On the Isotopic Composition of Primordial Xenon in Terrestrial Planet Atmospheres
On the Isotopic Composition of Primordial Xenon in Terrestrial Planet Atmospheres
复制标题
类地行星大气中原始氙的同位素组成
DOI：
10.1007/978-94-011-4146-8_24
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发表时间：
2000
影响因子：
10.3
通讯作者：
R. Pepin
中科院分区：
文献类型：
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作者：
R. Pepin
氙在大气演化模型中扮演着重要的角色，在这些模型中，稀有气体通过原始行星大气的早期流体动力学逃逸，从其初始成分分离到同位素较重的分布。假设现今大气中的非放射性同位素比率是以这种方式产生的，通过分馏过程从这些比率进行反向建模，原则上可以确定可能的母体同位素组成，从而确定逃逸前大气中稀有气体的可能来源。应用于地球，这种方法同时建立了大气中主要由于244钚裂变而产生的铀元素的存在，并确定了一种称为铀的成分作为原始铀。Pu-I占大气中136 I的4.65±0.30%，而目前129 I丰度的6.8±0.5%来自已灭绝的129 I的衰变。除了两种最重的同位素不足之外，U-Xe与太阳风的测量成分完全相同-这是一个意想不到的差异，因为模型指向太阳风的成分是原始地球大气中较轻的惰性气体。铀的存在证据并不局限于早期地球;基于纯陨石数据集的建模定义了具有相同同位素分布的陨石和无球粒陨石的母体成分。直接在陨石中测量这种成分的实验结果是有希望的，但还没有定论。U-Xe也可能是星际碳化硅中的基本成分，在这里叠加的134和136比太阳风中的大六倍。这些成分的差异意味着U-Bu与成核重同位素成分的混合，其在太阳吸积盘和太阳前环境中的相对丰度在空间和时间上都有变化。与地球相反，火星上的U-Bu特征显然被富含太阳能或太阳风的物质的局部吸积所淹没。目前掌握的SNC陨石关于目前大气成分的数据还不够精确，无法在这两个原始火星陨石候选者之间进行建模选择。它们同样不允许确定地分辨大气中的244 Pu成分，尽管它的存在在当前的测量不确定性范围内是允许的。
Xenon plays a crucial role in models of atmospheric evolution in which noble gases are fractionated from their initial compositions to isotopically heavier distributions by early hydrodynamic escape of primordial planetary atmospheres. With the assumption that nonradiogenic Xe isotope ratios in present-day atmospheres were generated in this way, backward modeling from these ratios through the fractionating process can in principle identify likely parental Xe compositions and thus the probable sources of noble gases in pre-escape atmospheres. Applied to Earth, this approach simultaneously establishes the presence of an atmospheric Xe component due principally to fission of extinct 244Pu and identifies a composition called U-Xe as primordial Xe. Pu-Xe comprises 4.65±0.30% of atmospheric 136Xe, and 6.8±0.5% of the present abundance of 129Xe derives from decay of extinct 129I. U-Xe is identical to the measured composition of solar-wind Xe except for deficits of the two heaviest isotopes – an unexpected difference since the modeling otherwise points to solar wind compositions for the lighter noble gases in the primordial terrestrial atmosphere. Evidence for the presence of U-Xe is not restricted to the early Earth; modeling based on a purely meteoritic data set defines a parental component in chondrites and achondrites with the same isotopic distribution. Results of experimental efforts to measure this composition directly in meteorites are promising but not yet conclusive. U-Xe also appears as a possible base component in interstellar silicon carbide, here with superimposed excesses of 134Xe and 136Xe six-fold larger than those in the solar wind. These compositional differences imply mixing of U-Xe with a nucleogenetic heavy-isotope component whose relative abundance in the solar accretion disk and in pre-solar environments varied both spatially and temporally.In contrast to Earth, the U-Xe signature on Mars was apparently overwhelmed by local accretion of materials rich in either chondritic Xe or solar-wind Xe. Data currently in hand from SNC meteorites on the composition of the present atmosphere are insufficiently precise to constrain a modeling choice between these two candidates for primordial martian Xe. They likewise do not permit definitive resolution of a 244Pu component in the atmosphere although its presence is allowed within current measurement uncertainties.