We analyze recently published nitrogen and hydrogen isotopic data to constrain the initial volatile abundances on Saturn's giant moon Titan. The nitrogen data are interpreted in terms of a model of non-thermal escape processes that lead to enhancement in the heavier isotope. We show that these data do not, in fact, strongly constrain the abundance of nitrogen present in Titan's early atmosphere, and that a wide range of initial atmospheric masses (all larger than the present value) can yield the measured enhancement. The enrichment in deuterated methane is now much better determined than it was when Pinto et al. (1986. Nature 319, 388-390) first proposed a photochemical mechanism to preferentially retain the deuterium. We develop a simple linear theory to provide a more reliable estimate of the relative dissociation rates of normal and deuterated methane. We utilize the improved data and models to compute initial methane reservoirs consistent with the observed enhancement. The result of this analysis agrees with an independent estimate for the initial methane abundance based solely on the present-day rate of photolysis and an assumption of steady state. This consistency in reservoir size is necessary but not sufficient to infer that methane photolysis has proceeded steadily over the age of the solar system to produce large quantities of less volatile organics. Our analysis indicates an epoch of early atmospheric escape of nitrogen, followed by a later addition of methane by outgassing from the interior. The results also suggest that Titan's volatile inventory came in part or largely from a circum-Saturnian disk of material more reducing than the surrounding solar nebula. Many of the ambiguities inherent in the present analysis can be resolved through Cassini-Huygens data and a program of laboratory studies on isotopic and molecular exchange processes. The value of, and interest in, the Cassini-Huygens data can be greatly enhanced if such a program were undertaken prior to the prime phase of the mission.

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从氮和甲烷的同位素丰度看泰坦的挥发性清单

我们分析了最近公布的氮和氢同位素数据，以限制土星巨大卫星泰坦上的初始挥发性丰度。氮数据根据非热逃逸过程模型进行解释，该模型导致较重同位素的增强。我们表明，这些数据实际上并没有强烈限制泰坦早期大气中存在的氮丰度，并且广泛的初始大气质量（均大于当前值）可以产生测量的增强。氘代甲烷的富集度现在比 Pinto 等人时更好地确定。(1986. Nature 319, 388-390) 首先提出了优先保留氘的光化学机制。我们开发了一个简单的线性理论，以更可靠地估计正态甲烷和氘代甲烷的相对解离速率。我们利用改进的数据和模型来计算与观察到的增强一致的初始甲烷储层。该分析的结果与仅基于当前光解速率和稳态假设对初始甲烷丰度的独立估计一致。储层大小的这种一致性是必要的，但不足以推断甲烷光解在太阳系的整个年龄阶段稳定进行，以产生大量挥发性较低的有机物。我们的分析表明，这是一个早期大气中氮逸出的时代，随后是通过从内部放气而添加的甲烷。结果还表明，泰坦' 不稳定的库存部分或主要来自土星周围的物质盘，比周围的太阳星云还原性更强。通过卡西尼-惠更斯数据和一项关于同位素和分子交换过程的实验室研究计划，可以解决当前分析中固有的许多含糊不清的问题。如果在任务的主要阶段之前进行这样的计划，卡西尼 - 惠更斯数据的价值和兴趣可以大大提高。