How primitive are the gases in Titan's atmosphere?

Adv. Space Res. Vol. 7, No. 5, pp. (5)51—(5)54, 1987 Printed in Great Britain. All rights reserved.

0273—1177/87 $0.00 + .50 Copyright © COSPAR

HOW PRIMITIVE ARE THE GASES IN TITAN’S ATMOSPHERE? Tobias Owen Department of Earth and Space Sciences, State University of New York, Stony Brook, NY 11794, U.S.A.

The basic characteristics of Titan’s atmosphere are still not well—known. The surface pressure of 1.5 bars is contributed by a mixture of nitrogen, methane, and possibly argon whose proportions remain to be accurately defined /1/. Nitrogen is certainly the principal constituent. The amount of methane must lie between 1 and 6 per cent, but an exact determination is frustrated by the f~t that ~thane can condense in Titan’s atmosphere. The possible presence of primordial ~°Ar and Ar has been suggested from its cosmic abundance and its likely incorporation in the icy planetesimals that accreted to form the satellite /2/. Given that approximately 20% of the present atmospheric nitrogen content has been lost through deposition or escape /3,4/, the assumption that cosmic abundances were preserved in the ice—trapping process leads to a prediction of about 10 per cent argon in the present atmosphere. Uncertainties in the mean molecular weight derived from the radio occultation permit as much as 18 per cent /1/. The upper limit on neon in Titan’s atmosphere is less than one per cent /5/ which tells us that the atmosphere cannot be captured /2/. Instead, it must be secondary, produced by devolatization of gases originally trapped in the ices from which the satellite accreted. But what were those gases? How different were they from the gases we now find in Titan’s atmosphere? The purpose of this note is to examine a new determination of the abundance of deuterium in Titan’s methane, to see what this can tell us about the pre—accretion characteristics of the atmospheric gases. In addition to the two or possibly three principal gases, Titan’s atmosphere contains hydrogen, several simple hydrocarbons and nitriles as well as carbon monoxide and carbon dioxide (see 6,4 for summaries). While the nitriles, hydrocarbons and hydrogen are apparently produced by photochemical reactions and/or particle bombardment of nitrogen and methane /3/, the oxides require a different pathway /7/. Carbon dioxide could be formed from CH 4 by means of reactions with OH, which first produce CO /7/. Alternatively, the CO could be primitive, one of the gases that was trapped in the ices that formed the satellite /8/. Even in this case, however, OH is still required to form CO2. The source of the OH for both of these schemes is thought to be H2O entering Titan’s atmosphere from the outside as meteorites or icy debris from the Satu1n system. Alternatively, it could be produced by electron bombardment of CO to produce O( D) which would then attack CH4 and produce OH /7/. Two alternative sources have also been proposed for the nitrogen on Titan. It is either the product of the photodissociation of ammonia, or it too may be primordial. In the former case, it is necessary to postulate an early, efficient atmospheric greenhouse on Titan, sufficient to warm the satellite’s surface to temperatures compatible with a high ammonia vapor pressure (T > 150 K) /9/. This is not an impossible requirement, particularly in view of the evidence for an early, active sun /10/. The second alternative — that nitrogen was directly trapped in the ices that formed the satellite — avoids any such requirement, but does depend on the presence of N2 in the environment in which the condensation of the ices that ultimately accreted to Form the satellite occurred. This environment is commonly assumed to be the proto—Saturnian nebula, a special local region of the primordial solar nebula. Prinn and Fegley /11/ have shown that in a giant proto—planetary nebula, the assumed dominance of CO and N2 can be modified by the locally higher temperature produced by formation of the planet to favor CH4 and NH3, respectively. The presence of CH4 in Titan’s atmosphere would seem to favor this latter condition, but the evidence is not yet decisive. Owen /2/ and Strobel /3/ pointed Out that both N2 and CM4 could form clathrate hydrates under conditions postulated for the proto—Saturnian nebula and thus the observed atmospheric abundances of these two gases could have come from this source, Subsequent laboratory work /12/ has shown that CO appears to form a clathrate in the same pressure and (5)51

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I. Owen

temperature range. Finally, it is now apparent that amorphous ice forming at temperatures below 135 K can trap large amounts of all of these gases, even without forming clathrates /13,14/. Neither process leads to the trapping of neon, unless temperatures are maintained below 25 K. Hence the absence of neon in Titan’s atmosphere is easily explained. The question then becomes, what gases were present in the environment(s) in which the ices formed? Some new evidence that can be used to answer this question has just become available from studies of deuterium on Titan. We have found that CH 3D/CH4 on Ti+t.~r~5as measured in the reflection spectrum near 1.6 pm, leads to a value of DIR = l.65_o~8x 10 , roughly 8 times higher than the value derived for Jupiter and Saturn and twic~the value found on Uranus /15,16/. This enrichment of DIR cannot be explained by post—accretion equilibrium with other hydrogen containing species, even if catalyzed, nor by the fractionation expected from hydrogen escape /17/. Pinto et al. /17/ showed that escape of hydrogen leads to an increase of DIR by a factor 1.7. They also found that catalysis on exposed metallic grains at temperatures of 500 K could produce another factor 2, but this process seems inherently unlikely at the orbit of Titan. Enrichment of atmospheric CH3D relative to CH3D dissolved in a C2H~ocean could four that times the the pre—formatlon primordial value deduced from produce another factor 1.3. It therefore5, seems value of DIN in Jupiter, Saturn,was andon311e meteorites. Titan’s methane theinorder of 8 x 1O~ As a result, we have postulated that there must be at least two distinct reservoirs of deuteriuin in the outer solar system /18/. The dominant reservoir is the deuterlum found in warm hydrogen gas, which can equilibrate with other hydrogen compounds and has t1~e primordial value of DIN “.~2 x io~, The second, much smaller (by a factor of ~lO ) reservoir consists of the hydrogen trapped in cold compounds that never became warm enough (T> 450 K) to equilibrate with the hydrogen. (At these low temperatures, the equilibrium reactions take longer than the age of the solar system to reach completion.) The methane on Titan appears to come from this second reservoir, as do the hydrogen—rich compounds found in carbonaceous chondrites (see 119/ for a review). The value of DIN on Titan is the same value (without the uncertainty) that is found in terrestrial sea water. Meteoritic ratios bracket this value, with some indication that water in hydrated silicates has a systematically lower D/H value than the hydrogen in organic compounds /20/. In their comprehensive review, Geiss and Reeves /19/ concluded that the observed enrichments of DIR in the solar system must result either from processes that occurred prior to solar system formation or from highly catalyzed (and undefined) subsequent chemistry. With the detection of this large enrichment in a low temperature environment, the evidence seems to point toward the former alternative. (Geiss and Reeves /19/ in fact suggested that a determination of DIR on outer planet satellites or comets could be used to discriminate between these two possibilities.) High enrichments of DIR are observed in molecules found in interstellar clouds, where they are explained by ion—molecule reactions /21,22/. Thus we are suggesting that the enrichment of deuterium in the methane on Titan is a result of processes that took place in the interstellar medium before the solar system was formed /18/. The implications of this conclusion for the origin of Titan’s atmosphere follow directly from the discussion given above. The methane that we find on Titan today has apparently survived its transition from the interstellar medium to the satellite’s atmosphere with relatively slight modifications. In particular, all of this methane was not reconstituted from CO and H 2 in the proto—Saturnian nebula in the high temperature zot~,e near the planet /l1~. If it had been, the D/H we find on Titan today would be 2—4 x lO~ instead of 1.6 x l0 . This means that equilibrium at T> 450 K was never fully achieved in the region of the proto—Saturnian nebula where Titan formed. The primordial value of D/H in solar nebula methane was probably even higher than this, since some mixing with higher temperature gas in the proto—Saturnian nebula presumably occurred. If much of the methane survived unaltered from the pre—solar nebula interstellar cloud, then we can reasonably expect that other gases in this cloud would also have been incorporated in the ices that formed Titan. This certainly implicates argon, as described in the introductory paragraph, and it also suggests that CO and N2 should be captured. Unfortunately, we don’t know the ratios of N2/NH3 and CO/CM4 in such clouds, since N2 and CM4 do not have permitted dipole transitions. Using the values for HN2+ and HCO+ in Orion and in the molecular cloud TMC—l tabulated by Irvine at al. /23/, we find N2/C0’~’lO~. (But see Linke et al. /24/ who suggest a value of’~l in a different cloud) The NM3 abundances in the same locations give N2/NH3’~3Oand 400, respectively. The situation with methane is even less well constrained. There seems to be general agreement that only 10—15 per cent of interstellar carbon is in the form of CO. But how the rest is apportioned among methane, interstellar grains, and atomic carbon is simply not known at present. Huntress /25/ suggests CO/CR4 >10 as a good lower limit from chemical models

Primitive Gases in Titan’s Atmosphere

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for gas phase reactions. Given these uncertainties and the known variations in abundances from one cloud to the next, it is impossible to come up with a unique set of starting conditions. All we can say is that CO and N 2 must have been the dominant gases in the outer solar nebula, and this dominance evidently persisted in the zone of the proto— Saturnian nebula from which Titan formed. In accord with the cautionary remarks of Prinn and Fegley /11/, the radial mixing in this sub—nebula was apparently inadequate to bring all of it to equilibrium with conditions near the planet. This perspective opens the possibility that the methane, nitrogen, and carbon monoxide we find on Titan today are all primordial gases. Unfortunately, the present abundance ratios on Titan tell us nothing about the primordial values because of atmospheric escape, deposition, and solution in the seas expected on the satellite’s surface /3,26,4,17/. One implication of this view (originally pointed out by Samuelson et al. /7/) is that the surface of Titan should be covered by a layer of solid CO2, in addition to the global ocean of ethane it may support /26/. The point here is that the amount of CO we now find in the atmosphere would be only a tiny fraction of the amount originally present, since the formation of CO2 via the reaction CO + OH~CO2 would steadily deplete the CO. According to Samuelson et al. /7/, a CO mole fraction of 0.2 in the primitive atmosphere of Titan could have been converted to CO2 well within the lifetime of the solar system. The CO2 would condense to form dry ice on the surface of Titan, equivalent to a layer with a thickness of one to two meters. Thus there is no way of recovering the original CO content from the present abundance of this gas. A further implication of this result is that other examples from the rich list of molecules found in interstellar clouds may also be locked in the ices of Titan. The same should be true for icy comet nuclei, if these too were formed in low temperature environments far enough from forming planets so interstellar conditions could be preserved. Pursuing this logic, it appears that unmodified, low temperature ice should be a rich reservoir of organic material wherever it is found in the solar system. The dark materials we see on the surfaces of some satellites — lapetus and Umbriel, to cite two very different examples — may be manifestations of the existence of this mixture /27/. The presence of C0~and N2+ in comet spectra, along side emissions from hydrocarbons and nitriles, and the dark coatings on old comet nuclei also seem to support this hypothesis. The determination of DIR in cometary molecules would be a critical test, since large enrichments are predicted. A search for argon in the atmosphere of Titan provides an excellent test of the ice trapping hypothesis, as has been stressed elsewhere /2/, but would not establish the connection with the interstellar medium. Unfortunately, both of these observations are difficult, but there is some prospect that values of D/R may emerge from mass spectra recorded during the recent spacecraft flybys of Halley’s Comet, and the Rubble Space Telescope will be able to make this measurement on new comets. REFERENCES 1.

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