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

Abstract

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) ®rst 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 su cient 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. #

Protosolar Nitrogen

“…On Titan, we know only that atmospheric nitrogen reveals evidence of massive early escape (Marten et al 1997;Lunine, Yung, & Lorenz 1999;Lammer et al 2000), while the data for Venus (Hoffman, Oyama, & von Zahn 1980) are not yet precise enough to allow what will ultimately be a highly informative comparison with Earth.…”

An estimate of the D/H ratio in Jupiter and Saturn's regular icy satellites – Implications for the Titan Huygens mission

“…Up until now, two physical processes have been proposed to explain the D-enrichment in the methane of Titan. The first process, described by Pinto et al (1986) and Lunine et al (1999), suggests that the photodissociated methane is replenished by evaporation from a reservoir located on the surface. Lunine et al (1999) calculated that the solar photolysis of the initially non-D-enriched CH 4 over a 4.5 byr timescale can explain today's observed D/H ratio in methane, assuming a closed system for the surface-atmospheric methane system.…”

“…The first process, described by Pinto et al (1986) and Lunine et al (1999), suggests that the photodissociated methane is replenished by evaporation from a reservoir located on the surface. Lunine et al (1999) calculated that the solar photolysis of the initially non-D-enriched CH 4 over a 4.5 byr timescale can explain today's observed D/H ratio in methane, assuming a closed system for the surface-atmospheric methane system. The second process, proposed by Mousis et al (2002b), is based on the scenario of the formation of Titan described by Paper I and reported in Sect.…”

Crossed Molecular Beam Study on the Formation of Phenylacetylene and Its Relevance to Titan’s Atmosphere

“…12 Finally, PAHs are thought to be crucial building blocks leading to the formation of the orange-reddish haze layers on Saturn's moon Titan. [13][14][15][16][17][18][19][20][21] But how are PAHs formed in these extreme environments? Multiple experimental studies have been completed in the last decades, where PAHs and nanosized soot particles were observed ranging from the hydrocarbon rich flame chemistry studies [22][23][24] to the shock wave experiment of Mimura 25 and the laser ablation experiment of graphite under different quenching atmospheres by Jäger et al 26 Chemical reaction networks that model the formation of PAHs in combustion flames [27][28][29] and in the interstellar medium 30 stress the importance of the phenylacetylene molecule (C 6 H 5 CCH) in the growth of PAHs starting from an initial hydrogen abstraction/acetylene addition sequence via the phenyl radical.…”