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Stability of Methane Clathrate Hydrates Under Pressure: Influence On Icarus 205 (2010) 581–593 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus Stability of methane clathrate hydrates under pressure: Influence on outgassing processes of methane on Titan Mathieu Choukroun a,*, Olivier Grasset b, Gabriel Tobie b, Christophe Sotin a,b a Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., MS 79-24, Pasadena, CA 91109, United States b UMR-CRNS 6112 Planétologie et Géodynamique, Université de Nantes, 2, rue de la Houssinière, 44322 Nantes Cedex 3, France article info abstract Article history: We have conducted high-pressure experiments in the H2O–CH4 and H2O–CH4–NH3 systems in order to Received 6 March 2009 investigate the stability of methane clathrate hydrates, with an optical sapphire-anvil cell coupled to a Revised 22 July 2009 Raman spectrometer for sample characterization. The results obtained confirm that three factors deter- Accepted 12 August 2009 mine the stability of methane clathrate hydrates: (1) the bulk methane content of the samples; (2) the Available online 25 August 2009 presence of additional gas compounds such as nitrogen; (3) the concentration of ammonia in the aqueous solution. We show that ammonia has a strong effect on the stability of methane clathrates. For example, a Keywords: 10 wt.% NH solution decreases the dissociation temperature of methane clathrates by 14–25 K at pres- Titan 3 sures above 5 MPa. Then, we apply these new results to Titan’s conditions. Dissociation of methane clath- Experimental techniques Interiors rate hydrates and subsequent outgassing can only occur in Titan’s icy crust, in presence of locally large Geophysics amounts of ammonia and in a warm context. We propose a model of cryomagma chamber within the Geological processes crust that provides the required conditions for methane outgassing: emplacement of an ice plume trig- gers the melting (if solid) or heating (if liquid) of large ammonia–water pockets trapped at shallow depth, and the generated cryomagmas dissociate surrounding methane clathrate hydrates. We show that this model may allow for the outgassing of significant amounts of methane, which would be sufficient to maintain the presence of methane in Titan’s atmosphere for several tens of thousands of years after a large cryovolcanic event. Published by Elsevier Inc. 1. I. Introduction concentration of the icy fraction of the planetesimals that formed Titan is probably lower than 5 wt.%, which may nevertheless sig- Titan has a bulk density of 1881 kg mÀ3 (e.g. Sohl et al., 2003, nificantly affect the internal structure and the thermal profile. and references therein), which implies a silicate fraction of 50– Titan’s methane concentration in the upper atmosphere is 1.4% 70% for densities within 2700 and 4000 kg mÀ3, the remainder (Niemann et al., 2005; Waite et al., 2005; Coustenis et al., 2007). In being mostly H2O. Thermal evolution models suggest a differenti- the troposphere, the mixing ratio of methane increases and reaches ated internal structure (Grasset and Sotin, 1996; Grasset et al., 5% close to the surface (Niemann et al., 2005). The vertical profile 2000; Sohl et al., 2003; Tobie et al., 2005; Sotin et al., 2009), with of methane concentration suggests a total mass of 2.8 Â 1020 gof a silicate core overlaid by a 700 km-thick H2O-dominated mantle. CH4, which is consistent with the post-Voyager estimations The gravity data obtained during the Cassini–Huygens prime mis- (Lunine and Stevenson, 1987). Owing to photochemistry driven sion support these models for the H2O mantle (Rappaport et al., by solar UV (e.g. Yung et al., 1984; Toublanc et al., 1995), the current 2008). Due to the high pressures and relatively low temperatures methane amounts in the atmosphere would disappear in a time existing inside Titan, several phases of H2O are theoretically stable span of 10–100 Myr. Such a short lifetime of methane in Titan’s and likely segregated in three layers because of the large density atmosphere implies the existence of replenishment processes, contrast between them: the low-pressure phase ice Ih, liquid which involve the emission of CH4 from surface or deep-seated res- water, and high-pressure polymorphs ice V and ice VI. From solar ervoirs. Prior to the Cassini–Huygens mission, several potential nebula condensation and satellite formation models (e.g. Alibert methane reservoirs or methane sources had been identified. The and Mousis, 2007; Hersant et al., 2008), primordial ammonia physical state of methane in these foreseen reservoirs is liquid, or gaseous. These reservoirs are summarized in Table 1, and the likelihood of methane replenishment is assessed from the Cas- * Corresponding author. Fax: +1 818 393 4878. sini–Huygens observations whenever available. E-mail addresses: [email protected] (M. Choukroun), Olivier. [email protected] (O. Grasset), [email protected] (G. Tobie), Chris The earliest conceptual models of Titan’s surface mostly focused [email protected] (C. Sotin). on the thermodynamic equilibrium of gaseous methane with liquid 0019-1035/$ - see front matter Published by Elsevier Inc. doi:10.1016/j.icarus.2009.08.011 582 M. Choukroun et al. / Icarus 205 (2010) 581–593 Table 1 Comparison of the reservoirs and sources of methane for atmospheric replenishment on Titan foreseen before the Cassini–Huygens mission with the Cassini observations and relevant experimentalwork. Y/ Reservoir/source Location Exchange Cassini/experimental evidence Comment N N Global ocean/seasa,b Surface Evaporation No oceansc N Lakesd Surface Evaporation Small lakese Present-dayf, does not explain CH4 Y? Porous regolithg 0–1 km Compaction/ Gas release on landingh Large porosity evap. Y? Deep methane 0–2 km Diffusion/evap. No relevant data oceani k,l YCH4 clathrate 0– Cryovolcanism Experimental: stable at depth Cassini: potentially cryovolcanic Possible throughout history hydratesj 1000 km featuresm,n o 40 p,h N Serpentinization 1000 km CO2 reduction Outgassing of Ar Early ages (stops at 2 Gy) N Cometsq Surface Evaporation Few impact craters at presentr Early ages (before 1 Gy) a Sagan and Dermott (1982). b Lunine et al. (1983). c Tomasko et al. (2005). d Flasar (1998). e Stofan et al. (2007). f Lorenz et al. (2008). g Kossacki and Lorenz (1996). h Niemann et al. (2005). i Stevenson (1992). j Lunine and Stevenson (1987). k Loveday et al. (2001). l Hirai et al. (2001). m Sotin et al. (xxxx). n Lopes et al. (2007). o Atreya et al. (2006). p Waite et al. (2005). q Zahnle et al. (1992). r Lorenz et al. (2007). reservoirs consisting of a mixture of methane and ethane. These sug- state or as clathrate hydrate. The boiling point of pure liquid meth- gested the presence of a global hydrocarbon-dominated ocean, or at ane is at a temperature of 110–130 K depending on the pressure least large-scale seas (e.g. Sagan and Dermott, 1982; Lunine et al., conditions, which temperatures are reached at very shallow depth 1983). The global ocean hypothesis has been ruled out by the first on Titan, thus restricting the methane amounts potentially trapped ground-based radar and infrared observations of Titan’s surface, as a liquid in a porous regolith. Such a methane reservoir cannot be which have shown reflectance variations (e.g. Muhleman et al., neglected (e.g., Stevenson, 1992; Kossacki and Lorenz, 1996), but 1990; Griffith, 1993; Coustenis et al., 1995). Observations of Titan’s the storing capability of ice, i.e. the wetting properties and perme- surface by the Cassini spacecraft (Stofan et al., 2007) and the Huy- ability, as well as the processes allowing methane to be supplied to gens probe (Tomasko et al., 2005) have shown the absence of exten- the atmosphere, remain poorly constrained. Thus the likelihood of sive liquid hydrocarbon bodies, the lakes of the North and South this replenishment mechanism is difficult to address in the current Polar regions excepted. According to the estimation of Lorenz et al. state of knowledge. (2008), Titan’s surface lakes may not contain more than Methane could also be trapped at depth on Titan in the form of 3 Â 105 km3 of liquid, which correspond to 1.2 Â 1020 g of methane clathrate hydrate. Clathrate hydrates are non-stoichiometric inclu- – assuming that the lakes are uniquely composed of liquid methane. sion compounds with an ice lattice forming molecular cages, in This corresponds to less than half of the total atmospheric methane which gases are trapped. Their stability at the low pressure–low mass at present-day, and therefore it is insufficient to sustain meth- temperature conditions relevant to the presolar nebula is such that ane in the atmosphere on geological timescales. clathrate hydrates are believed to be the primordial source of the Atmospheric methane could also originate from exogenic reser- volatiles trapped in icy satellites (e.g., Lunine and Stevenson, voirs, like cometary material, which would have fallen onto Titan’s 1985; Gautier and Hersant, 2005; Lunine et al., 2009). Methane surface and released methane upon heating during the impact (Zah- clathrate hydrates are also stable up to very high pressures of nle et al., 1992; Griffith and Zahnle, 1995). The high impact rates that 42 GPa (Hirai et al., 2003), with phase transitions that have been were expected in the early ages of Titan between its formation and documented at 0.8–1 GPa (Loveday et al., 2001; Hirai et al., 2001; the end of the Late Heavy Bombardment might indeed have contrib- Shimizu et al., 2002; Choukroun et al., 2007) and 2 GPa (Loveday uted to the atmospheric methane budget at that time. However, the et al., 2001; Hirai et al., 2001). This very broad stability domain impact rate has tremendously decreased since then.
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