The absence of endogenic methane on Titan and its implications for the origin of atmospheric nitrogen

Icarus 204 (2009) 637–644

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The absence of endogenic methane on Titan and its implications for the origin of atmospheric nitrogen Christopher R. Glein a,*, Steven J. Desch a, Everett L. Shock a,b a b

School of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, AZ 85287-1404, USA Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA

a r t i c l e

i n f o

Article history: Received 10 April 2009 Revised 20 June 2009 Accepted 23 June 2009 Available online 27 June 2009 Keywords: Titan Saturn, satellites Satellites, atmospheres Atmospheres, evolution Cosmochemistry

a b s t r a c t We calculate the D/H ratio of CH4 from serpentinization on Titan to determine whether Titan’s atmospheric CH4 was originally produced inside the giant satellite. This is done by performing equilibrium isotopic fractionation calculations in the CH4–H2O–H2 system, with the assumption that the bulk D/H ratio of the system is equivalent to that of the H2O in the plume of Enceladus. These calculations show that the D/H ratio of hydrothermally produced CH4 would be markedly higher than that of atmospheric CH4 on Titan. The implication is that Titan’s CH4 is a primordial chemical species that was accreted by the moon during its formation. There are two evolutionary scenarios that are consistent with the apparent absence of endogenic CH4 in Titan’s atmosphere. The first is that hydrothermal systems capable of making CH4 never existed on Titan because Titan’s interior has always been too cold. The second is that hydrothermal systems on Titan were sufficiently oxidized so that C existed in them predominately in the form of CO2. The latter scenario naturally predicts the formation of endogenic N2, providing a new hypothesis for the origin of Titan’s atmospheric N2: the hydrothermal oxidation of 15N-enriched NH3. A primordial origin for CH4 and an endogenic origin for N2 are self-consistent, but both hypotheses need to be tested further by acquiring isotopic data, especially the D/H ratio of CH4 in comets, and the 15N/14N ratio of NH3 in comets and that of N2 in one of Enceladus’ plumes. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction The presence of methane (CH4) in the atmosphere of Saturn’s largest moon Titan is a long-standing mystery (Owen, 1982, 2000; Lunine et al., 1989; Coustenis, 2005; Owen and Niemann, 2009). Because Titan’s atmospheric CH4 should be completely photolyzed over a geologically short timeframe (Yung et al., 1984), it is generally thought that a geophysical mechanism, such as cryovolcanism, has replenished the atmosphere by transporting CH4 from a subsurface clathrate hydrate reservoir to the atmosphere (Loveday et al., 2001; Tobie et al., 2006). While this model may have some observational support (Lopes et al., 2007), there is much more uncertainty concerning the origin of CH4 itself on Titan. Some have argued that the CH4 is primordial, having been accreted by the satellite during its formation (Hersant et al., 2004, 2008; Mousis et al., 2009b). Others have championed the idea that the CH4 was produced by geochemical reactions that occurred inside the moon (Atreya et al., 2006; Owen et al., 2006). The latter hypothesis can be tested by performing equilibrium isotopic frac-

* Corresponding author. Fax: +1 480 965 8102. E-mail address: [email protected] (C.R. Glein). 0019-1035/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2009.06.020

tionation calculations to determine whether the deuterium/hydrogen (D/H) ratio of Titan’s atmospheric CH4 is consistent with an endogenic origin. This is one goal of the present communication (it should be noted that Mousis et al. (2009a) have also been pursuing this goal using a similar strategy). Another objective of the present paper is to investigate the relationship between the geochemistry of C and that of N in Titan’s interior, which leads to the possibility that the N2 in Titan’s atmosphere may be a hydrothermally generated species.

2. The debate There has been controversy concerning the origin of Titan’s CH4 because each of the two leading hypotheses in the literature has various strengths and weaknesses. The primordial hypothesis (Hersant et al., 2004, 2008; Mousis et al., 2009b) is attractive because many comets contain CH4 (Gibb et al., 2003), which implies that CH4 was present in at least some regions of the early outer Solar System. Other icy planetesimals, including those that formed Titan, may have also contained CH4. However, even if CH4 was present during the formation of Titan, it may not have been incorporated into Titan if, for example, the temperature of Saturn’s circumplanetary disk in Titan’s feeding zone was too high for the

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efficient condensation of CH4 into icy materials (Owen et al., 2006). The lack of 36,38Ar, Kr, and Xe in Titan’s atmosphere seems to indicate that some volatiles were not trapped and accreted efficiently (Niemann et al., 2005); the same could also be true for CH4. It should be noted, however, that the paucity of 36,38Ar, Kr, and Xe in Titan’s atmosphere may not be indicative of conditions during Titan’s formation, and may instead be a consequence of processes on Titan, such as the trapping of noble gases in organic aerosols (Jacovi and Bar-Nun, 2008) or clathrate hydrates (Osegovic and Max, 2005; Thomas et al., 2008). The endogenic hypothesis (Atreya et al., 2006; Owen et al., 2006) invokes a familiar terrestrial phenomenon. It has been argued that some abiogenic CH4 is produced in water–rock systems on Earth (Charlou et al., 2002; Sherwood Lollar et al., 2002; Proskurowski et al., 2008). Therefore, it can be inferred that CH4 could have formed on Titan if suitable geochemical conditions existed there. For instance, if Titan accreted a large amount of iron–nickel metal, like the parent bodies of chondrites did (Brearley and Jones, 1998), plenty of reducing potential would have been available to facilitate CH4 formation. It is likely that Titan accreted a large quantity of carbon mainly in the forms of the relatively involatile materials CO2 ice and organic matter, both of which are abundant in a number of comets (Kissel and Krueger, 1987; Bockelée-Morvan et al., 2004) and chondrites (chondrites contain CO2 as carbonate minerals; Brearley and Jones, 1998; Alexander et al., 2007). By analogy with Earth, some of this carbon could have been reduced to CH4 in subseafloor hydrothermal systems (Atreya et al., 2006), assuming that Titan experienced at least some water–rock differentiation, and has a rocky core that is large enough to provide sufficient radiogenic heating for the maintenance of hydrothermal systems (Sohl et al., 2003; Tobie et al., 2006). A potential problem with this scenario is that hydrothermal circulation may be obstructed on Titan; geophysical modeling indicates that a layer of high-pressure ice that is several hundred kilometers thick should exist between a water ocean and a rocky core (Sohl et al., 2003; Tobie et al., 2006). However, the ocean may have been in direct contact with the core during Titan’s early history (Lunine and Stevenson, 1987; Atreya et al., 2006). It is also possible that thermal anomalies at the surface of the core caused localized melting of high-pressure ice and subsequent hydrothermal circulation.

aWM ¼ RH2 O =RCH4 ;

3.1. Model The main goal of this work is to test the plausibility of the endogenic hypothesis by performing thermodynamic calculations. Such calculations are useful, but are restricted by the assumption of chemical equilibrium, which cannot be verified for Titan’s interior (the non-equilibrium case is discussed in Section 4). We focus here on the D/H ratio of Titan’s atmospheric CH4. The formation of CH4 inside Titan can be envisaged to have taken place via the following net geochemical reactions (Zolotov et al., 2005; Owen et al., 2006)

3Fe0 ðsÞ þ 4H2 OðlÞ ! Fe3 O4 ðsÞ þ 4H2 ðaqÞ;

ð1Þ

CO2 ðaqÞ þ 4H2 ðaqÞ ! CH4 ðaqÞ þ 2H2 OðlÞ;

ð2Þ

and

ð3Þ

where s, l, and aq stand for solid, liquid, and aqueous phases, respectively. Before proceeding, it is worth clarifying that, contrary to popular belief, the oxidation of Fe2+-bearing silicate minerals by H2O (Oze and Sharma, 2005) is unlikely to be the main source of endogenic H2 on Titan. The reason for this is that the majority of the Fe in the planetesimals that coalesced into Titan was most likely

ð4Þ

where R denotes D/H ratio, and can be represented by the following polynomial (Horibe and Craig, 1995): aWM = 1.0997 + 8456T2 + 0.9611  109T4  27.82  1012T6 (T stands for absolute temperature) from 273 to 643 K. The D/H ratios in Eq. (4) correspond to final, equilibrium values, which are unknown. However, if we assume that the D/H ratio of Titan’s primordial H2O (Rprim H2 O ) was the same as that of H2O in Enceladus’ plume (and in cometary comae), we can use isotopic mass balance to account for the unknowns. This constraint can be expressed as

Rprim H2 O ¼

3. The methane–water–molecular hydrogen system

2Corganic ðsÞ þ 4H2 ðaqÞ ! 2CH4 ðaqÞ;

present as iron–nickel metal and sulfide (not as ferrous silicates), as in relatively unaltered chondrites (Brearley and Jones, 1998) and cometary particles (Zolensky et al., 2006). The above reactions show that the source of H in hydrothermally produced CH4 is H2O. Consequently, the D/H ratio of Titan’s atmospheric CH4 would be related to that of Titan’s internal H2O if the CH4 was made in hydrothermal systems. The D/H ratio of Titan’s water ice has not been measured (Niemann et al., 2005), so a suitable estimate must be obtained. Recently, the D/H ratio of the H2O in Enceladus’ plume was þ1:5  104 (Waite et al., 2009). Here, it is asdetermined to be 2:90:7 sumed that this value can serve as a proxy for the D/H ratio of H2O on Saturn’s other regular (native) satellites, including Titan. Application to Titan is supported by the similar uncompressed densities of Titan and Enceladus (1600 kg m3; Johnson and Lunine, 2005; Porco et al., 2006), which suggests that these moons are compositionally similar. Moreover, the D/H ratio of H2O in Enceladus’ plume appears to be characteristic of pristine, well-mixed outer Solar System water ice, since it is remarkably similar to that of H2O in Comets Halley, Hyakutake, and Hale-Bopp (3  104; Altwegg and Bockelée-Morvan, 2003; Waite et al., 2009). The D/H ratio of CH4 in the atmosphere of Titan is þ0:15  104 (Bézard et al., 2007), which is significantly less 1:320:11 than that of H2O in the plume of Enceladus. Hence, if Titan’s H2O has a similar D/H ratio as that of Enceladus, Titan’s atmospheric CH4 can have an endogenic origin only if there was a sufficient degree of H–D isotope fractionation between CH4 and H2O in Titan’s interior. We calculate the isotopic fractionation at thermodynamic equilibrium using experimentally acquired data. The equilibrium isotopic fractionation factor between H2O and CH4 (aWM) is

nCH3 D þ nHDO þ nHD ; 4nCH4 þ 2nH2 O þ 2nH2

ð5Þ

where n designates number of moles. This equation only approximates isotopic mass balance because it neglects multiply deuterated species in the numerator, and all deuterated species in the denominator. We include H2 in Eq. (5) because CH4 production is accompanied by H2 generation in hydrothermal systems (Horita and Berndt, 1999; Charlou et al., 2002; Proskurowski et al., 2008; McCollom and Bach, 2009). Note that Eq. (5) assumes that CH4 and H2 are derived entirely from H2O, as indicated by Eqs. (1)–(3). Thus, it should be viewed as a simplified representation since it does not account for other H-bearing species, such as organic compounds (see Section 4). The equation can be transformed into a more useful form by treating the final mole ratios of CH4/H2O and H2/H2O as free parameters, and by specifying RCH4 ¼ nCH3 D = 4nCH4 ; RH2 O ¼ nHDO =2nH2 O , and RH2 ¼ nHD =2nH2 . After substituting these quantities into Eq. (5), the equation reads

Rprim H2 O ¼

2ðnCH4 =nH2 O ÞRCH4 þ RH2 O þ ðnH2 =nH2 O ÞRH2 : 2ðnCH4 =nH2 O Þ þ 1 þ ðnH2 =nH2 O Þ

ð6Þ

Eq. (6) contains three unknowns: the D/H ratios of CH4, H2O, and H2. In order to calculate these quantities, the equilibrium isotopic fractionation factor between H2O and H2 (aWH) must be specified. Experimental data indicate that aWH can be represented by

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the following polynomial (Horibe and Craig, 1995): aWH = 1.0473 + 201,036T2 + 2.060  109T4 + 0.180  1015T6 from 273 to 643 K. The corresponding analytic expression is

aWH ¼ RH2 O =RH2 :

ð7Þ

We can now predict the D/H ratio of hydrothermally produced CH4, as a function of temperature, nCH4 =nH2 O , and nH2 =nH2 O , by solving Eqs. (4), (6), and (7) analytically, which yields

RCH4 ¼

½2ðnCH4 =nH2 O Þ þ 1 þ ðnH2 =nH2 O ÞRprim H2 O 2ðnCH4 =nH2 O Þ þ aWM þ ðaWM =aWH ÞðnH2 =nH2 O Þ

:

ð8Þ

3.2. Results The results of our thermodynamic calculations are shown in Fig. 1. Only a limited region of the parameter space is depicted because most of the parameters give solutions that rapidly approach an asymptote. This asymptote corresponds to the case where both nCH4 =nH2 O and nH2 =nH2 O approach small values. In this case, the D/H ratio of CH4 goes to

RCH4 ¼ Rprim H2 O =aWM :

ð9Þ

This is the mathematical solution that is most relevant to the origin of Titan’s CH4, because it is unlikely that the CH4/H2O and H2/H2O ratios in hydrothermal fluids on Titan would have been high enough to alter the result appreciably. We verified this by performing a couple of sensitivity tests using the full analytical solution (Eq. (8)). In the first test, we explored the sensitivity of the

Fig. 1. D/H ratio comparisons. The solid, black curve corresponds to the predicted D/H ratio of endogenic CH4 on Titan for the case where nH2 O  nCH4 and nH2 O  nH2 . The solid, green curve shows the effect of the CH4/H2O ratio on the D/H ratio of hydrothermally produced CH4 by arbitrarily assuming that nCH4 =nH2 O ¼ 0:1 and nH2 =nH2 O ¼ 0, and the solid, red curve shows the effect of the H2/H2O ratio on the D/ H ratio of hydrothermally produced CH4 by arbitrarily assuming that nCH4 =nH2 O ¼ 0:1 and nH2 =nH2 O ¼ 0:1. The solid, blue line denotes the D/H ratio of H2O in the plume of Enceladus (Waite et al., 2009), and the solid, orange line designates the D/H ratio of CH4 in the atmosphere of Titan (Bézard et al., 2007). Notice that the solid, black curve and the solid, orange line never intersect, despite the wide temperature range that is considered in this figure. This means that isotopic equilibrium in hydrothermal systems is incapable of reproducing the D/H ratio of Titan’s atmospheric CH4. The dashed lines illustrate the outcome of the combination of 1r errors in the determinations of the D/H ratio of Enceladus’ H2O and Titan’s CH4 that bring the black and orange lines closest together. Even in this case, the lines still do not cross. VSMOW stands for Vienna Standard Mean Ocean Water. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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solution to the CH4/H2O ratio by assuming that nCH4 =nH2 O ¼ 0:1 and nH2 =nH2 O ¼ 0. We found that the D/H ratio of CH4 increases as the ratio of CH4/H2O increases (Fig. 1). Nevertheless, the magnitude of this effect only becomes noticeable when the CH4/H2O ratio is greater than 102. In our second sensitivity test, we sought to determine how the inclusion of H2 would affect the solution. In this test, it was assumed that nCH4 =nH2 O ¼ 0:1 and nH2 =nH2 O ¼ 0:1. We found that the D/H ratio of CH4 increases as the ratio of H2/H2O increases (Fig. 1). However, as in the first test, the effect is essentially negligible when the H2/H2O ratio is less than 102. It is evident, then, that Eq. (9) should provide an accurate representation of the D/H ratio of hydrothermally produced CH4 on Titan, unless putative hydrothermal fluids on Titan were exceptionally rich in CH4, H2, or both. Our incomplete understanding of the geochemistry of Titan’s interior certainly permits many (perhaps exotic) possibilities, but it is notable that hydrothermal fluids at terrestrial analogue sites, such as the Lost City (Proskurowski et al., 2006, 2008) and Rainbow (Charlou et al., 2002) submarine hydrothermal systems, have relatively low CH4/H2O and H2/H2O ratios that are on the order of 105 and 105–104, respectively. Fig. 1 shows that if hydrothermal systems on Titan were H2Odominated, the D/H ratio of hydrothermally produced CH4 would be much higher than that of Titan’s atmospheric CH4 (about 560– 810‰ higher, actually). We conclude that this large discrepancy effectively rules out an endogenic origin for Titan’s atmospheric CH4. For the sake of completeness, we examined the effects of formal errors in the determinations of the D/H ratio of Titan’s CH4 (Bézard et al., 2007) and Enceladus’ H2O (Waite et al., 2009). We found that, even if we assume that Titan’s CH4 is as D-enriched and Enceladus’ H2O is as D-depleted as the 1r errors allow, the discrepancy between the D/H ratio of hydrothermally produced CH4 and that of atmospheric CH4 would still be quite large (130– 310‰; Fig. 1). A major shortcoming of the endogenic hypothesis is that the equilibrium isotopic fractionation factor between H2O and CH4 is simply not large enough to compensate for the expected high D/ H ratio of accreted H2O, regardless of temperature (even at a very low temperature of 200 K, we extrapolate a D/H ratio of endogenic CH4 of 2.0  104; temperatures above 643 K make matters even worse for the endogenic hypothesis, since RCH4 ! Rprim H2 O as T ! 1). We estimate that the D/H ratio of primordial H2O would need to be less than 1.7  104 for there to be a solution that would be compatible with the endogenic hypothesis. There are no readily apparent reasons why the D/H ratio of Titan’s bulk H2O should be expected to be so much smaller than that of H2O in Enceladus’ plume (Horner et al., 2008). On the other hand, as mentioned in Section 3.1, there are compelling reasons why they may be similar or identical. Our conclusion that Titan’s atmospheric CH4 has a D/H ratio that is inconsistent with an endogenic origin is likely to be robust, provided that isotopic equilibrium between CH4 and H2O would be attained (or closely approached) in hydrothermal systems on Titan. In the absence of analytical data, we believe that isotopic equilibrium is a reasonable assumption (the non-equilibrium case is taken up in Section 4, however). It is also worth pointing out that, because our calculations are thermodynamic in nature, our results are both path- and time-independent. This means that they do not depend on the specific reactions that would produce CH4, or the potential duration of CH4 production inside Titan. Lastly, we wish to emphasize that our conclusion is general, and does not depend on hydrothermal fluids on Titan having CH4/H2O and H2/H2O ratios that would be similar to those in hydrothermal systems on Earth. Indeed, it can be inferred by examining Fig. 1 that, because of the asymptote, there is no combination of CH4/H2O and H2/ H2O ratios that would yield CH4 with a D/H ratio that is the same as that of Titan’s CH4.

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4. Other factors While illuminating, our simplistic calculations neglected four potentially important phenomena: (1) the effect of atmospheric photochemistry and escape; (2) the possibility of kinetic isotopic fractionation during CH4 generation; (3) the effect of pressure; and (4) the isotopic composition of primordial organic matter. Each of these complexities would change details of our results, but as discussed in this section, these complexities are unlikely to overturn our conclusion. Above, we compared the predicted D/H ratio of hydrothermally produced CH4 to that of Titan’s present-day atmospheric CH4. But, this may not be the proper comparison because atmospheric photochemistry and escape may have enriched Titan’s atmospheric CH4 in D over time (Pinto et al., 1986; Cordier et al., 2008). Strictly speaking, the current D/H ratio of atmospheric CH4 represents an upper limit for the D/H ratio of Titan’s bulk CH4. This means that the discrepancy between the D/H ratio of hydrothermally produced CH4 and that of Titan’s CH4 may be larger than what is shown in Fig. 1. Therefore, the evolution of Titan’s atmosphere makes the endogenic hypothesis even less plausible. All of our calculations assumed that the isotopic fractionation between CH4 and H2O can be described by equilibrium thermodynamics. Yet, one could argue that chemical kinetics rather than thermodynamics may control the fractionation. In this case, the D/H ratio of endogenic CH4 would be expected to be much lower than that of primordial H2O because a kinetic isotope effect would produce CH4 faster than CH3D. If the kinetic isotope effect were strong enough, the D/H ratio of hydrothermally produced CH4 might be consistent with that of Titan’s atmospheric CH4. But, how likely is this? We can answer this question by defining an empirical H2O–CH4 kinetic isotopic fractionation factor (bWM) as

bWM ¼ RH2 O =RCH4 :

ð10Þ

The endogenic hypothesis would be consistent with the D/H ratio of Enceladus’ H2O and Titan’s CH4 if bWM were 2.9/1.32 = 2.2. We can test the hypothesis of extreme kinetic isotopic fractionation during the formation of CH4 by calculating the D/H ratio of CH4 in hydrothermal fluids on Earth using the above bWM value. The predicted D/H ratio can then be compared to measured D/H ratios to determine whether the large bWM value that is required by the endogenic hypothesis is consistent with the isotope geochemistry of water–rock systems that may be analogous to those on Titan. The D/H ratio of H2O in terrestrial submarine hydrothermal systems is essentially the same as that of H2O in seawater (Proskurowski et al., 2006), so RH2 O ¼ 1:5575  104 . Using this value, we compute that the dD of CH4 (referenced to Vienna Standard Mean Ocean Water) would be 550‰ if bWM = 2.2. This value is much more negative than that of CH4 in submarine hydrothermal systems on Earth, which generally ranges between 140‰ and 95‰ (Proskurowski et al., 2006). The implication is that if there were reaction pathways in suboceanic hydrothermal systems on Titan that were similar to those in terrestrial submarine hydrothermal systems, the production of CH4 on Titan would have only a weak kinetic isotope effect, which would be insufficient from the perspective of the endogenic hypothesis. On the other hand, it should be mentioned that severely D-depleted CH4 does occur on Earth (bWM = 1.6–1.7; Sherwood Lollar et al., 2008), but even this, the most D-poor CH4 that has been found in terrestrial hydrothermal systems, has a bWM that is considerably less than that which is needed to make the endogenic hypothesis consistent with the D/H ratio of Titan’s CH4. Also, the relevance of this CH4 to Titan is questionable because it does not occur in a marine environment, which is likely to be global in extent on Titan (Lorenz et al., 2008a). All of

this suggests that an extreme H2O–CH4 kinetic isotope effect on Titan is a remote possibility, although we cannot rule out this possibility completely, since we cannot verify that utterly unfamiliar geochemical processes have not occurred inside Titan. It is also important to consider the effect of pressure on the equilibrium isotopic fractionation factor between H2O and CH4 because hydrothermal systems hosted within a rocky core on Titan would be at much higher pressures (10–35 kbar; Lunine and Stevenson, 1987) than the pressures to which the fractionation factor used in this study corresponds (1–210 bar; Horibe and Craig, 1995). To the best of our knowledge, no one has studied pressure effects on H–D isotope fractionation in the CH4–H2O system. Thus, we cannot determine the fractionation factor under conditions that are truly representative of Titan. It seems likely, however, that high pressures would not shift the curves in Fig. 1 by a large enough amount so that endogenic CH4 would be as D-poor as atmospheric CH4. This is because, in a single fluid phase, the difference in molar volume between HDO and H2O is likely to be mostly canceled out by a corresponding difference in molar volume between CH3D and CH4, minimizing the pressure dependence of the fractionation factor (Horita et al., 2002). Overall, a detailed investigation of the effect of pressure would probably not change our conclusion that Titan’s atmospheric CH4 does not have an endogenic origin, although such a study would certainly be beneficial, as it would allow us to make sure that pressure does indeed have only a minimal effect. In our isotopic mass balance equation (Eq. (5)), it was assumed that the bulk D/H ratio of the modeled system is that of accreted H2O only. However, water–rock systems on Titan would almost certainly contain organic matter, another source of H + D. Although chondritic and cometary organic matter is rich in C, our idealized representation of it (see Eq. (3)) is not technically correct because the material does contain some H (Kissel and Krueger, 1987; Alexander et al., 2007). If subjected to hydrothermal conditions, Titan’s primordial, carbonaceous rocks may be expected to produce CH4, as in Eq. (3). However, the most primitive organic material that has been found in carbonaceous chondrites is highly D-enriched, with a bulk dD of roughly +3000‰ (D/H = 6  104; Alexander et al., 2007). If Titan accreted organic material that had a similar D enrichment (which was probably unavoidable given that this material appears to be so isotopically primitive; Alexander et al., 2007), that material would increase the bulk D/H ratio of hydrothermal fluids beyond that of primordial H2O, which would increase the D/H ratio of hydrothermally produced CH4 beyond that which is shown in Fig. 1. Therefore, thermally processed organic matter would make CH4 with a D/H ratio that is even less consistent with that of Titan’s CH4.

5. Implications Equilibrium isotopic fractionation calculations demonstrate that Titan’s atmospheric CH4 has a D/H ratio that is dramatically less than what would be possible if the CH4 were produced inside the moon. Above, we considered several scenarios that would complicate the calculations, but deduced that those complexities would most likely not alter our conclusion – Titan’s CH4 does not show isotopic evidence of an endogenic origin. Where, then, did the CH4 originally come from? The primordial hypothesis may provide a satisfying answer to this question. According to this model, presolar, D-rich CH4 ices were vaporized in the solar nebula, and subsequently exchanged some H/D with isotopically light H2 in the gas phase (Mousis et al., 2002). In time, the cooling of the nebula led to the formation of clathrate hydrates containing CH4 (Hersant et al., 2004, 2008; Mousis et al., 2009b). Icy planetesimals containing those clathrates were eventu-

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ally accreted by Titan in a somewhat warm (>50 K) Saturnian subnebula, where CO, 36Ar, and N2 were largely present as untrapped gases (Mousis et al., 2009b), consistent with the deficiency of CO and 36Ar in Titan’s atmosphere (Niemann et al., 2005; de Kok et al., 2007). While this model does illustrate a mechanism that can reproduce the D/H ratio of Titan’s CH4, the argument is not entirely compelling because the validity of the model cannot yet be tested, as the D/H ratio of solar nebula CH4 is presently unknown. A crucial test of the primordial hypothesis will thus be the determination of the D/H ratio of CH4 in comets (e.g., Gulkis and Alexander, 2008). Why is there no evidence of endogenic CH4 in Titan’s atmosphere, and what does its absence imply about the geochemistry of Titan’s interior? The following discussion offers three possible explanations for the first state of affairs: (1) sufficiently hot hydrothermal systems never existed on Titan, so CH4 could not be produced; (2) CH4 was produced in hydrothermal systems, but that CH4 has not been able to get into Titan’s atmosphere, unlike primordial CH4; and (3) C-bearing compounds other than CH4 formed in hydrothermal systems. These scenarios are discussed in Section 5.1, and a key implication stemming from the third one is detailed in Section 5.2. 5.1. Possible reasons why Titan’s atmosphere does not contain endogenic methane While it is generally assumed that Titan has a fully differentiated interior (Sohl et al., 2003; Tobie et al., 2006), this is by no means guaranteed. The case of Callisto demonstrates that not all large icy satellites experience complete water–rock separation (Anderson et al., 2001). A weakly differentiated Titan would be expected to have a relatively cold interior (like Callisto; Nagel et al., 2004), and as a result, would also be expected to be an unfavorable setting for the production of CH4 (owing to slow reaction rates at low temperatures; Seewald et al., 2006). A rule of thumb among terrestrial geochemists, which is guided by observations, is that CH4 formation is generally kinetically inhibited at temperatures below 300 °C (Shock, 1990). Thus, it does not seem likely (although we cannot strictly rule out the possibility) that CH4 could be generated inside a weakly differentiated Titan in the presence of cold ices and maybe some liquid water. On the other hand, hydrothermal systems capable of making CH4 may have existed on Titan, as the inferred presence of N2 in Enceladus’ plume (Waite et al., 2009) suggests that moon once had a hot interior (Matson et al., 2007; Glein et al., 2008), and it is difficult to imagine that hot hydrothermal systems existed on tiny Enceladus but not on huge Titan, especially if the main source of heat on early Enceladus was short-lived radioactivity (Schubert et al., 2007). It is notable that the case of Callisto contradicts this reasoning, as Callisto is large but partially differentiated, but the Jovian and Saturnian systems certainly had different accretional and tidal histories. Because the evidence is equivocal, it is possible that Titan never had hydrothermal systems that were hot enough for the production of CH4. This hypothesis can be tested soon. The key quantity is the moment-of-inertia factor of Titan, which will be derived using gravity data from the Cassini spacecraft (Rappaport et al., 1997). The absence of a rocky core would point to a lack of hot hydrothermal systems on Titan. In contrast, the presence of even a small core would suggest that such systems may have existed. Another possible explanation for the lack of endogenic CH4 in Titan’s atmosphere is that endogenic CH4 was trapped in Titan’s interior, perhaps in the form of a clathrate hydrate (Loveday et al., 2001). This scenario assumes that the source of Titan’s atmospheric CH4 is a shallow reservoir of primordial CH4, and that a deep reservoir of hydrothermally produced CH4 does not contribute CH4 to the atmosphere because it is too far from the surface-

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atmosphere system. Indeed, a rocky core inside Titan may be covered by a clathrate–ice–liquid water layer that could be nearly 1000 km thick (Sohl et al., 2003). We can determine whether the transport of CH4 over this long distance would be prohibitive by considering the outgassing of 40Ar. Argon-40 is a product of the radioactive decay of 40K, which is present in silicate minerals, salts, or aqueous species in Titan’s interior (Engel et al., 1994). The fact that 40Ar was detected in Titan’s atmosphere means that there are processes on Titan that transport internally generated gases to the atmosphere (Waite et al., 2005; Niemann et al., 2005). While CH4 may have a different outgassing efficiency than 40Ar does, it seems unreasonable to think that deep-seated CH4 would be immune to outgassing whereas 40Ar is not. Instead, it seems more likely that if endogenic CH4 existed, some of it would have gotten into Titan’s atmosphere. Once there, the CH4 would contribute a D/H ratio that is very different from the observed value. The third possible reason why Titan’s atmosphere does not contain hydrothermally produced CH4 is that CH4 formation was not thermodynamically favorable in hydrothermal systems on Titan. At chemical equilibrium, the partitioning of C between CH4 and CO2 is a function of temperature, pressure, and oxidation state (Shock, 1992). The formation of CH4 is favored by low temperatures, high pressures, and reducing conditions. We are neglecting CO because only a small amount of CO is normally present at equilibrium in hydrothermal fluids (Shock and McKinnon, 1993). CH4 would not have been produced in hydrothermal systems on Titan if those systems were sufficiently oxidized. Thus, the absence of endogenic CH4 in Titan’s atmosphere may imply that Titan had (and could still have) oxidized hydrothermal systems. It would not be surprising if oxidized materials are present deep within Titan. An oxidized interior would be consistent with the high inferred water/rock mass ratio of the satellite (0.8; Sohl et al., 2003); the first-order trend that is observed in the Solar System is that where there is more water, there is more oxidation. As an example, the upper mantle of Earth (Frost and McCammon, 2008) is more oxidized than the mantle of Io (Zolotov and Fegley, 2000) and that of the Moon (Wadhwa, 2008), two dry bodies. In addition, degree of oxidation correlates with degree of aqueous alteration in meteorites, with CI chondrites being the most aqueously altered and oxidized (Rubin et al., 1988). This is because H2O is the most abundant potential oxidizing agent in the Solar System. Overall, geochemical conditions that favor the formation of CO2 may be expected to exist in the interior of H2O-rich Titan. This scenario can be examined quantitatively by constructing a plot of the equilibrium predominance fields of CH4 and CO2 in aqueous solution as a function of oxidation state and temperature (Fig. 2). A predominance field encompasses the geochemical conditions at which an indicated species has the highest activity (concentration for neutral species) in a modeled system. Fig. 2 was made using the methodology of Glein et al. (2008), and corresponds to a total pressure of 5 kbar, the maximum pressure at which the thermodynamic data for aqueous species in the SUPCRT92 database are valid (Shock et al., 1989; Shock and Helgeson, 1990). Because the considered pressure is lower than those below the surface of a rocky core inside Titan (>10 kbar; Lunine and Stevenson, 1987), the positions of the curves in Fig. 2 should be regarded as an approximation to the situation that may exist on Titan. Higher-pressure thermodynamic data will need to be obtained to pinpoint the positions of the curves with respect to hydrothermal systems on Titan. Curves corresponding to the mineral redox buffers fayalite–magnetite–quartz (FMQ) and magnetite–hematite (MH) were included in Fig. 2 to provide geochemical context for the oxidation state variable, hydrogen fugacity (fH2). Note that Scott et al. (2002) also explored the speciation of C inside large icy satellites, such as Titan.

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Fig. 2. Equilibrium geochemistry of C and N in hydrothermal fluids on Titan. The blue curve indicates where CH4 and CO2 have equal activities, and the red curve designates where NH3 and N2 have equal activities. In the region above the blue/red curve, CH4/NH3 is the predominant C/N species, while CO2/N2 is the predominant C/ N species in the region below the blue/red curve. The formation of reduced species is favored by lower temperatures and higher hydrogen fugacities, whereas the formation of oxidized species is favored by higher temperatures and lower hydrogen fugacities. The green curve labeled FMQ refers to the fayalite–magnetite–quartz buffer, and the orange curve labeled MH denotes the magnetite– hematite buffer. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

CO2 would have been more stable than CH4 over a range of conditions in hydrothermal systems on Titan. Indeed, Fig. 2 shows that the formation of CO2 is more thermodynamically favorable than that of CH4 over a large region of fH2-T space, which widens at lower hydrogen fugacities and higher temperatures. It is possible that hydrothermal systems on Titan were somewhere in that zone. It is likely that Titan’s hydrothermal systems would have been more oxidized than the FMQ buffer, which provides a good description of the oxidation state of Earth’s upper mantle (Kasting et al., 1993). The rationale for this is that Titan has far more H2O (oxidant) than Earth does. It is more difficult to set an upper limit on the degree of oxidation of Titan’s rocky interior. Zolotov and Shock (2003a) inferred that sulfate-bearing hydrothermal fluids on Europa could be as oxidized as the MH buffer. If hydrothermal systems on Titan had a Europa-like oxidation state, CO2 would have been more stable than CH4 even in slightly warm (>60 °C) fluids (Fig. 2). Titan’s rocky interior could have been driven to the MH buffer by pervasive water–rock reactions and H2 escape (Zolotov and Shock, 2003b). 5.2. The potential existence of endogenic molecular nitrogen in Titan’s atmosphere A major implication of the possible existence of oxidized hydrothermal systems in Titan’s interior is the production of N2 from the oxidation of NH3. N2 is the most thermodynamically stable form of N in oxidized (but not oxygenated) water–rock systems, as shown in Fig. 2. It should be noted that the location of the boundary between the predominance fields of NH3 and N2 depends on the initial concentration of NH3 in an aqueous solution (Glein et al., 2008). In Fig. 2, it is assumed that primordial waters on Titan had an NH3/H2O mole ratio of 9  103, the ratio of NH3/H2O in Enceladus’ plume (Waite et al., 2009), which is similar to the ratios measured in several comets (5–15  103; Bockelée-Morvan et al., 2004). Fig. 2 shows that the predominance fields of CO2 and N2 overlap almost completely. It can also be seen that all geochemical condi-

tions that favor the formation of CO2 over that of CH4 also favor the formation of N2 over that of NH3 (Fig. 2). Therefore, it is reasonable to suspect that there would be endogenic N2 on Titan if oxidized, CO2-bearing hydrothermal systems existed there. What is so intriguing about this is that N2 is indeed present on Titan, in the form of a massive, N2-dominated atmosphere (Niemann et al., 2005). This raises the possibility that Titan’s atmospheric N2 was originally produced in hydrothermal systems. There are several additional arguments that support this hypothesis, which are presented below. First, a relevant geochemical analogue may exist in Enceladus. The inferred presence of N2 in Enceladus’ plume (Waite et al., 2009) can be explained by the oxidation of primordial NH3 to N2 in hydrothermal systems on early Enceladus (Matson et al., 2007; Glein et al., 2008). The production of N2 could well be an inescapable consequence of the geochemical evolution of geologically active icy worlds. The CH4/N2 mole ratio of Enceladus’ plume (0.83) is also illuminating (we assume that the somewhat ambiguous mass 28 species in the E5 INMS spectrum corresponds to N2 only; details can be found in Waite et al., 2009). The partial pressure of N2 in Titan’s near-surface atmosphere is 1.4 bar (Niemann et al., 2005). If we assume that both CH4 and N2 are delivered to Titan’s atmosphere by the outgassing of clathrate hydrates (Tobie et al., 2006) having the same CH4/N2 ratio as those on Enceladus (Kieffer et al., 2006), we can estimate the total amount of CH4 that has been injected into the surface-atmosphere system. This quantity turns out to be equivalent to 1.2 bar (0.83  1.4 bar), which agrees, to within an order-of-magnitude, with the assessment of Lorenz et al. (2008b), who estimated that the observable organic inventory of Titan is equivalent to about 0.10–0.13 bar CH4. This consistency suggests that putative cryovolcanic gases on Titan may be compositionally similar to those on Enceladus, and may thus contain both CH4 and N2, which are, of course, the dominant constituents of the atmosphere of Titan (Niemann et al., 2005). Second, the 15N/14N ratio of endogenic N2 may be consistent with that of Titan’s atmospheric N2 (5.46  103; Niemann et al., 2005). Chemical modeling of ion–molecule reactions in dense molecular cloud cores suggests that interstellar NH3 ice may be heavily enriched in 15N (Charnley and Rodgers, 2002), with the fractionation most sensitive to the initial N0/N2 mole ratio of the reacting gas (Rodgers and Charnley, 2008). The model of Rodgers and Charnley (2008) predicts that such reactions would produce NH3 ice having the same 15N/14N ratio as that of Titan’s N2 if 70% of interstellar N were initially N2. It follows that it is possible that presolar NH3 ice had an isotopic composition that was very similar to that of Titan’s atmospheric N2. The existence of NH3 in a number of comets suggests that presolar NH3 was present in the early outer Solar System (Lunine and Gautier, 2004). The corollary is that if Titan accreted NH3 (as did Enceladus; Waite et al., 2009), the relatively high 15N/14N ratio of Titan’s atmospheric N2 could be explained by the hydrothermal oxidation of 15N-rich NH3 to N2. And third, a hydrothermal model for the formation of Titan’s N2 may be consistent with the amount of N2 in Titan’s atmosphere (1.4 bar; Niemann et al., 2005). We can estimate the maximum amount of N2 that could have been made from the hydrothermal oxidation of NH3 by assuming that all of Titan’s primordial NH3 was converted to N2. To perform this calculation, we also assume that the mass of H2O on Titan is 6.1  1022 kg (Sohl et al., 2003), and that the NH3/H2O mole ratio of the ices that Titan accreted was the same as that of Enceladus’ plume (9  103; Waite et al., 2009). Using these values, we compute a theoretical yield of hydrothermally produced N2 of 70 bar. Therefore, it is likely that Titan started with more than enough NH3 to account for all of the N2 in its atmosphere. In fact, as little as 2% (1.4/70) of Titan’s primordial NH3 would have needed to be converted to N2 in this model

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(the conversion efficiency would have been greater than 2% if some endogenic N2 is still trapped in Titan’s interior). At this time, it is not possible to quantify the efficiencies of the mechanisms of the production and transport of N2 inside Titan. Thermal evolution models that include hydrothermal circulation will need to be developed to constrain the extent and duration of potential hydrothermal activity on Titan. The outgassing of 40Ar will also need to be studied to gain a better understanding of the transport of gases from Titan’s interior to its atmosphere. Altogether, it is evident that more work must be done to rigorously test the hypothesis of endogenic N2 on Titan, although the arguments presented above show that this model is plausible. We close by mentioning that there are other hypotheses for the origin of Titan’s atmospheric N2. The two that are consistent with the low 36Ar/N2 mole ratio of Titan’s atmosphere (Niemann et al., 2005) involve the conversion of NH3 to N2 by photochemistry (Atreya et al., 1978) or shock chemistry (McKay et al., 1988). It should be pointed out that part of the appeal of the Atreya et al. (1978) and McKay et al. (1988) models is that they put N2 into Titan’s atmosphere early in the history of the moon. This is crucial if it is assumed that the 15N/14N ratio of Titan’s primordial NH3 was identical to that of atmospheric N2 on Earth, because early atmospheric escape must then be invoked to account for the high 15 N/14N ratio of Titan’s N2 relative to that of Earth’s N2 (Lunine et al., 1999; Niemann et al., 2005). However, it must be stressed that the assumption that the 15N/14N ratio of accreted NH3 was the same as that of terrestrial N2 is now challenged by the calculations of Rodgers and Charnley (2008), as described above. If Titan accreted 15N-enriched NH3, then early atmospheric fractionation may no longer be required to explain the isotopic composition of Titan’s N2, and as a consequence, the geologically recent outgassing of isotopically heavy N2 becomes a very real possibility (at least on isotopic grounds). This underscores the need to test the validity of the Rodgers and Charnley (2008) model by measuring the 15N/14N ratio of NH3 in comets (e.g., Gulkis and Alexander, 2008). Another key test for the endogenic N2 hypothesis will be the determination of the 15N/14N ratio of N2 in a plume of Enceladus. Because Enceladus lacks an atmosphere, the 15N/14N ratio of its N2 is likely to be a result of geochemical processes only, rather than a potentially ambiguous combination of geochemical and atmospheric processes. Little Enceladus may thus hold a vital clue for solving one of the great mysteries of Titan – the origin of its atmospheric N2. Acknowledgments We wish to thank Misha Zolotov and Bill McKinnon for many enlightening discussions about the origin and evolution of icy satellites, including Titan. We are especially grateful to Hunter Waite, who was willing to share the most recent INMS data from Enceladus with one of us (C.R.G.) prior to publication. We would also like to thank Franck Hersant and the other reviewer for their thoughtful critiques. This research would not have been possible without the tireless efforts of many who have contributed to the CassiniHuygens mission. Partial funding for this work was provided by the NASA Astrobiology Institute at Arizona State University. References Alexander, C.M.O., Fogel, M., Yabuta, H., Cody, G.D., 2007. The origin and evolution of chondrites recorded in the elemental and isotopic compositions of their macromolecular organic matter. Geochim. Cosmochim. Acta 71, 4380–4403. doi:10.1016/j.gca.2007.06.052. Altwegg, K., Bockelée-Morvan, D., 2003. Isotopic abundances in comets. Space Sci. Rev. 106, 139–154. doi:10.1023/A:1024685620462. Anderson, J.D., Jacobson, R.A., McElrath, T.P., Moore, W.B., Schubert, G., Thomas, P.C., 2001. Shape, mean radius, gravity field, and interior structure of Callisto. Icarus 153, 157–161. doi:10.1006/icar.2001.6664.

643

Atreya, S.K., Donahue, T.M., Kuhn, W.R., 1978. Evolution of a nitrogen atmosphere on Titan. Science 201, 611–613. doi:10.1126/science.201.4356.611. Atreya, S.K., Adams, E.Y., Niemann, H.B., Demick-Montelara, J.E., Owen, T.C., Fulchignoni, M., Ferri, F., Wilson, E.H., 2006. Titan’s methane cycle. Planet. Space Sci. 54, 1177–1187. doi:10.1016/j.pss.2006.05.028. Bézard, B., Nixon, C.A., Kleiner, I., Jennings, D.E., 2007. Detection of 13CH3D on Titan. Icarus 191, 397–400. doi:10.1016/j.icarus.2007.06.004. Bockelée-Morvan, D., Crovisier, J., Mumma, M.J., Weaver, H.A., 2004. The composition of cometary volatiles. In: Festou, M.C., Keller, H.U., Weaver, H.A. (Eds.), Comets II. Univ. of Arizona Press, Tucson, pp. 391–423. Brearley, A.J., Jones, R.H., 1998. Chondritic meteorites. In: Papike, J.J. (Ed.), Planetary Materials, Rev. Mineral., vol. 36. Mineral. Soc. of Am., Washington DC, pp. 1– 398. Charlou, J.L., Donval, J.P., Fouquet, Y., Jean-Baptiste, P., Holm, N., 2002. Geochemistry of high H2 and CH4 vent fluids issuing from ultramafic rocks at the Rainbow hydrothermal field (36°140 N, MAR). Chem. Geol. 191, 345–359. doi:10.1016/ S0009-2541(02)00134-1. Charnley, S.B., Rodgers, S.D., 2002. The end of interstellar chemistry as the origin of nitrogen in comets and meteorites. Astrophys. J. 569, L133–L137. doi:10.1086/ 340484. Cordier, D., Mousis, O., Lunine, J.I., Moudens, A., Vuitton, V., 2008. Photochemical enrichment of deuterium in Titan’s atmosphere: New insights from CassiniHuygens. Astrophys. J. 689, L61–L64. doi:10.1086/595677. Coustenis, A., 2005. Formation and evolution of Titan’s atmosphere. Space Sci. Rev. 116, 171–184. doi:10.1007/s11214-005-1954-2. de Kok, R., and 12 colleagues, 2007. Oxygen compounds in Titan’s stratosphere as observed by Cassini CIRS. Icarus 186, 354–363. doi: 10.1016/ j.icarus.2006.09.016. Engel, S., Lunine, J.I., Norton, D.L., 1994. Silicate interactions with ammonia–water fluids on early Titan. J. Geophys. Res. 99, 3745–3752. doi:10.1029/93JE03433. Frost, D.J., McCammon, C.A., 2008. The redox state of Earth’s mantle. Annu. Rev. Earth Planet. Sci. 36, 389–420. doi:10.1146/annurev.earth.36.031207.124322. Gibb, E.L., Mumma, M.J., Dello Russo, N., DiSanti, M.A., Magee-Sauer, K., 2003. Methane in Oort cloud comets. Icarus 165, 391–406. doi:10.1016/S00191035(03)00201-X. Glein, C.R., Zolotov, M.Y., Shock, E.L., 2008. The oxidation state of hydrothermal systems on early Enceladus. Icarus 197, 157–163. doi:10.1016/ j.icarus.2008.03.021. Gulkis, S., Alexander, C., 2008. Composition measurements of a comet from the Rosetta orbiter spacecraft. Space Sci. Rev. 138, 259–274. doi:10.1007/s11214008-9335-2. Hersant, F., Gautier, D., Lunine, J.I., 2004. Enrichment in volatiles in the giant planets of the Solar System. Planet. Space Sci. 52, 623–641. doi:10.1016/j.pss.2003.12. 011. Hersant, F., Gautier, D., Tobie, G., Lunine, J.I., 2008. Interpretation of the carbon abundance in Saturn measured by Cassini. Planet. Space Sci. 56, 1103–1111. doi:10.1016/j.pss.2008.02.007. Horibe, Y., Craig, H., 1995. D/H fractionation in the system methane–hydrogen– water. Geochim. Cosmochim. Acta 59, 5209–5217. doi:10.1016/0016-7037(95) 00391-6. Horita, J., Berndt, M.E., 1999. Abiogenic methane formation and isotopic fractionation under hydrothermal conditions. Science 285, 1055–1057. doi:10.1126/science.285.5430.1055. Horita, J., Cole, D.R., Polyakov, V.B., Driesner, T., 2002. Experimental and theoretical study of pressure effects on hydrogen isotope fractionation in the system brucite–water at elevated temperatures. Geochim. Cosmochim. Acta 66, 3769– 3788. doi:10.1016/S0016-7037(02)00887-6. Horner, J., Mousis, O., Alibert, Y., Lunine, J.I., Blanc, M., 2008. Constraints from deuterium on the formation of icy bodies in the jovian system and beyond. Planet. Space Sci. 56, 1585–1595. doi:10.1016/j.pss.2008.04.010. Jacovi, R., Bar-Nun, A., 2008. Removal of Titan’s noble gases by their trapping in its haze. Icarus 196, 302–304. doi:10.1016/j.icarus.2008.02.014. Johnson, T.V., Lunine, J.I., 2005. Saturn’s moon Phoebe as a captured body from the outer Solar System. Nature 435, 69–71. doi:10.1038/nature03384. Kasting, J.F., Eggler, D.H., Raeburn, S.P., 1993. Mantle redox evolution and the oxidation state of the Archean atmosphere. J. Geol. 101, 245–257. Kieffer, S.W., Lu, X., Bethke, C.M., Spencer, J.R., Marshak, S., Novrotsky, A., 2006. A clathrate reservoir hypothesis for Enceladus’ south polar plume. Science 314, 1764–1766. doi:10.1126/science.1133519. Kissel, J., Krueger, F.R., 1987. The organic component in dust from Comet Halley as measured by the PUMA mass spectrometer on board Vega 1. Nature 326, 755– 760. doi:10.1038/326755a0. Lopes, R.M.C., and 43 colleagues, 2007. Cryovolcanic features on Titan’s surface as revealed by the Cassini Titan Radar Mapper. Icarus 186, 395–412. doi: 10.1016/ j.icarus.2006.09.006. Lorenz, R.D., Stiles, B.W., Kirk, R.L., Allison, M.D., del Marmo, P.P., Iess, L., Lunine, J.I., Ostro, S.J., Hensley, S., 2008a. Titan’s rotation reveals an internal ocean and changing zonal winds. Science 319, 1649–1651. doi:10.1126/science.1151639. Lorenz, R.D., and 15 colleagues, 2008b. Titan’s inventory of organic surface materials. Geophys. Res. Lett. 35, L02206. doi: 10.1029/2007GL032118. Loveday, J.S., Nelmes, R.J., Guthrie, M., Belmonte, S.A., Allan, D.R., Klug, D.D., Tse, J.S., Handa, Y.P., 2001. Stable methane hydrate above 2 GPa and the source of Titan’s atmospheric methane. Nature 410, 661–663. doi:10.1038/35070513. Lunine, J.I., Gautier, D., 2004. Coupled physical and chemical evolution of volatiles in the protoplanetary disk: A tale of three elements. In: Festou, M.C., Keller, H.U., Weaver, H.A. (Eds.), Comets II. Univ. of Arizona Press, Tucson, pp. 105–113.

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Lunine, J.I., Stevenson, D.J., 1987. Clathrate and ammonia hydrates at high pressure: Application to the origin of methane on Titan. Icarus 70, 61–77. doi:10.1016/ 0019-1035(87)90075-3. Lunine, J.I., Atreya, S.K., Pollack, J.B., 1989. Present state and chemical evolution of the atmospheres of Titan, Triton and Pluto. In: Atreya, S.K., Pollack, J.B., Matthews, M.S. (Eds.), Origin and Evolution of Planetary and Satellite Atmospheres. Univ. of Arizona Press, Tucson, pp. 605–665. Lunine, J.I., Yung, Y.L., Lorenz, R.D., 1999. On the volatile inventory of Titan from isotopic abundances in nitrogen and methane. Planet. Space Sci. 47, 1291–1303. doi:10.1016/S0032-0633(99)00052-5. Matson, D.L., Castillo, J.C., Lunine, J., Johnson, T., 2007. Enceladus’ plume: Compositional evidence for a hot interior. Icarus 187, 569–573. doi:10.1016/ j.icarus.2006.10.016. McCollom, T.M., Bach, W.G., 2009. Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. Geochim. Cosmochim. Acta 73, 856–875. doi:10.1016/j.gca.2008.10.032. McKay, C.P., Scattergood, T.W., Pollack, J.B., Borucki, W.J., Van Ghyseghem, H.T., 1988. High-temperature shock formation of N2 and organics on primordial Titan. Nature 332, 520–522. doi:10.1038/332520a0. Mousis, O., Gautier, D., Coustenis, A., 2002. The D/H ratio in methane in Titan: Origin and history. Icarus 159, 156–165. doi:10.1006/icar.2002.6930. Mousis, O., Lunine, J.I., Pasek, M., Cordier, D., Waite, J.H., Mandt, K.E., Lewis, W.S., Nguyen, M.J., 2009a. Is serpentinization the source of Titan’s atmospheric methane? Lunar Planet. Sci. XL (abstract 1182). Mousis, O., and 10 colleagues, 2009b. Clathration of volatiles in the solar nebula and implications for the origin of Titan’s atmosphere. Astrophys. J. 691, 1780–1786. doi: 10.1088/0004-637X/691/2/1780. Nagel, K., Breuer, D., Spohn, T., 2004. A model for the interior structure, evolution, and differentiation of Callisto. Icarus 169, 402–412. doi:10.1016/ j.icarus.2003.12.019. Niemann, H.B., and 17 colleagues, 2005. The abundances of constituents of Titan’s atmosphere from the GCMS instrument on the Huygens probe. Nature 439, 779–784. doi: 10.1038/nature04122. Osegovic, J.P., Max, M.D., 2005. Compound clathrate hydrate on Titan’s surface. J. Geophys. Res. 110, E08004. doi:10.1029/2005JE002435. Owen, T., 1982. The composition and origin of Titan’s atmosphere. Planet. Space Sci. 30, 833–838. doi:10.1016/0032-0633(82)90115-5. Owen, T.C., 2000. On the origin of Titan’s atmosphere. Planet. Space Sci. 48, 747– 752. doi:10.1016/S0032-0633(00)00040-4. Owen, T., Niemann, H.B., 2009. The origin of Titan’s atmosphere: Some recent advances. Philos. Trans. R. Soc. London, Ser. A 367, 607–615. doi:10.1098/ rsta.2008.0247. Owen, T.C., Niemann, H., Atreya, S., Zolotov, M.Y., 2006. Between heaven and Earth: The exploration of Titan. Faraday Discuss. 133, 387–391. doi:10.1039/ b517174a. Oze, C., Sharma, M., 2005. Have olivine, will gas: Serpentinization and the abiogenic production of methane on Mars. Geophys. Res. Lett. 32, L10203. doi:10.1029/ 2005GL022691. Pinto, J.P., Lunine, J.I., Kim, S.-J., Yung, Y.L., 1986. D to H ratio and the origin and evolution of Titan’s atmosphere. Nature 319, 388–390. doi:10.1038/319388a0. Porco, C.C., and 24 colleagues, 2006. Cassini observes the active south pole of Enceladus. Science 311, 1393–1401. doi: 10.1126/science.1123013. Proskurowski, G., Lilley, M.D., Kelley, D.S., Olson, E.J., 2006. Low temperature volatile production at the Lost City hydrothermal field, evidence from a hydrogen stable isotope geothermometer. Chem. Geol. 229, 331–343. doi:10.1016/j.chemgeo.2005.11.005. Proskurowski, G., Lilley, M.D., Seewald, J.S., Früh-Green, G.L., Olson, E.J., Lupton, J.E., Sylva, S.P., Kelley, D.S., 2008. Abiogenic hydrocarbon production at Lost City hydrothermal field. Science 319, 604–607. doi:10.1126/science.1151194. Rappaport, N., Bertotti, B., Giampieri, G., Anderson, J.D., 1997. Doppler measurements of the quadruple moments of Titan. Icarus 126, 313–323. doi:10.1006/icar.1996.5661. Rodgers, S.D., Charnley, S.B., 2008. Nitrogen superfractionation in dense cloud cores. Mon. Not. R. Astron. Soc. Lett. 385, L48–L52. doi:10.1111/j.17453933.2008.00431.x. Rubin, A.E., Fegley, B., Brett, R., 1988. Oxidation state in chondrites. In: Kerridge, J.F., Matthews, M.S. (Eds.), Meteorites and the Early Solar System. Univ. of Arizona Press, Tucson, pp. 488–511.

Schubert, G., Anderson, J.D., Travis, B.J., Palguta, J., 2007. Enceladus: Present internal structure and differentiation by early and long-term radiogenic heating. Icarus 188, 345–355. doi:10.1016/j.icarus.2006.12.012. Scott, H.P., Williams, Q., Ryerson, F.J., 2002. Experimental constraints on the chemical evolution of large icy satellites. Earth Planet. Sci. Lett. 203, 399–412. doi:10.1016/S0012-821X(02)00850-6. Seewald, J.S., Zolotov, M.Y., McCollom, T., 2006. Experimental investigation of single carbon compounds under hydrothermal conditions. Geochim. Cosmochim. Acta 70, 446–460. doi:10.1016/j.gca.2005.09.002. Sherwood Lollar, B., Westgate, T.D., Ward, J.A., Slater, G.F., Lacrampe-Couloume, G., 2002. Abiogenic formation of alkanes in the Earth’s crust as a minor source for global hydrocarbon reservoirs. Nature 416, 522–524. doi:10.1038/416522a. Sherwood Lollar, B., Lacrampe-Couloume, G., Voglesonger, K., Onstott, T.C., Pratt, L.M., Slater, G.F., 2008. Isotopic signatures of CH4 and higher hydrocarbon gases from Precambrian Shield sites: A model for abiogenic polymerization of hydrocarbons. Geochim. Cosmochim. Acta 72, 4778–4795. doi:10.1016/ j.gca.2008.07.004. Shock, E.L., 1990. Geochemical constraints on the origin of organic compounds in hydrothermal systems. Origins Life Evol. Biosphere 20, 331–367. doi:10.1007/ BF01808115. Shock, E.L., 1992. Chemical environments of submarine hydrothermal systems. Origins Life Evol. Biosphere 22, 67–107. doi:10.1007/BF01808019. Shock, E.L., Helgeson, H.C., 1990. Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Standard partial molal properties of organic species. Geochim. Cosmochim. Acta 54, 915– 945. doi:10.1016/0016-7037(90)90429-O. Shock, E.L., McKinnon, W.B., 1993. Hydrothermal processing of cometary volatiles – Applications to Triton. Icarus 106, 464–477. doi:10.1006/icar.1993.1185. Shock, E.L., Helgeson, H.C., Sverjensky, D.A., 1989. Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Standard partial molal properties of inorganic neutral species. Geochim. Cosmochim. Acta 53, 2157–2183. doi:10.1016/0016-7037(89)903414. Sohl, F., Hussmann, H., Schwentker, B., Spohn, T., Lorenz, R.D., 2003. Interior structure models and tidal Love numbers of Titan. J. Geophys. Res. 108, 5130. doi:10.1029/2003JE002044. Thomas, C., Picaud, S., Mousis, O., Ballenegger, V., 2008. A theoretical investigation into the trapping of noble gases by clathrates on Titan. Planet. Space Sci. 56, 1607–1617. doi:10.1016/j.pss.2008.04.009. Tobie, G., Lunine, J.I., Sotin, C., 2006. Episodic outgassing as the origin of atmospheric methane on Titan. Nature 440, 61–64. doi:10.1038/nature04497. Wadhwa, M., 2008. Redox conditions on small bodies, the Moon and Mars. In: MacPherson, G.J. (Ed.), Oxygen in the Solar System, Rev. Mineral. Geochem., vol. 68. Mineral. Soc. of Am, Washington DC, pp. 493–510. Waite, J.H., and 21 colleagues, 2005. Ion and neutral mass spectrometer results from the first flyby of Titan. Science 308, 982–986. doi: 10.1126/science.1110652. Waite, J.H.15 colleagues, 2009. Liquid water on Enceladus from observations of ammonia and 40Ar in the plume. Nature 460, 487–490. doi:10.1038/ nature08153. Yung, Y.L., Allen, M., Pinto, J.P., 1984. Photochemistry of the atmosphere of Titan: Comparison between model and observations. Astrophys. J. Suppl. Ser. 55, 465– 506. doi:10.1086/190963. Zolensky, M.E., and 74 colleagues, 2006. Mineralogy and petrology of Comet 81P/ Wild 2 nucleus samples. Science 314, 1735–1739. doi: 10.1126/ science.1135842. Zolotov, M.Y., Fegley, B., 2000. Eruption conditions of Pele volcano on Io inferred from chemistry of its volcanic plume. Geophys. Res. Lett. 27, 2789–2792. doi:10.1029/2000GL011608. Zolotov, M.Y., Shock, E.L., 2003a. Energy for biologic sulfate reduction in a hydrothermally formed ocean on Europa. J. Geophys. Res. 108, 5022. doi:10.1029/2002JE001966. Zolotov, M.Y., Shock, E.L., 2003b. Aqueous oxidation of parent bodies of carbonaceous chondrites and Galilean satellites driven by hydrogen escape. Lunar Planet. Sci. XXXIV (abstract 2047). Zolotov, M.Y., Owen, T., Atreya, S., Niemann, H.B., Shock, E.L., 2005. An endogenic origin of Titan’s methane. Eos Trans. AGU (Fall Meet. Suppl.) 86 (52) (abstract P43B-04).