The D/H Ratio in Methane in Titan: Origin and History
The D/H Ratio in Methane in Titan: Origin and History
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泰坦中甲烷的 D/H 比：起源和历史
DOI：
10.1006/icar.2002.6930
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发表时间：
2002
期刊：
影响因子：
3.2
通讯作者：
A. Coustenis
中科院分区：
文献类型：
--
作者：
O. Mousis;D. Gautier;A. Coustenis
摘要 我们对土卫六大气中观测到的 CH4 的 D/H 比值提出了新的解释。使用土星亚星云的湍流演化模型（O. Mousis et al. 2002, Icarus156, 162–175），我们表明，与当前情况相反，在泰坦观测到的太阳值方面的氘富集不可能发生在亚星云中。相反，我们认为，在泰坦中测量的 D/H 比值是在冷却的太阳星云中通过氢与 CH3D 的同位素热交换获得的，CH3D 源自星云中蒸发的星际甲烷富含 D 的冰。同位素交换速率随温度降低，并在 200 K 左右完全受到抑制。随后，甲烷以 60 K 形成的笼形水合物的形式被捕获在 10 AU 左右的结晶冰中，并并入形成泰坦核心的星子中。随后，氮-甲烷气氛因水合物的分解而被排出（Mousis 等，2002）。通过使用太阳星云的湍流演化模型（O. Mousis et al. 2000, Icarus148, 513–525），我们重建了 CH4 中 D/H 的整个故事，从早期太阳星云中的高值（在前太阳云中获得）到今天在土卫六大气中测量到的值。考虑到土卫六中 D/H 比率的最后两次测定——从地面观测中获得的 D/H=(7.75±2.25)×10−5 (Orton 1992, In: Symposium on Titan, ESA SP-338, 81–85)，以及从 ISO 观测中获得的 D/H=(8.75+3.25−2.25)×10−5 (Coustenis 等人) al. 2002，提交出版）——我们推断出早期外太阳星云中甲烷的 D/H 比上限约为 3×10−4。我们的方法与几位作者提出的设想一致，即土卫六大气中的甲烷是由位于土卫六内部的高压 CH4 笼形水合物储层不断补充的。如果这种情况是正确的，那么 2004 年至 2008 年卡西尼-惠更斯任务航天器上的雷达、成像系统和其他遥感仪器对卫星的观测应该会揭示出表面的局部破坏和预测的除气的其他特征。
Abstract We propose a new interpretation of the D/H ratio in CH4 observed in the atmosphere of Titan. Using a turbulent evolutionary model of the subnebula of Saturn (O. Mousis et al. 2002, Icarus156, 162–175), we show that in contrast to the current scenario, the deuterium enrichment with respect to the solar value observed in Titan cannot have occurred in the subnebula. Instead, we argue that values of the D/H ratio measured in Titan were obtained in the cooling solar nebula by isotopic thermal exchange of hydrogen with CH3D originating from interstellar methane D-enriched ices that vaporized in the nebula. The rate of the isotopic exchange decreased with temperature and became fully inhibited around 200 K. Methane was subsequently trapped in crystalline ices around 10 AU in the form of clathrate hydrates formed at 60 K, and incorporated into planetesimals that formed the core of Titan. The nitrogen–methane atmosphere was subsequently outgassed from the decomposition of the hydrates (Mousis et al. 2002). By use of a turbulent evolutionary model of the solar nebula (O. Mousis et al. 2000, Icarus148, 513–525), we have reconstructed the entire story of D/H in CH4, from its high value in the early solar nebula (acquired in the presolar cloud) down to the value measured in Titan's atmosphere today. Considering the two last determinations of the D/H ratio in Titan—D/H=(7.75±2.25)×10−5 obtained from ground-based observations (Orton 1992, In: Symposium on Titan, ESA SP-338, pp. 81–85), and D/H=(8.75+3.25−2.25)×10−5, obtained from ISO observations (Coustenis et al. 2002, submitted for publication)—we inferred an upper limit of the D/H ratio in methane in the early outer solar nebula of about 3×10−4. Our approach is consistent with the scenario advocated by several authors in which the atmospheric methane of Titan is continuously replenished from a reservoir of clathrate hydrates of CH4 at high pressures, located in the interior of Titan. If this scenario is correct, observations of the satellite to be performed by the radar, the imaging system, and other remote sensing instruments aboard the spacecraft of the Cassini–Huygens mission from 2004 to 2008 should reveal local disruptions of the surface and other signatures of the predicted outgassing.
影响因子：
3.2
作者：
V. Mannings;A. Boss;S. Russell
通讯作者：
V. Mannings;A. Boss;S. Russell