chemical evolution on the giant planets and titan

Ads'. Space Re~. Vol.4, No.12, pp.51-58, 1984 0273-1177/84 $0.00 + .50 Printed in Great Britain. All rights reserved. Copyright © COSPAR

CHEMICAL EVOLUTION ON THE GIANT PLANETS A N D TITAN

John Caldwell and Tobias Owen

Department of Earth and Space Sciences, State University of New York, Stony Brook, NY 11794, U.S.A.

ABSTRACT

We summarize the current status of atmospheric chemistry in the atmospheres of the outer solar system with special emphasis on the question of HCN formation on Jupiter, differences between polar and equatorial compositions on Jupiter, the coloration of the Great Red Spot, and the unique environment of Titan.

INTRODUCTION

Investigations of chemical evolution in the atmospheres of the outer planets and their satellites have a long history, beyinning with the early ideas of Rupert Wildt /i/ followed by Harold Urey /2,3/. They were constrained only by the early spectroscopic result that methane (CH 4) was universally present in all such atmospheres and ammonia (NH 3) was present on Jupiter, and by general considerations of densities that suggested that hydrogen and helium were dominant constituents. The first quantitative models of abundances in planetary atmospheres by Lewis /4/ assumed that thermo-chemical equilibrium prevailed throughout. These models were consistent with the early spectra.

However, modern planetary research has shown with absolute certainty that thermo-chemical equilibrium is not consistent with actual atmospheres as the number of identified species grew explosively during the last decade. This explosion was triggered by the application of Fourier Transform Spectroscopy to planetary observations.

The primary contributor has been the infrared interferometer-spectrometers (IRIS) aboard the Voyager 1 and 2 spacecraft. A valuable supplement has been high resolution infrared spectroscopy through several "windows" of transparency from i to 14 um in the terrestrial atmosphere. A third contributor has been the International Ultraviolet Explorer (IUE) spacecraft which uses conventional echelle spectrographs. The IUE observations are particularly important as a quantitative check on infrared studies because it is difficult to separate the effects of temperature and abundance in infrared emission, whereas ultraviolet absorption coefficients are, to first order, insensitive to temperature. Finally, the ultraviolet spectrometers (UVS) on the Voyagers have also been extremely useful in defining the chemical composition of these atmospheres.

This new information has transformed the subject from one dominated by hypothesis and specu- lation to a field in which it is possible to describe what is actually happening. Spectros- copy can give us identities and abundances of most but not all of the simplest substances that are being formed. Nevertheless, there are still many aspects of this general problem that are poorly defined. Even in the case of some of the simpler molecules, such as N_, our data are not very good. We also know that more complex compounds are present as well is the ones that have been identified, even though we don't yet know exactly what they are.

In this short note, we shall briefly review the present status of our knowledge about the chemistry in the atmospheres of the outer solar system with emphasis on those of Jupiter and Titan. We shall not attempt to cite every paper since Wildt and Urey that has been signifi- cant in defining the present understanding, but will reference representative recent work that contains citations of earlier seminal publications.

OVERVIEW

Four fully reduced volatile compounds are of particular importance in the atmospheric chemistry of the giant planets. These are: water (H20), N'H 3, phosDhine (PH3) and ~1%, where the list is in order of increasing volatility. However, because the temperatures of the giant planets generally decrease with increasing distance from the Sun, these substances

51

52 J. Caldwell and T. Owen

are condensed to differing degrees on the different planets. Therefore, the abundances which are visible and available to participate in photochemistry vary greatly from planet to planet.

On Jupiter, CH~ is not subject to condensation. It is uniformly mixed throughout the atmosphere, up to very high altitudes where photodlssociation occurs. Because CH~ is dissociated only by photons with wavelengths less than 145 nm, it is well protected by the Rayleigh scattering of the dominant constituent, H 2. PH 3 does not condense, but it is strongly depleted in the upper atmosphere because it can be dissociated by photons with wavelengths as long as 230 nm. At such wavelengths, the available Solar photons are much more numerous and the protection from Rayleigh scattering is much less than at 145 nm.

~ is depleted at high altitudes from two effects: photodissociation at wavelengths up to 230 nm, and condensation. The familiar, alternating bright/dark, belt/zone structure visible on Jupiter is caused by latitudinally variable ~{3 cirrus clouds. However, gas phase NH 3 is also clearly visible in Jupiter's spectrum. H20 freezes at very low altitudes but gas phase H20 is again detectable. Because of condensation, the mixing ratios of NH 3 and H20 are both poorly determined, particularly the latter.

On Saturn, the major difference with respect to Jupiter is that N}I 3 is almost completely frozen out. The NH 3 forms thick clouds, and does not participate in photochemistry. This latter point is convincingly proven by the absence of pre-dissociation bands of NH 3 from 210 to 230 nm in Saturn's ultraviolet spectrum, although such bands are prominent on Jupiter. CH~ and PH 3 are available for photochemistry on Saturn.

In the atmospheres of Uranus and Neptune, all four of these gases can condense. The highest altitude cloud layer is probably CH~, but there is also gas phase CH~ above the clouds. No other major constituents except the ever-present H 2 are visible; only CH 4 is available for photochemistry unless some N 2 (currently undetectahle) is present.

Titan is the only satellite in the solar system with a massive atmosphere. Many specific molecules have been identified there. The reddish color of this icy satellite is also a clue that chemical reactions are taking place, transforming simple gases into more complex substances. The satellite is small enough that hydrogen can easily escape, so the atmosphere we see today is highly evolved. As part of this evolution, Titan's atmosphere has produced a large amount of material that must be deposited on the satellite's cold, invis- ible surface.

FIVE ATMOSPHERES

Jupiter

Jupiter is the largest, closest, and most colorful of the outer planets. It is also the best studied. The subtle colors in its clouds by themselves provide an indication that chemistry is occurring in its atmosphere, since clouds formed by condensation of the major atmospheric constituents would simply be white.

On Jupiter, the atmosphere is dominated by the presence of a nearly cosmic abundance of hydrogen. There is an internal heat source that keeps the temperature well above 1000°K in the deep atmosphere, below the region where photochemistry is taking place. Strong vertical convection thus ensures that complex compounds created in the upper atmosphere will be reduced to simpler substances when they are swept down to the high temperature region, leading to an endless cycle of creation and destruction.

A summary of our current understanding of the composition of Jupiter's atmosphere is given in Table i. Saturn is included for comparison. The compounds above the dashed line are those which are expected to form from a cosmic mixture of the elements at a temperature of 300°K. Evidently, a large amount of chemical activity is indeed taking place in the atmospheres of these two planets.

Chemical Evolution on the Giant Planets 53

TABLE 1 Approximate atmospheric abundances

Gas Jupiter Saturn

H 2 1 i CH4 i - 2 x 10 -3 "~ 2 x 10 -3 NH 3 2 x i0 -~ > 2 x 10 -5 H20 10 -6 __

C2H 2 8 x 10 -7 i x 10 -7 C2N 6 4 x 10 -5 7.5 x 10 -6 CO 3 x 10 -9 HCN 2 x 10 -9 < 7TI0 -9 GeH 4 6 x i0 -I0 PH 3 4 x 10 -7 3 x 10 -6

C3H 4 yes C3H 8 yes yes

In fact it now appears that the atmosphere of Jupiter does not reproduce cosmic abundances even for the major uncondensed volatile, CH 4. As Gautier and Owen /5/ have stressed, the enrichment of [C]/[H] on Jupiter as determined by the best Voyager measurements appears to preclude models for the formation of the planet by simple condensation from the primordial Solar nebula. The planet must have grown in a two-stage process - first forming a large ice-rock core and then attracting an envelope of gas from the nebula.

The present atmosphere is thus a mixture of a secondary, degassed (or volatilized) atmosphere produced by the core and an envelope of solar-composltion gas captured from the primordial nebula. We therefore expect all elements less volatile than hydrogen, helium, and neon to be enriched in the atmosphere. So far, methane is our best example of this enrichment. But the Galileo entry probe will provide a much more rigorous test of this model in 1988 by measuring abundances of many more gases, at low altitudes where disequilibrating influences are small and where water and anlnonia do not condense.

Given the starting mixture of major volatiles, what chemical evolution will occur? Several disequilibrating mechanisms have been identified for Jupiter: at high altitudes, photochemistry /6/ and particle bombardment /7/; at intermediate altitudes, lightning discharge /8/; at lower altitudes, pyrolization /9/. In this note, we cannot review all of these mechanisms in detail. But we want to give special attention to the problem of hydrogen cyanide (HCN) production and to the characteristics of the chemistry occurring in the auroral zone near the planet's north pole.

HCN has been observed in absorption in the spectrum of Jupiter by Tokunaga et el. /i0/. Because it is in absorption, and not emission, contrary to the thermal infrared signatures of several hydrocarbons (CH4; ethane [C2H6]; acetylene [C2H2]), most of the HCN must be present in the upper troposphere or lower stratosphere. Its altitude distribution is dis- similar to those of C2H 2 and C2H6, which are produced at high altitudes, where CH~ photochemistry occurs. It is somewhat similar, however, to the altitude distribution of NH 3.

Kaye and Strobel /ii/ have produced a quantitative model which agrees with the observations of Tokunaga et el. In this model, NH 2 radicals produced by the photodissociation of NH3, and C2H 3 radicals produced by the addition of H to C2H2, combine to form an isomer of C2H5N which is called both aziridine and also ethyleneimlne. The C2115N in turn is photodisso- ciated to produce HCN. The details of the model depend to a great extent on some reaction rates which are not well known, and on vertical mixing rates which themselves are difficult to estimate. Therefore, the model can only be regarded as a plausible but not unique mechanism for HCN production. (An independent, nonphotochemical process, lightning discharge, will be discussed below.) In a separate study, F.aye and Strobel /12/ did conclude that an alternate photochemical path, in which methyl amine (CH3NH 2) is an intermediate product, is much less likely than the process involving C2H5N.

Neither CH3NH 2 nor C2H5N has been detected on Jupiter, although if it is their photodissociation which leads to HCN, then they must exert some effect on the ultraviolet spectrum of the planet. Thorough searches have been made. Unfortunately, the predicted amounts are so much smaller than known strong uv absorbers, such as NH 3 and C2H2, that it has not been possible to place meaningful limits on the observed abundances of C2H5 N and CH3NH 2 /13/.


The amount of the phosphorus analog to C2H5N , phosphirane (C2H5P) is predicted to be less than that of C~H5N , and the C2H5P would be formed deeper than C2HbN, if at all. Therefore HCP, the phosphorus analog to IICN, is not expected to be observable on Jupiter, in agreement with current observations.

An alternative process for HCN production has been suggested by Bar-Nun et al. /8/. These authors showed that a laboratory simulation of the Jovian atmosphere in which the gases are subjected to an electric discharge led to the formation of both HCN and C2H2, among other products. Their proposal is that lightning discharges on Jupiter may account for the observed abundance of HCN. This is consistent with recent estimates of the energy available from Jovian thunderbolts /14/.

A further recent development in Jupiter studies is that the amounts of hydrocarbons produced at high altitudes appear to be very different at different latitudes. In retrospect, this is not difficult to understand, since the solar photon flux is clearly much greater at the equator of Jupiter than at its poles, and the precipitation of charged particles from the Jovian magnetosphere is much larger near the poles than at the equator. Both energizing agents can induce disequilibrium chemistry, and the resulting steady state concentrations of products can certainly be different.

The recognition of this possibility comes from a combination of ground-based and spacecraft infrared results. Caldwell et al. /15,16/ have shown that there are small, local regions on Jupiter, which are physically very near the magnetic poles, which are brighter at 8 um wave- length than any other places on Jupiter. Their interpretation of this result is that the bright regions correspond to areas where charged particles have heated the atmosphere, and that the observed excess radiation is due to thermal emission from the v~ fundamental band of the dominant trace stratospheric molecule, CH 4.

The Voyager IRIS instrument had observed the north polar bright region in 1979. The observations were taken because the UVS instrument was making auroral observations, and the two spectrometers are co-aligned. Originally, nothing unusual was recognized in the infrared spectrum of this region. However, with retrospective knowledge of the ground-based infrared polar observations, Kim et al. /17/ discovered emission features caused by three molecules that do not appear at other locations on Jupiter. These are (with estimated mixing ratios): ethylene, C2H 4 (7 x 10-9); methyl acetylene, C3H 4 (3 x 10-9); and benzene, C6H 6 (2 x 10-9). The latter detection is unique in the entire astronomical literature. Although these molecules are observed in the part of Jupiter with the highest stratospheric temperature, and their emissivities are therefore greater there, quantitative calculations indicate that the abundances of all three molecules are really greater near the magnetic poles than at the equator.

The reasons for this zenographic chemical variation are not known currently. What is measured is the steady state accumulation of the trace constituents, which results from a balance between creative and destructive processes. The excess accumulation of C2H~, C3H~ and C6H 6 at the Jovian poles may be the result of faster creation of these molecules by the bombardment of energetic particles from the magnetosphere, or of slower destruction because of the reduced solar ultraviolet photon flux with respect to that at the equator.

To emphasize the current indications of spatial variability on Jupiter, we note that Wagener et al. /13/ have modelled ultraviolet equatorial spectra from the IUE satellite, and find that, in addition to the major absorbers, NH 3 and C2H2, the fit to the data is improved with a mixing ratio of i x 10 -9 of allene (C3H ~) (not detectable at this concentration in the infrared) and, with somewhat less certainty, some amount of cyclopropane (C3H 6) less than 15 x 10 -9 .

Another continuing problem of considerable interest is the nature of chromophore(s) respon- sible for the color of Jupiter's Great Red Spot (GRS). No specific absorption in the spectrum of this feature has been found that could be used in such an identification. But it is possible to search for the presence of small molecules that could be intermediaries in the reactions leading to the red material. Thus Ferris and Benson /18/ have suggested a search for P2H~, since they found that this substance plays an important role in the conver- sion of PH 3 to P~.

Red P~was suggested by Prinn and Lewis /19/ to be the cause of the color of the GRS. Subse- quent laboratory studies suggest that phosphine photochemistry produces yellow P~ /20/, or no P4 /21/, but there are still disagreements about the applicability of these studies to Jupiter.

The Prinn-Lewis model makes the specific prediction that PH 3 should be notably enhanced above the GRS compared with neighboring regions on Jupiter. Kim and Owen /22/ have looked for this enhancement in IRIS spectra of the GRS. No evidence of the predicted enhancement is apparent, in agreement with earlier ground-based observations /23/.


However, Wagener and Caldwell /24/ have also observed the GRS with the IUE. They find stronger NH 3 absorption bands near 200 nm in the GRS than at neighboring longitudes. They conclude that this effect is consistent with rapid upward transport of NH 3 in the GRS, rendering the uv bands more visible. The ultraviolet enhancement of NH 3 and the infrared normality of PH 3 in the GRS are very new results which have not yet been fully studied. It should be noted that the ultraviolet data refer to very much higher altitudes than the infrared, so that simple comparisons are not meaningful.

Saturn

It has been remarked above that Saturn is different from Jupiter in that there is no gas phase NH 3 on Saturn available for photochemistry. PIt 3 does not condense there, however, and it has been detected by various observers. Kaye and Strobel /25/ have modelled the phosphine photochemistry in the atmosphere of Saturn, and predict potentially observable amounts of CH3PH 2 and HCP, and with much lower probability, P2 and P4. None of these compounds have been seen on Saturn as yet, however. A wide range of hydrocarbons has been observed there (Table i).

Uranus and Neptune

These cold giant planets have only CH~ to begin more complex chemistry, as all other volatiles except H 2 and He are frozen out at very low altitudes. For reasons which are still not satisfactorily explained, the stratosphere of Neptune is warmer than that of Uranus, even though Neptune is farther from the Sun. Molecular emissions from stratopsheric CH4 and C2H 6 are definitely observed on Neptune. There is also a possibility of C2H 2 on Neptune, although the latter must be observed very near the edge of the terrestrial opacity window at 14 ~m and the signal to noise ratio is very poor. No such emissions are observed on Uranus.

To date, no specific absorptions have been seen in the ultraviolet spectra of either planet, although C2H 2 has strong characteristic bands between 170 and 190 nm. For Uranus, recent observations suggest a mixing ratio of less than 10 -9 , although earlier spectra were also consistent with, but didn't prove, a mixing ratio of 3 x 10 -8 . Neptune has not yet been adequately observed below 200 nm.

Titan

The big advance in our knowledge about Titan's atmosphere that came from the observations made by Voyager i in November 1980 is still being digested. A large number of articles have been written giving the original results and providing interpretations. We will ex- plicitly reference only those which have been published since the excellent review by Hunten et al. /26/. Table 2 summarizes the current understanding of the abundances of trace constituents in Titan's atmosphere.

The abundances of the major constituents remain poorly defined. The mean molecular weight of the atmosphere is 28, and nitrogen has been detected by the Voyager UVS. But there is sufficient uncerta~nt~ iN the determination of the mean molecular weight that several per cent of some heavier, spectroscopically undetected gas could be present /27/. Argon is an excellent candidate for this additional constituent. If present, it would be primordial argon 36Ar and 3BAr rather than radiogenic ~0Ar. It may be possible to search for the argon resonance line at 104.8 nm from Earth orbit with greater sensitivity than the Voyager UVS had. Otherwise it will be necessary to wait for a probe that enters the satellite's atmosphere to establish the proportions of the major constituents.

The abundance of CH4 is also not well known. Although its infrared spectral features are very prominent, quantitative studies are hampered by the presence of the ubiquitous aerosol that totally obscures Titan's surface at visible wavelengths. The fact that CH~ will condense in the satellite's atmosphere also contributes to the difficulty.

Taken together, these considerations suggest the following range of values for the major constituents: N 2 (80-99%); CH~ (1-16%), Ar (0-15%). The main points concerning the origin and evolution of Titan's atmosphere may be summarized as follows:

i. It is of secondary origin. If it has been captured from the proto-Saturnlan or the primordial solar nebula, the abundance of neon would be almost as great as the atomic abundance of nitrogen in accord with their abundances in the Sun. Instead, an upper limit of < i% was set from UVS observations.

2. The source of the N 2 - Titan's main atmospheric constituent - remains ambiguous. Two possibilities suggest themselves: dissociation of ~3 or release of trapped primordial N 2. Which of these was dominant depends on the composition of the gases in the proto-Saturnian nebula and the ease with which they were incorporated in the ices that formed the satellite. Initial concern about the need to maintain a high surface temperature (~ 150°K) for a long


TABLE 2 The atmosphere of Titan

Trace Constituents (parts per million)

Hydrogen (H 2) 2000 ppm

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Hydrocarbons

Acetylene (C2H2) 2 Ethylene (C2H ~) 0.4 Ethane (C2H 6) 20 Diacetylene (C~H2) 0.I - 0.01 Methylacetylene (C3H ~) 0.03 Propane (C3H 8) 20

Nitrogen Compounds

Cyanogen (C2N2) Hydrogen Cyanide (HCN) Cyanoacetylene (HC3N)

0.i - 0.01

0.2 0.i - 0.01

Oxygen Compounds

Carbon Monoxide (CO) Carbon Dioxide (CO 2)

50 - 150 0.0015

enough time to get NH 3 into the atmosphere where it could be dissociated have been amelio- rated by an awareness of the enhanced solar UV flux that was available at that time /28/. On the other hand, formation of N 2 clathrate hydrates provides an easy way of trapping and then releasing nitrogen into the atmosphere. Detection of Ar would demonstrate that at least the argon clathrate had been formed.

3. A similar ambiguity exists regarding the source(s) of CO and CO 2. The CO could be formed either from ice coming from outside the satellite or it too could have been trapped as a clathrate from the nebula and subsequently released. In either case, the formation of CO 2 requires the presence of OH, which must arise from incoming ice.

4. Subsequent chemistry in the atmosphere is driven by solar UV and precipitating electrons from Saturn's magnetosphere. Electrons are the dominant source of energy for these reactions, by a ratio of i0:i. An extensive review of possible reactions has been published by Yung et al. /29/. These authors are able to account for all the compounds that have been discovered to date.

5. Both the chemical calculations and laboratory studies designed to simulate conditions on Titan have demonstrated that C2H 6 is the major gas phase product. Lunine et al. /30/ and Flasar pointed out that under the temperature and pressure conditions on Titan, C2H 6 will condense, raising the prospect of seas or even a global ocean of this substance. Alter- natively, the additional chemistry that is producing the aerosol may take the reactions further, so that a solid layer of organic material roughly half a kilometer deep may have accumulated on Titan.

In conclusion, we emphasize that the special relevance of Titan to the problem of chemical evolution on the primitive Earth lies in the fact that it is small enough for hydrogen to escape yet cold enough to keep water vapor out of the atmosphere. Hence it has remained a reducing environment over the lifetime of the solar system. The products of the chemistry occurring in this environment can accumulate on Titan's surface, where they are available for future analysis. The low temperature and subsequent absence of liquid H20 make Titan drastically different from the early Earth, but reactions in our planet's atmosphere must also have occurred in the absence of an abundant source of oxygen. Hence some of the initial steps in prebiotic chemistry on Earth are occurring on Titan today. The challenge is to see how far these steps proceed and what the end products are - whether there are any preferred pathways to complexity in a totally natural setting.


ACKNOWLEDGEMENTS

John Caldwell acknowledges the support of NASA grants NAG5276 and NAGWI60 and Tobias Owen acknowledges the support of NASA grant NGR33015141 and contract 953614 during the course of the research reported here and during the writing of this review.

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chemical evolution on the giant planets and titan

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