The origin of ’s : some recent advances

By Tobias Owen1 & H. B. Niemann2 1University of Hawaii, Institute for , 2680 Woodlawn Drive, Honolulu, HI 96822, USA 2Laboratory for , Goddard Fight Center, Greenbelt, MD 20771, USA

It is possible to make a consistent story for the origin of Titan’s atmosphere starting with the birth of Titan in the subnebula. If we use nuclei as a model, Titan’s and could easily have been delivered by the that makes up ∼50% of its . If Titan’s atmospheric is derived from that ice, it is possible that Titan and comet nuclei are in fact made of the same protosolar ice. The noble abundances are consistent with relative abundances found in the atmospheres of and , the , and the . Keywords: Origin, atmosphere, composition, noble ,

1. Introduction In this note, we will assume that Titan originated in Saturn’s subnebula as a result of the of icy : particles and larger lumps made of ice and . Alibert & Mousis (2007) reached this same point of view using an evolutionary, turbulent model of Saturn’s subnebula. They found that planetesimals made in the solar according to their model led to a huge overabundance of CO on Titan. We obviously have no direct measurements of the composition of these planetes- imals. We can use as a guide, always remembering that comets formed in the solar nebula where conditions must have been different from those in Saturn’s subnebula, e.g., much colder. We can deduce the composition of the solar nebula at Saturn’s distance from the sun by studying the atmosphere of Saturn itself, as this giant presumably captured both particles and gas during its formation (Pollack et al. 1996). We therefore begin with a brief discussion of Saturn before moving on to Titan.

2. Saturn

The atmosphere of Saturn is >99% hydrogen and . In Saturn’s H2, we find D/H = 1.9 × 10−5 (Lellouch et al. 2001) compared to the value of 1.5 ± 0.1 × 10−5 in atomic hydrogen in the local (Linsky 2003). Except for trace constituents (see Atreya et al. 1999), the only other gas detected with certainty in −4 Saturn’s atmosphere is methane, with a mixing ratio of CH4/H2 + He = 7 × 10 (Flasar et al. 2005). The value of D/H in Saturn’s methane is the same as the value of this ratio in the H2 (Lellouch et al. 2001). These numbers are all consistent with

Article submitted to Royal Society TEX Paper 2 T. Owen & H. B. Niemann models for Saturn’s formation in which the methane—or at least the in it— was delivered by icy planetesimals that formed at below 37 K (Owen et al. 1999; Gautier et al. 2001a, b; Owen & Encrenaz 2006; Gautier & Hersant 2005). There is no indication from accurate observations of Saturn’s atmosphere— so far—that there was anything different about the solar nebula from which this planet formed compared with conditions at as deduced from the Probe measurements in Jupiter’s atmosphere (Niemann et al. 1996). However, it is essential to understand that we have only the abundance of carbon to use in the evaluation of different possibilities. Thus we are unable to evaluate models that suggest strongly nonsolar abundances relative to carbon in the local solar nebula and hence in Saturn. In particular, we cannot test the interesting suggestion by Gautier & Hersant (2005) that nitrogen may be deficient and enhanced in Saturn’s atmosphere. These discrepancies would arise from a hypothetical depletion of H2O in the solar nebula at Saturn’s distance from the sun. Absent accurate observations of additional atmospheric abundances, we will simply assume that the composition of the solar nebula at Saturn was essentially identical with its composition at Jupiter. This then furnishes our reference for studying Titan.

3. Titan’s atmosphere We are immediately struck by the dominance of nitrogen in Titan’s atmosphere. Earth is the only other body in the with an atmospheric greater than a few µbars that exhibits this characteristic. Unlike Earth, we find a highly on Titan with methane instead of CO2 and free H2 instead of O2. This mixture of gases resembles the putative composition of the outer solar nebula although not in these proportions. However, it is clear that Titan’s atmosphere is secondary, produced by of the icy planetesimals that formed the satellite. If Titan had collected its atmosphere directly from the solar nebula or from Saturn’s subnebula, its atmosphere would contain slightly more Ne than N, while Ar/N would equal 1/30 according to modern solar abundances (Grevesse et al. 2005). Instead, on Titan today we find Ne/N < 10−8 and Ar/N = 1.5 × 10−7 (Niemann et al. 2005).

(a) Methane The primitive reducing conditions in Titan’s atmosphere today are a direct result of the 94 K surface (Fulchignoni et al. 2005). The of ice at 94 K is so low (<10−2 µbars; List 1958) that we expect no oxidation of methane from this source. Thus any original methane or methane made on the planet will not be converted to CO2 as it must be on any warm inner planet in any . We expect the ratio of carbon to nitrogen to be 20 ± 10, as observed in the atmosphere of and in the reconstituted (Owen & Bar- Nun 1995, 2000). This expectation is consistent with the observed depletion of nitrogen in comets (Geiss 1988), attributed to the difficulty in trapping N2 in ice (Owen & Bar-Nun 1995). N2 is assumed to be the dominant carrier of nitrogen in the outer solar nebula following conditions in the interstellar medium where atomic N is also prevalent (van Dishoeck et al. 1993). Delivery of to Earth and

Article submitted to Royal Society The origin of Titan’s atmosphere 3

Venus by icy planetesimals apparently preserved this nitrogen depletion, resulting in the observed value of C/N = 20 ± 10 compared with the solar value of C/N = 4 (Grevesse et al. 2005). Interstellar abundances predict NH3/N2 ≈ 10 with NH3 assumed to be the dominant condensed nitrogen compound. These values again to C/N ≈ 20, in good agreement with abundances found on comets, Venus and Earth (Owen & Bar-Nun 1995). Thus we might predict 4 ≤ C/N ≤ 20 ± 10 on Titan. Instead we observe C/N < 0.04 (Niemann et al. 2005), well outside this domain. On Earth, the carbon missing from the atmosphere is primarily bound up in carbonate rocks with some contribution from buried organic carbon (e.g., coal and petroleum). Where is the missing carbon on Titan? Another forceful argument that to this same question is the relatively short lifetime of methane in Titan’s atmosphere. is destroying methane in Titan’s upper atmosphere with subsequent reactions ultimately leading to escape or producing aerosols that are continually precipitating to the satellite’s surface. The rate of this conversion is such that the present complement of will be gone in 10–20 × 106 years (Strobel 1982; Yung et al. 1984). Thus the methane in the atmosphere requires a source. That source is not in the famous and ; it must be inside Titan. A detailed model for episodic renewal of atmospheric methane by release from a clathrate cap on a putative subsurface has been developed in detail by Tobie et al. (2006). In this model, the methane in the subsurface ocean was originally brought to the satellite as clathrate , becoming a major component of the primitive atmosphere. It then formed a layer of clathrate at the surface of the ocean, from which it can escape into the atmosphere (Tobie et al. 2006). Manufacture of some of the methane on Titan could have occurred in a two-step process: water-rock reactions such as serpentinization produce hydrogen followed by Fischer-Tropsch type (FTT) reactions with CO and CO2 to produce methane (Owen et al. 2005; Niemann et al. 2005). In the of the Oort comet Hale-Bopp, CO and CO2 were respectively 20 and 6 times as abundant as CH4 (Bockel´ee-Morvan et al. 2004). The total carbon in comets is larger still, owing to the carbon-containing grains in their nuclei (Geiss 1988). Thus there should be plenty of carbon for the hypothesized FTT reactions. However, these reactions require , whereas the first step in this scenario—serpentinization—does not. Hence, it is more likely that this scenario will simply produce hydrogen. Furthermore, if it was preserved in the planetesimals that formed Titan, the 1% methane in Hale- Bopp’s coma would provide far more of this gas than is necessary to account for an adequate reservoir to supply Titan’s atmosphere (cp., NH3 in §3b). Thus it seems more likely that direct delivery was responsible for the methane we see today. In principle, it might be possible to determine the relative importance of en- dogenous and exogenous methane on Titan by comparing ratios with those in protosolar = cometary methane. However, we already know that the carbon in Titan’s methane is fractionated: 12C/13C = 85 = ± 1 vs. 89.0 (VPDB), presum- ably by preferential escape of the isotope from the atmosphere (Niemann et al. 2005). Hence D/H in the methane must be higher than the starting value (presum- ably the D/H in cometary methane) just as we anticipate that cometary methane has a higher value of 12C/13C than the value we find in atmospheric methane on Titan today. Thus we can conclude that today’s methane is not protosolar, unlike

Article submitted to Royal Society 4 T. Owen & H. B. Niemann the methane we find in comets. The same problem would confront an effort to com- pare D/H in Titan’s ice with D/H in contemporary atmospheric methane: the ice may well be protosolar, the methane certainly isn’t.

(b) Nitrogen As we have seen, nitrogen must have arrived on Titan as an easily trapped condensed compound, most likely NH3. This is apparently what happened in me- teorites, comets and the inner (Owen & Bar-Nun 1995; Owen et al. 2001; Meibom et al. 2007). In the case of the meteorites and Earth, this conclusion is sup- ported by studies of 15N/14N (Owen et al. 2001; Meibom et al. 2007). In contrast, N2, the dominant form of nitrogen in the outer solar nebula, was most probably included in the hydrodynamic collapse of surrounding nebula gases that brought H2 and He to Saturn. Alternatively, Gautier & Hersant (2005) have suggested that NH3 was the dominant form of nitrogen captured by Saturn, in which case the ratio of 15N/14N in Saturn’s nitrogen should be similar to the one on Earth. The comets tell us that icy planestimals formed at T > 32 K will not contain N2. This again supports our scenario for the origin of Titan’s nitrogen:

• The solar nebula contains nitrogen primarily in two forms: N2 plus ∼10% NH3.

• Saturn acquires all forms of nitrogen in the solar nebula.

• The icy planetesimals that accreted to form Titan predominantly captured NH3.

Cometary comae contain about 1% nitrogen from NH3 (Bockel´ee-Morvan et al. 2004). If this holds true for comet nuclei as well, and if comets indeed have compo- sitions closely similar to the planetesimals that formed Titan, we can see that Titan ice (∼0.5 of the total mass of the satellite) could hold several times the amount of degassed to form the original atmosphere. The NH3 in that atmosphere would have been photochemically converted to N2 (Atreya et al. 1978). On Jupiter, and thus most likely in the sun, 15N/14N = 2.7 × 10−3 This nitrogen comes pre- 15 14 dominantly from N2. On Earth, and in all varieties of meteorites, N/ N = 3.6 × 10−3. This nitrogen comes from some condensed nitrogen compound, probably 15 14 −3 NH3. On Titan, N/ N = 6.0 × 10 . The most straightforward interpretation of this high value for Titan is the loss of 14N through and upper atmosphere chemistry. The fractionation of the observed in Titan’s nitrogen suggests that the original amount of atmospheric N2 was considerably larger than the present one (Lunine et al. 1999). 36 Assuming that Ar degassed to the same extent as NH3, we can then calculate 36 36 −7 the value of Ar/N in the original atmosphere. Today, Ar/N2 is 2.8 × 10 . Correcting for a factor ∼1.5 to obtain the original N2 abundance produced from 36 7 NH3, we find N/ Ar ∼10 . On Earth N/36Ar = 6 × 104. Some part of this large difference with Titan could be attributed to the warmer temperature at which Titan’s formed in the Saturn subnebula, compared with the comets that brought 36Ar to Earth (if they did!).

Article submitted to Royal Society The origin of Titan’s atmosphere 5

(c) Hydrogen

The important ratio of D/H has been measured in methane (CH3D/CH4) as +0.15 −4 D/H = 1.32−0.11 × 10 by Bezard et al. (2007), while D/H in hydrogen (H2) determined by the Gas Chromatograph Mass Spectrometer (GCMS) is approximately twice as large (the exact value has not yet been established). The atmospheric H2 may be a product of the photolysis of methane. The dif- ference in the values of D/H for CH4 and H2 would then be ascribed to a reaction- driven enrichment of D in the H2 at the expense of the D in methane (Coustenis et al. 2007). Alternatively, the hydrogen may be leaking out from Titan’s interior, the excess gas from reactions that are the first step in the possible production of methane described above. The classic example of such a reaction is serpentinization that would produce H2 from water melted from Titan’s ice interacting with rocks (Owen et al. 2005; Niemann et al. 2005). In this case the higher value of D/H in atmospheric H2 would require a higher value in Titan’s H2O. The problem is to know how, when and where this process occurred. This ambiguity in the explana- tion for the difference between D/H in methane and in H2 could be resolved if we could measure D/H in Titan’s ice directly. It would be especially interesting if the value of D/H in Titan’s ice is indeed the value measured in atmospheric H2 as that value may well be within the error bars of ground-based observations of D/H in cometary H2O (Meier & Owen 1999). (For example, the measurement of D/H in Comet Hyakutake was 2.9 ± 1.0 × 10−4; Bockel´ee-Morvan et al. 1998). In this case there would be a strong presumption that the ice making up ∼50% of the mass of Titan is identical with the ice in comet nuclei. Cometary ice is most likely protosolar, in which case so is its value of D/H. The only way to avoid this would be through a sequence in which the ice is warmed until it vaporizes, allowing cometary H2O to exchange isotopes with solar nebula H2, and then the vapor re-freezes, forming ice with a lower value of D/H. For this to happen, the temperature in the outer solar nebula would have to be about 200 K or more. However, this warming would also lead to complete loss of CO from comets, which is not observed (e.g., Bockel´ee-Morvan et al. 2004). Thus a coincidence between the value of D/H in Titan’s atmospheric H2 with the value measured in cometary H2O could mean that Titan’s ice is also protosolar— that it also never became warm enough to go through this “exchange” . This possibility makes the determination of D/H in both the H2 and in the ice especially interesting.

(d) Noble gases Ne has not been detected on Titan, as mentioned previously. It is not expected to be present owing to the extreme difficulty in trapping this highly volatile gas. The production of radiogenic 40Ar on Titan has already been discussed in Niemann et al. (2005). Its main relevance here is that it demonstrates there are pathways from the rocky interior of Titan through the thick ice to the atmosphere. These pathways could be followed by the primordial noble gases and by methane as well (cf. §3a). If Titan’s atmosphere follows the relative abundances of 36Ar and the isotopes of and xenon in the sun, or even in the atmospheres of Earth, Mars, Venus

Article submitted to Royal Society 6 T. Owen & H. B. Niemann and in the meteorites, it is not surprising that the GCMS did not detect Kr and Xe. The upper limits on both of these gases in Titan’s atmosphere were 10−8 (Nie- mann et al. 2005). That means that 36Ar/Kr > (2.8±0.3×10−7)/10−8), = >28±3. On both the sun and Venus, 36Ar/84Kr ∼2 × 103. This means that if Titan had captured the noble gases in solar abundances, our upper limit was 140 times too high to detect krypton. Owen & Bar-Nun (1995, 2000) have suggested that the heavy noble gases on Earth and Mars were brought to those planets by icy planetesimals. On Earth 36Ar/84Kr = 28; on Mars it is 20.5±2.5 (Bogard et al. 1998). Given the uncertainties in these determinations, it might be possible that the Martian value overlaps the Titan upper limit. However, there is no trace of krypton in the GCMS mass spectra (Niemann et al. 2005). Xenon is approximately 10 times less abundant than Kr in the sun (Grevesse et al. 2005) and in the atmospheres of Earth and Mars (Bogard et al. 1998). Xenon has not yet been measured on Venus and is nearly equal to krypton in the meteorites. Without detecting Kr, it is therefore unlikely that Xe would be detectable if Titan noble gas abundances follow those of the sun or Earth, Mars, and the meteorites. Of course, there are other possibilities. If 36Ar were strongly depleted on Titan by atmospheric escape or by some difficulty in trapping this gas in icy planetesimals compared with Kr and Xe, (e.g., Gautier & Hersant 2005), then 36Ar/(Kr, or Xe) could be even smaller than on Mars. In this case, because we did detect 36Ar, we should have detected Kr. Since we didn’t achieve this detection, we can conclude with some confidence that there has been no selective depletion of 36Ar relative to Kr greater than that on Mars. But suppose 36Ar diffuses into the atmosphere more readily than Kr (and Xe). Or suppose one or both Kr and Xe are trapped as clathrates at the bottom of the putative methane ocean (Tobie et al. 2006). Then atmospheric 36Ar would appear highly enriched relative to the other two elements, again giving us no chance to detect them in the GCMS spectra. The conclusion of this discussion is that it is not surprising that the GCMS did not detect krypton and xenon unless there was a unique depletion of 36Ar in the atmosphere. Thomas et al. (2007) have proposed the formation of clathrate on Titan’s surface as a sink for krypton and xenon, while Osegovic & Max (2005) have calculated that compound clathrates could explain the absence of xenon and presumably krypton as well. The arguments presented above show that the special conditions required for clathrate formation are not required to explain the upper limits on these two gases given the detection threshold of the GCMS. Jacovi & Bar-Nun (2008) have demonstrated in laboratory experiments that aerosols made from C2H2 and HCN can trap the heavy noble gases in proportions consistent with the GCMS results. This process is also not required.

4. Future work It will be many years before we return to Titan with still more capability than Cassini-Huygens was able to achieve. Meanwhile, there are many more things to do. There must be a laboratory investigation of the hypothesis that photolysis of

Article submitted to Royal Society The origin of Titan’s atmosphere 7

CH4 can lead to enrichment of D in the H2 that is produced. This can be done in the next few years. We may hope that future studies of comets will improve our understanding of the starting mix of volatiles on Titan. Measuring the ratio of D/H in cometary CH4 and other H-containing , searching for the heavy noble gases and measuring their abundances and isotope ratios in comets are all critical goals. The value of N/36Ar and the xenon isotope ratios in comets will be especially interesting. All of these measurements, along with many others, will be made by the Rosetta mission when it reaches comet Churyumov-Gerasimenko in 2012. We will need missions to other types of comets as well—especially pristine comets entering the inner solar system for the first time—before we know true “cometary” abundances. We await all these results with great interest. We thank Ingo Mueller-Wodarg for the invitation to this conference. We are grateful to D. Gautier for many helpful discussions and to O. Mousis for his comments and references.

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