Icarus 225 (2013) 856–861

Contents lists available at SciVerse ScienceDirect

Icarus

journal homepage: www.elsevier .com/ locate/ icarus

Note

On the possible noble gas deficiency of Pluto’s atmosphere

Olivier Mousis a,b,⇑, Jonathan I. Lunine c, Kathleen E. Mandt d, Eric Schindhelm e, Harold A. Weaver f,S. Alan Stern e, J. Hunter Waite d, Randy Gladstone d, Audrey Moudens g

a Université de Franche-Comté, Institut UTINAM, CNRS/INSU, UMR 6213, Besançon Cedex, Franceb Université de Toulouse, UPS-OMP, CNRS-INSU, IRAP, 14 Avenue Edouard Belin, 31400 Toulouse, Francec Center for Radiophysics and Space Research, Space Sciences Building Cornell University, Ithaca, NY 14853, USAd Space Science and Engineering Division, Southwest Research Institute, San Antonio, TX 78228, USAe Southwest Research Institute, 1050 Walnut Street, Boulder, CO 8030223, USAf Space Department, Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, USAg LERMA, Université de Cergy-Pontoise, Observatoire de Paris, ENS, UPMC, UMR 8112 du CNRS, 5 mail Gay Lussac, 95000 Cergy Pontoise Cedex, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 December 2012Revised 15 March 2013Accepted 17 March 2013Available online 4 April 2013

Keywords:Pluto, SurfacePluto, AtmosphereIcesTritonTrans-neptunian objects

0019-1035/$ - see front matter � 2013 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.icarus.2013.03.008

⇑ Corresponding author at: Université de Franche-CoINSU, UMR 6213, Besançon Cedex, France.

E-mail address: olivier.mousis@obs-besancon.fr (O

We use a statistical–thermodynamic model to investigate the formation and composition of noble-gas-rich clathrates on Pluto’s surface. By considering an atmospheric composition close to that of today’sPluto and a broad range of surface pressures, we find that Ar, Kr and Xe can be efficiently trapped in clath-rates if they formed at the surface, in a way similar to what has been proposed for Titan. The formation onPluto of clathrates rich in noble gases could then induce a strong decrease in their atmospheric abun-dances relative to their initial values. A clathrate thickness of order of a few centimeters globally averagedon the planet is enough to trap all Ar, Kr and Xe if these noble gases were in protosolar proportions inPluto’s early atmosphere. Because atmospheric escape over an extended period of time (millions of years)should lead to a noble gas abundance that either remains constant or increases with time, we find that apotential depletion of Ar, Kr and Xe in the atmosphere would best be explained by their trapping in clath-rates. A key observational test is the measurement of Ar since the Alice UV spectrometer aboard the NewHorizons spacecraft will be sensitive enough to detect its abundance �10 times smaller than in the caseconsidered here.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

Gas hydrates, or clathrates, may exist throughout the Solar Sys-tem. Comparison of predicted stability fields of clathrates withconditions in various planetary environments suggest that thesestructures could be present in the Martian permafrost (Mussel-white and Lunine, 1995; Thomas et al., 2009; Swindle et al.,2009; Herri and Chassefière, 2012; Mousis et al., 2013), on thesurface and in the interior of Titan (Tobie et al., 2006; Mousisand Schmitt, 2008), and in other icy satellites (Prieto-Ballesteroset al., 2005; Hand et al., 2006). It has also been suggested thatthe activity of many comets could result from the destabilizationof these ices (Marboeuf et al., 2010, 2011, 2012a). On Earth, thedestabilization of significant masses of CO2 and CH4 potentiallystored in clathrates buried in seabeds and permafrost is regardedas a possible aggravating factor in future global warming (clathrategun hypothesis – Kennett et al., 2003). Broadly speaking, clathrates

ll rights reserved.

mté, Institut UTINAM, CNRS/

. Mousis).

are thought to have taken part in the assemblage of the buildingblocks of many bodies of the Solar System and may be in otherplanetary systems (Lunine and Stevenson, 1985; Mousis et al.,2002, 2006, 2009, 2010, 2011, 2012a; Mousis and Gautier, 2004;Alibert et al., 2005; Marboeuf et al., 2008; Madhusudhan et al.,2011; Johnson et al., 2012).

Clathrates have also been proposed to be at the origin of the no-ble gas deficiency measured in situ by the Huygens probe in theatmosphere of Titan (Osegovic and Max, 2005; Thomas et al.,2007, 2008; Mousis et al., 2011). In the case of Mars, importantquantities of argon, krypton and xenon are believed to be trappedin clathrates located in the near subsurface and their storage inthese structures could explain the measured two order of magni-tude drop between the noble gas atmospheric abundances in Earthand Mars (Mousis et al., 2012b). Here we investigate the possibilityof formation of clathrates rich in noble gases on Pluto’s surface. Todo so, we use the same statistical–thermodynamic model appliedto the case of Titan to determine the composition of clathrates thatmight form on Pluto and to investigate the possible consequencesof their presence on the atmospheric composition.

Table 1Parameters for Kihara and Lennard-Jones potentials.

Molecule rK�W (Å) �K�W/kB (K) aK�W (Å) Reference

N2 3.0993 133.13 0.3526 Herri and Chassefière (2012)Ar 2.9434 174.14 0.184 Herri and Chassefière (2012)Kr 2.9739 198.34 0.230 Parrish and Prausnitz (1972)Xe 3.32968 193.708 0.2357 Sloan and Koh (2008)

rK�W is the Lennard-Jones diameter, �K�W is the depth of the potential well, andaK�W is the radius of the impenetrable core, for the guest-water pairs.

Table 2Parameters of the dissociation curves for various single guest clathrate hydrates. A isdimensionless and B is in K. Constants for Kr and Xe are given for an Arrhenius lawmaking use of Napierian logarithm.

Molecule A B

N2 9.86 �728.58Ar 9.34 �648.79Kr 22.3934 �2237.82Xe 16.62 �3159

Table 3Assumed composition of Pluto’s atmosphere at the ground level. Noble gasabundances relative to N2 are assumed to be protosolar (Asplund et al., 2009).

Species, K Mole fraction, xK

N2 9.31 � 10�1

Ar 6.92 � 10�2

Kr 4.90 � 10�5

Xe 4.79 � 10�6

1 10 100 1000Pressure (Pa)

70

75

80

85

90

95

100

105

110

Tem

pera

ture

(K)

Equilibrium curves of clathrates

Cooling of the atmosphere

Fig. 1. From top to bottom: equilibrium curves of Xe-, Kr-, and Ar-dominatedclathrates (the latter two appear superimposed). The arrow pointing downrepresents the path followed by a cooling atmosphere with a surface pressure of2.4 Pa. When the cooling curve intercepts an equilibrium curve, then thecorresponding clathrate forms.

Note / Icarus 225 (2013) 856–861 857

2. The statistical–thermodynamic model

To calculate the relative abundances of guest species incorpo-rated in a multiple guest clathrate (hereafter MG clathrate) atgiven temperature and pressure, we use a model applying classicalstatistical mechanics that relates the macroscopic thermodynamicproperties of clathrates to the molecular structure and interactionenergies (van der Waals and Platteeuw, 1959; Lunine and Steven-son, 1985). It is based on the original ideas of van der Waals andPlatteeuw for clathrate formation, which assume that trapping ofguest molecules into cages corresponds to the three-dimensionalgeneralization of ideal localized adsorption.

In this formalism, the fractional occupancy of a guest moleculeK for a given type t (t = small or large) of cage (see Sloan, 1998;Sloan and Koh, 2008) can be written as

yK;t ¼CK;tPK

1þP

JCJ;tPJ; ð1Þ

where the sum in the denominator includes all the species whichare present in the initial gas phase. CK,t is the Langmuir constantof species K in the cage of type t, and PK is the partial pressure ofspecies K. This partial pressure is given by PK = xK � P (we assumethat the sample behaves as an ideal gas), with xK the mole fractionof species K in the initial gas, and P the total atmospheric gas pres-sure, which is dominated by N2. The Langmuir constant depends onthe strength of the interaction between each guest species and eachtype of cage, and can be determined by integrating the molecularpotential within the cavity as

CK;t ¼4pkBT

Z Rc

0exp �wK;tðrÞ

kBT

� �r2 dr; ð2Þ

where Rc represents the radius of the cavity assumed to bespherical, kB the Boltzmann constant, and wK,t(r) is the sphericallyaveraged Kihara potential representing the interactions betweenthe guest molecules K and the H2O molecules forming the sur-rounding cage t. This potential w(r) can be written for a sphericalguest molecule, as (McKoy and Sinanoglu, 1963)

wðrÞ ¼ 2z�r12

R11c r

d10ðrÞ þ aRc

d11ðrÞ� �

� r6

R5c r

d4ðrÞ þ aRc

d5ðrÞ� �" #

;

ð3Þ

with

dNðrÞ ¼ 1N

1� rRc� a

Rc

� ��N

� 1þ rRc� a

Rc

� ��N" #

: ð4Þ

In Eq. (3), z is the coordination number of the cell. This parameterdepends on the structure of the clathrate (I or II; see Sloan andKoh, 2008) and on the type of the cage (small or large). The Kiharaparameters a, r and � for the molecule-water interactions, given inTable 1, have been taken from the recent compilation of Sloan andKoh (2008) when available and from Parrish and Prausnitz (1972)for the remaining species.

Finally, the mole fraction fK of a guest molecule K in a clathratecan be calculated with respect to the whole set of species consid-ered in the system as

fK ¼bsyK;s þ b‘yK;‘

bsP

JyJ;s þ b‘P

JyJ;‘; ð5Þ

where bs and bl are the number of small and large cages per unit cellrespectively, for the clathrate structure under consideration, andwith

PK fK ¼ 1. Values of Rc, z, bs and bl are taken from Parrish

and Prausnitz (1972).In the present approach, the dissociation pressure of the MG

clathrate and the mole fractions of the trapped volatiles areindependently calculated. All mole fraction calculations areperformed at the dissociation pressure P ¼ Pdiss

mix of the clathrate,i.e. temperature and pressure conditions at which this ice forms.This dissociation pressure can be deduced from the dissociationpressure Pdiss

K of a pure clathrate of species K, as (Hand et al.,2006; Thomas et al., 2007):

Pdissmix ¼

XK

xK

PdissK

!�1

; ð6Þ

Fig. 2. From top to bottom: mole fractions of volatiles encaged in clathrates thatsuccessively form and calculated as a function of the surface pressure of N2. Noblegas abundances are assumed to be protosolar relative to N2 (Asplund et al., 2009).The clathrate composition is investigated from a gas phase composed of Ar, Kr, Xeand N2 (top panel), of Ar, Kr and N2 (middle panel), and of Ar and N2 (bottom panel).

858 Note / Icarus 225 (2013) 856–861

where xK is the atmospheric mole fraction of species K and PdissK its

dissociation pressure. PdissK derives from laboratory measurements

and follows an Arrhenius law (Miller, 1961) as

Fig. 3. Left: Simulated Ar I 104.8 nm airglow emission with a 3% mixing ratio

logðPdissÞ ¼ Aþ BT; ð7Þ

where Pdiss is expressed in Pa and T is the temperature in K. Theconstants A and B for N2 and Ar have been fitted to experimentaldata (Lunine and Stevenson, 1985; Sloan, 1998) and those for Xeand Kr have been taken from Fray et al. (2010) and Marboeufet al. (2012b), respectively (see Table 2).

3. Clathrate production on Pluto

Depending on its composition, the dissociation pressure of MGclathrate varies between �5.1 � 10�10 and 2.1 � 10�5 Pa at Pluto’saverage surface temperature (�50 K; Lellouch et al., 2000), a valuewell below the atmospheric surface pressure which lies in the0.65–2.4 Pa range (Elliot et al., 2003; Sicardy et al., 2003). Thisimplies that MG clathrate remains stable at the surface irrespectiveof Pluto’s seasonal variations and that its dissociation and reforma-tion cannot occur under the planet’s current atmospheric condi-tions. The only conditions on Pluto’s surface allowing MG clathrateformation from an initial inventory present in the atmosphererequire that the surface be hotter than the present-daytemperature, most likely at early epochs after the planet’sformation (see e.g. Fig. 1 of McKinnon (2002)) or during thecollisions that engendered the Pluto–Charon binary system (McKin-non and Mueller, 1988; McKinnon, 1989). An alternative possibilitywould be the recent or ancient release of hot ice from the interior ofPluto as the result of cryovolcanic events (Cook et al., 2007). It is dur-ing clathrate formation that substantial amounts of volatiles mighthave been sequestrated from the atmosphere into the surface.

Table 3 gives the composition of Pluto’s proto-atmosphere usedin our calculations. We made the conservative assumption that allnoble gases were initially present in the proto-atmosphere ofPluto, with Ar/N, Kr/N and Xe/N ratios assigned to be protosolar(Asplund et al., 2009). Because Pluto’s proto-atmosphere isexpected to be strongly dominated by N2 and that N2 clathrate isof structure II (Lunine and Stevenson, 1985; Sloan and Koh,2008), we show here calculations of the MG clathrate compositiononly for this structure. It is also important to note that our compo-sition calculations are only valid along the dissociation curve of theclathrate of interest (see Fig. 1).

From our calculations, we find that three kinds of clathrateswith distinct compositions might form on Pluto’s surface, each ofthem containing noble gases in different proportions. Fig. 2 showsthe composition of these clathrates computed for an atmosphericpressure ranging between 1 and 103 Pa. Note that these surface

. Right: Count rate images for Alice observation with a 3% mixing ratio.

Fig. 4. Emergent Ar I 104.8 and 106.7 nm brightness as a function of mixing ratio.

Table 4Relevant parameters of each airglow observation.

Observation di (km) ti (s) Xi (lrad)

PC–Airglow–Fill–0 1,116,455 6900 3.21PC–Airglow–Fill–2 819,350 600 4.70PC–Airglow–Fill–2 809,012 600 4.77PC–Airglow–Fill–2 798,261 600 4.84PC–Airglow–Fill–2 788,336 600 4.91PC–Airglow–Fill–2 780,204 360 4.97PC–Airglow–Appr–1a 611,833 1200 6.49PC–Airglow–Appr–1b 575,456 600 6.94PC–Airglow–Appr–2 525,813 4200 7.63PC–Airglow–Appr–3 408,242 3600 9.93PC–Airglow–Appr–4 318,118 3000 12.83

Note / Icarus 225 (2013) 856–861 859

pressures correspond to a MG clathrate equilibrium temperature inthe �80–100 K range. If a greater surface pressure is considered,then the MG clathrate will also form at a higher equilibrium tem-perature. A first layer forms from the gas phase compositiondepicted in Table 3. Irrespective of the pressure considered, themole fraction of Xe trapped in this clathrate is between �0.15and 0.76, i.e. a range of values that is �31,000–159,000 timeslarger than its atmospheric mole fraction. The mole fraction of Kris also substantially enhanced by a factor of �400–750 in clathratecompared to its atmospheric value. In contrast, the mole fraction ofAr trapped in clathrate evolves from a slight impoverishment(�0.5) to a moderate enrichment (�3) with increasing pressure,compared to its atmospheric value. The second clathrate layerforms once Xe is fully trapped in the first layer. In this case, onlyN2, Kr and Ar remain in the gas phase (the mole fractions of thesespecies trapped in the first layer remain negligible). The molefraction of Kr trapped in this clathrate is enhanced by a factor of400–4000 times compared to its atmospheric mole fraction inthe 1–103 Pa pressure range. The Ar mole fraction in this layer isalso found to be moderately enriched by a factor of �2–3,compared to its atmospheric mole fraction. In its turn, a thirdclathrate layer forms when Kr is fully trapped in the second layerand in this case only N2 and Ar remain in the coexisting gas phase.The fraction of Ar trapped in this clathrate remains constant irre-spective of the surface pressure considered and is found to be �3times larger than its atmospheric value.

If Ar, Kr and Xe were initially in protosolar abundances in theatmosphere of Pluto, the amount of clathrates needed for theirsequestration is relatively low. For example, if the three clathratelayers formed at their equilibrium temperatures1 for a surfacepressure of 2.4 Pa, their total equivalent thickness is of order�4 mm globally averaged on the planet, assuming a full clathrationefficiency and the presence of a structure II clathrate. Interestingly,calculations conducted in the case of formation of structure I clath-rate lead to similar conclusions. The noble gas trapping efficienciesare still very high but require an overall thickness of the three clath-rate layers of �2 cm. In both clathrate structures, the equivalentlayer of the Ar-dominated clathrate is more than three orders ofmagnitude thicker than those of Xe- and Kr-dominated clathrates.One must also note that the noble-gas-rich clathrates formed on Plu-to could consist in a mixture of structures I and II clathrates. Indeed,in our computations of the composition of structure II clathrates, wefind that the first layer formed is dominated by Xe, which is itselfpredicted to form a structure I clathrate (Sloan and Koh, 2008).

Fig. 5. Signal to noise ratio for the added Ar I 104.8 and 106.7 nm line emissionversus mixing ratio.

4. Competition with atmospheric escape

An alternative method for losing these noble gases from Pluto’satmosphere could be atmospheric escape. The atmosphere of Plutois expected to escape efficiently due to Pluto’s low gravity (Strobel,2008), though debate exists as to whether the escape rate is greaterthan subsonic (Tucker et al., 2012). If the escape of Pluto’s atmo-sphere is hydrodynamic (Trafton et al., 1997; Strobel, 2008), theoutflow of N2 provides enough energy to drag Ar, Kr and Xe fromthe atmosphere. The escape rate of these gases depends directlyon the escape rate of N2 (Hunten et al., 1987):

Fi ¼Xi

XN2

FN2

mc �mi

mc �mN2

� �; ð8Þ

where F is the escape flux, X is the abundance of the noble gas(subscript i) and N2, m is the mass of each constituent and mc is

1 The equilibrium temperatures of the first, second and third clathrate layers are�77, 76 and 76 K, respectively.

the critical mass which is also a function of the N2 escape rate (Hun-ten et al., 1987):

mc ¼ mN2 þkTFN2

bgXN2

; ð9Þ

where k is the Boltzmann’s constant, T is the temperature, b is thebinary diffusion coefficient given as b = ATs with A and s constantsdetermined through modeling or laboratory measurements, and gis the acceleration due to gravity.

860 Note / Icarus 225 (2013) 856–861

It is clear from Eq. (9) that the critical mass is always greaterthan the mass of N2, so constituents with mass less than N2 arealways subject to escape when hydrodynamic escape is occurring.Ar, Kr and Xe all have masses greater than N2, but the proposedescape rate of N2 is so high that the critical mass is several timesgreater than the mass of Xe and the last term in Eq. (8) has a valueof 1.0. This means that the escape rate of the noble gases relative tothe escape rate of N2, under hydrodynamic escape conditions, isequivalent to the abundance of the noble gas relative to N2 assuggested in Eq. (8). Therefore, at the highest possible escape ratefor the noble gases, the abundance of the noble gas relative to N2

will remain constant. In the case of escape rates that are subsonic,escape of the noble gases will be far less efficient due to insufficientenergy to drag the heavier molecules from the atmosphere. As aresult, the noble gas escape rate relative to N2’s rate of escape willbe less than the abundance of the noble gases relative to N2 in theatmosphere. Over time, the differential escape rates will lead to anincrease in the noble gas abundance in Pluto’s atmosphere. Thismeans that without trapping of Ar by clathrates, escape over anextended period of time (millions of years) will lead to a noblegas abundance that either remains constant or increases with time.Therefore, a depletion of Ar, Kr and Xe in the atmosphere can bestbe explained by trapping of the noble gases in clathrates.

5. Discussion

Using the noble gas abundances given in Table 3 and the fittinglaws of sublimation laboratory data proposed by Fray and Schmitt(2009), we find that the equilibrium temperatures of Ar, Kr and Xepure ices are �39.5, 42.5 and 55.3 K at the surface pressure of2.4 Pa, respectively. Given Pluto’s average surface temperature(�50 K), Xe appears stable on the surface in present-day condi-tions and the condensation of this noble gas should also induce adecrease of its atmospheric abundance. This implies that there isno way to disentangle Xe’s previous trapping in clathrate versusits simple condensation. This is fortunately not the case for Arand Kr, so the measurement of at least one of these two noblegases is critical for testing the scenario of clathrate trapping. TheUV spectrometer Alice aboard the New Horizons spacecraft willbe sensitive enough to detect an argon abundance that is �10times smaller than protosolar Ar/N in some circumstances (seeAppendix A).

Our scenario of the noble gas trapping in clathrates on Pluto ismotivated by the fact that the same interpretation was provided inthe case of Titan in order to account for its observed noble gas defi-ciency (Osegovic and Max, 2005; Thomas et al., 2007, 2008; Mousiset al., 2011). The situation here is much more favorable than in thecase of Titan since the clathrate layer required on Pluto’s surface is�104 times thinner. This is essentially due to the differencebetween the surface pressures since the two atmospheres are bothdominated by N2.

Note that our model considers only the trapping of the noblegases that were present in the early atmosphere. It does notexclude the possibility that the bulk of these noble gases is stillin the interior of Pluto. However, if there is outgassing, the noblegases released in the atmosphere should be trapped as well byclathrates. Our calculations are also valid irrespective of the source(primordial or radiogenic) of the noble gases potentially present inPluto’s atmosphere. Indeed, since Pluto is �half rock, a significantpart of the existing argon could result from the radiogenic decayof potassium-40.

Interestingly, similar calculations have been performed in thecase of Triton and they lead to conclusions as favorable as in thecase of Pluto. However, the surface temperature of Triton, �38 K(Tryka et al., 1994), is much lower than Pluto’s. It is low enough

that the condensation of the three noble gases as pure ices onthe surface would largely remove them from the atmosphere irre-spective of their sequestration in clathrate. Nonetheless, a massspectrometer directly sampling Triton’s atmosphere could inprinciple detect the atmospheric abundances of all three noblegases to assess if they were consistent with coexistent surface ices.Since the atmospheric abundances in coexistence with crustalclathrate are orders of magnitude smaller (probably too small fordetection), this provides an eventual test of the hypothesis in thecase of a future mission to the Neptune system.

6. Conclusions

By considering an atmospheric composition close to that oftoday’s Pluto and a broad range of surface pressures, we find thatAr, Kr and Xe can be efficiently trapped in clathrates if they formedat the surface. The formation of noble gas-rich clathrates on Plutocould then induce a strong decrease of their initial atmosphericabundances. A clathrate thickness of order of a few centimetersglobally averaged on the planet is indeed enough to trap Ar, Krand Xe irrespective of the clathrate structure, if they were inprotosolar proportions in the early Pluto’s atmosphere. We suggestthat the measurement of the Ar abundance by the Alice ultravioletspectrometer (Stern et al., 2008) aboard the New Horizons space-craft during Pluto’s flyby should provide a test of the validity ofour scenario.

Acknowledgments

O. Mousis acknowledges support from CNES. J.I. Lunine wassupported by a contract from JPL under the Distinguished VisitingScientist program. J.H. Waite acknowledges support from NASARosetta funding.

Appendix A. Estimating argon airglow emission for the NewHorizons mission

With the nominal Pluto encounter of the New Horizons missionfully planned, we wish to estimate the amount of Ar I that can bedetected by the Alice UVS instrument during approach. There willbe 11 Alice observations at close range on approach dedicated tomeasuring extended airglow emission, with integrations timesranging from 360 to 6900 s each, for a total of approximately 6 hof integration. The midpoint distances from Pluto for these obser-vations range from 1 � 106 km to 3 � 105 km. We use the plannedobservation times and their corresponding midpoint distances, aswell as the Alice effective area at the Ar I doublet (104.8,106.7 nm), to estimate the signal to noise ratio (SNR) for a givenAr I mixing ratio.

The Ar I brightness was determined using the Atmospheric Ultra-violet Radiance Integrated Code (hereafter AURIC) (Strickland et al.,1999). This was run with Ar I abundances of 0.1%, 0.3%, 1%, 3%, and10%, to produce volume production rates from photoelectron impactand photoexcitation processes. ‘‘Model 2’’ densities from Krasnopol-sky and Cruikshank (1999) were used for N2 and CH4 in the modelatmosphere (7.1 � 1013 cm�3 and 6.4 � 1011 cm�3, respectively),with laboratory measured cross sections and TIMED/SEE solar spec-tral irradiance for excitation. The Ar I brightnesses fed into the AliceUVS model thus are the emergent intensities upwelling through theN2/CH4 atmosphere to space.

Fig. 3 shows simulated brightness images for one of theapproach observations, along with the observation coordinates.Fig. 4 shows the curve of growth made with the brightnessestimates for each Ar I abundance simulated. We use this bright-ness relation to calculate the SNR obtained with the Alice airglow

Note / Icarus 225 (2013) 856–861 861

observations. The total counts obtained (C in photons) in the Aliceslit is just:

C ¼ 106

4pRAX

tiXi; ð10Þ

where R is the brightness (Rayleighs), and A is the effective area(0.1 cm2 at the Ar I doublet). Since each observation differs in length(ti in seconds) and is performed at different distances (leading todifferent solid angles), we sum the photons piecewise. Xi is theeffective solid angle of the emitting object visible in the slit. The dif-ferent exposure times (ti), midpoint distances from Pluto (di), andslit-filling solid angle for each observation are listed in Table 4.

For our SNR estimates we consider only the emission from thearea of Pluto’s solid disk. We assume a detector background darkrate of 0.02 counts per spatial/spectral element (as measured inflight) and the AURIC brightness estimates of the 104.8 and106.7 nm Ar I emission lines to calculate the SNR for a given mixingratio, shown in Fig. 5.

With these calculations we conclude that in 6 h of observing onfinal approach to Pluto, the Alice instrument should be able todetect Argon emission in Pluto’s atmosphere even at lower thansolar abundances. Assuming a number abundance of 0.25 ppmfor Ar I and 36 ppm for N2, i.e., at one tenth solar abundance, theSNR will be �16. Unfortunately, the measurements of Kr and Xeabundances in the atmosphere of Pluto is beyond the instrument’scapabilities.

References

Alibert, Y., Mousis, O., Benz, W., 2005. On the volatile enrichments and compositionof Jupiter. Astrophys. J. 622, L145–L148.

Asplund, M., Grevesse, N., Sauval, A.J., Scott, P., 2009. The chemical composition ofthe Sun. Annu. Rev. Astron. Astrophys. 47, 481–522.

Cook, J.C., Desch, S.J., Roush, T.L., Trujillo, C.A., Geballe, T.R., 2007. Near-infraredspectroscopy of Charon: Possible evidence for cryovolcanism on Kuiper Beltobjects. Astrophys. J. 663, 1406–1419.

Elliot, J.L. et al., 2003. The recent expansion of Pluto’s atmosphere. Nature 424, 165–168.

Fray, N., Schmitt, B., 2009. Sublimation of ices of astrophysical interest: Abibliographic review. Planet. Space Sci. 57, 2053–2080.

Fray, N., Marboeuf, U., Brissaud, O., Schmitt, B., 2010. Equilibrium data of methane,carbon dioxide, and xenon clathrate hydrates below the freezing point of water.Applications to astrophysical environments. J. Chem. Eng. 55, 5101–5208.

Hand, D.P., Chyba, C.F., Carlson, R.W., Cooper, J.F., 2006. Clathrate hydrates ofoxidants in the ice shell of Europa. Astrobiology 6, 463–482.

Herri, J.-M., Chassefière, E., 2012. Carbon dioxide, argon, nitrogen and methaneclathrate hydrates: Thermodynamic modelling, investigation of their stability inmartian atmospheric conditions and variability of methane trapping. Planet.Space Sci. 73, 376–386.

Hunten, D.M., Pepin, R.O., Walker, J.C.G., 1987. Mass fractionation in hydrodynamicescape. Icarus 69, 532–549.

Kennett, J.P., Cannariato, K.G., Hendy, I.L., Behl, R.J., 2003. Methane Hydrates inQuaternary Climate Change: The Clathrate Gun Hypothesis, AGU, Washington,DC, 216pp. http://dx.doi.org/10.1029/054SP.

Johnson, T.V., Mousis, O., Lunine, J.I., Madhusudhan, N., 2012. Planetesimalcompositions in exoplanet systems. Astrophys. J. 757, 192.

Krasnopolsky, V.A., Cruikshank, D.P., 1999. Photochemistry of Pluto’s atmosphereand ionosphere near perihelion. J. Geophys. Res. 104, 21979–21996.

Lellouch, E., Laureijs, R., Schmitt, B., Quirico, E., de Bergh, C., Crovisier, J., Coustenis,A., 2000. Pluto’s non-isothermal surface. Icarus 147, 220–250.

Lunine, J.I., Stevenson, D.J., 1985. Thermodynamics of clathrate hydrate at low andhigh pressures with application to the outer Solar System. Astrophys. J. Suppl.Ser. 58, 493–531.

Madhusudhan, N., Mousis, O., Johnson, T.V., Lunine, J.I., 2011. Carbon-rich giantplanets: Atmospheric chemistry, thermal inversions, spectra, and formationconditions. Astrophys. J. 743, 191.

Marboeuf, U. et al., 2008. Composition of ices in low-mass extrasolar planets.Astrophys. J. 681, 1624–1630.

Marboeuf, U., Mousis, O., Petit, J.-M., Schmitt, B., 2010. Clathrate hydrates formationin short-period comets. Astrophys. J. 708, 812–816.

Marboeuf, U., Mousis, O., Petit, J.-M., Schmitt, B., Cochran, A.L., Weaver, H.A., 2011.On the stability of clathrate hydrates in comets 67P/Churyumov–Gerasimenkoand 46P/Wirtanen. Astron. Astrophys. 525, A144.

Marboeuf, U., Schmitt, B., Petit, J.-M., Mousis, O., Fray, N., 2012a. A cometary nucleusmodel taking into account all phase changes of water ice: Amorphous,crystalline, and clathrate. Astron. Astrophys. 542, A82.

Marboeuf, U., Fray, N., Brissaud, O., Schmitt, B., Bockelée-Morvan, D., Gautier, D.,2012b. Equilibrium pressure of ethane, acetylene, and krypton clathratehydrates below the freezing point of water. J. Chem. Eng. 57, 3408–3415.

McKinnon, W.B., 1989. On the origin of the Pluto–Charon binary. Astrophys. J. 344,L41–L44.

McKinnon, W.B., 2002. On the initial thermal evolution of Kuiper Belt objects.Asteroids Comets Meteors: ACM 500, 29–38.

McKinnon, W.B., Mueller, S., 1988. Pluto’s structure and composition suggest originin the solar, not a planetary, nebula. Nature 335, 240–243.

McKoy, V., Sinanoglu, O., 1963. Theory of dissociation pressures of some gashydrates. J. Chem. Phys. 38 (12), 2946–2956.

Miller, S.L., 1961. The occurrence of gas hydrates in the Solar System. Proc. Natl.Acad. Sci. 47, 1798–1808.

Mousis, O., Gautier, D., 2004. Constraints on the presence of volatiles in Ganymedeand Callisto from an evolutionary turbulent model of the jovian subnebula.Planet. Space Sci. 52, 361–370.

Mousis, O., Schmitt, B., 2008. Sequestration of ethane in the cryovolcanic subsurfaceof Titan. Astrophys. J. 677, L67–L70.

Mousis, O., Gautier, D., Bockelée-Morvan, D., 2002. An evolutionary turbulent modelof Saturn’s subnebula: Implications for the origin of the atmosphere of Titan.Icarus 156, 162–175.

Mousis, O., Alibert, Y., Benz, W., 2006. Saturn’s internal structure and carbonenrichment. Astron. Astrophys. 449, 411–415.

Mousis, O. et al., 2009. Determination of the minimum masses of heavy elements inthe envelopes of Jupiter and Saturn. Astrophys. J. 696, 1348–1354.

Mousis, O., Lunine, J.I., Picaud, S., Cordier, D., 2010. Volatile inventories in clathratehydrates formed in the primordial nebula. Faraday Discuss. 147, 509–525.

Mousis, O., Lunine, J.I., Picaud, S., Cordier, D., Waite Jr., J.H., Mandt, K.E., 2011.Removal of Titan’s atmospheric noble gases by their sequestration in surfaceclathrates. Astrophys. J. 740, L9.

Mousis, O., Lunine, J.I., Madhusudhan, N., Johnson, T.V., 2012a. Nebular waterdepletion as the cause of Jupiter’s low oxygen abundance. Astrophys. J. 751, L7.

Mousis, O., Lunine, J.I., Chassefière, E., Montmessin, F., Lakhlifi, A., Picaud, S., Petit, J.-M., Cordier, D., 2012b. Mars cryosphere: A potential reservoir for heavy noblegases? Icarus 218, 80–87.

Mousis, O. et al., 2013. Volatile trapping in martian clathrates. Space Sci. Rev. 174,213–250.

Musselwhite, D., Lunine, J.I., 1995. Alteration of volatile inventories by polarclathrate formation on Mars. J. Geophys. Res. 100, 23301–23306.

Osegovic, J.P., Max, M.D., 2005. Compound clathrate hydrate on Titan’s surface. J.Geophys. Res. (Planets) 110, 8004.

Parrish, W.R., Prausnitz, J.M., 1972. Dissociation pressures of gas hydrates formedby gas mixtures. Ind. Eng. Chem. Process Des. Dev. 11 (1), 26–35 (Erratum:Parrish, W.R., Prausnitz, J.M., 1972. Ind. Eng. Chem. Process Des. Dev. 11(3),462).

Prieto-Ballesteros, O., Kargel, J.S., Fernández-Sampedro, M., Selsis, F., Martınez, E.S.,Hogenboom, D.L., 2005. Evaluation of the possible presence of clathratehydrates in Europa’s icy shell or seafloor. Icarus 177, 491–505.

Sicardy, B. et al., 2003. Large changes in Pluto’s atmosphere as revealed by recentstellar occultations. Nature 424, 168–170.

Sloan Jr., E.D., 1998. Clathrate Hydrates of Natural Gases. Marcel Dekker, New York.Sloan, E.D., Koh, C.A., 2008. Clathrate Hydrates of Natural Gases, third ed. CRC Press,

Taylor & Francis Group, Boca Raton.Stern, S.A. et al., 2008. ALICE: The ultraviolet imaging spectrograph aboard the New

Horizons Pluto–Kuiper Belt mission. Space Sci. Rev. 140, 155–187.Strickland, D.J. et al., 1999. Atmospheric ultraviolet radiance integrated code

(AURIC): Theory, software architecture, inputs, and selected results. J. Quant.Spectrosc. Radiat. Trans. 62, 689–742.

Strobel, D.F., 2008. N2 escape rates from Pluto’s atmosphere. Icarus 193, 612–619.Swindle, T.D., Thomas, C., Mousis, O., Lunine, J.I., Picaud, S., 2009. Incorporation of

argon, krypton and xenon into clathrates on Mars. Icarus 203, 66–70.Thomas, C., Mousis, O., Ballenegger, V., Picaud, S., 2007. Clathrate hydrates as a sink

of noble gases in Titan’s atmosphere. Astron. Astrophys. 474, L17–L20.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.

Thomas, C., Mousis, O., Picaud, S., Ballenegger, V., 2009. Variability of the methanetrapping in martian subsurface clathrate hydrates. Planet. Space Sci. 57, 42–47.

Tobie, G., Lunine, J.I., Sotin, C., 2006. Episodic outgassing as the origin ofatmospheric methane on Titan. Nature 440, 61–64.

Trafton, L.M., Hunten, D.M., Zahnle, K.J., McNutt Jr., R.L., 1997. Escape processes atPluto and Charon. In: Pluto and Charon, p. 475.

Tryka, K.A., Brown, R.H., Chruikshank, D.P., Owen, T.C., Geballe, T.R., de Bergh, C.,1994. Temperature of nitrogen ice on Pluto and its implications for fluxmeasurements. Icarus 112, 513–527.

Tucker, O.J., Erwin, J.T., Deighan, J.I., Volkov, A.N., Johnson, R.E., 2012. Thermallydriven escape from Pluto’s atmosphere: A combined fluid/kinetic model. Icarus217, 408–415.

van der Waals, J.H., Platteeuw, J.C., 1959. Clathrate solutions. Advances in ChemicalPhysics, vol. 2. Interscience, New York.