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On the volatile inventory of Titan from isotopic abundances in nitrogen and methane

Lunine, Jonathan I. and Yung, Yuk L. and Lorenz, Ralph D. (1999) On the volatile inventory of Titan from isotopic abundances in nitrogen and methane. Planetary and Space Science, 47 (10-11). pp. 1291-1303. ISSN 0032-0633. doi:10.1016/S0032-0633(99)00052-5.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription
Lunine, Jonathan I.0000-0003-2279-4131
Yung, Yuk L.0000-0002-4263-2562
Lorenz, Ralph D.0000-0001-8528-4644
Additional Information:© 1999 Elsevier Science Ltd. Received 9 November 1998; received in revised form 1 February 1999; accepted 22 February 1999. Available online 24 November 1999. The first author is most grateful to Dr Angioletta Coradini for hosting his sabbatical at the Istituto di Astrofisica Spaziale, Rome, during the preparation of this paper. Helpful and substantive referee suggestions, as well as those of Dr Daniel Gautier, improved the quality of the work. Support for the project and paper were provided by the Consiglio Nazionale della Ricerche and NASA's Atmospheres and Geoscience programs.
Group:UNSPECIFIED, Division of Geological and Planetary Sciences
Funding AgencyGrant Number
Consiglio Nazionale della RicercheUNSPECIFIED
NASA Atmospheres and Geoscience programsUNSPECIFIED
Issue or Number:10-11
Record Number:CaltechAUTHORS:20140827-134733578
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Official Citation:Jonathan I. Lunine, Yuk L. Yung, Ralph D. Lorenz, On the volatile inventory of Titan from isotopic abundances in nitrogen and methane, Planetary and Space Science, Volume 47, Issues 10–11, October–November 1999, Pages 1291-1303, ISSN 0032-0633, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48996
Deposited By: Ruth Sustaita
Deposited On:27 Aug 2014 21:03
Last Modified:10 Nov 2021 18:38

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