CLATHRATE HYDRATES: FTIR SPECTROSCOPY FOR ASTROPHYSICAL REMOTE DETECTION

Emmanuel Dartois∗, Mehdi Bouzit Institut d'Astrophysique Spatiale, UMR-8617

Université Paris-Sud, bâtiment 121, 91405 Orsay FRANCE

Bernard Schmitt

Institut de Planétologie et d'Astrophysique de Grenoble, UMR 5274, Bâtiment D de Physique, BP 53, 38041 Grenoble Cedex 9,

FRANCE

ABSTRACT Clathrate hydrate early entered the field of space astrophysics as they were suggested to explain the anomalous desorption behaviour of gaseous parent molecules in comets by providing a dissociation pressure curve intermediate between the pure compound sublimation equilibrium and the water ice phase. It is nowadays the source for many astrophysical investigations with application ranging from explaining giant planets (and/or their satellites) observed molecular abundances, the possible occurrence of Mars carbon dioxide/methane clathrates or ethane sequestration on the Saturn's moon Titan. We experimentally investigate near to mid-infrared spectroscopic signature of clathrates in the low temperature range adapted to these icy bodies of our solar system. We discuss the implications for space-based astrophysical satellites or probes remote sensing in order to constrain their possible presence.

Keywords: gas hydrates, astrophysics, comets, FTIR spectroscopy

∗ Corresponding author: Phone: +33 (0)169858638 Fax +33 (0)169858675 E-mail: emmanuel.dartois@ias.u-psud.fr

INTRODUCTION Clathrate hydrates may be important for the stability of gases in many astrophysical bodies (planets, satellites, comets...) as they provide a trapping mechanism playing a role in the preservation in the solid state of these molecules at temperatures higher than expected, avoiding their early escape. Their occurrence would thus modify the absolute and relative composition of astrophysical (icy) bodies as well as increase preservation timescales, or e.g. provide late time injection of gaseous species in planetary atmospheres. Many laboratory studies examined their thermodynamic or kinetic behaviour, but one way to confirm their presence in astrophysical bodies will come from remote infrared

spectroscopy observations by telescopes or space probes. Methane, carbon dioxide and carbon monoxide clathrate crystals were produced in our laboratory, and the specific fingerprints betraying these clathrate hydrates presence recorded by infrared spectroscopy at relevant astrophysical temperatures (many previous spectroscopic experiments focused on e.g. Raman spectroscopy and/or neutron diffraction studies). In particular, we show that the trapped methane molecules display a gaseous-like behaviour at low temperature in the water cages. Because the vibrational spectra recorded are unique to methane, carbon monoxide and carbon dioxide clathrate hydrates, they represent an identification pattern for low temperature astrophysical icy bodies, such as planets, comets and/or interstellar

Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

grains. To allow a comparison of experimental work with astrophysical observations, some fundamental (and difficult) questions have to be addressed in parallel to the spectroscopic studies that will be presented. In particular the formation kinetic under realistic astrophysical conditions is essential and will be conditioned by the labile water ice network interaction/reconstruction. As many physical interactions are possible with ice, and not necessarily involving the formation of a crystallographic system such as clathrate hydrates, it is of importance to be able to constrain their abundances in astrophysical media, to understand if they represent, once observed, a widespread phenomena in astrophysical media or only a local state in a few objects. EXPERIMENTS A dedicated evacuable enclosed cell was built to study at first the low-temperature methane clathrate hydrate infrared spectrum. The cell in copper, gold-coated, was attached and thermally coupled to a liquid He-transfer cold finger, placed in an high-vacuum, evacuated cryostat (P < 10-7 mbar). Thick infrared transmitting MgF2, sapphire or ZnSe windows can be sealed with indium gaskets to the two lateral ports. The spectrometer infrared beam monitors the clathrate hydrate spectrum. The cell possesses a soldered stainless steel injection tube at the bottom for the injection of gas or its evacuation. A 3D drawing of the cell is shown in Fig.1. The formation of the hydrate follows a procedure that satisfies both its nucleation and prevents the sample from becoming optically thick to the infrared beam. Water vapour is first injected into the evacuated cell maintained at a temperature just above the water's triple point. Because of the gradual increase of pressure in the cell during injection, liquid water condenses out on the windows, and guest is injected immediately after at moderate pressures, in large excess. The cell is then cooled in the range 180 to 260K to form a hexagonal ice film as monitored by the profile change of the high-wavenumber side of the ice OH-stretching mode not hidden in the spectrum by the gaseous guest molecule absorptions. The cell is maintained in this state, a few bars above the clathrate hydrate stability line at this temperature, typically during two days. The guest diffuses into the thin ice film (1 to 100 microns thick) present on the two interfaces, kept under pressure,

allowing the hydrate nucleation from the solid phase.

Guest Texp/K Pexp/MPa

Type

CH4 240 0.4 I CO2 220 0.4 I CO 180 5 I

Table 1. Experimental formation conditions.

Figure 1: Low-temperature, moderate-pressure closed cell used to nucleate and characterize the clathrate hydrate. The top is attached to a liquid He transfer cold finger. The lower injection tube is used both to inject gaseous water and guest, as well as to evacuate it once the hydrate is formed. The IR transmitting windows offer a 5 mm radius aperture to the infrared beam. In a next step, the cell temperature is then lowered using liquid He. The lowest temperature reached (5.6K) is driven by the thermal escape through the long injection tube connected to room temperature at the other side and exposed cell parts. The gaseous guest is progressively evacuated from the cell. At this stage, care is taken to follow a path in the P-T phase diagram close to, but just below, the guest vaporisation and sublimation curves, to stay above the expected clathrate hydrate dissociation curve. In these experiments, the successful formation of a clathrate hydrate can only be checked at this stage, when all the excess gas has been evacuated while maintaining the lowest temperature. The FTIR spectra are recorded with a

Bruker IFS~66v at a resolution of 0.16 to 0.5 cm-1, with a globar IR source, KBr beamsplitter, and an HgCdTe detector cooled at LN2. The excursions to record the spectra at the highest temperatures are short enough to prevent the eventual and progressive clathrate hydrate dissociation, a limit set by monitoring in previous experiments at higher temperatures. RESULTS Methane

Figure 2: Temperature-dependent spectra of methane clathrate hydrate in the region of the CH stretching mode (A) and in the first combination modes region (B). A pure CH4 crystalline ice spectrum is shown for comparison, in its two phases below 20.9K (cubic phase II) and just above (phase I), for comparison. Spectra are shifted vertically for clarity. The temperature dependent methane clathrate hydrate infrared spectra in the CH stretching mode region are displayed in Fig.1. In addition, for the combination modes the spectra of pure CH4 ice in its two low temperature phases are shown for

comparison. The striking feature is the observation of sub-structures as compared to typical methane ice bands, resembling gaseous methane rovibrational lines, but shifted to the red. The methane is quasi free to rotate at low temperature, as already observed from neutron diffraction data for the rotational transitions in the ground state [1,2,3]. As the temperature increases this ability is partially quenched by collisions with the water ice cage potential surface, and then remains two broad bands assigned to methane trapped in the small and large water cages of the type I clathrate type. Details on the assignments can be found in [4] for the stretching modes and [5] for the near infrared combination modes. Carbon dioxide

Figure 3: Temperature-dependent spectra of carbon dioxide clathrate hydrate in the region of the anti-symmetric stretching mode for the 13CO2 isotopologue (A) and in the Fermi resonance combination modes (B). A pure CO2 crystalline ice spectrum is shown for comparison. Spectra are shifted vertically for clarity.

Carbon dioxide clathrate hydrate is different from methane in many respects as shown in Fig.3. The guest size is larger, the molecule linear, and the rotational ability hindered. One sees in the anti-symmetric mode only two broad bands corresponding to the two unequal cages in which CO2 is trapped. The main vibrational band had been recorded previously at low temperature [6] and we saturated the known bands to access both to the isotopologues transitions as well as the combinations modes [7], in Fermi resonance (Fig.3, panel B). Note that the forbidden antisymmetric stretching-mode overtone (2ν3), activated in the carbon dioxide simple hydrate, is absent in the clathrate hydrate Carbon monoxide The carbon monoxide clathrate forms a type I clathrate whereas its size would predict a more stable type II clathrate such as found for N2 or O2. Davidson et al. [8], using X ray diffraction patterns, have shown that it crystallizes in the type I. To check the clathrate type we formed under our experimental conditions, the spectra of a co-mixed CH4/CO (1:13) clathrate, prepared at 180K under 7 MPa of total pressure, and recorded at 10K was examined in the CH4 stretching mode region and CO unique vibrational transition. The CH4 mode band positions correspond to the ones measured in previous pure CH4 CLH experiments, forming a type I CLH [4], different from the type II CLH stretching modes [9]. Methane is used as a probe of the structure formed and we confirm the formation of a type I clathrate structure whereas, as said above, simple guest size estimates would favour a type II clathrate hydrate, revealing interactions of this molecule with its water network during clathrate formation. The observed cage vibrational downshift with respect to pure CO ice lies within 5 cm-1. The temperature dependent wavenumber separation between the two enclathrated CO vibrational transitions in the two distinct type I clathrate cages is less than a wavenumber at low temperature, implying that the spectral simplification for detailed spectroscopic analysis of the individual profiles is a difficult task. The dynamics of the CO molecules in the cages is affected very differently from 5K to 140K. Above about 30K, the molecule is extremely mobile in the cages, as revealed by the infrared profile, significantly different from CO entrapped in water ice (simple hydrate).

Figure 4: Temperature-dependent spectra of the carbon monoxide clathrate hydrate. The progressive conversion of the absorption from a solid ice spectrum (low temperature, narrower bands) to rotational diffusion (wide absorption band profile) occurs with a barrier of about 17K. A pure CO ice film spectrum at 10K is added to show the vibrational shift induced by carbon monoxide molecule enclathration. The insert shows a close-up on the central part of the absorption. DISCUSSION Solar system context During its early phase, our solar system nebular disk is generally assumed to have experienced hotter temperatures than recorded today at the same distances. The protosolar nebula then progressively cooled down, condensing materials that formed the planets, asteroids and comets. Following this cooling phase at different distances from the young sun, if sufficient water molecules were available, the adiabats crossed the stability curves for the occurrence of clathrate hydrates [10,11]. At this point the thermodynamical conditions to trap species into a clathrate hydrate phase are met. At the considered low pressures encountered in the solar system at this epoch, the crossing took place at low temperature, below about 100K, depending on the considered guest species. Eventually, comets during their formation may have thus partly formed clathrate phases. Later in the evolution of our solar system, other processes could have triggered the formation of

clathrates, often at much higher pressures like encountered in planets or their satellites. This has led to the proposal that methane clathrate hydrate may be present in Mars subsurface [12] or ethane clathrate in Titan [13]. The most important species that may play a role in the formation of solar system clathrates are expected to be small abundant molecules detected in e.g. the coma of comets or planetary atmospheres (like the CH4, CO2, and CO molecules investigated in this study). One of the issues is that, up to now, only models have predicted clathrates hydrates in space and astrophysicists should endeavour to observe them directly. The possible observation of clathrates rely principally on their remote sensing using space probes and/or following a specific influence they might have on observed chemical equilibria. The experiments presented here record the band profiles and expected vibrational shifts in a temperature range spanning the trans-Neptunian objects (30-60K) as well as the Jovian and Saturnian expected radiative equilibrium temperatures (100-150K). Clathrate hydrates spectra of the guests main vibrational modes, generally sufficient for their characterisation, have to be complemented by combination and overtone spectral regions for astrophysical purposes. Spectral surveys of planetary/small body ice bands are indeed mainly carried out in the near-infrared to maximise the reflectance flux, the illuminating source being reflected sunlight, but also because entering the thermal emission regime of solar system objects at lower wavenumbers would deform or mask the features. Observing clathrates The unequivocal confirmation of clathrate hydrate detection requires spectroscopic means. Whereas in the laboratory, in addition to the infrared, they can be probed by e.g. Raman or neutron-scattering spectroscopy, these techniques are difficult or impossible to implement in space probes. To distinguish between a clathrate hydrate crystal and another chemical-ice mixture displaying a potentially comparable thermodynamic behaviour, infrared spectroscopy represents a major remote sensing means. One should be able to record the CLH fingerprints by observing the two distinct cages infrared spectra with a characteristic profile. Methane

The observation of methane hydrate rovibrational transitions at low temperatures, or of the cage-field induced shift at higher temperature constitutes a necessary step in its astrophysical identification. Remote observations of methane clathrate hydrate for high-temperature planetary surfaces can best be performed using enclathrated methane overtone and combination modes when the ν3 stretching mode is saturated. The spectra show that, at temperatures higher than about 40-60 K, rovibrational lines are broadened (as compared to the sharper low temperature transitions). The higher temperatures are more adequate to describe the outer solar system water-dominated ice surfaces (100 - 120 K in the Jovian and Saturnian regions). The red shift and profile induced by the two overlapping cages transitions should still be observable through remote sensing. Outer bodies, such as Trans Neptunian Objects (TNO) as well as some comets, before entering the inner solar system, do possess lower surface temperatures (30-60K) that are low enough to be able to possibly distinguish the rotational substructures. Solar system objects where solid methane transitions has been found include Pluto, Makemake, Eris, Sedna, Quaoar, Triton [14,15,16,17,18,19,20,21,22]. Pluto's methane transitions observed vibrational shifts are better explained by an effect of a N2 ice matrix trapping the CH4 molecules [23,24]. The Neptune's moon Triton, probably a captured TNO, displays the same behaviour involving a nitrogen matrix-shift [25,26,27]. On the surface of the TNO Makemake, shifts of the methane bands are observed [28], but they do not correspond to a CLH phase. Methane is also a still-debated constituent of the Martian atmosphere [29]. Its presence may be the result of the dehydration of a recently exposed clathrate subsurface reservoir [30,12]. Remote sensing observations are then challenging, as they need to be made during the surface release associated with the dehydration process. Cometary ices are still elusive except when ejected and thus modified. No methane absorptions in ejected grains have been reported following Hale-Bopp observations [31] and the "Deep impact" mission to comet Tempel 1 [32].

The CLH water ice network is also to be searched for in astrophysics. Especially for the smallest bodies such as comets, the observable physical state of the ice (e.g., amorphous versus crystalline) is an additional side constraint to the specific transitions associated with trapped molecules, despite the difficulty to distinguish between crystalline ice and clathrate with the water infrared signature only. Carbon dioxide CO2 is present on many solar system icy bodies. It is observed as an almost pure phase on the surface of many satellites from near-infrared spectra sampling the second Fermi resonances (Ariel, Umbriel, Titania; [33], Triton; [34]). On Triton the position and shape of the bands are consistent with pure CO2 ice, although very small shifts remain unexplained [34]. In other objects, the CO2 stretching mode in the mid-infrared reflectance spectra displays blue-shifted bands with respect to pure CO2 (Ganymede, Callisto [35,36], Phoebe, Iapetus, and Hyperion [37,38]). These shifts can find their origin in the formation of simple hydrates (or complexes), e.g. Chaban et al. [39] or by CO2 physisorbed on refractory solids (such as minerals [40]). Mars is a planet with a CO2 dominated surface and planetary atmosphere. It represents a very specific case, because in the temperature range encountered at different latitude and seasons the pure CO2 ice sublimation curve and the clathrate hydrate stability curve crosses in a P,T diagram. Seasonal variations in surface temperature and atmospheric pressure can in principle induce local and seasonal formation and decomposition of CO2 clathrate hydrate (e.g. [41,42]). In addition, the adsorption energy for CO2 on crystalline ice is rather high (E_A/k~2553K [43]; E_A/k~2394K [44]). Then, if cold water ice particles are present in clouds or on the surface and their size distribution is dominated by very small particles displaying a high surface-to-volume ratio, CO2 adsorption may compete with the two previous phases mentioned. Considering these thermodynamical proximities, together with the kinetics behaviour of carbon dioxide interaction with ice (e.g. [45,46]), a spectroscopic discrimination is therefore an essential resource for distinguishing which phase actually occurs. Carbon monoxide

One of the possible occurrences of CO clathrate in comets was recently calculated and discussed by [47]. The authors considered the formation of clathrate during the comet journey along its orbit, subjected to heating and cooling. The formation of clathrates is then evaluated from thermodynamic stability curves. The heat of dissociation of the cages, their thermal conductivity, their formation/dissociation, and the rate of conversion of H2O ice into clathrates were not taken into account at this stage, implying they all behave in favour of clathration, in a time shorter than the characteristic time scales of the thermal variations induced during one comet period. Even with such ideal kinetics, the CO clathration in Jupiter family comets would require the equilibrium pressure of CO clathrate hydrate to lie one order of magnitude lower than expected. Experimental constraints have to be set on this curve, but under this favourable situation, only the upper part of the comet nucleus (less than a few percents) can be clathrated. In a more recent model applied to short period comets [48], the authors show that a layer of multiple guests clathrate can be stabilised in the subsurface of all short period comets, as a function of the thermal wave variations in the subsurface during the comet revolution around the Sun. This would stabilise a few percents of the upper layers of the comet. With the starting considered mixture of gases (CO, CH4, CO2, H2S), the clathration implies strong modifications of the comet crust composition with respect to the initial gas mixture. In particular, hydrogen sulphide should dominate the cages occupancies. Such a clathration would alter the immediate release of molecules observed in the coma of short period comets, or alternatively proceed from a permeation/transition to a clathrate shell of the molecules initially present in the bulk nucleus in another form. Such a scheme does imply the clathration occurs due to the comet crust heating wave and not at the very early stage during the formation of comets. This early epoch thus remains one of the most important to constrain the actual bulk composition of comets, but the presence of clathrate at the surface may alter the release of volatiles thought to be more directly linked to the bulk composition of the comet. One output of the model will be the expected outgassing profile of volatiles that could be eventually measured by the Rosetta mission, thus constraining the structural type of ice existing in the interior of Comet 67P/Churyumov

Gerasimenko. A foreseen distinction should then appear between long and short period comets, and such characteristic differences may allow assessing indirectly the influence of CLH. The outcrop of such a CLH layer would also be essential to search for using reflectance spectroscopy. Perspectives Clathrates have been studied in an astrophysical context with various degrees of knowledge. Their presence can be inferred using modelling, taking into account proper thermodynamics and kinetics or, as discussed above, monitored directly using spectroscopy. The most widely used constraint is linked to thermodynamics, defining the clathrate stability boundaries. It often only sets optimistic limits in models, as it is a necessary condition, when the formation kinetics is neglected. Clathrate hydrates formation may be especially favoured thermodynamically on some planetary surfaces, with notably higher temperatures and pressures than in other objects such as comets. Models, principally based on thermodynamic stability boundaries from phase diagrams, have predicted clathrate hydrates in astrophysical objects [47, 48, 49, 50, 51, 52]. Rare gas abundances in giant planets and the late release of volatiles from comets have been linked to clathrate hydrate nucleation during the proto-solar nebula phase [53, 54, 10, Hersant2008, 47]. Whereas clathrate hydrates are possibly stable against dissociation in various environments, the kinetics for their formation is another step forward to their understanding. At low temperatures, sub-microns ice grains may offer a high surface-to-volume ratio and the build-up of hydrate cages may proceed by adsorption/reconstruction of the ice network. The mixing rate, or renewal of the ice surface, will thus strongly influence the formation kinetics [42, 45]. By contrast, and because the low temperature diffusion of small molecules like e.g. methane into ice is inefficient, methane diffusion, at the temperatures encountered when comets formed, into bulk ice to form the clathrate is probably extremely hampered. Another possibility to form clathrates at low temperature that may also exist in some astrophysical environments is their crystallization directly from water-rich amorphous ice mixtures. Such exothermic process should be

kinetically much more favourable than direct gas-ice interaction, but rely on the detailed balance between trapped gas escape flux within the reorganizing ice matrix and time scale for crystallization. Kinetics is probably the key factor controlling the evolution of astrophysical bodies. Spectroscopy is, as discussed previously, essential to assess the possible presence of clathrate hydrates, and must be evaluated concomitantly with thermodynamic and kinetic experiments. CLH have up to now escaped detection in astrophysical objects, as the expected band doubling is absent from spectra and the observed shifts are different from that expected for CLH. In many objects the water ice does not even dominate the inventory of surface-observed species. If the surface has been replenished in methane recently by out-gassing from the interior, where the species may be trapped as clathrate hydrates, a very strong driving force must be at work. In any case, additional observations must be performed to constrain their presence and establish their relevance. Many alternative physical interactions with water ice are possible at low temperatures, not necessarily involving clathrate hydrates. Spectroscopy should help by revealing whether clathrate hydrates are widespread in astrophysical media or exist only locally in a few objects. REFERENCES [1] Prager M, Press W. Methane clathrate: CH4 quantum rotor state dependent rattling potential. J. Chem. Phys. 2006;125(21):214703-214703-7. [2] Tse JS, Ratcliffe CI, Powell BM, Sears VF and Handa YP. Rotational and Translational Motions of Trapped Methane. Incoherent Inelastic Neutron Scattering of Methane Hydrate. J. Phys. Chem. A 1997;101: 4491-4495. [3] Sears VF, Powell BM, Tse JS, Ratcliffe CI and Handa YP. Motion of CH4 molecules in D2O clathrate from incoherent inelastic neutron scattering. Physica B: Physics of Condensed Matter, 1992;180:658-660. [4] Dartois E, Deboffle D. Methane clathrate hydrate FTIR spectrum. Implications for its cometary and planetary detection. Astron. Astrophys. 2008;490:L19-L22.

[5] Dartois E, Deboffle D, Bouzit M. Methane clathrate hydrate infrared spectrum. II. Near infrared overtones, combination modes and cages assignments. Astron. Astrophys. 2010;514: id.A49. [6] Fleyfel F, Devlin J-P. Carbon Dioxide Clathrate Hydrate Epitaxial Growth: Spectroscopic Evidence for Formation of the Simple Type-II CO2 Hydrate. J. Phys Chem. 1991; 95(5):3811-3815. [7] Dartois E, Schmitt B. Carbon dioxide clathrate hydrate FTIR spectrum. Near infrared combination modes for astrophysical remote detection. Astron. Astrophys. 2009;504:869-873. [8] Davidson DW, Desando, MA, Gough SR, Handa YP, Ratcliffe CI, Ripmeester JA, Tse JS. A clathrate hydrate of carbon monoxide. Nature 1987;328(6129):418-419. [9] Dartois E. CO clathrate hydrate: Near to mid-IR spectroscopic signatures. Icarus 2011;212(2): 950-956. [10] Iro N., Gautier D., Hersant F., Bockelée-Morvan D., Lunine J. I. An interpretation of the nitrogen deficiency in comets. Icarus 2003; 161(2), 511-532. [11] Lunine JI, Stevenson DJ. Thermodynamics of clathrate hydrate at low and high pressures with application to the outer solar system. Astrophys. J. Suppl. 1985;58:493–531. [12] Chassefière E. Metastable methane clathrate particles as a source of methane to the martian atmosphere. Icarus 2009;204(1):137-144. [13] Mousis O, Lunine JI, Thomas C, Pasek M, Marbœuf U, Alibert Y, Ballenegger V, Cordier D, Ellinger Y, Pauzat F, Picaud S. Clathration of Volatiles in the Solar Nebula and Implications for the Origin of Titan's Atmosphere. Astrophys. J. 2009;691:1780-1786. [14] Cruikshank DP, Pilcher CB, Morrison D. Pluto - Evidence for methane frost. Science 1976; 194:835-837. [15] Metz WD. The Coldest Planet: Methane Ice Found on Pluto. Science 1976;192:362. [16] Cruikshank DP., Apt J. Methane on Triton - Physical state and distribution. Icarus 1984;58: 306-311. [17] Barkume KM, Brown ME, Schaller EL. Near Infrared Spectroscopy of Icy Planetoids. Bulletin of the American Astronomical Society 2005;37:738. [18] Barucci MA, Cruikshank DP, Dotto E, Merlin F, Poulet F, Dalle Ore C, Fornasier S, de Bergh C.

Is Sedna another Triton? Astron. Astrophys. 2005;439:L1-L4. [19] Trujillo CA, Brown ME, Rabinowitz DL, Geballe TR. Near-Infrared Surface Properties of the Two Intrinsically Brightest Minor Planets: (90377) Sedna and (90482) Orcus. Astrophys. J. 2005;627:1057-1065. [20] Licandro J, Pinilla-Alonso N, Pedani M, Oliva E, Tozzi GP, Grundy WM. The methane ice rich surface of large TNO 2005 FY_9: a Pluto-twin in the trans-neptunian belt? Astron. Astrophys. 2006;445:L35-L38. [21] Dumas C, Merlin F, Barucci MA, de Bergh C, Hainault O, Guilbert A, Vernazza P, Doressoundiram A. Surface composition of the largest dwarf planet 136199 Eris (2003 UB313). Astron. Astrophys. 2007;471:331-334. [22] Schaller EL, Brown ME. Detection of Methane on Kuiper Belt Object (50000) Quaoar Astrophys. J. Letters 2007;670:L49-L51. [23] Douté S, Schmitt B, Quirico E, Owen TC, Cruikshank DP, de Bergh C, Geballe TR, Roush TL. Evidence for Methane Segregation at the Surface of Pluto Icarus 1999;142:421-444. [24] Grundy WM, Buie MW. Distribution and Evolution of CH4, N2, and CO Ices on Pluto's Surface: 1995 to 1998. Icarus 2001;153(2):248-263. [25] Cruikshank DP, Roush TL, Owen TC, Geballe TR, de Bergh C, Schmitt B, Brown RH, Bartholomew MJ. Ices on the surface of Triton. Science 1993;261:742-745. [26] Quirico E., Douté S., Schmitt B., de Bergh C., Cruikshank D.P., Owen T.C., Geballe T. R., Roush T. L. Composition, Physical State, and Distribution of Ices at the Surface of Triton. Icarus 1999;139:159-178. [27] Grundy WM, Young LA. Near-infrared spectral monitoring of Triton with IRTF/SpeX I: establishing a baseline for rotational variability Icarus 2004;172:455-465. [28] Brown ME, Barkume KM, Blake GA, Schaller EL, Rabinowitz DL, Roe HG, Trujillo C A. Methane and Ethane on the Bright Kuiper Belt Object 2005 FY9. Astron. J. 2007;133:284-289. [29] Formisano V, Atreya S, Encrenaz Th, Ignatiev N, Giuranna M. Detection of Methane in the Atmosphere of Mars. Science 2004;306:1758-1761. [30] Chastain BK, Chevrier V. Methane clathrate hydrates as a potential source for martian atmospheric methane. Plan. Space Sci. 2007;55:1246-1256.

[31] Lellouch E, Crovisier J, Lim T, Bockelee-Morvan D, Leech K, Hanner MS, Altieri B, Schmitt B, Trotta F, Keller HU. Evidence for water ice and estimate of dust production rate in comet Hale-Bopp at 2.9 AU from the Sun. Astron. Astrophys. 1998;339:L9-L12. [32] Sunshine JM, Groussin O, Schultz PH, A'Hearn MF, Feaga LM, Farnham TL, Klaasen KP. The distribution of water ice in the interior of Comet Tempel 1. Icarus 2007;191(2):73-83. [33] Grundy WM, Young LA, Spencer JR, Johnson RE, Young EF, Buie MW. Distributions of H2O and CO2 ices on Ariel, Umbriel, Titania, and Oberon from IRTF/SpeX observations Icarus 2006;184(2):543-555. [34] Quirico E, Douté S, Schmitt B, de Bergh C, Cruikshank DP, Owen TC, Geballe TR, Roush TL. Composition, Physical State, and Distribution of Ices at the Surface of Triton. Icarus 1999;139:159-178. [35] Hibbitts CA, McCord TB, Hansen GB. Distributions of CO2 and SO2 on the surface of Callisto. J. Geophys. Res. 2000;105:2254. [36] McCord TB et al. Non-water-ice constituents in the surface material of the icy Galilean satellites from the Galileo near-infrared mapping spectrometer investigation. J. Geophys. Res. 1998;103(E4):8603-8626. [37] Buratti BJ, 28 colleagues Cassini VIMS observations of Iapetus: Detection of CO2. Astrophys. J. 2005;622(2):L149-L152. [38] Clark RN, et al. Compositional mapping of Saturn’s moon Phoebe with Imaging Spectroscopy. Nature 2005;435:66-69. [39] Chaban GM, Bernstein M, Cruikshank DP. Carbon dioxide on planetary bodies: Theoretical and experimental studies of molecular complexes. Icarus 2007;187(2):592-599. [40] Hibbitts C. A., Szanyi J. Physisorption of CO2 on non-ice materials relevant to icy satellites. Icarus 2007;191(1):371-380. [41] Longhi J. Phase equilibrium in the system CO2-H2O: Application to Mars. Journal of Geophysical Research (Planets) 2006;111:6011. [42] Schmitt B, Mulato L, Douté S. The Formation and Detectability of CO2 Clathrate Hydrate on Mars. Third International Conference on Mars Polar Science and Exploration, 2003;8073. [43] Andersson PU, Nâgârd MB, Witt G, Pettersson JBC. Carbon Dioxide Interactions with Crystalline and Amorphous Ice Surfaces. J. Phys. Chem. A 2004; 108(21):4627-4631.

[44] Galvez O, Ortega IK, Maté B, Moreno MA, Martin-Llorente B, Herrero VJ, Escribano R, Gutiérrez PJ. A study of the interaction of CO2 with water ice. Astron. Astrophys. 2007;472:691-698. [45] Schmitt, B. La surface de la glace: structure, dynamique et interactions, implications astrophysiques Thesis 1986, Université de Grenoble. [46] Adamson AW, Jones BR. Physical adsorption of vapors on ice. IV. Carbon dioxide. Journal of Colloid and Interface Science 1971;37 (4):831-835. [47] Marboeuf U, Mousis O, Petit J-M, Schmitt B. Clathrate Hydrates Formation in Short-Period Comets. Astrophys. J. 2010;708:812-816. [48] Marboeuf U, Mousis O, Petit J-M, Schmitt B, Cochran AL, Weaver HA. On the stability of clathrate hydrates in comets 67P/Churyumov-Gerasimenko and 46P/Wirtanen. Astron. Astrophys. 2011;525:144-147. [49] Prieto-Ballesteros O, Kargel JS, Fernández-Sampedro M, Selsis F, Martínez ES, Hogenboom DL. Evaluation of the possible presence of clathrate hydrates in Europa's icy shell or seafloor. Icarus 2005;177:491-505. [50] Sotin C, Jaumann R, Buratti BJ, Brown, RH, Clark RN, Soderblom LA, Baines KH, Bellucci G, Bibring J-P, Capaccioni F, and 16 coauthors. Release of volatiles from a possible cryovolcano from near-infrared imaging of Titan. Nature 2005; 435:786-789. [51] Kargel JS, Beget J, Furfaro R, Prieto-Ballesteros O, Palmero-Rodriguez JA. Roles of Clathrate Hydrates in Crustal Heating and Volatile Storage/Release on Earth, Mars, and Beyond. American Geophysical Union, Fall Meeting 2007, abstract #GC14A-01. [52] Mousis O, Schmitt B. Sequestration of Ethane in the Cryovolcanic Subsurface of Titan. Astrophys. J. 2008;677:L67-L70. [53] Lunine JI, Stevenson, DJ. Thermodynamics of clathrate hydrate at low and high pressures with application to the outer solar system. Astrophys. J. Supplement Series 1985;58:493-531. [54] Gautier D, Hersant F, Mousis O, Lunine JI. Enrichments in Volatiles in Jupiter: A New Interpretation of the Galileo Measurements. Astrophys. J. 2001;550:L227-L230. [55] Hersant F, Gautier D, Tobie G, Lunine JI. Interpretation of the carbon abundance in Saturn measured by Cassini. Plan. Space Sci. 2008;56(8):1103-1111.