Planetary and Space Science 54 (2006) 1188–1196

Compositional constraints on giant planet formation

Tobias Owena,�, Therese Encrenazb

aInstitute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USAbLESIA, Observatoire de Paris, F-92195 Meudon, France

Accepted 4 May 2006

Available online 14 August 2006

Abstract

Using Ockham’s razor as a guide, we have tried to find the simplest model for the formation of giant planets that can explain current

observations of atmospheric composition. While this ‘‘top-down’’ approach is far from sufficient to define such models, it establishes a

set of boundary conditions whose satisfaction is necessary. Using Jupiter as the prototype, we find that a simple model for giant planet

formation that begins with a solar nebula of uniform composition and relies on accretion of low temperature icy planetesimals plus

collapse of surrounding solar nebula gas supplies that satisfaction. We compare the resulting predictions of elemental abundances and

isotope ratios in the atmospheres of the other giants with those from contrasting models and suggest some key measurements to make

further progress.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Giant planets; Origin; Solar nebula; Abundances

1. Introduction

One of the most surprising results from the GalileoMission to Jupiter was the discovery that most of themeasured heavy element abundances were enhanced by thesame factor of 371 over solar values, relative to hydrogen,regardless of their chemical properties (Owen et al. 1999).Among the nine elements measured, there were threeexceptions (O, He and Ne) whose depleted abundanceswere explained by local (O) or internal (He, Ne) processes.This analysis was based on the solar abundances publishedby Anders and Grevesse (1989). More recent studies of thesolar lower atmosphere using non-LTE and 3D hydro-dynamical models have provided far better fits to therelevant absorption lines in the solar spectrum thanprevious work. As a result, we now have a new, morereliable set of solar abundances that incidentally decreasethe sun’s metallicity, bringing it in line with conditions inthe solar neighborhood (Asplund et al., 2005; Grevesseet al., 2005). Furthermore, it is now possible to relate the

observed photospheric abundances to the protosolarabundances, taking into account the effects of radiativeacceleration and gravitational settling (Turcotte et al.,1998; Turcotte and Wimmer-Schweingruber, 2002). Thesecorrections lead to a 12% increase in presently observedabundances, which are still well below the values tabulatedby Anders and Grevesse (1989). The decrease in abun-dances is not uniform, with argon and nitrogen beingmost strongly affected. We therefore find the enrichmentof heavy elements on Jupiter to be 472 times their originalabundances in the sun (Table 1). The proto-solarmass fraction of heavy elements is also smaller; it is nowzo ¼ 0:0132 compared to the value of z ¼ 0:019 for thepresent sun cited by Anders and Grevesse (1989).Although further changes in solar abundances may be

anticipated, we have elected to use the new values tabulatedby Grevesse et al. (2005) because of the major analyticalimprovements cited by these authors. The consequentdecrease in the measured metallicity of the sun simplyemphasizes the fundamental Galileo finding: all heavyelements except O, He and Ne whose abundances relativeto hydrogen were determined on Jupiter are enrichedcompared with the abundances on the sun.

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0032-0633/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.pss.2006.05.030

�Corresponding author. Tel.: +1808 9568007.

E-mail addresses: owen@ifa.hawaii.edu (T. Owen),

therese.encrenaz@obspm.fr (T. Encrenaz).

The relatively low values for the enrichment of Kr andXe suggest the possibility of systematic error in thedetermination of the abundances for these two rare gases,but we have no basis yet for reaching that conclusion.

Even during the time when only carbon was known to beenriched on the giant planets, icy planetesimals wereinvoked as the carriers of the extra heavy elements (e.g.,Pollack and Bodenheimer, 1989; Pollack et al., 1996). TheGalileo Probe data suggest that these planetesimals musthave been formed at surprisingly low temperatures. Valuesless than 40K for either clathrates, adsorption orcondensation are required to capture Ar and N2 (Owenet al., 1999; Gautier et al., 2001). N2 is necessary to explainthe observed ratio of 15N=14N ¼ 2:3� 0:3� 10�3 (Fouchetet al., 2000; Owen et al., 2001; Fouchet et al., 2004; Abbaset al., 2004). Owen and Bar-Nun (1995) had predicted thatN2 would be the dominant source of N on Jupiter, basedon the probable prevalence of N and N2 in the ISM (vanDishoeck et al., 1993). But they had not anticipated that Nwould be as enriched as carbon, as no known comets ormeteorites exhibit this parity, although it would exist intheir hypothetical Type III Comets. Accordingly, in ourprevious discussion of this subject, we suggested that theicy planetesimals that enriched Jupiter might be calledSolar Composition Icy Planetesimals or SCIPs, to distin-guish them from ordinary comets, which are the only icyplanetesimals whose composition we know (Owen andEncrenaz, 2003). We now want to see if the simpleaccretion and addition of a similar mass of these SCIPsto collapsing envelopes of solar nebula gas of varying masswill allow us to explain the observed abundances andisotope ratios in the giant planets.

Whatever other characteristics it may explain, a success-ful model for the origin and present state of the giantplanets must be able to account for the variations incomposition that we observe as we move from Jupiterto Neptune. Following Ockham’s advice, our goal inthis paper is to find the simplest model that satisfiesthe constraint posed by the abundances and isotoperatios of the elements measured in the giant planetatmospheres.

We first estimate the expected enrichment of heavyelements in a simple core-accretion model, and we compareit to observations of the giant planets (Section 2). Next,we compare our predictions with other models and wediscuss possible diagnostics for discriminating among them(Section 3). Finally, we discuss the implications of ourmodel for the existence of SCIPs and their possibleevolution (Section 4). Our conclusions complete the article.

2. The Ockham approach

In our first paper mentioning this subject (Owen andEncrenaz, 2003), we suggested that the observed uniformenrichment of heavy elements on Jupiter was simply anecessary consequence of the accretion-collapse model forgiant planet formation (Mizuno, 1980; Pollack et al., 1996),provided that the planetesimals that carried in the heavyelements were formed at temperatures below 40K. Tosummarize very briefly, in this model, a massive coreaccretes from icy planetesimals, producing an outgassedatmosphere in the process. Once a critical mass is reached,the surrounding solar nebula gas undergoes a hydrody-namic collapse that is accompanied and followed by theinfall of additional planetesimals. This critical mass may beas low as �5M� (Hubickyj et al., 2005) or as high as20M� or more (Pollack et al., 1996; Wuchterl, 2005),depending on the atmospheric opacity (Hubickyj et al.,2005). The final result is a giant planet of mass M

consisting of a solar composition component of mass Msc

that is enriched by a mass Mi of icy planetesimals that mayinclude both excess mass in the core and in the infallingdebris: M ¼Msc þM i.Here we are assuming that the icy planetesimals are

SCIPs: they exhibit a solar composition of heavy elements;they do not contain solar abundances of H and He.Depending on their temperature of formation, they mayalso lack Ne. Essentially all the hydrogen is in the form ofH2O. These conditions conform to the Galileo measure-ments of element abundances on Jupiter. They are basedon the further assumption that the initial molecular

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Table 1

Element abundance enrichments in Jupiter using the new results of Grevesse et al. (2005)

Element (X) X/H (Anders and

Grevesse, 1989)

X/H (Grevesse et al.,

2005)

Enrichment in Jupiter

versus the solar valuea,bEnrichment in Jupiter versus the

solar value (revised value)

He 0.097 0.085

C 3.62 (�4) 2.45 (�4) 3.370.8a 4.371.1

N 1.12 (�4) 6.02 (�5) 3.071.1a 4.971.9

S 1.85 (�5) 1.38 (�5) 2.770.7a 3.370.8

Ar 3.62 (�6) 1.51 (�6) 2.570.5b 5.471.1

Kr 1.61(�9) 1.91 (�9) 2.770.5b 2.170.4

Xe 1.68 (�10) 1.86 (�10) 2.670.5b 2.170.5

aWong et al., 2004.bMahaffy et al., 2000a.

T. Owen, T. Encrenaz / Planetary and Space Science 54 (2006) 1188–1196 1189

composition of the SCIPs closely follows that of theinterstellar cloud fragment that produced the solar nebula.

The sum of all heavy elements on the planet is thenz0Msc þ fM i, where z0 is the mass fraction of heavyelements in a proto-solar composition gas and f is thefraction of heavy elements in the icy planetesimals. Usingthe abundances from Grevesse et al. (2005), the value of f is0.95. For our standard of comparison, we use zo Mp, whereMp is the mass of an equivalent planet of proto-solarcomposition that is numerically equal to the measuredmass M of the planet in question. Then the enrichment inheavy elements that we seek to calculate is

E ¼ ½z0Msc þ fM i�=½z0Mp�. (1)

We can consider two limiting cases:

(a) Mi ¼ 0: there is no extra icy component. Then Msc ¼

Mp and E ¼ 1, as specified.(b) z0Msc5fMi: in this case there is a very small nebula

component and we can write

E � ½fM i�=½z0Mp�. (2)

There are at least three ways this condition can occur:there is simply insufficient gas available in the surroundingnebula, the icy core fails to achieve critical mass, or thecollapsing gas has been depleted in heavy elements becauseof formation of the icy core. In the latter case, we mustreplace z0 in z0Msc by z0, where z0 is the mass fraction ofheavy elements in the collapsing gas and z0ooz0. Theultimate limit is set by the constraint that MipMp. ThusEmax ¼ f =z0 ¼ 70.

This approach leaves Mi as a free parameter. However,one of the original discoveries of the accretion collapsemodel was the finding that all four giant planets couldbegin with cores of approximately the same mass, about10M� (Mizuno 1980). We will test this assumption asdescribed below, finding that a range of nominal values ofMi between 8.5 and 13M� satisfies the observations of allfour planets. Note that the Ockham approach does notdistinguish between the core and the infalling planetesi-mals. Mi represents the total mass of SCIPs required toproduce the observed enrichments of C (on all giantplanets) and, on Jupiter, N, S, Ar, Kr, and Xe.

In the following calculations, there are three underlyingassumptions: (1) all heavy elements are equally trapped inthe ices; (2) at least the volatile elements in the ices arehomogeneously mixed with the external layers of the planetduring the collapse and post-collapse phases of the planet’sformation; (3) the surrounding solar nebula gas isreplenished in heavy elements, despite formation of thecore. The first assumption is actually not what we expectfrom observations of comets and laboratory studies, and isdiscussed in more detail below. If the second assumption isnot justified, Eq. (1) gives us a lower limit on the expectedenrichment, as the outer layers (where the E factor ismeasured) can only be depleted in heavy elements with

respect to the inner layers. Assumption (3) could bejustified if the planets migrate through the nebula as theyform (e.g., Alibert et al., 2005a, b), if the feeding zone ismixed with surrounding gas through turbulence in thenebula, or if infall from the interstellar medium continuesduring formation of the planet. If assumption (3) is notsatisfied, we have z0Msc ¼ 0, and we must use Eq. (2).We now consider the application of the Ockham

approach to the planets. Our best case is obviously Jupiter,where we have the most data. Here Mp ¼ 318M�. ThenEq. (1) gives us M i ¼ 12ðþ10;�6Þ for E ¼ 4� 2.Were the SCIPs the building blocks for all the giant

planets, as Ockham would suggest, or just for Jupiter? Animmediate test of the ‘‘Ockham approach’’ is to see how wellit fits the available observations of the other giant planets.Here we face a severe problem. At present, the only heavyelement whose abundance we know well on these otherworlds so far is the carbon in methane, as methane is theonly non-condensing gas besides hydrogen and heliumwhose global abundance can be measured by remote sensing.Thus we are forced to rely on spectroscopic values of C/H.Saturn is the best example as methane has been recently

measured by ISO (Lellouch et al., 2001) and confirmedindependently by observations with the CIRS instrumenton the Cassini Spacecraft (Flasar et al., 2005; Orton, 2005).Both of these measurements confirmed the earlier resultfrom Voyager IRIS spectra (Courtin et al., 1984), namely,C/H on Saturn is 971.5 times the new solar value. Themuch lower value of C/H proposed by Kerola et al. (1997)is apparently incorrect.Using Eq. (1) with Mp ¼ 95 Mp and z0 ¼ 0:0132, we find

M i ¼ 11� 2M� , for the observed value of E ¼ 9� 1:5�solar.The agreement with the Jovian value for Mi is pleasing,

but we clearly need more than this if we are to prove thatSCIPs were the source of Saturn’s heavy element enrich-ment. We need to know the abundance of at least oneadditional heavy element that can only be trapped in solarabundance relative to carbon at a very low temperature,i.e. N or Ar. As we will see, the nitrogen isotope ratiowould also be definitive. Unfortunately, we are unlikely toobtain these measurements any time soon, as they requireatmospheric probes.We turn next to Uranus and Neptune where we confront

the same situation as for Saturn, made worse by theuncertainty in the methane abundances. The observationsyield a range of C=H ¼ 40ðþ15;�10Þ � solar for Uranusand 55� 15� solar for Neptune (Baines et al., 1995),corrected for the solar abundances of Grevesse et al.(2005). We find nominal values of M i ¼ 8:5ðþ2:5;�2:0ÞM� for Uranus and Mi ¼ 13� 3M� for Neptune.All of these results are summarized in Table 2. The near

constancy of Mi while M varies by a factor of 25 is striking.It suggests that Mi may indeed represent the masses of theoriginal cores of these planets, and that the contributionfrom infalling planetesimals after core formation was notespecially significant.

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3. Comparisons with other approaches

3.1. The solar nebula

We have assumed that the composition of the solarnebula is solar and approximately homogenous. Yet thereis an implicit assumption in the accretion-collapse modelthat there was a local increase of solids in the solar nebuladisk to start the process that was well beyond what wouldexpected from a uniform distribution of solar abundancematerial (Lissauer, 1987). Hence some increase in heavyelement abundances is expected in order to form the giantplanets (Hueso and Guillot, 2003). One solution to this hasbeen to assume an enhancement of H2O at Jupiter’s orbit,which is seen as the final position of the ‘‘snow line’’, wherewater first condenses. Water vapor from the inner solarsystem could diffuse outward and be ‘‘cold trapped’’ at 5.2AU, according to Stevenson and Lunine (1988) and Cyr etal. (1998). This could be consistent with the models ofGautier et al. (2001), Hersant et al. (2004) and Alibert et al.(2005a, b) which require an enrichment of H2O greaterthan 10 times the solar value at Jupiter in order to make theclathrate hydrates they postulate as the carriers of theheavy elements. On the other hand, Guillot (1999) hasstated that models of Jupiter’s interior based on the GalileoProbe data ‘‘y rule out water abundances larger than 10times solar in Jupiter’s deep atmosphere’’. Consistent withthis, Bezard et al. (2002) have set an upper limit of 9� thesolar value of O/H on Jupiter from their study of CO in theatmosphere. Models invoking clathrates are thus at theedge of acceptability. In the case of core-migration, theneed for local density enhancements is obliterated accord-ing to Alibert et al. (2004, 2005a). In contrast to all of theseinvestigations, Lodders (2004) has suggested that it is aconcentration of carbon, rather than H2O, that providesthe necessary extra mass. However, her model is based onher interpretation that Jupiter’s H2O abundance asmeasured by the Galileo probe is a global value, whereasit is most commonly interpreted as a lower limit to the truevalue (Niemann et al. 1998; Atreya et al. 1999). Amongother problems, this interpretation fails to explain whySaturn appears to exhibit the same depletion of H2O asJupiter when examined by remote sensing (de Graauw et al.1997) even though Saturn’s carbon is much more enhancedthan Jupiter’s. Meteorology solves this problem simply.Thus Lodders’ conclusion that oxygen and hence H2O are

under-abundant on Jupiter compared with other measuredheavy elements cannot be supported by available data.

3.2. The composition of planets

3.2.1. Total heavy element abundances

The total heavy element abundance of each planet(z0Msc þ fM i) predicted by various models is given inTable 3. There is some ambiguity here because the differentauthors have used different values for solar abundances inmaking their calculations. However, the overall disagree-ments are larger than this difference would explain.The major discrepancy for Jupiter and Saturn is with

Boss (2000, 2001, 2002) who has developed models forgiant planet formation based on disk instabilities that growinto protoplanets. These models will produce planets withsolar abundances unless some extra, external source ofheavy elements is postulated. Boss has suggested that hismodel could reproduce the results found on Jupiter if theplanet were struck by a sufficient number of comets after itsformation, and the cometary material stayed in the upperatmosphere where it would be observable by spectroscopy.However, the troposphere and interior of Jupiter aremixing, as evidenced by the depletion of helium and neon(Niemann et al., 1998), and the presence of CO (Bezard etal., 2002), PH3 , AsH4 and GeH4 (Bezard et al., 1988; Nolland Larson, 1991) in the troposphere. Furthermore, thismodel does not explain why Saturn exhibits a carbonenrichment twice that of Jupiter. Comparison withGuillot’s models shows that in the case of Jupiter theagreement is quite good, whereas for Saturn, the Ockhamprediction is a factor 2 lower although marginallyconsistent in view of the uncertainties. Finally, ourpredictions also contradict Marley’s statement that internalmodels of Uranus and Neptune can be built with coressmaller than 1M� and even without any initial core. BothBoss and Marley fail to predict the CH4 enrichmentsmeasured on these two planets.

3.2.2. Isotopes

As already mentioned above, the deduction that nitrogenin Jupiter must have been delivered in the form of N2 issupported by the low value of the nitrogen isotope ratio:15N=14N ¼ 2:3� 0:3� 10�3. The fact that N appears atleast as enhanced as C on Jupiter is one of the argumentsrequiring low temperature planetestimals. Conversely,Ockham would suggest that the nitrogen throughout the

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Table 2

Mass of SCIPs (Mi) required in planets of mass M to produce observed

enrichment E

M (M�) Mi (M�) E

Jupiter 318 12 (+10, �6) 472��

Saturn 95 1172 9.071.5

Uranus 14 8.5 (+2.5/–2.0) 40 (+15, �10)

Neptune 17 1373 55715

Table 3

Total abundance of heavy elements in giant planets as predicted by

different models

Planet Ockham Boss, 2002 Guillot, 2004 Marley, 1999

Jupiter 16 (+10, �6) 6 o19 11–16

Saturn 1272 2 24710 11–16

Uranus 8.5 (+2.5, �2.0) 0.3 o1.5

Neptune 1374 0.3 o1.5

T. Owen, T. Encrenaz / Planetary and Space Science 54 (2006) 1188–1196 1191

outer nebula must have been predominantly (10:1) N2.Thus this low value of the isotope ratio should be found inthe nitrogen on all of the giant planets, thereby signallingthe presence of SCIPs. In contrast, Hersant et al. (2004)predict that a major portion of Saturn’s nitrogen wasdelivered as NH3, which would move the isotope ratiotoward the value found in our atmosphere and mostcommonly in the meteorites, viz. 15N=14N ¼ 3:70� 10�3.

In addition to its intrinsic interest, D/H in theseatmospheres provides another way to constrain total heavyelement abundances, if they were indeed contributed mainlyby SCIPs. As first pointed out by Hubbard andMacFarlane(1980) (hereafter HM) the value of D/H in the atmospheresof these planets depends on their total heavy elementcomplements, again assuming complete mixing of volatiles.This is because D/H in the compounds making upcondensed matter has a higher value than D/H in hydrogengas. We now know these two values with higher precisionthan HM: D/H in solar nebula H2 ¼ 2:0� 0:35� 10�5

(Geiss and Gloeckler, 2003) and D/H in Oort comet H2O,our best guess for the icy planetesimals, is 3:2� 0:3� 10�4

(Meier and Owen, 1999; Bockelee-Morvan et al., 2005). Weassume that the icy planetesimals are SCIPs, so H2O isessentially the only hydrogen carrier. Then the massfraction of hydrogen they contain is 1�f, or �5%.

Following the formalism of HM, we can then write

D

H

� �p

¼D

H

� �s

X eMe þ jX iM i

X eMe þ X iM i

� �, (3)

where Xe is the mass fraction of hydrogen in an atmo-spheric envelope of mass Me and X i ¼ 1Ff is the massfraction of hydrogen in the icy component of the planet,Mi, where Mi has the same meaning as in Eq. (1). j ¼ 16,the ratio of ðD=HÞice to (D/H)protosolarH. The mass fractionof hydrogen will vary somewhat from planet to planet (e.g.,Conrath and Gautier, 2000), but for our purposes, it issufficient to set X e ¼ 0:8. Using the values of Mi fromTable 1, we can calculate ðD=HÞp for each planet, which wecompare to the observed values in Table 4. EvidentlyOckham does rather well. The fits to Uranus and Neptuneare better than those of HM, who found ðD=HÞp410�10�5 for both planets. The improvement results primarilyfrom the smaller amount of hydrogen in SCIPs comparedwith that in a mixture of CH4, NH3 and H2O ices, asassumed by HM.

The agreement between calculation and observationlends support to the values of Mi deduced to fit theobserved values of E, an encouraging consistency for theOckham approach. Concurrently, this agreement appearsto preclude models in which much more mass of heavyelements is added to the planets, unless that mass containsno hydrogen. In particular, we note that models for Uranusand Neptune in which the abundance of water is greaterthan or at least equal to 100� solar (Lodders and Fegley,1994; Hersant et al., 2004) would lead to a value ofðD=HÞpX9� 10�5, at the upper bound of present observa-

tional errors. Adding 15M� of SCIPs to Saturn to fit thegravitational moment J4 (Guillot, 2004) leads to ðD=HÞp ¼

3� 10�5, which is again at the upper edge of uncertainty inthe observations.

3.2.3. Abundances of the elements

The predictions of element abundances on Jupiter byOckham and predictions by Hersant et al. (2004) andAlibert et al. (2005a, b) are very similar. The majordifference between our model and the latter two is causedby the assumption of these authors that the measuredelements were delivered to the planets in volatile com-pounds trapped as clathrate hydrates, whereas Ockhamsuggests they arrived through a combination of volatilecondensation and trapping in amorphous ice and by directincorporation of non-volatile grains in planetesimals.Subsequent heating of these planetesimals in the giantplanet interiors produced the well-mixed ‘‘solar composi-tion’’ of volatiles we observe in the atmospheres today.This difference in the process by which volatiles are

trapped appears in the prediction for the abundance ofsulfur in the clathrate hydrate models, which is approxi-mately twice the value found on the planet.To account for this discrepancy, both Hersant et al.

(2004) and Alibert et al. (2005a,b) have suggested that theH2S/H2 ratio was about 0.57 solar S/H at 5AU whenclathrates were created. Hersant et al. (2004) explain thatthis deficiency ‘‘yimplies that a part of sulfur present inthe early nebula was consumed in the hot inner nebulay’’by reactions that formed FeS, MgS, SiS, etc. They go on toconclude that ‘‘This value [0.57� solar] should have beenthe same throughout the whole turbulent nebula, as long asH2S was not clathrated or did not condense’’. However, ithas been pointed by Atreya et al. (2003) and by Owen andEncrenaz (2003) that this partitioning of sulfur between avolatile and a refractory component should be detectableamong the inner planets, while the depletion in the outernebula should appear in the composition of comets. In fact,S/Si is solar on Earth and in the meteorites. S/O is indeedabout half the solar value in the volatile compoundsdetectable in the coma of Comet Hale-Bopp, but so is C/O,so that C/S is � solar (Bockelee-Morvan et al., 2005).

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Table 4

Predicted and observed D/H in giant planet atmospheres

(D/H)p (D/H)obs

Jupiter 2.1� 10�5 2.670.7� 10�5a

2.2470.04b

2.070.5c

Saturn 2.2 1.70 (+0.75, �0.45) 10�5

Uranus 4.6 5.5 (+3.5, �1.5) 10�5d

Neptune 6.8 6.5 (+2.5, �1.5) 10�5d

(see Owen and Encrenaz (2003) for additional observed values).aMahaffy et al. (2000b).bLellouch et al. (2001).cGeiss and Gloeckler (2003)dFeuchtgruber et al. (1999).

T. Owen, T. Encrenaz / Planetary and Space Science 54 (2006) 1188–11961192

These observations suggest the possibility that the produc-tion of refractory materials in the inner nebula did notpropagate to the outer nebula. Ockham assumes thatsulfur, like carbon, was present in the ISM and in the outernebula in many forms, including being a component ofsome grains, all of which were captured in the SCIPs.Volatilization in the interior of Jupiter would then lead tothe observed solar abundances in the planet’s atmosphere.

The contrast among the models becomes even moreacute on the other three giants. Ockham predicts uniformenrichment on each planet: the same values of E given inTable 2 apply to every element. In contrast, Hersant et al.(2004) predict widely different values for different elements,resulting from the conditions under which they formclathrates and the local supply of ice, which is thought tovary radially in the nebula. In the case of Saturn, Ockham’sapproach provides a better agreement for C/H than theHersant model. As mentioned above, the low value of CH4

inferred by Kerola et al (1997) from the near-IR spectrumof Saturn and matched here by Hersant et al. contradictsall thermal IR results from Voyager, ISO and Cassini.However, while the latest confirmation by Cassini that C/H¼ 971.5 times solar on Saturn appears to contradict theHersant model, Gautier (private communication, 2005) hasremarked that changing the assumed ratio of CH4/CO inthe solar nebula from their original estimate (they haveassumed that all the carbon in the nebula is only in thesetwo compounds) would allow their model to achieve theCassini value of C/H. A possible argument against this‘‘fix’’ is that most of the carbon in the outer nebula mayhave been in the form of grains and organic compounds, asis found to be the case in the ISM and in comets (Bockelee-Morvan et al., 2005). In that case, both the originalprediction and the ‘‘fix’’ would be questionable. TheOckham approach seems to disagree with publishedmeasurements of NH3 and H2S (Briggs and Sackett,1989), while Hersant et al.’s values are closer. However,this model again requires the sulfur abundance in the outersolar nebula to be uniformly reduced to 0.57 solar to fit thechosen Saturnian value of H2S. In fact, the abundance ofH2S on Saturn is only indirectly inferred from the shape ofthe radio continuum, which depends upon several possibleabsorbers. The abundance of NH3 is also derived fromEarth-based observations of the microwave spectrum,which turned out to be badly wrong at Jupiter (Niemannet al., 1998), where some unknown process is depletingNH3 in the upper troposphere (Atreya and Owen, 1995).

On Uranus and Neptune we again have only C/H towork with, and the uncertainties in the measured valuesmake any forceful comparisons with predictions impossi-ble. Both Ockham and Hersant et al. (2004) are consistentwith the existing estimates of C/H, as indeed they must asthey are pegged to those estimates! For S, the agreementwith Ockham’s approach is more marginal; however, as forSaturn, the S measurement is very uncertain (de Pater etal., 1991). Hersant et al. (2004) predict an enrichment of Nof less than 10� solar, with Kr and Ar not enriched at all

and Xe enriched as much as C. Rather than postulating anoriginal depletion of NH3 to explain the microwavespectrum, Ockham would favor an internal depletion ofthis compound, e.g., through solution in an ionic ocean(Podolak and Marley, 1991; Hubbard, 1997), and uniformenrichments of all elements to the same degree as C.

3.2.4. Some critical observational tests

If nothing else, we hope the Ockham approachemphasizes the possibility that a very simple set ofassumptions can explain the available observations ofatmospheric composition as well as if not better than muchmore complex models. There is no need to invoke majordepletion of sulfur in the outer solar nebula, radialvariations in the nebula’s composition, burying of excesswater or other inconvenient species in planetary cores. Yetwe fully recognize that the seeming success of thissimplicity may well result simply from a lack of sufficientknowledge.Unfortunately, it is not yet possible to use the potential

power of isotope ratios and elemental abundances todiscriminate rigorously among available models for plane-tary formation and solar nebula composition and struc-ture. To progress, we obviously need more data on moreelements in all of these atmospheres. A major step forwardwill come in 2012–13 when the Juno spacecraft measuresthe global H2O abundance on Jupiter (Bolton et al., 2003).This will be the first test of possible water variations inthe nebula that could then have led to the incorporationof 410� solar water on Jupiter, in flagrant violation ofOckham’s predictions. Beyond Juno, we need a family ofouter planet probes (Owen, 2004). Relatively shallowprobes—5–10 bar—can give us the abundances and iso-topes of hydrogen, helium, all of the other noble gases andcarbon. In the case of Saturn, such a probe should also giveus 15N/14N. This is a rich harvest and would providesufficient information to distinguish among various modelsfor the solar nebula and the origins and internal structuresof these planets.In particular, measurements of the Ar/H ratio in Saturn,

Uranus and Neptune will allow us to determine whether ornot all giant planets formed from planetesimals thataccreted at very low temperature, as appears to be thecase for Jupiter. If the answer were yes, it would follow theSCIPs were indeed the building blocks for all the giantplanets and thus the most abundant solid material in theearly solar system.

4. SCIPs—where are they now?

If these solar composition icy planetesimals were soabundant in the early solar system, why don’t we see themas leftovers from planet formation today? There are at leasttwo possible answers to this question:

(a) We do see them but we do not know it. A natural placeto look for them would be among the comets in the

ARTICLE IN PRESST. Owen, T. Encrenaz / Planetary and Space Science 54 (2006) 1188–1196 1193

Oort cloud or the Kuiper Belt. To a remote observer,the indication that a given icy planetesimal was in fact aSCIP would be the high (solar) proportion of N and Arrelative to C or O. Unfortunately, we cannot use theCO abundance in comets for this test because it appearsthat this molecule is contributed by more than onesource, (Krankowsky, 1991; Bockelee-Morvan et al.,2005). Thus the CO abundance is not a reliableindication of conditions of cometary origin. Therehave been some measurements of N2 in Oort cloudcomets, reported as abundances of N2

+ in comet tails.The corresponding values of N2 range from zero to asmall fraction of the solar ratio (Cochran et al., 1999,2000). These results are consistent with upper limits oncometary Ar (Weaver et al., 2002). Perhaps thesecomets started out as SCIPs, but lost near-surface N2

and Ar through heating during the planetary encoun-ters that expelled them to the Oort cloud, or in thenebula during the T-Tauri stage of the sun before theywere expelled. In that case, we may still find SCIPsamong the KBOs, if these objects never experiencedthat heating.

(b) Alternatively, just as the rocks that built the innerplanets have vanished, leaving only the similar butisotopically different asteroids and meteorites behind,perhaps giant planet formation was so efficient that allthe SCIPs were consumed, and comets are reallysomething different.

5. Conclusions

We conclude that the Ockham approach despite itssimplicity provides an adequate picture of the limitedfraction of reality we have at our disposal. It challengessome of the more complex models, although the ultimateresolution of apparent differences will require moreinformation than we presently have. Ockham lends supportto the accretion plus collapse scenario for giant planetformation in our solar system, in contrast to the develop-ment of protoplanets from disk instabilities, and favors amodel of the outer solar nebula in which the elements—including oxygen—maintain solar abundances. This con-straint on the composition of the outer nebula in turnappears to be consistent with the composition of comets.Our calculations suggest an excess icy planetesimalcomponent that might have been simply an initial corewith a nominal mass ranging from 8.5–13 terrestrial massesfor all giant planets, a remarkably constant value in view ofthe wide range of their total masses.

There are several implications of these results. Mostimportant is the presence of solid material formed attemperatures below 40K at 5.2 AU. In fact this same low-temperature material must have been spread throughoutthe outer nebula and must have rapidly accreted to sizeslarge enough to avoid significant losses of Ar and N2 beforethe nebula warmed above 30–40K. SCIPs may have been

the first material to define the midplane of the solar nebula.Perhaps they were present from the beginning, formed atvery low temperatures during the collapse of the interstellarcloud fragment that made the solar nebula, or they wereformed from low temperature grains migrating inwardfrom the outer solar system (Hueso and Guillot, 2005).They must then rapidly accrete to form those giant planetcores. The history of this process may be hidden in the icesof the most primitive of the Kuiper Belt Objects.

Acknowledgements

We thank S. Atreya, D. Gautier, T. Guillot, andF. Hersant for helpful comments.

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