The Astrophysical Journal, 701:L39–L42, 2009 August 10 C 2009.
doi:10.1088/0004-637X/701/1/L39
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
FORMATION CONDITIONS OF ENCELADUS AND ORIGIN OF ITS METHANE RESERVOIR O. Mousis1,2 , J. I. Lunine1 , J. H. Waite, Jr.3 , B. Magee3 , W. S. Lewis3 , K. E. Mandt3 , D. Marquer4 , and D. Cordier5,6 2
1 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA Universit´e de Franche-Comt´e, Institut UTINAM, CNRS/INSU, UMR 6213, 25030 Besan¸con Cedex, France;
[email protected] 3 Center for Excellence in Analytical Mass Spectroscopy, Southwest Research Institute, San Antonio, Texas, USA 4 Universit´ e de Franche-Comt´e, Chrono-Environnement, CNRS/INSU, UMR 6249, 25030 Besan¸con Cedex, France 5 Institut de Physique de Rennes, CNRS, UMR 6251, Universit´ e de Rennes 1, Campus de Beaulieu, 35042 Rennes, France 6 Ecole Nationale Sup´ erieure de Chimie de Rennes, Campus de Beaulieu, 35700 Rennes, France Received 2009 April 3; accepted 2009 June 22; published 2009 July 24
ABSTRACT We describe a formation scenario of Enceladus constrained by the deuterium-to-hydrogen ratio (D/H) in the gas plumes as measured by the Cassini Ion and Neutral Mass Spectrometer. We propose that, similarly to Titan, Enceladus formed from icy planetesimals that were partly devolatilized during their migration within the Kronian subnebula. In our scenario, at least primordial Ar, CO, and N2 were devolatilized from planetesimals during their drift within the subnebula, due to the increasing temperature and pressure conditions of the gas phase. The origin of methane is still uncertain since it might have been either trapped in the planetesimals of Enceladus during their formation in the solar nebula or produced via serpentinization reactions in the satellite’s interior. If the methane of Enceladus originates from the solar nebula, then its D/H ratio should range between ∼4.7 × 10−5 and 1.5 × 10−4 . Moreover, Xe/H2 O and Kr/H2 O ratios are predicted to be equal to ∼7 × 10−7 and 7 × 10−6 , respectively, in the satellite’s interior. On the other hand, if the methane of Enceladus results from serpentinization reactions, then its D/H ratio should range between ∼2.1 × 10−4 and 4.5 × 10−4 . In this case, Kr/H2 O should not exceed ∼10−10 and Xe/H2 O should range between ∼1 × 10−7 and 7 × 10−7 in the satellite’s interior. Future spacecraft missions, such as Titan Saturn System Mission, will have the capability to provide new insight into the origin of Enceladus by testing these observational predictions. Key words: planets and satellites: formation – planets and satellites: individual (Enceladus) is supported by the recent deuterium-to-hydrogen (D/H) mea−4 surement in H2 O in the satellite’s plume (D/H = 2.9+1.5 −0.7 ×10 ), which is close to the cometary value (Waite et al. 2009). Furthermore, the hypothesis of a partial devolatilization of planetesimals in Saturn’s subnebula has already been formulated in order to account for the origin of Titan and the composition of its current atmosphere (Alibert & Mousis 2007; Mousis et al. 2009b). Thus, in the case of Titan, it has been proposed that H2 O, NH3 , CO2 , and CH4 remained trapped in the building blocks of the satellite during their migration within the subnebula, while CO and N2 were devolatilized. Since Enceladus orbits Saturn at a closer distance than Titan, and because the gas temperature and pressure conditions increase within the subdisk with decreasing distance from Saturn (Mousis et al. 2002; Alibert & Mousis 2007), the devolatilization of its planetesimals should be greater than Titan’s. Here, we aim to provide observational tests that may allow characterization of the importance of the devolatilization undergone by the building blocks of Enceladus during their migration within Saturn’s subnebula. Our attention is focused on the origin of CH4 , which is directly tied to the magnitude of this devolatilization. We show that the fractions of Kr and Xe trapped in the interior of Enceladus will vary as a function of the CH4 origin, i.e., trapping in the solar nebula or production via serpentinization reactions in the interior of Enceladus. These noble gases have the ability to be incorporated in H2 S- and CH4 dominated clathrates initially trapped in the building blocks of Enceladus at the time of their formation. Moreover, we demonstrate that the resulting D/H ratio in CH4 differs as a function of its source. The future investigation of these predictions will place important constraints on the devolatilization process that the building blocks of Enceladus might have undergone in Saturn’s subnebula and on the origin of the methane detected in the satellite.
1. INTRODUCTION The composition of the gas plume emanating from Enceladus’ southern pole has been measured five times by the Ion and Neutral Mass Spectrometer (INMS) instrument aboard the Cassini spacecraft. From these data, Waite et al. (2009) inferred that the composition of the plume is dominated by H2 O vapor, a few percent of CO2 , CH4 , NH3 , H2 S, and organic compounds ranging from C2 H2 to C6 H6 . Signatures of possible CO, N2 , and H2 are seen but the presence of such species cannot be confirmed from the data alone. 40 Ar has also been detected and is probably the decay product of 40 K (Waite et al. 2009). Among the compounds observed by the INMS instrument, at least H2 O, NH3 , H2 S, and CO2 are expected to be primordial (Waite et al. 2009). Indeed, the possible presence of N2 can be explained as a result from the thermal decomposition of NH3 in the interior of Enceladus (Matson et al. 2007). The measured CO is likely the product of fragmentation of primordial CO2 during collection by the INMS (Waite et al. 2009). Moreover, the origin of CH4 and high-order hydrocarbons is uncertain because these compounds might have been trapped by the building blocks7 of Enceladus at the time of their formation (Waite et al. 2009) or might also result from hydrothermal reactions in the interior of the satellite (Matson et al. 2007). In the present work, we propose that Enceladus formed from icy planetesimals initially produced in the solar nebula that, once embedded in the subnebula of Saturn, have been partly devolatilized due to the increasing gas temperature and pressure conditions during their migration inward within the subdisk. The idea of a solar nebula origin for the building blocks of Enceladus 7
Building blocks designate the planetesimals that accreted together to form Enceladus.
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Table 1 Gas Phase Abundances in the Solar Nebula Species X O C N S Ar Kr Xe H2 O
(X/H2 )
Species X
(X/H2 )
1.16 × 10−3 5.82 × 10−4 1.60 × 10−4 3.66 × 10−5 8.43 × 10−6 4.54 × 10−9 4.44 × 10−10 4.43 × 10−4
N2 NH3 CO CO2 CH3 OH CH4 H2 S
4.05 × 10−5 4.05 × 10−5 2.21 × 10−4 3.16 × 10−5 6.31 × 10−6 3.16 × 10−6 1.83 × 10−5
Notes. Elemental abundances derive from Lodders (2003). Molecular abundances result from the distribution of elements between refractory and volatile components.
2. FORMATION OF ENCELADUS’ BUILDING BLOCKS We briefly recall the formation conditions of planetesimals in Saturn’s feeding zone as detailed in Mousis et al. (2009b). We assume that the gas phase abundances of all elements are solar and that they are distributed between refractory and volatile components, with CO:CO2 :CH3 OH:CH4 = 70:10:2:1, H2 S:H2 = 0.5 × (S:H2 ) and N2 :NH3 = 1:1 in the nebula gas phase (see Table 1 for the list of the gas-phase abundances of the main volatile compounds). The process by which volatiles are trapped in icy planetesimals, illustrated in Figure 1, is calculated using the stability curves of hydrates, clathrates, and pure condensates, and the thermodynamic path detailing the evolution of temperature and pressure at 9.5 AU in the solar nebula, corresponding to the position of Saturn. The cooling curve intercepts the equilibrium curves of the different ices at particular temperatures and pressures. For each ice considered, the domain of stability is the region located below its corresponding equilibrium curve. The clathration process stops when no more crystalline water ice is available to trap the volatile species. As a result of the assumed solar gas-phase abundance for oxygen, ices formed in the outer solar nebula are composed of a mix of clathrates, hydrates, and pure condensates which are, except for CO2 and CH3 OH,8 produced at temperatures ranging between 20 and 50 K. Once formed, the different ices agglomerated and incorporated into the growing planetesimals. Figure 1 illustrates the case where the efficiency of clathration is only of ∼25%. Here, either only a part of the clathrates cages have been filled by guest molecules, either the diffusion of clathrated layers through the planetesimals was too slow to enclathrate most of the ice, or the poor trapping efficiency was the combination of these two processes. In this case, only NH3 , H2 S, Xe, and CH4 form NH3 -H2 O hydrates and H2 S-5.75H2 O, Xe-5.75H2 O, and CH4 -5.75H2 O clathrates. Due to the deficiency in accessible water in icy planetesimals, all CO, Ar, Kr, and N2 form pure condensates in the feeding zone of Saturn. 3. NOBLE GASES TRAPPING IN ENCELADUS AS A FUNCTION OF METHANE INCORPORATION We investigate the fraction of noble gases that can be incorporated in Enceladus at the time of its formation, provided that Ar, Kr, and Xe were in solar abundances in the initial gas 8 CO is the only species that crystallizes at a higher temperature than its 2 associated clathrate in the solar nebula. Moreover, we consider only the formation of CH3 OH pure ice because no experimental data concerning the equilibrium curve of its associated clathrate have been reported in the literature.
Figure 1. Formation sequence of the different ices in Saturn’s feeding zone. Equilibrium curves of ammonia monohydrate, clathrates (solid lines), and pure condensates (dotted lines), and cooling curve of the solar nebula at the heliocentric distance of 9.5 AU, assuming a clathration efficiency of 25%. The bottom and top arrows designate, respectively, the maximum temperatures at which the building blocks of Enceladus can be heated during their migration within the Saturn’s subnebula if methane observed in the plumes is primordial or if it is produced in the satellite.
phase of Saturn’s feeding zone. These results can be useful to constrain the formation of Enceladus because the nature of the trapped species in the interior of the satellite will depend on the temperature at which its building blocks have been devolatilized within the subdisk. Mousis et al. (2009b) have calculated the relative abundances of guests that can be incorporated in H2 S-, Xe-, and CH4 dominated clathrates at the temperature and pressure formation conditions in the solar nebula reported in Figure 1. They have followed the method described by Lunine & Stevenson (1985) and Thomas et al. (2008), which uses classical statistical mechanics to relate the macroscopic thermodynamic properties of clathrates to the molecular structure and interaction energies. Table 2 represents the fraction Fi of volatiles incorporated in these clathrates relative to their initial fraction available in the nebula gas. These calculations show that CO, N2 , and Ar are poorly trapped in clathrates because their relative abundances in these structures are orders-of-magnitude lower than those found in the solar nebula. On the other hand, substantial amounts of Xe and Kr are trapped in H2 S- and CH4 -dominated clathrates, respectively. In particular, because the Kr/CH4 ratio is larger in CH4 -dominated clathrate than in the solar nebula (FKr > 1), most of Kr is incorporated in this clathrate, thus preventing the formation of its pure condensate at lower temperature. Two different maximum devolatilization temperatures can be envisaged for the building blocks of Enceladus. In the first case, similarly to Titan we assume that the methane detected in the plumes is primordial. This corresponds to the hypothesis that the devolatilization temperature of planetesimals never exceeded ∼50 K during their drift within the subdisk. In the second case, we assume that methane is not primordial and has been produced in the interior of the satellite. Here, the maximum devolatilization temperature is then of ∼75 K because a higher value would not be compatible with the presence of primordial CO2 in Enceladus (see Figure 1). In the first case, since most of Xe is trapped as a Xe-dominated clathrate in the building blocks of Enceladus, the corresponding
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FORMATION CONDITIONS OF ENCELADUS AND ORIGIN OF ITS METHANE RESERVOIR
Table 2 Abundance of Volatile i in Clathrate Relative to Initial Abundance in the Nebula (from Mousis et al. 2009b) Clathrate
Species
fi
Fi
H2 S-dominated
CO2 Xe CH4 CO Kr Ar N2
3.12 × 10−6 4.36 × 10−6 5.45 × 10−6 2.13 × 10−7 3.57 × 10−9 8.74 × 10−9 4.04 × 10−7
1.81 × 10−6 0.18 3.16 × 10−5 1.76 × 10−8 1.43 × 10−5 1.90 × 10−8 1.83 × 10−7
Xe-dominated
CH4 CO Kr Ar N2
7.28 × 10−2 5.18 × 10−5 7.82 × 10−6 1.08 × 10−6 6.68 × 10−4
1.02 × 10−5 1.04 × 10−10 7.65 × 10−7 5.69 × 10−11 7.32 × 10−9
CH4 -dominated
CO Kr Ar N2
1.74 × 10−3 1.67 × 10−3 6.43 × 10−5 3.94 × 10−3
2.49 × 10−5 1.16 2.41 × 10−5 3.07 × 10−4
Notes. fi is defined as the molar ratio of i to X in X-dominated clathrate and Fi is the ratio of fi to the initial ratio (i to X) in the solar nebula. Calculations are performed at temperature and pressure conditions given by the intersection of the cooling curve at 9.5 AU and the stability curves of the considered clathrates (see Figure 1). Only the species that are not yet condensed or trapped prior the epoch of clathrate formation are considered in our calculations.
Xe/H2 O ratio in the satellite is given by (Mousis & Gautier 2004): Σ(9.5AU ; TX , PX ) xX , (1) yX = xH2 O Σ(9.5AU ; TH2 O , PH2 O ) where xX and xH2 O are the molar mixing ratios of species X and H2 O with respect to H2 in the nebula, respectively. Σ(9.5AU ; TX , PX ) and Σ(9.5AU ; TH2 O , PH2 O ) are the surface density of the nebula at the current heliocentric distance of Saturn at the formation epoch of X-dominated clathrate, and at the epoch of condensation of water, respectively. These values are 1146.9 g cm−2 , 829.1 g cm−2 , 688.2 g cm−2 , 659.2 g cm−2 for water, H2 S-, Xe-, and CH4 -dominated clathrates, respectively. On the other hand, the remaining fraction of Xe and essentially all Kr are trapped in H2 S- and CH4 -dominated clathrates, respectively. The corresponding fractions relative to water are then given by: zi = fi × yX ,
(2)
where fi is the molar ratio of i to X in X-dominated clathrate given in Table 2. From Equations (1) and (2), we infer that Xe/H2 O and Kr/H2 O ratios are respectively equal to ∼7 × 10−7 and 7 × 10−6 in Enceladus if methane is primordial. In the second case, Kr/H2 O should not be higher than ∼10−10 in Enceladus because CH4 -dominated clathrates have been dissociated during the migration of the planetesimals within the subdisk. On the other hand, depending on the value of the devolatilization temperature of planetesimals, which translates into the preservation or not of the Xe-dominated clathrates, the Xe/H2 O ratio in Enceladus should range between ∼1 × 10−7 and 7 × 10−7 . 4. WHAT IF SERPENTINIZATION WERE THE SOURCE OF METHANE OBSERVED IN ENCELADUS? We examine the possibility that CH4 is the result of serpentinization reactions, as proposed by Atreya et al. (2006) in the
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Table 3 D/H Ratio in Methane as a Function of its Postulated Origin Origin
Range of D/H Values
Serpentinization reactions Solar nebula
2.1 × 10−4 –4.5 × 10−4 4.7 × 10−5a –1.5 × 10−4
Notes. a Minimum D/H ratio acquired by the primordial methane of Titan in case of strong photochemical enrichment in the atmosphere (see the text).
case of Titan. We first estimate the D/H value in the methane of Enceladus, assuming it was produced in its interior from the association of CO2 or carbon grains with the H2 formed during the hydrothermal alteration of peridotite (Moody 1976; Atreya et al. 2006; Oze & Sharma 2007), and we compare this value to the one acquired by methane if it had originated from the solar nebula. In terrestrial oceans, hydrothermal fluids and seawater interact with peridotite via the following summary reaction (Moody 1976; Atreya et al. 2006): peridotite(olivine/pyroxene) + water → serpentine + brucite + magnetite + hydrogen.
(3)
With a bulk density of 1610 kg m−3 , thermal evolution models suggest that Enceladus is most likely a differentiated body with a large rocky core surrounded by a water ice shell that may be liquid at depth (Schubert et al. 2007). In our calculations, we assume that this liquid layer constitutes the water reservoir that is in contact with peridotite during serpentinization reactions. Under these conditions, residual water is deuterium-enriched at the expense of the initial reservoir of free water and the fractionation factor, α, between OH-bearing minerals and water is then (L´ecuyer et al. 2000): f
αr−w =
Rr
f
Rw
.
(4)
The effect on the D/H ratio of the residual water of a D/H fractionation between the initial water and hydrated peridotite can be readily tested with the following mass balance equation that describes a batch equilibrium mechanism of both hydration and isotopic fractionation between given masses of rock and water (L´ecuyer et al. 2000): Mwi Xwi Rwi + Mri Xri Rri = Mwf Xwf Rwf + Mrf Xrf Rrf ,
(5)
where M is the mass, X the mass fractions of hydrogen in f f water (Xwi = Xw = 1/9) or rock (Xr = 4/277), R the D/H ratios before hydration reactions (i) and at batch equilibrium (f). If we neglect the amount of water incorporated in primordial f peridotites of Enceladus (Xri = 0) and postulate that Mwi Mw f f f f and Mw .Xw Mr .Xr , which is a reasonable assumption,9 then Equation (5) can be expressed as Rrf αr−w Rwi .
(6)
9 The CH /H O volume ratio observed in the plumes is ∼0.5% during the 4 2 E2 encounter (Waite et al. 2006) and is representative of the value existing in the hypothetical internal ocean (Waite et al. 2009). Assuming that all H2 involved in the formation of CH4 results from the reaction of peridotite and f f water, the fraction of used water is only of ∼1% and the term Mw .Xw is more f f than 11 times greater than the term Mr .Xr .
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We consider two extreme values, 0.95 and 1.03, of the hydrogen fractionation αr−w between the serpentine and the free-water reservoir in the literature based on laboratory and field data made at temperatures ranging between 298 and 773 K (Vennemann & O’Neil 1996). We also postulate that the D/H ratio in the methane initially released from the interior is that acquired by hydrated rocks once equilibrium is reached during serpentinization reactions. Hence, the D/H ratio acquired by hydrated rocks would be preserved in the H2 produced from the alteration of peridotite and used in the recombination of CH4 . Table 3 summarizes the range of predicted values for D/H in CH4 produced within Enceladus, assuming that D/H ratio in the primordial water reservoir is the value measured by Cassini (Waite et al. 2009). This value is compared to the one acquired by methane if it was initially incorporated by the building blocks of Enceladus during their formation in the nebula. In this case, since it was established that the methane of Titan originates from the solar nebula (Mousis et al. 2009a) and since the building blocks of the two satellites share a common origin, the D/H ratio in Enceladus’ methane should range between the minimum D/H ratio initially acquired by Titan if a significant photochemical enrichment of deuterium occurred during the evolution of its atmosphere (Cordier et al. 2008) and the value observed today in Titan by Cassini (B´ezard et al. 2007). From these values, we infer that the D/H ratio in CH4 produced by serpentinization should be enriched by a factor of 1.4–9.6 relative to D/H in methane originating from the nebula. 5. DISCUSSION A word of caution must be given about our estimate of the D/H ratio in CH4 produced via serpentinization. It is based on the idea that no D-fractionation occurred between the produced H2 and serpentine, and between the produced CH4 and H2 . Unfortunately, these points are still not well established in the literature and future laboratory work is required to assess their validity. Recent INMS measurements during the 2008 October 9 encounter with the best signal quality to date have shown no clear sign of Kr in the Enceladus’ plume. Instrument sensitivity and observed signal to noise constrain the determination of Kr/H2 O to an upper limit of < 10−5 . However, subsequent encounters with Enceladus’ plume may enhance the signal-to-noise to a
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level at which Kr can be identified without ambiguity. On the other hand, the INMS instrument is unable to directly measure Xe as its observable mass range is capped at 100 D. As noted in Waite et al. (2009), the measured H2 in recent encounters is most likely a product of H2 O interactions with the instrument’s antechamber walls due to the high spacecraft velocity relative to the plume. Therefore, the amount of endogenous H2 and its D/H ratio remains undetermined. Future Cassini encounters are currently planned at slower spacecraft speed that may allow separate assessments of H2 in the plume from that synthesized inside the INMS instrument. In this case, determinations of D/H in H2 during the Cassini mission remain a remote possibility that could tell if this species is produced by serpentinization reactions. Measurement of D/H in CH4 cannot be made with INMS due to overlapping signals of several species in this portion of the mass spectrum. This measurement requires an instrument with mass resolution (M/ΔM) greater than 6000 and, thus, must wait for a future spacecraft mission, such as Titan Saturn System Mission actually studied by NASA and ESA. Support from the Cassini project is gratefully acknowledged. REFERENCES Alibert, Y., & Mousis, O. 2007, A&A, 465, 1051 Atreya, S. K., Adams, E. Y., Niemann, H. B., Demick-Montelara, J. E., Owen, T. C., Fulchignoni, M., Ferri, F., & Wilson, E. H. 2006, Planet. Space Sci., 54, 1177 B´ezard, B., Nixon, C. A., Kleiner, I., & Jennings, D. E. 2007, Icarus, 191, 397 Cordier, D., Mousis, O., Lunine, J. I., Moudens, A., & Vuitton, V. 2008, ApJ, 689, L61 L´ecuyer, C., Simon, L., & Guy, F. 2000, Earth Planet. Sci. Lett., 181, 33 Lodders, K. 2003, ApJ, 591, 1220 Lunine, J. I., & Stevenson, D. J. 1985, ApJS, 58, 493 Matson, D. L., Castillo, J. C., Lunine, J., & Johnson, T. V. 2007, Icarus, 187, 56 Moody, J. 1976, Lithos, 9, 125 Mousis, O., & Gautier, D. 2004, Planet. Space Sci., 52, 361 Mousis, O., Gautier, D., & Bockel´ee-Morvan, D. 2002, Icarus, 156, 162 Mousis, O., Lunine, J. I., Pasek, M., Cordier, D., Waite, J. H., Mandt, K. E., Lewis, W. S., & Nguyen, M.-J. 2009a, Icarus, submitted Mousis, O., et al. 2009b, ApJ, 691, 1780 Oze, C., & Sharma, M. 2007, Icarus, 186, 557 Schubert, G., Anderson, J. D., Travis, B. J., & Palguta, J. 2007, Icarus, 188, 345 Thomas, C., Picaud, S., Mousis, O., & Ballenegger, V. 2008, Planet. Space Sci., 56, 1607 Vennemann, T. W., & O’Neil, J. R. 1996, Geochim. Cosmochim. Acta, 60, 2437 Waite, J. H., et al. 2006, Science, 311, 1419 Waite, J. H., et al. 2009, Nature, in press
