15 rue Notre Dame des Pauvres, BP 20, 54501, Vandoeuvre-lès-Nancy, France.
Sci Rep. 2020 Apr 2;10(1):5796. doi: 10.1038/s41598-020-62650-3.
The origin of terrestrial volatiles remains one of the most puzzling questions in planetary sciences. The timing and composition of chondritic and cometary deliveries to Earth has remained enigmatic due to the paucity of reliable measurements of cometary material. This work uses recently measured volatile elemental ratios and noble gas isotope data from comet 67P/Churyumov-Gerasimenko (67P/C-G), in combination with chondritic data from the literature, to reconstruct the composition of Earth's ancient atmosphere. Comets are found to have contributed ~20% of atmospheric heavy noble gases (i.e., Kr and Xe) but limited amounts of other volatile elements (water, halogens and likely organic materials) to Earth. These cometary noble gases were likely mixed with chondritic - and not solar - sources to form the atmosphere. We show that an ancient atmosphere composed of chondritic and cometary volatiles is more enriched in Xe relative to the modern atmosphere, requiring that 8-12 times the present-day inventory of Xe was lost to space. This potentially resolves the long-standing mystery of Earth's "missing xenon", with regards to both Xe elemental depletion and isotopic fractionation in the atmosphere. The inferred Kr/HO and Xe/HO of the initial atmosphere suggest that Earth's surface volatiles might not have been fully delivered by the late accretion of volatile-rich carbonaceous chondrites. Instead, "dry" materials akin to enstatite chondrites potentially constituted a significant source of chondritic volatiles now residing on the Earth's surface. We outline the working hypotheses, implications and limitations of this model in the last section of this contribution.
地球挥发性物质的起源仍然是行星科学中最令人困惑的问题之一。由于对彗星物质的可靠测量匮乏，球粒陨石和彗星向地球输送物质的时间和成分一直成谜。这项研究利用最近测量的67P/丘留莫夫－格拉西缅科彗星（67P/C-G）的挥发性元素比率和稀有气体同位素数据，结合文献中的球粒陨石数据，来重建地球古代大气的成分。研究发现，彗星为地球大气贡献了约20%的重稀有气体（即氪和氙），但对其他挥发性元素（水、卤素以及可能的有机物质）的贡献有限。这些彗星稀有气体可能与球粒陨石来源而非太阳来源混合形成了大气。我们表明，由球粒陨石和彗星挥发性物质组成的古代大气相对于现代大气而言，氙的含量更高，这意味着现今大气中氙含量的8至12倍已经散失到太空。这在氙元素损耗和大气同位素分馏方面，可能解决了长期存在的地球“失踪氙”之谜。推断出的初始大气的氪/氢和氙/氢表明，地球表面的挥发性物质可能并非完全由富含挥发性物质的碳质球粒陨石在后期吸积过程中输送而来。相反，类似于顽火辉石球粒陨石的“干燥”物质可能是目前地球表面球粒陨石挥发性物质的重要来源。在本论文的最后一部分，我们概述了该模型的工作假设、意义和局限性。
