A primordial origin for the atmospheric methane of Saturn’s moon Titan

Abstract

The origin of Titan's atmospheric methane is a key issue for understanding the origin of the Saturnian satellite system. It has been proposed that serpentinization reactions in Titan's interior could lead to the formation of the observed methane. Meanwhile, alternative scenarios suggest that methane was incorporated in Titan's planetesimals before its formation. Here, we point out that serpentinization reactions in Titan's interior are not able to reproduce the deuterium over hydrogen (D/H) ratio observed at present in methane in its atmosphere, and would require a maximum D/H ratio in Titan's water ice 30% lower than the value likely acquired by the satellite during its formation, based on Cassini observations at Enceladus.Alternatively, production of methane in Titan's interior via radiolytic reactions with water can be envisaged but the associated production rates remain uncertain. On the other hand, a mechanism that easily explains the presence of large amounts of methane trapped in Titan in a way consistent with its measured atmospheric D/H ratio is its direct capture in the satellite's planetesimals at the time of their formation in the solar nebula.In this case, the mass of methane trapped in Titan's interior can be up to ∼1,300times the current mass of atmospheric methane.

Constraints from Comets on the Formation and Volatile Acquisition of the Planets and Satellites

“…Furthermore, if Titan had formed at such high temperatures, the D/H in water ice of Titan would be lower than that of OCCs (Owen 2008), which disagrees with the D/H measurement at Enceladus (Mousis et al 2009a). As shown in Table 4, we can assume that Titan and Enceladus formed under similar conditions and that Titan's D/H in H 2 O is also similar to that of OCCs (Mousis et al 2009a). A D/H similar to that of OCCs requires very low formation temperatures, like those found in the PSN, and excludes the possibility of formation temperatures above 100 K. This suggests that N 2 and CO could have initially been trapped in the building blocks of Titan.…”

“…While each of these measurements provides insight into the evolution of Titan's atmosphere, the most important values for evaluating Titan's formation are the three that have been determined to represent primordial composition of the building blocks of Titan (italicized in Table 4). The D/H in water at Titan can be presumed to be in the same range as Xe/N Xe < 5 × 10 −9c -2 .58 × 10 −6d was determined for Enceladus (Waite et al 2009;Mousis et al 2009a). The limited range of values for primordial D/H in methane is based on limits placed by the evolution of 12 C/ 13 C in Titan's methane (Mandt et al 2009(Mandt et al , 2012, while the upper limit for 14 N/ 15 N in Titan's nitrogen inventory is based on the maximum possible fractionation allowed due to escape processes (Mandt et al 2014).…”

“…Fortes and Stofan 2005;Osegovic and Max 2005;. Furthermore, if Titan had formed at such high temperatures, the D/H in water ice of Titan would be lower than that of OCCs (Owen 2008), which disagrees with the D/H measurement at Enceladus (Mousis et al 2009a). As shown in Table 4, we can assume that Titan and Enceladus formed under similar conditions and that Titan's D/H in H 2 O is also similar to that of OCCs (Mousis et al 2009a).…”

“…Prinn and Fegley 1989), although some may have formed from building blocks formed earlier in the PSN that migrated into the subnebula (e.g. Mousis et al, 2009aMousis et al, , 2009bMandt et al 2014). Others are thought to be planetesimals that were captured by the planet (e.g.…”

“…Satellites that are formed early in the process of giant planet formation are predicted have a low ice-to-rock ratio compared to those that formed later, so the high ice-to-rock ratio of most of the giant planet satellites is interpreted to mean that they formed later in the process of planetary formation (Canup and Ward 2006). However, it is also possible that these satellites formed at a greater distance from their parent planet and migrated to their current position (Mousis et al 2009a;Crida and Charnoz 2012). Comparing the primordial composition and isotopic ratios of noble gases and carbon, nitrogen, hydrogen and oxygen to solar composition and the composition of comets provides important clues as to the formation conditions of these satellites.…”