Nitrogen Genesis on Titan

NITROGEN GENESIS ON TITAN

Patricia Mills Department of Earth Sciences, University of Northern Colorado, Greeley, CO 80634

ABSTRACT

The genesis of Titan’s predominately nitrogen (N2) atmosphere has long been debated. Two primary models address the origin of N2 in Titan’s atmosphere suggesting that either the gas was accreted onto Titan as N2 hydrates and then out-gassed or that the N2 is a result of photolytically converted ammonia hydrates. A third model proposes biogenically produced N2 from an ammonia ocean or sea. Each model has problems that inhibit any conclusion as to the

origin of Titan’s N2. Perhaps in 2004, when the Cassini-Huygens mission investigates Titan’s atmosphere, a definite answer can be found.

“It is not so important that you get the right answer; it is most important that you ask the right question.”

David J. Stevenson

INTRODUCTION

Titan, Saturn’s largest satellite, is a distinctive moon. Titan is the only satellite with a substantial atmosphere, the main constituent of which is molecular nitrogen (N2).

Earth is the only other body in the solar system whose atmosphere is primarily N2.

Further, a complex set of hydrocarbons and nitriles exists in Titan’s atmosphere, a feature also unique to Titan.

The presence of nitrogen (Broadfoot, et al., 1981), methane and hydrocarbons

(Maguire, et al., 1981), hydrogen (Samuelson et al., 1981) and nitriles (Kunde et al.,

1981) were confirmed by the Voyager I spacecraft in 1980. Since then, a great deal scientific study has been conducted in an attempt to unravel the origin and evolution of

Titan’s atmosphere. The chemical composition and processes in Titan’s atmosphere, according to Khare and Sagan, (1986) are thought to be similar to Earth’s early atmosphere, raising the possibility of the formation of amino-acids, the so-called

“building blocks for the origin of life” (Khare and Sagan, 1986). The genesis of Titan’s

1 N2 atmosphere remains an open question and has been the subject of intense debate over the last two decades. If the origin of Titan’s N2 can be determined, a great deal of insight into the composition and evolution of the solar system may be gained.

Owen (1981) notes that Titan’s atmosphere is primarily the result of devolatilization of ice and rock that accreted during the moon’s formation. Two molecules are capable of producing an N2 atmosphere: molecular nitrogen and ammonia.

As a result, two well-publicized prospects for the origin of N2 in Titan’s atmosphere have been proposed (Wayne, 1985). In addition, a new model will also be addressed.

The first theory for the presence of nitrogen on Titan, involves N2 trapped in ices and subsequently melted by internal heating and released into the atmosphere: the second theory involves ammonia (NH3) as the primary molecule with N2 resulting from photolysis of NH3. The third theory suggests that the N2 is biogenically created. Each scenario requires a different set of conditions for atmospheric N2 to predominate in

Titan’s atmosphere. This paper investigates the genesis of N2 in Titan’s atmosphere by reviewing what is currently known about the satellite and then reviewing each of the models and the conditions needed for the evolution of Titan’s atmosphere.

PHYSICAL PROPERTIES OF NITROGEN

Nitrogen is the fifth-most common element in the solar system (Lewis, 1997). It is a colorless, odorless and tasteless gas with a melting point of 63° K and a boiling point of 78° K. Molecular nitrogen is almost inert largely because of the strength of the N-N bond. However, once the bond is broken, either by photochemistry or by lightning oxidation (nitrogen fixation), the resultant N ions and N atoms become the catalysts for

2 nitrile development. Nitriles such as hydrogen cyanide are precursor compounds to the formation of life. (Wäser, Trueblood, 1976).

ATMOSPHERE REVEALED: THE VOYAGER MISSIONS

In 1980, the Voyager I spacecraft flew near Titan. The first data relayed to Earth included images of the satellite, which unfortunately revealed little. Titan was completely blanketed with a high altitude haze layer approximately 100 km above a dense aerosol layer that completely obscured any surface features in the visible wavelengths. “The moon resembled a fuzzy tennis ball” (Owen, 1982). See Figures 1 and 2.

The most interesting and revealing data came from the other instruments aboard

Voyager I: the Infrared Imaging Spectrometer (IRIS), the Ultraviolet Spectrometer

(UVS) and Radio Science. The results of the expedition confirmed N2 as the major component of Titan’s atmosphere. The UVS detected N2, N, and N ions at 3600 km above the surface (Broadfoot, 1981). The IRIS detected Hydrogen Cyanide (HCN)

Cyanoacetylene (HC3N), Cyanogen (C2N2) and other hydrocarbon molecules in the haze layer (See Table 1), (Kunde et al, 1981). The radio science occultations revealed a temperature and pressure scale commensurate with a predominately N2 atmosphere

(Lindal, G.F. et al, 1982).

The observed N2 emission in Titan’s upper atmosphere is significant beyond the mere identification of an N2 atmosphere. The dissociation of N2 by electrical bombardment during Titan’s convergence with Saturn’s magnetosphere may provide the mechanism by which Titan’s nitriles are formed. According Broadfoot, et al., (1981),

3 about 40 % of the N atoms produced in the exosphere escape; the remaining atoms react with hydrocarbons to form HCN in the inversion layer, in amounts identifiable to IRIS.

Examination of the IRIS data also revealed an equatorial surface temperature range from 94° K to 97° K (Samuelson, et al., 1981). These IRIS data are consistent with the surface temperature occultation measurements, indicating a surface temperature of

94° K which is consistent with the pure N2 presumed to exist on Titan (Lindal, et al,

1982).

Titan’s atmospheric mean molecular weight was measured to be 28.6 amu, which is near the mass of N2 (28 amu). These data implied that most of the satellite’s atmosphere is made up of N2, but that heavier atom could also be present, with argon

(39.9 amu) being the most likely candidate. Thus, Titan’s atmosphere is presumed to consist of N2 along with methane (CH4) (16 amu) and argon (Morrison, Gregory, 1985).

Voyager I did not detect argon and neon directly in the atmosphere. According to

Owen, (1982) the fact that argon was not detected by UVS in the upper atmosphere means that the concentration of argon is less than 6%. So, the presence of argon is not ruled out by the Voyager data. Argon levels are of interest because if N2 was trapped in water-ice during the accretion (growth by external addition) of Titan, then Ar would be expected to be trapped similarly. The melting point of argon is 83.3° K. Neon however, would not be trapped in the same manner because the gas condenses at too low a temperature (< 20° K). Thus, the neon found on Titan would be negligible. (Gross,

1974).

The lack of neon in Titan’s atmosphere is not surprising. Its presence in significant amounts would indicate a captured primordial atmosphere, which according to

4 Atreya and Pollack (1989), most likely did not occur. Large amounts of neon would be expected in a captured atmosphere because the solar-nebula concentrations of neon and nitrogen are nearly identical. As a result, the concentrations of the two gases would more likely be 67 % neon and 33% molecular nitrogen. Further, since the molecular weight of neon is heavier than Titan’s escape velocity, most of the neon would remain (Owen,

1982).

The presence of H2 was also confirmed by the UVS on Voyager I. The molecular hydrogen produced strong emissions in the Lyman α region of the spectrometer. The

Voyager results indicate H2 is 0.002% of Titan’s lower atmosphere (Strobel and

Shemansky, 1982).

The cumulative results of the Voyager measurements impute N2 as the dominant component in Titan’s atmosphere with smaller amounts of CH4, possibly argon and H2 as a minor constituent. The bulk of what is known about Titan comes from Voyager I data. The physical properties are detailed in Table I and the atmospheric composition is detailed in Table II. Figure 3 illustrates the vertical structure of Titan’s atmosphere.

The radio occultation experiments established the atmospheric temperature (94

0.7 K) and pressure at (~ 1.5 bars), the radius at (2575  0.5 km), and a mean density of

1.881  0.002 g cm-3 (Lindal, 1982).

Lindal (1982) interpreted the radio occultation measurements using abundant, a small amount of CH4 and hydrocarbons in their calculations. They determined the atmosphere’s structure as well. From the surface, the temperature decreased with height yielding a minimum temperature at the tropopause (42 km) of 74  0.5 º K and a pressure of 130 mb. From there, the temperature increased with altitude reaching 170  15 º K at

5 the 200 km level where the temperature remained stable into the higher levels of the atmosphere (Hunten, 1984).

Although much is currently known about Titan many questions remain unanswered. The lack of knowledge about Titan’s surface and interior precludes understanding the formation and evolution of the moon. The confirmation that there is, in fact, N2 in Titan’s atmosphere is just the beginning of an understanding of the complexities of this moon’s history, surface, geologic properties and atmosphere.

THE ORIGIN OF NITROGEN ON TITAN

The ultimate source of N2 on Titan is a significant issue because it may provide clues about the creation and evolution of Titan as a whole. In addition, much could be revealed about the formation of the outer solar system including temperatures, pressures, gas abundances and the nature of the proto-Saturn nebula. Furthermore, possibilities for the existence of life on Titan may come from an understanding of the origin of N2 on

Titan.

Three models will be explored for the genesis of N2 on Titan. The first model postulates “nitrogen form ammonia” (Lewis, 1971, Atreya, 1978, Hunten, 1984); the second model suggests “nitrogen from nitrogen hydrates” (Gross, 1974, Strobel, 1981,

Owen, 1982); the third model involves “biogenic nitrogen” (Fortes, 2000).

Each model has problems that inhibit the formation of any conclusion as to the origin of N2 on Titan. The first problem concerns all three models. Was the dominant species in the circumplanetary disk from which Titan was formed N2 or NH3? Currently there is no Earth-based means to determine the disk’s composition. The second problem

6 relates specifically to the “nitrogen from ammonia” theory: in order for ammonia to result in N2, substantial atmospheric heating, well beyond Titan’s current temperature, must have existed for a significant period of the moon’s history. The third problem concerns

“nitrogen from nitrogen hydrates”: there should be a higher abundance of CO relative to the amount of nitrogen found in the atmosphere. The fourth problem relates to “biogenic nitrogen.” This model not only requires an H2O-NH3 sub-surface ocean, but it demands the presence of some sort of anaerobic life during Titan’s past, neither of which can currently be proved.

NITROGEN FROM AMMONIA

The most commonly accepted theory of the origin of N2 on Titan is that NH3 hydrates and methane clathrates were accreted onto Titan (Stevenson, 1986, Hunten,

1984). Lunine and Stevenson (1984) estimated that up to 15% of the total ice amassed on

Titan is ammonia hydrates and the remaining ice is methane clathrates. A hydrate is a molecule such as NH3 bonded to H20 ice, whereas clathrates are lattice-work ice structures usually made form H2O which creates “cages” that can be filled with gas molecules. Once entrapped, the gasses are free to move within the ice-cage (Lunine and

Stevenson, 1984).

Whether ammonia was the dominant nitrogen-bearing molecule in Saturn’s circumplanetary disk is a crucial question in determining the origin of N2 on Titan. Prinn and Fegley (1981) proposed one model that supports ammonia formation on Titan. In their model, Titan was most likely formed within a circumplanetary disk surrounding

Saturn, within the solar system nebula. According to Prinn and Fegley, the disk gas

7 densities were higher and temperatures were colder in the outer disk, thus favoring the production of water-ices, ammonia and methane; near proto-Saturn, N2 and CO were more prevalent because the temperature was higher and the density lower (Prinn, Fegley,

1981, Lunine, 1993). In the circumplanetary disk, Lunine (1993) estimates that the nebular gas density could have been 5 to 8 orders of magnitude higher than in the solar nebula. As a result, higher rates of molecular collisions occurred allowing the gases to stabilize and form ices (Lewis, 1997). In the Saturn satellite disk, water-ices were thought to be formed at 160º K, and ammonia hydrates formed between 100º K and 160º

K. At around 100º K, methane clathrates formed; N2 hydrates formed at about 60º K. At

40º K, methane condensed directly. Lewis estimates that Titan formed between 45º K and 100 º K, which allows for any of the molecules to accrete as hydrates or clathrates.

However, because the pressures in the circumplanetary disk were thought to be high, the reduced molecules such as methane and ammonia would more likely form (Lewis, 1997 – see Figure 4).

Thus, according to Prinn, Fegley, Lewis and Lunine, Titan’s ice composition should be primarily water, ammonia and methane. Stevenson (1986) notes, however, that currently there are no accretion models that conform to Titan’s present-day conditions and further observational data are necessary to confirm the origin of Titan’s N2 atmosphere.

If ammonia molecules were trapped in ices on Titan, an N2 atmosphere would result from photolysis of the NH3. For the ammonia model to produce the present day N2 atmosphere, Titan would have to be at least 50º K warmer than its current temperature of

94º K for 4% of its history (Strobel, 1981).

8 Photolysis of NH3 undergoes several chemical reactions prior to becoming N2.

(See Table III.) Atreya et al. (1978) produced a model for the production of N2 from

NH3. The primary problem with the ammonia model is that when N2H4 (reaction 5 in

Table III) is produced at Titan’s current surface temperature, the molecule condenses, halting the production of N2. According to Atreya and Strobel (1981), reaction 5 requires temperatures in excess of 150º K in order to prevent N2H4 from condensing. This raises serious questions about possible mechanisms that could have warmed Titan’s atmosphere sufficiently to produce the current supply of N2. Possible energy sources for warming

Titan include accretion heating, shock impacts and lightning.

Lunine (1989) contends that Titan was hot (up to 600º K) during its formative years. He asserts that cooling form accretion heating could have taken 108 to 109 years, allowing sufficient time for the photolytic reactions to occur, producing Titan’s current atmosphere. Calculations made by Lewis (1971) indicate that radiogenic heating of

Titan’s interior following accretion would have been sufficient to raise the atmospheric temperature to 173º K, warm enough to complete the photolytic reactions. The hot interior would have melted the ammonia ices into a magma. Stevenson (1986) goes on to suggest that out-gassing would have released the ammonia and methane into Titan’s atmosphere through cracks in the surface or possibly from cryovolcanism.

Shock impacts from comets may have heated Titan sufficiently to produce the current N2 atmosphere according to Jones and Lewis (1987). Zahnle et al. (1991) suggest that stray Uranus-Neptune plantesimals or perturbed Kuiper belt objects may have contributed significantly to Titan’s atmosphere. Because of Titan’s distance from Saturn, the velocity at which the plantesimals traveled, coupled with gas drag, slowed the objects

9 sufficiently to prevent atmospheric erosion and, in fact, probably added material to the atmosphere. This could explain Titan’s deuterium/hydrogen (D/H) ratio (Zahnle, 1991,

Coustenis, 1991).

Titan has an unexplained, abnormally high D/H ration: about 4.2 x 10-4 as

-5 compared to the planets: 2-3.4 x 10 . Mono-dueterated methane, CH3D, was identified in the Voyager infrared spectra by Kim and Caldwell in 1982 (Coustenis, 1991). Although several models exist for the origin of deuterium on Titan, delivery via comet impacts is one plausible explanation. Wyckoff (1991) reports the existence of the isotope on Comet

Halley; thus it is conceivable that if most comets share a common origin, the isotope could have been present on the icy bodies that impacted Titan (Zahnle, 1991).

Jones and Lewis (1991) estimate from impact shocks during a period of heavy bombardment early in Titan’s history, that at least twice the amount of N2 currently in

Titan’s atmosphere could have been converted from ammonia by shock heating. Jones and Lewis modeled shock-effects in 59 compounds of H, C, N, O and S. Shock temperatures of 1200º K and 2500º K were used because Borucki (1984) estimated the quench temperatures of the tested compounds to be between 2200º K and 3000º K. Gas mixtures representing Titan’s atmosphere and primordial gas composition of CH4, H2O and NH3 were examined. The conversion of NH3 to N2 was found to be very efficient.

The authors postulate that heating due to impacts is a plausible solution for the production of N2 from NH3 on Titan.

Another method of heating Titan’s atmosphere includes lightning strikes.

Lightning discharges can produce shock heating of the gases through which they travel.

Voyager I had the ability to detect lightning on Titan but none was found (Desch, Kaiser,

10 1990). Desch and Kaiser estimate lightning occurrence on Titan to be less than on Earth, with the total discharge on Titan estimated to be 103 times smaller than those on the terrestrial planet. The work of Borucki et al. (1984) indicates that because of the convective energy potential in Titan’s atmosphere, about 4 x 10 3 g cm-3 of compounds could be synthesized by lightning throughout its existence; this amount is close to the current amount of N2 observed in Titan’s atmosphere.

Thus the problem of heating Titan sufficiently to complete the photolysis of ammonia to molecular nitrogen could theoretically be solved by any of the above mechanisms. According to Lewis and Jones (1991), either from of shock heating

(lightening and impact) is capable of warming the surrounding gas to extreme temperatures: in excess of 10,000º K for lightning, and 5000º K for impact heating.

However, no evidence exists to prove that either of these mechanisms actually occurred on Titan.

NITROGEN FROM NITROGEN HYDRATES

The N2 in Titan’s atmosphere could have been produced by N2 hydrates (N2 + 7

H2O) (Strobel, 1982). Strobel proposed a cold accretion model in which Titan was formed at-or-below 60º K. At such low temperatures, N2 could have sublimated into N2 hydrates. As Titan warmed to its present temperature, N2 would have been out-gassed into its atmosphere from internal heating.

However, if N2 hydrates accreted directly onto Titan, then (based on predicted solar nebula contents), carbon monoxide should be more prevalent in the satellite’s atmosphere (Atreya, 1978, Lunine and Stevenson, 1984). Lunine (1989) notes that CO

11 should have been trapped in ice much more readily than N2, but less than methane.

However, if CO were the dominant carbon species, as in this model, there would be little methane available to be trapped. Lunine estimates that the CO-to-CH4 ratio would be about 500, and the CO-to-N2 ratio to be about 10. Owen (1982), who also supported the possibility of accreted N2 hydrates, noted that the lack of CO needed to be justified.

Lunine and Stevenson stated that if the N2 hydrates were the source of N2 on Titan, over the lifetime of the satellite, 10 bars of CO would have had to have been destroyed, for which there is no current explanation. Strobel (1982) suggests that the lack of CO resulted from a slow radial mixing of gases. Owen (1982) supports Strobel’s hypothesis but the support is based on the probable existence of argon in Titan’s atmosphere.

According to Owen, the [N/Ar] ratio in the solar nebula should be approximately

22, yielding an [N2/Ar] ratio of 11. The [N2/Ar] ratio proposed for Titan is only 7, based on calculations made from Voyager data (Owen, 1982), which, if correct, suggests that nitrogen is “missing” from Titan. Owen (1982) accounts for the missing N2 through nitrogen escape from the exosphere (outermost layer of the atmosphere). He estimates that 20% of Titan’s N2 has escaped over its history and that roughly 50% more has been lost by conversion to nitriles. The true [N2/Ar] ratio will be measured by the Cassini-

Huygens mission in 2004.

BIOGENIC NITROGEN

Based on the work of Lunine and Stevenson (1987) Fortes (1999) proposes a scenario in which all methane and molecular nitrogen found on Titan are produced biogenically. This model presumes a > 50 km deep sub-surface ocean consisting of 15%

12 ammonia and 85% water, at a pressure of 3-5 bars and temperature of 235º K. Fortes further presumes life to exist in this aquifer.

Fortes cites archaebacteria, known as a methanogen, to be one possible organism to inhabit Titan’s aquifer. He states that methanogens reduce CO and CO2 under anaerobic conditions to produce methane. The methanogens could thus possibly deliver a steady supply of CH4 to Titan’s atmosphere through cracks in its icy surface. Fortes estimates that 4 x 1011 mol-yr-1 of methane could be supplied. In addition, de-nitrifying bacteria could oxidize ammonia under anaerobic conditions by consuming nitrate ions thereby producing N2. The N2 could then be out-gassed through cracks in the surface the ocean, creating Titan’s current atmosphere.

While the idea of a biogenic origin for Titan’s atmosphere is tantalizing, there is no evidence of life on Titan. Fortes claims that there is very little chance that the Cassini-

Huygens project will be able to identify any chemical markers indicating life on Titan, so his model will be very hard to prove.

OBSERVATIONAL TESTS

Two tests exist which could help determine the origin of N2 on Titan: the bulk density of the satellite and its atmospheric nitrogen-to-argon ratio.

An indicator that Titan may have warmed sufficiently to allow N2 to be produced from ammonia is the bulk density of Titan that predicts a rock/ice ratio of 55/45.

According to Stevenson (1986), most satellites in the outer solar system have a rock/ice ratio of 40/60, but the vast majority of planetary moons are much smaller than Titan.

Although it is possible that Titan may have been formed with this anomalous high

13 rock/ice ratio, Stevenson (1986) postulates that the ratio is a result of high temperatures on this large satellite resulting from accretion heating. The high temperatures evaporated enough H2O from Titan to change the ratio to its current state. Indeed, Titan, Ganymede and Callisto are all of similar size and are the three largest planetary moons: all three have rock/ice ratios of about 60/40 rather than the 40/60 ratio found in other smaller icy satellites of the solar system. The three giant moons are all thought to have experienced accretion heating and evaporation (Stevenson, 1986).

The second test involves determining the [N2/Ar] ratio in Titan’s atmosphere, which will be measured by the Cassini-Huygens probe in 2004. If the ratio is found to be

>1%, then the N2 was likely accreted onto Titan. If the ratio is < 1%, then ammonia was probably the accreted gas. Argon is similar to nitrogen in its ability to form hydrates or to be enclathrated (Lunine and Stevenson, 1985). Assuming their even distribution in the ices throughout Titan’s core, N2 and Ar should have out-gassed at similar rates (Owen,

1982, Strobel, 1982, Lunine, 1989). Despite the fact that Ar was not measured in Titan’s atmosphere by Voyager instruments, the satellite’s atmospheric molecular weight does suggest the presence of argon.

WHAT CASSINI WILL UNFOLD IN 2004

The Cassini-Huygens mission, a joint project involving NASA and ESA (European

Space Agency), will place the Cassini orbiter about Saturn and send the Huygens probe into Titan’s atmosphere in 2004. The project goals for Titan are (JPL, NASA, 2000):

 Determine the temperature structure within the atmosphere

14  Determine the horizontal and vertical distribution of minor atmospheric components (especially organic molecules)

 Determine the horizontal and vertical distribution of the aerosols and clouds

 Determine noble gas abundance and isotopic ratios (especially deuterium to hydrogen)

 Construct a three-dimensional model of the atmospheric circulation

 Characterize the chemical composition and physical phase configuration of the surface on all spatial scales accessible to probe and orbiter instruments

 Determine the vertical atmospheric structure, gas-phase composition as function of altitude, cloud composition and structure verses altitude, and wind velocity

 Quantify the coupled surface-atmosphere interactions, chemical and physical, on seasonal and geologic time scales

 Understand the long-term evolution of Titan, in particular to quantify the processes by which Titan’s current volatile inventory was acquired, and provide some constraints on the initial inventory

 Determine the origin, nature, distribution, and possible evolution of organic compounds in the three parts of Titan’s “geofluid”, namely its gas phase (atmosphere), its liquid phase (ocean), and its solid phase (sedimentary deposits and atmospheric aerosols)

 Determine the nature and role of chemical couplings between the three parts.

In addition to the Huygens goals as stated above, the Cassini orbiter will map Titan’s surface and lower atmosphere with radar, near-infrared, and visible wavelength instruments. Titan’s uppermost atmosphere will also be explored along with the moon’s interaction with Saturn’s magnetosphere.

15 CONCLUSION

The “ammonia to N2” model seems the most plausible given what is known about

Titan. Based on the chemistry of the reductive and oxidized molecules, the circumplanetary nebula conditions could have been conducive to the conversion of NH3 to N2 and CO to CH4. The temperatures and pressures determined necessary for the production of reduced gases appears convincing. The heating mechanisms needed to photochemically change ammonia into molecular nitrogen could have existed based on crater evidence from other Saturn satellites indicating heavy comet or meteor bombardment in Saturn’s vicinity. The bulk density supports some form of heating of the moon.

Each model, regardless of which is correct, has generated important questions about Titan and its atmosphere resulting in further research and exploration. Future work includes finding explanations for the low CO levels and the high D/H ratios on Titan.

Both of these issues could help determine the origin of Titan’s atmosphere. The Cassini-

Huygens mission will likely unravel some of Titan’s mysteries as the probe uncovers the moon from its thick, blanketed atmosphere. Until then, the genesis of Titan’s atmosphere will remain uncertain.

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