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The role of Fischer–Tropsch catalysis in the origin of methane-rich Titan
Icarus 178 (2005) 154–164 www.elsevier.com/locate/icarus
The role of Fischer–Tropsch catalysis in the origin of methane-rich Titan Yasuhito Sekine a,∗ , Seiji Sugita b , Takafumi Shido c , Takashi Yamamoto c , Yasuhiro Iwasawa c , Toshihiko Kadono d , Takafumi Matsui b a Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan b Department of Complexity Science and Engineering, Graduate School of Frontier Science, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa,
Chiba 227-8562, Japan c Department of Chemistry, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan d Institute for Research on Earth Evolution, Japan Agency for Marine–Earth Science and Technology, 2-15 Natsushima, Yokosuka,
Kanagawa 237-0061, Japan Received 12 August 2004; revised 4 March 2005 Available online 17 May 2005
Abstract Fischer–Tropsch catalysis, which converts CO and H2 into CH4 on the surface of iron catalyst, has been proposed to produce the CH4 on Titan during its formation process in a circum-planetary subnebula. However, Fischer–Tropsch reaction rate under the conditions of subnebula have not been measured quantitatively yet. In this study, we conduct laboratory experiments to determine CH4 formation rate and also conduct theoretical calculation of clathrate formation to clarify the significance of Fischer–Tropsch catalysis in a subnebula. Our experimental result indicates that the range of conditions where Fischer–Tropsch catalysis proceeds efficiently is narrow (T ∼ 500–600 K) in a subnebula because the catalysts are poisoned at temperatures above 600 K under the condition of subnebula (i.e., H2 /CO = 1000). This suggests that an entire subnebula may not become rich in CH4 but rather that only limited region of a subnebula may enriched in CH4 (i.e., CH4 -rich band formation). Our experimental result also suggests that both CO and CO2 are converted into CH4 within time significantly shorter than the lifetime of the solar nebula at the optimal temperatures around 550 K. The calculation result of clathration shows that CO2 -rich satellitesimals are formed in the catalytically inactive outer region of subnebula. In the catalytically active inner region, CH4 rich satellitesimals are formed. The resulting CH4 -rich satellitesimals formed in this region play an important role in the origin of CH4 on Titan. When our experimental data are applied to a high-pressure model for subnebula evolution, it would predict that there should be CO2 underneath the Iapetus subsurface and no thick CO2 ice layer on Titan’s icy crust. Such surface and subsurface composition, which may be observed by Cassini–Huygens mission, would provide crucial information on the origin of icy satellites. 2005 Elsevier Inc. All rights reserved. Keywords: Fischer–Tropsch catalysis; Titan; Cassini–Huygens mission; Circum-planetary subnebula; Methane
1. Introduction Titan has a dense atmosphere dominated by N2 and CH4 . However, CO are thought to be the main reservoir of gasphase carbon in the interstellar medium and solar nebula (e.g., Pollack et al., 1994). Thus, methane-producing reactions from CO in the circum-planetary subnebula are important in understanding the origin of the CH4 -rich atmosphere * Corresponding author. Fax: +81 3 5841 8318.
E-mail address: [email protected] (Y. Sekine). 0019-1035/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2005.03.016
on Titan. Prinn and Fegley (1981, 1989) show the importance of conversion of CO and N2 into CH4 and NH3 in a circum-planetary subnebula using a thermochemical equilibrium model. In their studies, a CH4 -rich subnebula is formed through both gas-phase reactions and catalytic reactions (e.g., Fischer–Tropsch catalysis). Then, Owen (1982) and Lunine and Stevenson (1985) argue that CH4 formed in the subnebula was trapped in clathrate hydrates and incorporated into Titan. Furthermore, Atreya et al. (1978) and McKay et al. (1988) show that N2 in the present atmosphere of Titan may be a product of NH3 photolysis and shock heat-
Fischer–Tropsch catalysis for the origin of CH4 on Titan
ing by meteoritic impacts. This scenario is consistent with the present atmospheric composition of Titan. However, there is large uncertainty in the conversion rate of CO into CH4 in a subnebula. In gas-phase reactions, CO are energetically inhibited by the high activation barrier to break the CO bond. This prohibits gas-phase reactions from proceeding at low temperatures (<2000 K) and pressures (
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lar, the subnebular gas pressure is a very important factor. If the gas pressure in a subnebula is relatively high (i.e., more than 10−3 bar at 500 K) (Prinn and Fegley, 1989; Coradini et al., 1989; Mosqueira and Estrada, 2003), the CH4 becomes thermochemically stable. Then, the CH4 produced in a subnebula will contribute to the CH4 on Titan. In this study, we use such a thick subnebula model, which hereafter we call the high-pressure model for subnebula evolution, as a reference model. However, if the gas pressure is as low as the solar nebula (i.e., less than 10−5 bar at 500 K) (Mousis et al., 2002), the CH4 production does not proceed efficiently in a subnebula. Then, the CH4 derived from molecular cloud becomes the dominant source for the CH4 on Titan. The detail of stability of CH4 in such low-pressure subnebula models will be discussed below in Section 4.3. Although these different evolution models may predict similarly CH4 -rich atmosphere on Titan, other chemical characteristics (e.g., crustal composition) of resulting icy satellites might be very different depending on the subnebula model. In fact, Cassini–Huygens observation may reveal such key chemical/geologic evidence. However, the relation between possible chemical/geological evidence and subnebula evolution is not understood well yet. Then, in this study, we also attempt to clarify the relation between subnebula evolution models and possible chemical/geological observations based on the experimental results. In the following, we describe the experimental system (Section 2), show the experimental results on Fischer– Tropsch catalysis (Section 3), calculate the clathrate hydrate formation from the subnebular gases using our experimental data (Section 4), and provide observational propositions for Cassini–Huygens mission (Section 5).
2. Experimental We use iron powder as catalyst because iron is much more abundant in the solar composition than other catalysts for Fischer–Tropsch catalysis, such as Ru, Co, and Ni (Anders and Grevesse, 1989). The grain size and purity of catalyst used in the experiments is ∼250 µm (60 mesh) and 99.998%, respectively. In the following, we treat the CH4 formation rate as a specific reaction rate (product molecules per reaction site (i.e., surface atom) per second). In order to measure the number of surface sites, we conduct gas adsorption (BET) experiments using Kr gas as adsorbate. The BET experiments indicate the surface area and the number of surface site per unit weight of the iron powder used in our experiments are 1.5(±0.1) × 10−1 m2 /g and 1.5(±0.1) × 1018 site/g, respectively. A schematic diagram of the experimental system is shown in Fig. 1. We use a gas chromatograph (GC) (GC-8A, Shimadzu Corp.) for the analysis. We use a porous polymer reagent, Porapak Q (GL Science), which is used for separating light hydrocarbons from its gas mixtures. The electric current is fixed at 100 mA. The temperatures at the
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Fig. 2. Methane production normalized by the number of surface sites as a function of reaction time. The experimental condition for this run is: H2 /CO = 1000, P = 0.53 bar, and T = 550 K. Fig. 1. Schematic diagram of experimental system to measure Fischer–Tropsch catalysis rate.
GC column and the injection port are fixed at 55 and 70 ◦ C, respectively. A diaphragm pump is used to circulate the gas in the closed-loop system. Prior to each measurement of reaction rate, the catalyst surface is cleaned with H2 gas at T = 700 K and P ∼ 0.3 bar for 1.5 h. This pre-treatment ensures that oxide iron is reduced to metallic iron before the main experiment. Then, we exhaust the reducing gases for 1.5 h using a diffusion pump. Finally, we introduce reactant gas mixture to the reaction cell. After a prescribed length of time, a part of the gas in the reaction cell is extracted and analyzed with the GC. We analyzed the gas mixtures after 4–9 different reaction times for each experimental condition.
3. Results 3.1. Methane formation rate Fig. 2 shows the CH4 production per surface site as a function of reaction time at P = 0.53 bar and T = 550 K. The experimental result indicates that the CH4 production increases linearly with time under a given reaction condition. The CH4 formation rate (molecules/site/s) under this reaction condition is obtained by a least-square fit of the data points. Fig. 3 shows the CH4 formation rate as a function of temperature (i.e., Arrhenius plot). As stated in Section 1, Vannice (1975) obtains an empirical law of CH4 formation rate via Fischer–Tropsch catalysis as a function of temperature and partial pressures of CO and H2 based on the experimental data at H2 /CO = 0.6–15.1. In the previous study by Prinn and Fegley (1989), they extrapolate this empirical law
Fig. 3. Methane formation rate normalized by the number of surface sites as a function of temperature. The solid circles, squares, diamonds, and triangles indicate our experimental results of H2 /CO = 1000 and P = 0.53, 0.30, 0.16, and 0.09 bar, respectively. The broken lines show an power law based on extrapolation of data by Vannice (1975) at H2 /CO = 1000, and P = 0.53, 0.30, 0.16, and 0.09 bar, respectively. The solid lines are the fitting lines (an exponential function) of our experimental data points (T = 500, 525, and 550 K) determined by the least square method.
to the nebular conditions (e.g., H2 /CO = 1200) and estimate CH4 production in the nebular conditions. Catalytical efficiency at H2 /CO = 1000 using the empirical law obtained by Vannice (1975) is also shown in Fig. 3. Then, our experimental data and extrapolated catalytical efficiency based on the empirical law under the same conditions of temperature and partial pressures of CO and H2 are compared. At temperatures lower than 550 K, the CH4 formation rate increases exponentially as temperature rises. This temperature range
Fischer–Tropsch catalysis for the origin of CH4 on Titan
is close to the temperature range (513–553 K) where the industrial laboratory data by Vannice (1975) are obtained. Comparison between our experimental data and the extrapolated results of the empirical law based on Vannice (1975) indicates that the gradients of the two results are very similar. In other words, the activation energies obtained in our experiments (75–109 kJ/mol) agree well with industrial laboratory data (∼88–96 kJ/mol). However, the absolute CH4 formation rates of our experimental results are 10–30 times the extrapolated results of the empirical power law based on data by Vannice (1975). This may be caused by the difference in H2 /CO ratio of the reactant gases. Our experiments are under H2 -rich conditions to simulate a subnebula condition (H2 /CO = 1000), while H2 /CO = 0.6–15.1 were used by Vannice (1975). This difference suggests that the CH4 formation rates estimated by the empirical power law based on Vannice (1975) would significantly underestimate the production in this reaction range in a circum-planetary subnebula. 3.2. Poisoning At temperatures higher than 550 K in Fig. 3, CH4 formation rates deviate from an exponential trends and decrease as temperature rises. This is most likely caused by “poisoning” of catalyst. The poisoning is a gradual loss of catalytic activity through conversion of surface carbide to unreactive graphitic carbon (e.g., Krebs et al., 1979). When poisoning occurs on the surface of catalyst, the slope of the CH4 production decreases gradually with time. The effect of poisoning has been investigated extensively in catalytic chemistry (e.g., Krebs et al., 1979). According to Krebs et al. (1979), poisoning of catalyst is more difficult to occur when reactant gas has a higher H2 /CO ratio (i.e., H2 -rich condition). In planetary science, theoretical calculations suggest that poisoning of catalyst may occur in the solar nebula (Kress and Tielens, 2001). However, it has not been clear if poisoning of iron catalyst may occur at very high H2 /CO conditions, such as the nebular gas. Fig. 4 demonstrates that catalytic poisoning occurs under the conditions of subnebula (i.e., H2 /CO = 1000); the CH4 production decreases as a function of reaction time at T = 600 K and 650 K. Comparison between the result at 600 and that at 650 K indicates that the effect of poisoning increases with temperature. The poisoning is not observed at temperatures below 550 K. This may be because the conversion rate of carbide into graphitic carbon is so slow at lower temperatures that the carbide is removed as CH4 by hydrogenation before the formation of graphitic carbon. After the gradual decrease, CH4 formation rate approaches a steady state around 1 × 104 –2 × 104 s. Since a typical time scale of subnebula evolution is much longer than that of poisoning processes, the steady-state rate after poisoning is more relevant as the actual rate of conversion from CO into CH4 in a subnebula. Because the CH4 formation rate in Fig. 3 is that
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Fig. 4. Methane production per surface site as a function of reaction time. The reaction temperatures are 600 (solid circles) and 650 K (open triangles).
in a steady state (i.e., poisoned rate), it decreases as temperature rises above ∼550 K. 3.3. Conversion of CO2 into CH4 Recently, both infrared observations of the interstellar ices (e.g., Gibb et al., 2000) and a theoretical study on cosmic-ray ionization reaction in the solar nebula (e.g., Aikawa et al., 1999) indicate that a large amount of CO2 ice is present in the outer solar nebula. Such CO2 ice will evaporate and exist in the gas phase in a warm subnebula in addition to CO. In the industrial chemistry, it is known that CO2 are also hydrogenated by H2 gas on the surface of iron catalyst through Fischer–Tropsch-type reaction (e.g., Gieshoff et al., 1994). However, similarly to CO, the conversion rate of CO2 into CH4 under subnebula conditions has not been investigated yet. Thus, we also conduct experiments under the condition of high H2 /CO2 ratio (H2 /CO2 = 1000) that simulates subnebula conditions. Fig. 5 shows our experimental data of CH4 formation rates of CO and CO2 at 550 K as a function of pressure. At this temperature, the poisoning did not occur in both CO and CO2 . Our data indicate that the CH4 formation rate increases with pressure and that the gradients of the data points for CO and CO2 are similar to each other. However, the absolute values of the conversion rate of CO2 are about 0.01 times those of CO. This result indicates that the time scales of CH4 formation from CO and CO2 are different and that CO are hydrogenated into CH4 on the surface of Fe much faster than CO2 in a subnebula.
4. Discussion In this section, first we estimate the time scale of the conversion of CO into CH4 in a subnebula and discuss the gas
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Fig. 5. Methane formation rate normalized by the number of surface site at temperature of 550 K as a function of pressure. The conversion rates of CO (solid circles) and those of CO2 (open triangles) are shown. The time scale for the conversion of CO into CH4 in the subnebula is also shown. Here, it is assumed that about 10% of the cosmic component of Fe was present as metal in the nebula and that the particles were spherical and 1 µm in radius. The gray zones indicate the time scale of Titan formation (Mosqueira and Estrada, 2003) and the lifetime of the solar nebula (Prinn and Fegley, 1989).
composition of subnebula using our experimental results. Then, we calculate the chemical composition incorporated in clathrate hydrate formed in a subnebula and discuss the role of Fischer–Tropsch catalysis for the origin of CH4 -rich atmosphere on Titan. 4.1. Subnebula chemistry Our experimental result indicates that the CH4 production due to Fischer–Tropsch catalysis in a subnebula does not simply increase exponentially with temperature as assumed by Prinn and Fegley (1989). Our data rather show that it proceeds most efficiently around T = 550 K and loses its efficiency at either higher or lower temperatures. Here, we estimate the time scale for CO to be converted into CH4 through Fischer–Tropsch catalysis in a subnebula region where temperature is about 550 K. We define the CO lifetime in this study as tlife =
[CO] , [site] × rate
where rate is the CH4 formation rate per surface site of catalyst. The parameters [CO] and [site] are the number density of CO per unit volume of subnebula and the number of surface sites of metallic iron per unit volume of subnebula, respectively. We can estimate the number density [site] of surface site of catalyst from the size and shape of the dust and the abundance of metallic iron in a nebula. We assume that about 10% of the cosmic component of Fe was present as metal in the solar nebula (Pollack et al., 1994) and that the particles were spherical and 1 µm in radius. The number
density [CO] of carbon monoxide is given by the solar composition (Anders and Grevesse, 1989), assuming that all the carbon atoms are in the form of CO. The result of calculation on the lifetime of CO is also shown in Fig. 5. Although a number of models of a subnebula have been proposed (e.g., Pollack et al., 1976; Prinn and Fegley, 1981; Coradini et al., 1989; Mousis et al., 2002), the nebula pressure at the catalytically optimal temperature of 550 K has not been understood well. Prinn and Fegley (1981, 1989) suggest a dense subnebula model, in which the pressure is about 1–10 bars at 550 K, while Coradini et al. (1989) suggest a thin subnebula model, in which the pressure is about 10−3 bar at 550 K. As shown in Fig. 5, our result indicates that the conversion of CO into CH4 proceeds very rapidly around 550 K in both thick and thin subnebula models relative to the time scale of Titan formation (e.g., Mosqueira and Estrada, 2003) and the lifetime of solar nebula (Prinn and Fegley, 1989). When the pressure becomes lower than 10−5 bar, however, an equilibrium calculation by Mousis et al. (2002) shows that CO become more abundant than CH4 at 550 K. In such a low-pressure subnebula, the conversion of CO into CH4 may not proceed efficiently. Prinn and Fegley (1989) assume that Fischer–Tropsch catalysis proceeds in a wide range of temperature (400– 1500 K) in a subnebula and argue that an entire subnebula becomes rich in CH4 . Our data, however, strongly suggest that CH4 production through Fischer–Tropsch catalysis is limited at temperatures around 550 K in a subnebula. At temperatures below ∼450 K (a typical evaporation temperature of organics (Kouchi et al., 2002)), the organics and H2 O ice would cover the iron dust (Greenberg, 1998). If this occurs, the catalytic reaction does not occur. At temperatures above ∼600 K, the poisoning of the catalyst prevents active CH4 production. This result indicates that the total amount of CH4 produced in a whole subnebula is reduced drastically from the prediction by Prinn and Fegley (1989). Our experimental data suggest that the subnebula is simply divided in three regions: (1) the high-temperature region in which the CH4 production through Fischer–Tropsch catalysis is prevented by poisoning, (2) the CH4 -rich inner region where the catalysis is efficient, and (3) the outer low temperature region where the catalysis does not proceed. 4.2. Clathrate formation As a subnebula cools with time, the gas species are incorporated in clathrate hydrate (Lunine and Stevenson, 1985). If such volatile species incorporated into clathrate are left on the surface of icy satellites, they could be observed by space probes. However, the composition of volatiles trapped in clathrate is not always the same as that of subnebular gas. Thus, our experimental result cannot be compared directly with observational data of icy satellites. The calculation of clathrate formation is necessary before we apply our data to the observation of icy satellites. We calculate clathrate formation using theoretical models developed in the previ-
Fischer–Tropsch catalysis for the origin of CH4 on Titan
ous studies (van der Waals and Platteeuw, 1959; McKoy and Sinano˘glu, 1963; Lunine and Stevenson, 1985). We will describe an outline of this theory in Appendix A briefly. A precise estimate of clathrate formation in the subnebula needs to take into account the time evolution of temperature, pressure, and surface density of the subnebula. However, the purpose in this section is not to investigate a precise composition of clathrate but to estimate the effect of Fischer– Tropsch catalytic reaction on the composition of satellitesimals. Thus, in this study, we just calculate the equilibrium composition of clathrate hydrate in the subnebula As described above in Section 4.1, we simply divide a subnebula in three regions. However, the gas composition of the region where the poisoning occurs is expected to be similar to that of the low temperature region. In the following, thus, we describe the clathrate formation in the two regions of a subnebula: the catalytically inactive outer region and the catalytically active inner region. 4.2.1. Outer region of a subnebula We assume that the volatile composition in the outer region of a subnebula is similar to that of the solar nebula at 10 AU and that the volatile species are initially in the gas phase. As mentioned in Section 3.3, CO2 are thought to be abundant in the outer solar nebula (e.g., Aikawa et al., 1999; Gibb et al., 2000). Our calculation indicates that CO2 -rich satellitesimals will be formed in such a region in a subnebula. The model of relative abundance of gas molecules in the outer region of a subnebula in this study is given as follows. The abundances of elements in a subnebula are assumed to be the same as the Solar-System abundances given by Anders and Grevesse (1989). We consider both rock components (SiO2 and MgO: ∼15% of total oxygen) and organics. The fractional abundance of organic carbon is assumed to be 55% of total carbon (Pollack et al., 1994), and the ratio of C:O:N included in organics is assumed to be 1:0.5:0.12 (Jessberger et al., 1988). We assume CO:CO2 :CH4 = 10:10:1, based on the calculation result of molecular abundances in the solar nebula at 10 AU (Aikawa et al., 1999). In this study, all the nitrogen is assumed to be in the form of N2 except for that included in organics. Table 1a lists the gas-phase abundances in an outer subnebula. Table 1 Gas abundances (mole fraction) relative to H2 in nebula
H2 O CO2 CO CH4 N2 Ar Kr Xe
(a) CO:CO2 :CH4 = 10:10:1, organics 55%
(b) CO:CO2 :CH4 = 0:0:1, organics 0%
7.80 × 10−4 1.55 × 10−4 1.55 × 10−4 1.55 × 10−5 7.22 × 10−5 7.24 × 10−6 3.26 × 10−9 3.37 × 10−10
1.45 × 10−3 0 0 7.24 × 10−4 1.12 × 10−4 7.24 × 10−6 3.26 × 10−9 3.37 × 10−10
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Another important factor controlling the satellitesimal composition is lower critical decomposition point of CO2 clathrate. Miller and Smythe (1970) estimate this point to be about the temperature of 120 K and the pressure of 1 × 10−5 bar based on an extrapolation of laboratory data of the dissociation pressure of CO2 . Because the vapor pressure of CO2 ice is less than the dissociation pressure of CO2 clathrate below the temperature of this point, the CO2 clathrate is not stable and converts to CO2 ice. When CO2 partial gas pressure is less than the pressure of this critical point, the temperature of CO2 ice condensation is always higher than that of CO2 clathrate formation and CO2 clathrate also becomes unstable. Thus, CO2 clathrate is stable only above 120 K and above 1 × 10−5 bar. This partial pressure of CO2 , 1 × 10−5 bar, is approximately corresponding to 0.1 bar of H2 pressure in the outer region of subnebula. Fig. 6a shows the dissociation pressures of clathrate hydrates as a function of temperature for different gas species. In the outer region of subnebula, CO2 clathrate becomes unstable at less than about 0.1 bar of H2 gas pressure. Fig. 7a shows that the abundance of volatiles incorporated in clathrate relative to the number of clathrate cages when CO2 partial gas pressure is higher than 1 × 10−5 bar. Xenon is fully depleted from the gas phase; all the xenon in a nebula is incorporated in clathrate. At temperatures higher than 120 K, most of the cages of clathrate are occupied with CO2 . Because CO2 ice is more stable than CO2 -baring clathrate at temperatures less than 120 K, most of the clathrate cages are occupied with CH4 . When CO2 partial gas pressure is lower than 1 × 10−5 bar, CH4 also becomes dominant volatile species in the clathrate even above 120 K. However, the bulk composition of volatile in solid is richer in CO2 than CH4 (CO2 /CH4 = 10), because most CO2 exist in the form of ice, not gas. Consequently, satellitesimals formed in the outer subnebula will be rich in CO2 regardless of condensation temperature. If the volatile species are not in a gas phase but in a solid phase in the outer region of subnebula, the volatile composition of satellitesimals is similar to that of icy bodies in the solar nebula around 10 AU. According to theoretical calculation about the volatile composition in the solar nebula (e.g., Aikawa et al., 1999), the amount of CO2 is larger than that of CH4 in the icy bodies at 10 AU. Thus, even if the volatile species are in a solid phase in outer region of subnebula, the satellitesimals will contain much CO2 rather than CH4 . 4.2.2. Catalytically active region in a subnebula In the inner region of a subnebula where the subnebular temperature rises to 500–600 K, Fischer–Tropsch catalysis occurs efficiently. We do not consider the presence of organics in this region because the organics are most likely to evaporate at temperatures higher than ∼450 K and decomposed into gas molecules, such as CH4 , CO, CO2 , H2 O, and NH3 (Kouchi et al., 2002). Our experimental result indicates
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(a)
(b)
Fig. 6. Dissociation pressures of clathrate hydrate of several volatiles as functions of temperature. (a) The results for the subnebular gas compositions of Table 1a. (b) The results for the gas compositions of Table 1b.
(a)
(b)
Fig. 7. Fraction of volatile species incorporated in clathrate cages for the subnebular gas composition. (a) The result for the subnebular gas composition of type (a) (outer subnebula), and that of type (b) (inner subnebula) in Table 1.
that CO and CO2 convert into CH4 in a short time in this temperature range. Table 1 lists the gas-phase abundances in this inner region of a subnebula. Fig. 6b shows the dissociation pressures of clathrate hydrates as functions of temperature for different gas species. Fig. 7b shows the abundance of volatiles incorporated in clathrate relative to the number of clathrate cages. In a CH4 rich nebula, most of the cages of clathrate are occupied with CH4 . This result suggests that volatile compositions trapped in the satellitesimals would be rich in CH4 in this inner region. The amount of CH4 in CH4 -rich satellitesimals is 10–200 times that in CO2 -rich satellitesimals formed in the outer region.
The result of clathrate calculation based on our experimental data on the catalysis indicates that volatile composition in satellitesimals varies depending on radial distance from the central planet. The CH4 -rich satellitesimals are formed in an inner region, while CO2 -rich satellitesimals are formed in the outer region of a subnebula. Finally, we briefly discuss how CH4 in the satellitesimals are preserved as Titan forms. At the early stage of Titan’s accretion, such inner CH4 -rich satellitesimals would form Titan’s embryo. CH4 contained in deep inside of Titan may be degassed as clathrate dissociation is induced by crustal movements (Lunine and Stevenson, 1987; Loveday et al., 2001). At the end of accretion stage of Titan, these
Fischer–Tropsch catalysis for the origin of CH4 on Titan
satellitesimals would collide onto Titan at more than its escape velocity. Such hypervelocity impact evaporates some fraction of impactor and target. In a high temperature vapor, a fraction of CH4 in the impactor may be oxidized to CO and CO2 . However, in the case of a vertical impact of water-ices at the velocity of less than 7 km/s, the maximum temperature of the impact vapor cloud is estimated to be significantly less than 2000 K using a planer impact approximation (e.g., Melosh, 1989). In reality, most of impacts are oblique one. In an oblique impact, the temperature of impact cloud becomes lower than in a vertical impact. Since 2000 K is quench temperature of gas-phase reaction of most simple carbon molecules (Fegley et al., 1986; Zahnle, 1990; Gerasimov et al., 1998), a large part of CH4 is not expected to be dissociated through an impact but will be supplied to surface of Titan at the end of accretion stage.
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entire subnebula stays similar to that of the solar nebula because the gas pressure in a subnebula might be too low (as low as that of solar nebula) to allow Fischer–Tropsch reaction to proceed efficiently (Section 4.1) (Mousis et al., 2002). The other type of model is that the entire subnebula becomes rich in CH4 , when the subnebula is very convective (Prinn, 1990) and well mixed through its lifetime. Based on the currently available data, however, we cannot tell which model is the case for subnebula evolution. Nevertheless, Cassini– Huygens mission may provide key information on the origin of CH4 on Titan and the chemical evolution of the circumsaturnian subnebula. In this section, based on the results obtained in this study, we discuss the relation between these subnebula models and possible chemical/geologic observation by Cassini–Huygens spacecraft. 5.1. Iapetus composition
4.3. Uncertainty in Fischer–Tropsch catalysis in a subnebula There are a couple of possibilities that Fischer–Tropsch catalysis does not occur efficiently anywhere in a subnebula. First, we assume that metallic iron grain exists in a subnebula in this study. However, metal grain may have already formed oxides or troilite FeS in the high-temperature region in the solar nebula before the formation of the central gas planet. Lauretta et al. (1996) and Fegley (2000) show that FeS formation in the solar nebula is rapid at 600–700 K (e.g., ∼1010 s at 700 K for 0.1 µm radius metal grain) relative to the lifetime of the solar nebula. If Fe grains undergo such a high temperature, CH4 formation through Fischer–Tropsch catalysis is prevented by the conversion to FeS. Second, as described in Section 4.1, when the subnebula is geometrically thin as suggested by a turbulent model by Mousis et al. (2002), CH4 production through Fischer–Tropsch catalysis may not proceed efficiently because of the low pressure. Then, the gas composition of a subnebula becomes uniform and similar to that of the solar nebula. If these possibilities occur, the volatile composition in satellitesimals formed in the entire subnebula becomes rich in CO2 and characteristic geologic evidence would be left on the surface of Titan. Such evidence may be observed by spacecrafts, such as Cassini and Huygens. The possibility of such observation is discussed in further detail below in Section 5.
5. Prospects for Cassini–Huygens observations When the result of the experiments in this study is applied to the high-pressure model for subnebula evolution (Section 1), it would strongly suggest that CH4 -rich region is formed only in the inner region of the subnebula. However, there are two other types of models for the subnebula chemistry. One type of model is that the composition of the
Iapetus is located at ∼ 60RS from Saturn, where the subnebular temperature is not thought to have increased to temperatures necessary for efficient Fischer–Tropsch catalysis during its formation period (e.g., Coradini and Magni, 1984; Coradini et al., 1989). If the entire subnebula becomes rich in CH4 , Iapetus should contain a large amount of CH4 as Titan does. However, the other two models suggest that the building materials for Iapetus should contain a large amount of CO2 and only a small amount of CH4 . The visible and infrared spectrometers on the Cassini orbiter will investigate the surface composition of Iapetus. If Cassini observations provide evidence for subsurface CO2 on Iapetus, such as CO2 -rich crater floors found on the surface of Callisto (Hibbitts et al., 2002), it will support the other two models. 5.2. Titan composition The surface property of Titan will also provide important information on whether Titan is formed by CH4 -rich or CO2 -rich satellitesimals. These satellitesimals are thought to form Titan and other icy satellites by collisional accretion. Although the formation of Titan has not been understood completely, theoretical studies suggest that Titan had a thick proto-atmosphere during its accretion stage (e.g., Lunine and Stevenson, 1982; Kuramoto and Matsui, 1994). Kuramoto and Matsui (1994) indicate that when the accretion time is less than 105 years, the surface temperature becomes higher than 500 K and that a thick proto-atmosphere is formed by the evaporation of icy satellitesimals. Recent theoretical study on the evolution of subnebula and on the formation of satellites around gas planets indicates that the accretion time of Titan is about 104 –105 years (Mosqueira and Estrada, 2003). In this study, we assume that Titan had a hot and thick proto-atmosphere during its accretion stage. When the entire subnebula was rich in CO and CO2 but poor in CH4 as the solar nebula was, the CO2 -rich satellitesimals must have formed Titan. If this is the case, a large
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amount of CO2 would have been degassed during the accretion stage of Titan. As the atmospheric temperature decreases with time, CO2 condenses and may form a thick CO2 ice crust over the surface of Titan. Coradini et al. (1989) suggest that Titan may be formed from the satellitesimals not only in the inner region but also the outer region of the subnebula by migration of the satellitesimals. Actually, the mass of Titan is more than 90% of total mass of saturnian satellite system. It may have collected solid matter from the entire subnebula region. Our results suggest that CH4 -rich satellitesimals are formed in the inner region and CO2 -rich satellitesimals are formed in the outer region of subnebula. If CO2 -rich satellitesimals in the outer region were accreted on Titan by migration, CO2 became one of the major components in the proto-atmosphere during accretion stage and then a thick CO2 icy crust was formed on the surface after the accretion stage. However, even if this is the case, CH4 -rich satellitesimals formed in the inner subnebula are important for the origin of CH4 on Titan because they would contain CH4 as much as 10–200 times that in the outer satellitesimals. Then, the surface composition of satellites may be affected by the subsequent processes after the accretion. UV reaction, for instance, may form organic haze and ices in the atmosphere, and they also deposit on the surface (e.g., Sagan and Thompson, 1984; Yung et al., 1984). Impact cratering may excavate the materials buried under the ground to the surface and expose them to the atmosphere. If Huygens probe in Cassini mission observe a thick CO2 icy crust under the organic deposits, it may indicate that the entire or a part of Titan is formed by CO2 -rich satellitesimals. When the circum-saturnian subnebula contained a significant amount of CH4 , Titan could have been formed by CH4 rich satellitesimals. Then, Titan surface should not have a thick CO2 icy crust. If our model of CH4 -rich satellitesimals is correct chemical/geologic evidence that supports CO2 rich subsurface of Iapetus and no thick CO2 layer over the surface of Titan would be observed by spacecrafts, such as Cassini and Huygens.
6. Conclusions The experimental data indicate that the conversion of CO into CH4 via Fischer–Tropsch catalysis is efficient only around 550 K at subnebular condition. At temperatures higher than 600 K, our experimental data indicate that the catalyst suffers from poisoning even at high H2 /CO ratio (i.e., 1000) and that the CH4 formation rate decreases as temperature rises. At temperatures lower than 550 K, catalytic efficiency is lower because of Arrhenius factor (i.e., exp(−E/kT )). At temperatures between 500 and 600 K, both CO and CO2 are converted into CH4 in a short period of time relative to the lifetime of the solar nebula. This result suggests that Fischer–Tropsch catalysis proceeds efficiently
within a narrow region of a subnebula and that an entire subnebula may not necessarily become rich in CH4 . Our calculation based on our experimental results suggests that the chemical compositions of volatile in satellitesimals are considerably different depending on the radial distance from the central gas planet. Outside the catalytically active inner region, we assume that the gas composition of a subnebula will be similar to that of the solar nebula. In such a region, our calculation indicates that CO2 -rich satellitesimals are formed. However, CH4 -rich satellitesimals are formed from the CH4 -rich nebula gas region, where Fischer– Tropsch catalysis proceeds efficiently. The results of the experiments and calculations in this study indicate that observation of the surface composition of Titan and the subsurface composition of Iapetus may provide key information on the chemical evolution of the circum-saturnian subnebula because these compositions are sensitive to the distribution of CH4 , CO, and CO2 in the subnebula. If the high-pressure model for subnebula evolution is correct, the subsurface CO2 on Iapetus should be observed, but thick CO2 ice layer over Titan’s icy crust should not be observed. In contrast, if the efficiency of radial mixing in a subnebula is high (Prinn, 1990), both the subsurface CO2 on Iapetus and thick CO2 ice layer on Titan should not be observed. If the gas pressure in a subnebula is low (Mousis et al., 2002), both the subsurface CO2 on Iapetus and the CO2 ice crust over Titan should be observed. Thus, observation of subsurface (e.g., crater floor) composition of Iapetus and surface composition of Titan by Cassini–Huygens mission should deserved particular attention in the light of subnebular evolution.
Acknowledgments One of the authors (Y.S.) thanks T. Sasaki and Y. Namai for useful discussions on the experiments and K. Kuramoto for fruitful discussions and suggestions. The authors also thank to O. Mousis and an anonymous reviewer for careful reviews and their valuable comments. This research is partially supported by Grant in Aid from Japan Society for the Promotion.
Appendix A The thermodynamic properties and the theory of formation and dissociation of the clathrate hydrate have been developed by van der Waals and Platteeuw (1959) and McKoy and Sinano˘glu (1963). Lunine and Stevenson (1985) applied these theories to solar nebula and circum-planetary subnebula conditions. Their assumptions are adopted in this study and are as follows: (a) The chemical potential of the lattice is independent of the guest occupation; (b) the encaged molecules are localized in the cavities, and a cavity cannot hold more than one guest molecule; (c) guest molecules rotate
Fischer–Tropsch catalysis for the origin of CH4 on Titan
freely within the cage; (d) guest molecules do not interact with each other; (e) classical statistical mechanics are valid. According to van der Waals and Platteeuw (1959), the chemical potential of the water-ice in clathrate hydrate is given by β
µH2 O = µH2 O
ysj + vl ln 1 − ylj , + kT vs ln 1 − j
j
(A.1) β µH2 O ,
where k, T , vs , vl , ysj , and ylj are chemical potential of the empty clathrate hydrate lattice, Boltzmann’s constant, temperature, the number of cage sites per water molecule for the small and large cages of the clathrate hydrate, and the probabilities of finding a molecule of j in the small and large cages, respectively. At the phase boundary (ice + gas = clathrate), the chemical potential of the water-ice in clathrate is required to be equal to that of ordinary ice (µiH2 O ). The probability of finding a molecule of j in the small and large cages is given by yij =
1+
Cij Pj , k Cik Pk
(A.2)
where Cij , and Pj are Langmuir constant and partial pressure of gas molecule j , respectively. Langmuir constants depend on the molecular properties through the spherically averaged potential energy of the guest molecule in clathrate cage. In this paper, we use the equations in McKoy and Sinano˘glu (1963) for the spherically averaged potential energy and fit the results of dissociation pressures of various gas species to existing laboratory data (Lunine and Stevenson, 1985).
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The role of Fischer–Tropsch catalysis in the origin of methane-rich Titan Yasuhito Sekine a,∗ , Seiji Sugita b , Takafumi Shido c , Takashi Yamamoto c , Yasuhiro Iwasawa c , Toshihiko Kadono d , Takafumi Matsui b a Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan b Department of Complexity Science and Engineering, Graduate School of Frontier Science, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa,
Chiba 227-8562, Japan c Department of Chemistry, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan d Institute for Research on Earth Evolution, Japan Agency for Marine–Earth Science and Technology, 2-15 Natsushima, Yokosuka,
Kanagawa 237-0061, Japan Received 12 August 2004; revised 4 March 2005 Available online 17 May 2005
Abstract Fischer–Tropsch catalysis, which converts CO and H2 into CH4 on the surface of iron catalyst, has been proposed to produce the CH4 on Titan during its formation process in a circum-planetary subnebula. However, Fischer–Tropsch reaction rate under the conditions of subnebula have not been measured quantitatively yet. In this study, we conduct laboratory experiments to determine CH4 formation rate and also conduct theoretical calculation of clathrate formation to clarify the significance of Fischer–Tropsch catalysis in a subnebula. Our experimental result indicates that the range of conditions where Fischer–Tropsch catalysis proceeds efficiently is narrow (T ∼ 500–600 K) in a subnebula because the catalysts are poisoned at temperatures above 600 K under the condition of subnebula (i.e., H2 /CO = 1000). This suggests that an entire subnebula may not become rich in CH4 but rather that only limited region of a subnebula may enriched in CH4 (i.e., CH4 -rich band formation). Our experimental result also suggests that both CO and CO2 are converted into CH4 within time significantly shorter than the lifetime of the solar nebula at the optimal temperatures around 550 K. The calculation result of clathration shows that CO2 -rich satellitesimals are formed in the catalytically inactive outer region of subnebula. In the catalytically active inner region, CH4 rich satellitesimals are formed. The resulting CH4 -rich satellitesimals formed in this region play an important role in the origin of CH4 on Titan. When our experimental data are applied to a high-pressure model for subnebula evolution, it would predict that there should be CO2 underneath the Iapetus subsurface and no thick CO2 ice layer on Titan’s icy crust. Such surface and subsurface composition, which may be observed by Cassini–Huygens mission, would provide crucial information on the origin of icy satellites. 2005 Elsevier Inc. All rights reserved. Keywords: Fischer–Tropsch catalysis; Titan; Cassini–Huygens mission; Circum-planetary subnebula; Methane
1. Introduction Titan has a dense atmosphere dominated by N2 and CH4 . However, CO are thought to be the main reservoir of gasphase carbon in the interstellar medium and solar nebula (e.g., Pollack et al., 1994). Thus, methane-producing reactions from CO in the circum-planetary subnebula are important in understanding the origin of the CH4 -rich atmosphere * Corresponding author. Fax: +81 3 5841 8318.
E-mail address: [email protected] (Y. Sekine). 0019-1035/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2005.03.016
on Titan. Prinn and Fegley (1981, 1989) show the importance of conversion of CO and N2 into CH4 and NH3 in a circum-planetary subnebula using a thermochemical equilibrium model. In their studies, a CH4 -rich subnebula is formed through both gas-phase reactions and catalytic reactions (e.g., Fischer–Tropsch catalysis). Then, Owen (1982) and Lunine and Stevenson (1985) argue that CH4 formed in the subnebula was trapped in clathrate hydrates and incorporated into Titan. Furthermore, Atreya et al. (1978) and McKay et al. (1988) show that N2 in the present atmosphere of Titan may be a product of NH3 photolysis and shock heat-
Fischer–Tropsch catalysis for the origin of CH4 on Titan
ing by meteoritic impacts. This scenario is consistent with the present atmospheric composition of Titan. However, there is large uncertainty in the conversion rate of CO into CH4 in a subnebula. In gas-phase reactions, CO are energetically inhibited by the high activation barrier to break the CO bond. This prohibits gas-phase reactions from proceeding at low temperatures (<2000 K) and pressures (
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lar, the subnebular gas pressure is a very important factor. If the gas pressure in a subnebula is relatively high (i.e., more than 10−3 bar at 500 K) (Prinn and Fegley, 1989; Coradini et al., 1989; Mosqueira and Estrada, 2003), the CH4 becomes thermochemically stable. Then, the CH4 produced in a subnebula will contribute to the CH4 on Titan. In this study, we use such a thick subnebula model, which hereafter we call the high-pressure model for subnebula evolution, as a reference model. However, if the gas pressure is as low as the solar nebula (i.e., less than 10−5 bar at 500 K) (Mousis et al., 2002), the CH4 production does not proceed efficiently in a subnebula. Then, the CH4 derived from molecular cloud becomes the dominant source for the CH4 on Titan. The detail of stability of CH4 in such low-pressure subnebula models will be discussed below in Section 4.3. Although these different evolution models may predict similarly CH4 -rich atmosphere on Titan, other chemical characteristics (e.g., crustal composition) of resulting icy satellites might be very different depending on the subnebula model. In fact, Cassini–Huygens observation may reveal such key chemical/geologic evidence. However, the relation between possible chemical/geological evidence and subnebula evolution is not understood well yet. Then, in this study, we also attempt to clarify the relation between subnebula evolution models and possible chemical/geological observations based on the experimental results. In the following, we describe the experimental system (Section 2), show the experimental results on Fischer– Tropsch catalysis (Section 3), calculate the clathrate hydrate formation from the subnebular gases using our experimental data (Section 4), and provide observational propositions for Cassini–Huygens mission (Section 5).
2. Experimental We use iron powder as catalyst because iron is much more abundant in the solar composition than other catalysts for Fischer–Tropsch catalysis, such as Ru, Co, and Ni (Anders and Grevesse, 1989). The grain size and purity of catalyst used in the experiments is ∼250 µm (60 mesh) and 99.998%, respectively. In the following, we treat the CH4 formation rate as a specific reaction rate (product molecules per reaction site (i.e., surface atom) per second). In order to measure the number of surface sites, we conduct gas adsorption (BET) experiments using Kr gas as adsorbate. The BET experiments indicate the surface area and the number of surface site per unit weight of the iron powder used in our experiments are 1.5(±0.1) × 10−1 m2 /g and 1.5(±0.1) × 1018 site/g, respectively. A schematic diagram of the experimental system is shown in Fig. 1. We use a gas chromatograph (GC) (GC-8A, Shimadzu Corp.) for the analysis. We use a porous polymer reagent, Porapak Q (GL Science), which is used for separating light hydrocarbons from its gas mixtures. The electric current is fixed at 100 mA. The temperatures at the
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Fig. 2. Methane production normalized by the number of surface sites as a function of reaction time. The experimental condition for this run is: H2 /CO = 1000, P = 0.53 bar, and T = 550 K. Fig. 1. Schematic diagram of experimental system to measure Fischer–Tropsch catalysis rate.
GC column and the injection port are fixed at 55 and 70 ◦ C, respectively. A diaphragm pump is used to circulate the gas in the closed-loop system. Prior to each measurement of reaction rate, the catalyst surface is cleaned with H2 gas at T = 700 K and P ∼ 0.3 bar for 1.5 h. This pre-treatment ensures that oxide iron is reduced to metallic iron before the main experiment. Then, we exhaust the reducing gases for 1.5 h using a diffusion pump. Finally, we introduce reactant gas mixture to the reaction cell. After a prescribed length of time, a part of the gas in the reaction cell is extracted and analyzed with the GC. We analyzed the gas mixtures after 4–9 different reaction times for each experimental condition.
3. Results 3.1. Methane formation rate Fig. 2 shows the CH4 production per surface site as a function of reaction time at P = 0.53 bar and T = 550 K. The experimental result indicates that the CH4 production increases linearly with time under a given reaction condition. The CH4 formation rate (molecules/site/s) under this reaction condition is obtained by a least-square fit of the data points. Fig. 3 shows the CH4 formation rate as a function of temperature (i.e., Arrhenius plot). As stated in Section 1, Vannice (1975) obtains an empirical law of CH4 formation rate via Fischer–Tropsch catalysis as a function of temperature and partial pressures of CO and H2 based on the experimental data at H2 /CO = 0.6–15.1. In the previous study by Prinn and Fegley (1989), they extrapolate this empirical law
Fig. 3. Methane formation rate normalized by the number of surface sites as a function of temperature. The solid circles, squares, diamonds, and triangles indicate our experimental results of H2 /CO = 1000 and P = 0.53, 0.30, 0.16, and 0.09 bar, respectively. The broken lines show an power law based on extrapolation of data by Vannice (1975) at H2 /CO = 1000, and P = 0.53, 0.30, 0.16, and 0.09 bar, respectively. The solid lines are the fitting lines (an exponential function) of our experimental data points (T = 500, 525, and 550 K) determined by the least square method.
to the nebular conditions (e.g., H2 /CO = 1200) and estimate CH4 production in the nebular conditions. Catalytical efficiency at H2 /CO = 1000 using the empirical law obtained by Vannice (1975) is also shown in Fig. 3. Then, our experimental data and extrapolated catalytical efficiency based on the empirical law under the same conditions of temperature and partial pressures of CO and H2 are compared. At temperatures lower than 550 K, the CH4 formation rate increases exponentially as temperature rises. This temperature range
Fischer–Tropsch catalysis for the origin of CH4 on Titan
is close to the temperature range (513–553 K) where the industrial laboratory data by Vannice (1975) are obtained. Comparison between our experimental data and the extrapolated results of the empirical law based on Vannice (1975) indicates that the gradients of the two results are very similar. In other words, the activation energies obtained in our experiments (75–109 kJ/mol) agree well with industrial laboratory data (∼88–96 kJ/mol). However, the absolute CH4 formation rates of our experimental results are 10–30 times the extrapolated results of the empirical power law based on data by Vannice (1975). This may be caused by the difference in H2 /CO ratio of the reactant gases. Our experiments are under H2 -rich conditions to simulate a subnebula condition (H2 /CO = 1000), while H2 /CO = 0.6–15.1 were used by Vannice (1975). This difference suggests that the CH4 formation rates estimated by the empirical power law based on Vannice (1975) would significantly underestimate the production in this reaction range in a circum-planetary subnebula. 3.2. Poisoning At temperatures higher than 550 K in Fig. 3, CH4 formation rates deviate from an exponential trends and decrease as temperature rises. This is most likely caused by “poisoning” of catalyst. The poisoning is a gradual loss of catalytic activity through conversion of surface carbide to unreactive graphitic carbon (e.g., Krebs et al., 1979). When poisoning occurs on the surface of catalyst, the slope of the CH4 production decreases gradually with time. The effect of poisoning has been investigated extensively in catalytic chemistry (e.g., Krebs et al., 1979). According to Krebs et al. (1979), poisoning of catalyst is more difficult to occur when reactant gas has a higher H2 /CO ratio (i.e., H2 -rich condition). In planetary science, theoretical calculations suggest that poisoning of catalyst may occur in the solar nebula (Kress and Tielens, 2001). However, it has not been clear if poisoning of iron catalyst may occur at very high H2 /CO conditions, such as the nebular gas. Fig. 4 demonstrates that catalytic poisoning occurs under the conditions of subnebula (i.e., H2 /CO = 1000); the CH4 production decreases as a function of reaction time at T = 600 K and 650 K. Comparison between the result at 600 and that at 650 K indicates that the effect of poisoning increases with temperature. The poisoning is not observed at temperatures below 550 K. This may be because the conversion rate of carbide into graphitic carbon is so slow at lower temperatures that the carbide is removed as CH4 by hydrogenation before the formation of graphitic carbon. After the gradual decrease, CH4 formation rate approaches a steady state around 1 × 104 –2 × 104 s. Since a typical time scale of subnebula evolution is much longer than that of poisoning processes, the steady-state rate after poisoning is more relevant as the actual rate of conversion from CO into CH4 in a subnebula. Because the CH4 formation rate in Fig. 3 is that
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Fig. 4. Methane production per surface site as a function of reaction time. The reaction temperatures are 600 (solid circles) and 650 K (open triangles).
in a steady state (i.e., poisoned rate), it decreases as temperature rises above ∼550 K. 3.3. Conversion of CO2 into CH4 Recently, both infrared observations of the interstellar ices (e.g., Gibb et al., 2000) and a theoretical study on cosmic-ray ionization reaction in the solar nebula (e.g., Aikawa et al., 1999) indicate that a large amount of CO2 ice is present in the outer solar nebula. Such CO2 ice will evaporate and exist in the gas phase in a warm subnebula in addition to CO. In the industrial chemistry, it is known that CO2 are also hydrogenated by H2 gas on the surface of iron catalyst through Fischer–Tropsch-type reaction (e.g., Gieshoff et al., 1994). However, similarly to CO, the conversion rate of CO2 into CH4 under subnebula conditions has not been investigated yet. Thus, we also conduct experiments under the condition of high H2 /CO2 ratio (H2 /CO2 = 1000) that simulates subnebula conditions. Fig. 5 shows our experimental data of CH4 formation rates of CO and CO2 at 550 K as a function of pressure. At this temperature, the poisoning did not occur in both CO and CO2 . Our data indicate that the CH4 formation rate increases with pressure and that the gradients of the data points for CO and CO2 are similar to each other. However, the absolute values of the conversion rate of CO2 are about 0.01 times those of CO. This result indicates that the time scales of CH4 formation from CO and CO2 are different and that CO are hydrogenated into CH4 on the surface of Fe much faster than CO2 in a subnebula.
4. Discussion In this section, first we estimate the time scale of the conversion of CO into CH4 in a subnebula and discuss the gas
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Fig. 5. Methane formation rate normalized by the number of surface site at temperature of 550 K as a function of pressure. The conversion rates of CO (solid circles) and those of CO2 (open triangles) are shown. The time scale for the conversion of CO into CH4 in the subnebula is also shown. Here, it is assumed that about 10% of the cosmic component of Fe was present as metal in the nebula and that the particles were spherical and 1 µm in radius. The gray zones indicate the time scale of Titan formation (Mosqueira and Estrada, 2003) and the lifetime of the solar nebula (Prinn and Fegley, 1989).
composition of subnebula using our experimental results. Then, we calculate the chemical composition incorporated in clathrate hydrate formed in a subnebula and discuss the role of Fischer–Tropsch catalysis for the origin of CH4 -rich atmosphere on Titan. 4.1. Subnebula chemistry Our experimental result indicates that the CH4 production due to Fischer–Tropsch catalysis in a subnebula does not simply increase exponentially with temperature as assumed by Prinn and Fegley (1989). Our data rather show that it proceeds most efficiently around T = 550 K and loses its efficiency at either higher or lower temperatures. Here, we estimate the time scale for CO to be converted into CH4 through Fischer–Tropsch catalysis in a subnebula region where temperature is about 550 K. We define the CO lifetime in this study as tlife =
[CO] , [site] × rate
where rate is the CH4 formation rate per surface site of catalyst. The parameters [CO] and [site] are the number density of CO per unit volume of subnebula and the number of surface sites of metallic iron per unit volume of subnebula, respectively. We can estimate the number density [site] of surface site of catalyst from the size and shape of the dust and the abundance of metallic iron in a nebula. We assume that about 10% of the cosmic component of Fe was present as metal in the solar nebula (Pollack et al., 1994) and that the particles were spherical and 1 µm in radius. The number
density [CO] of carbon monoxide is given by the solar composition (Anders and Grevesse, 1989), assuming that all the carbon atoms are in the form of CO. The result of calculation on the lifetime of CO is also shown in Fig. 5. Although a number of models of a subnebula have been proposed (e.g., Pollack et al., 1976; Prinn and Fegley, 1981; Coradini et al., 1989; Mousis et al., 2002), the nebula pressure at the catalytically optimal temperature of 550 K has not been understood well. Prinn and Fegley (1981, 1989) suggest a dense subnebula model, in which the pressure is about 1–10 bars at 550 K, while Coradini et al. (1989) suggest a thin subnebula model, in which the pressure is about 10−3 bar at 550 K. As shown in Fig. 5, our result indicates that the conversion of CO into CH4 proceeds very rapidly around 550 K in both thick and thin subnebula models relative to the time scale of Titan formation (e.g., Mosqueira and Estrada, 2003) and the lifetime of solar nebula (Prinn and Fegley, 1989). When the pressure becomes lower than 10−5 bar, however, an equilibrium calculation by Mousis et al. (2002) shows that CO become more abundant than CH4 at 550 K. In such a low-pressure subnebula, the conversion of CO into CH4 may not proceed efficiently. Prinn and Fegley (1989) assume that Fischer–Tropsch catalysis proceeds in a wide range of temperature (400– 1500 K) in a subnebula and argue that an entire subnebula becomes rich in CH4 . Our data, however, strongly suggest that CH4 production through Fischer–Tropsch catalysis is limited at temperatures around 550 K in a subnebula. At temperatures below ∼450 K (a typical evaporation temperature of organics (Kouchi et al., 2002)), the organics and H2 O ice would cover the iron dust (Greenberg, 1998). If this occurs, the catalytic reaction does not occur. At temperatures above ∼600 K, the poisoning of the catalyst prevents active CH4 production. This result indicates that the total amount of CH4 produced in a whole subnebula is reduced drastically from the prediction by Prinn and Fegley (1989). Our experimental data suggest that the subnebula is simply divided in three regions: (1) the high-temperature region in which the CH4 production through Fischer–Tropsch catalysis is prevented by poisoning, (2) the CH4 -rich inner region where the catalysis is efficient, and (3) the outer low temperature region where the catalysis does not proceed. 4.2. Clathrate formation As a subnebula cools with time, the gas species are incorporated in clathrate hydrate (Lunine and Stevenson, 1985). If such volatile species incorporated into clathrate are left on the surface of icy satellites, they could be observed by space probes. However, the composition of volatiles trapped in clathrate is not always the same as that of subnebular gas. Thus, our experimental result cannot be compared directly with observational data of icy satellites. The calculation of clathrate formation is necessary before we apply our data to the observation of icy satellites. We calculate clathrate formation using theoretical models developed in the previ-
Fischer–Tropsch catalysis for the origin of CH4 on Titan
ous studies (van der Waals and Platteeuw, 1959; McKoy and Sinano˘glu, 1963; Lunine and Stevenson, 1985). We will describe an outline of this theory in Appendix A briefly. A precise estimate of clathrate formation in the subnebula needs to take into account the time evolution of temperature, pressure, and surface density of the subnebula. However, the purpose in this section is not to investigate a precise composition of clathrate but to estimate the effect of Fischer– Tropsch catalytic reaction on the composition of satellitesimals. Thus, in this study, we just calculate the equilibrium composition of clathrate hydrate in the subnebula As described above in Section 4.1, we simply divide a subnebula in three regions. However, the gas composition of the region where the poisoning occurs is expected to be similar to that of the low temperature region. In the following, thus, we describe the clathrate formation in the two regions of a subnebula: the catalytically inactive outer region and the catalytically active inner region. 4.2.1. Outer region of a subnebula We assume that the volatile composition in the outer region of a subnebula is similar to that of the solar nebula at 10 AU and that the volatile species are initially in the gas phase. As mentioned in Section 3.3, CO2 are thought to be abundant in the outer solar nebula (e.g., Aikawa et al., 1999; Gibb et al., 2000). Our calculation indicates that CO2 -rich satellitesimals will be formed in such a region in a subnebula. The model of relative abundance of gas molecules in the outer region of a subnebula in this study is given as follows. The abundances of elements in a subnebula are assumed to be the same as the Solar-System abundances given by Anders and Grevesse (1989). We consider both rock components (SiO2 and MgO: ∼15% of total oxygen) and organics. The fractional abundance of organic carbon is assumed to be 55% of total carbon (Pollack et al., 1994), and the ratio of C:O:N included in organics is assumed to be 1:0.5:0.12 (Jessberger et al., 1988). We assume CO:CO2 :CH4 = 10:10:1, based on the calculation result of molecular abundances in the solar nebula at 10 AU (Aikawa et al., 1999). In this study, all the nitrogen is assumed to be in the form of N2 except for that included in organics. Table 1a lists the gas-phase abundances in an outer subnebula. Table 1 Gas abundances (mole fraction) relative to H2 in nebula
H2 O CO2 CO CH4 N2 Ar Kr Xe
(a) CO:CO2 :CH4 = 10:10:1, organics 55%
(b) CO:CO2 :CH4 = 0:0:1, organics 0%
7.80 × 10−4 1.55 × 10−4 1.55 × 10−4 1.55 × 10−5 7.22 × 10−5 7.24 × 10−6 3.26 × 10−9 3.37 × 10−10
1.45 × 10−3 0 0 7.24 × 10−4 1.12 × 10−4 7.24 × 10−6 3.26 × 10−9 3.37 × 10−10
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Another important factor controlling the satellitesimal composition is lower critical decomposition point of CO2 clathrate. Miller and Smythe (1970) estimate this point to be about the temperature of 120 K and the pressure of 1 × 10−5 bar based on an extrapolation of laboratory data of the dissociation pressure of CO2 . Because the vapor pressure of CO2 ice is less than the dissociation pressure of CO2 clathrate below the temperature of this point, the CO2 clathrate is not stable and converts to CO2 ice. When CO2 partial gas pressure is less than the pressure of this critical point, the temperature of CO2 ice condensation is always higher than that of CO2 clathrate formation and CO2 clathrate also becomes unstable. Thus, CO2 clathrate is stable only above 120 K and above 1 × 10−5 bar. This partial pressure of CO2 , 1 × 10−5 bar, is approximately corresponding to 0.1 bar of H2 pressure in the outer region of subnebula. Fig. 6a shows the dissociation pressures of clathrate hydrates as a function of temperature for different gas species. In the outer region of subnebula, CO2 clathrate becomes unstable at less than about 0.1 bar of H2 gas pressure. Fig. 7a shows that the abundance of volatiles incorporated in clathrate relative to the number of clathrate cages when CO2 partial gas pressure is higher than 1 × 10−5 bar. Xenon is fully depleted from the gas phase; all the xenon in a nebula is incorporated in clathrate. At temperatures higher than 120 K, most of the cages of clathrate are occupied with CO2 . Because CO2 ice is more stable than CO2 -baring clathrate at temperatures less than 120 K, most of the clathrate cages are occupied with CH4 . When CO2 partial gas pressure is lower than 1 × 10−5 bar, CH4 also becomes dominant volatile species in the clathrate even above 120 K. However, the bulk composition of volatile in solid is richer in CO2 than CH4 (CO2 /CH4 = 10), because most CO2 exist in the form of ice, not gas. Consequently, satellitesimals formed in the outer subnebula will be rich in CO2 regardless of condensation temperature. If the volatile species are not in a gas phase but in a solid phase in the outer region of subnebula, the volatile composition of satellitesimals is similar to that of icy bodies in the solar nebula around 10 AU. According to theoretical calculation about the volatile composition in the solar nebula (e.g., Aikawa et al., 1999), the amount of CO2 is larger than that of CH4 in the icy bodies at 10 AU. Thus, even if the volatile species are in a solid phase in outer region of subnebula, the satellitesimals will contain much CO2 rather than CH4 . 4.2.2. Catalytically active region in a subnebula In the inner region of a subnebula where the subnebular temperature rises to 500–600 K, Fischer–Tropsch catalysis occurs efficiently. We do not consider the presence of organics in this region because the organics are most likely to evaporate at temperatures higher than ∼450 K and decomposed into gas molecules, such as CH4 , CO, CO2 , H2 O, and NH3 (Kouchi et al., 2002). Our experimental result indicates
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(a)
(b)
Fig. 6. Dissociation pressures of clathrate hydrate of several volatiles as functions of temperature. (a) The results for the subnebular gas compositions of Table 1a. (b) The results for the gas compositions of Table 1b.
(a)
(b)
Fig. 7. Fraction of volatile species incorporated in clathrate cages for the subnebular gas composition. (a) The result for the subnebular gas composition of type (a) (outer subnebula), and that of type (b) (inner subnebula) in Table 1.
that CO and CO2 convert into CH4 in a short time in this temperature range. Table 1 lists the gas-phase abundances in this inner region of a subnebula. Fig. 6b shows the dissociation pressures of clathrate hydrates as functions of temperature for different gas species. Fig. 7b shows the abundance of volatiles incorporated in clathrate relative to the number of clathrate cages. In a CH4 rich nebula, most of the cages of clathrate are occupied with CH4 . This result suggests that volatile compositions trapped in the satellitesimals would be rich in CH4 in this inner region. The amount of CH4 in CH4 -rich satellitesimals is 10–200 times that in CO2 -rich satellitesimals formed in the outer region.
The result of clathrate calculation based on our experimental data on the catalysis indicates that volatile composition in satellitesimals varies depending on radial distance from the central planet. The CH4 -rich satellitesimals are formed in an inner region, while CO2 -rich satellitesimals are formed in the outer region of a subnebula. Finally, we briefly discuss how CH4 in the satellitesimals are preserved as Titan forms. At the early stage of Titan’s accretion, such inner CH4 -rich satellitesimals would form Titan’s embryo. CH4 contained in deep inside of Titan may be degassed as clathrate dissociation is induced by crustal movements (Lunine and Stevenson, 1987; Loveday et al., 2001). At the end of accretion stage of Titan, these
Fischer–Tropsch catalysis for the origin of CH4 on Titan
satellitesimals would collide onto Titan at more than its escape velocity. Such hypervelocity impact evaporates some fraction of impactor and target. In a high temperature vapor, a fraction of CH4 in the impactor may be oxidized to CO and CO2 . However, in the case of a vertical impact of water-ices at the velocity of less than 7 km/s, the maximum temperature of the impact vapor cloud is estimated to be significantly less than 2000 K using a planer impact approximation (e.g., Melosh, 1989). In reality, most of impacts are oblique one. In an oblique impact, the temperature of impact cloud becomes lower than in a vertical impact. Since 2000 K is quench temperature of gas-phase reaction of most simple carbon molecules (Fegley et al., 1986; Zahnle, 1990; Gerasimov et al., 1998), a large part of CH4 is not expected to be dissociated through an impact but will be supplied to surface of Titan at the end of accretion stage.
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entire subnebula stays similar to that of the solar nebula because the gas pressure in a subnebula might be too low (as low as that of solar nebula) to allow Fischer–Tropsch reaction to proceed efficiently (Section 4.1) (Mousis et al., 2002). The other type of model is that the entire subnebula becomes rich in CH4 , when the subnebula is very convective (Prinn, 1990) and well mixed through its lifetime. Based on the currently available data, however, we cannot tell which model is the case for subnebula evolution. Nevertheless, Cassini– Huygens mission may provide key information on the origin of CH4 on Titan and the chemical evolution of the circumsaturnian subnebula. In this section, based on the results obtained in this study, we discuss the relation between these subnebula models and possible chemical/geologic observation by Cassini–Huygens spacecraft. 5.1. Iapetus composition
4.3. Uncertainty in Fischer–Tropsch catalysis in a subnebula There are a couple of possibilities that Fischer–Tropsch catalysis does not occur efficiently anywhere in a subnebula. First, we assume that metallic iron grain exists in a subnebula in this study. However, metal grain may have already formed oxides or troilite FeS in the high-temperature region in the solar nebula before the formation of the central gas planet. Lauretta et al. (1996) and Fegley (2000) show that FeS formation in the solar nebula is rapid at 600–700 K (e.g., ∼1010 s at 700 K for 0.1 µm radius metal grain) relative to the lifetime of the solar nebula. If Fe grains undergo such a high temperature, CH4 formation through Fischer–Tropsch catalysis is prevented by the conversion to FeS. Second, as described in Section 4.1, when the subnebula is geometrically thin as suggested by a turbulent model by Mousis et al. (2002), CH4 production through Fischer–Tropsch catalysis may not proceed efficiently because of the low pressure. Then, the gas composition of a subnebula becomes uniform and similar to that of the solar nebula. If these possibilities occur, the volatile composition in satellitesimals formed in the entire subnebula becomes rich in CO2 and characteristic geologic evidence would be left on the surface of Titan. Such evidence may be observed by spacecrafts, such as Cassini and Huygens. The possibility of such observation is discussed in further detail below in Section 5.
5. Prospects for Cassini–Huygens observations When the result of the experiments in this study is applied to the high-pressure model for subnebula evolution (Section 1), it would strongly suggest that CH4 -rich region is formed only in the inner region of the subnebula. However, there are two other types of models for the subnebula chemistry. One type of model is that the composition of the
Iapetus is located at ∼ 60RS from Saturn, where the subnebular temperature is not thought to have increased to temperatures necessary for efficient Fischer–Tropsch catalysis during its formation period (e.g., Coradini and Magni, 1984; Coradini et al., 1989). If the entire subnebula becomes rich in CH4 , Iapetus should contain a large amount of CH4 as Titan does. However, the other two models suggest that the building materials for Iapetus should contain a large amount of CO2 and only a small amount of CH4 . The visible and infrared spectrometers on the Cassini orbiter will investigate the surface composition of Iapetus. If Cassini observations provide evidence for subsurface CO2 on Iapetus, such as CO2 -rich crater floors found on the surface of Callisto (Hibbitts et al., 2002), it will support the other two models. 5.2. Titan composition The surface property of Titan will also provide important information on whether Titan is formed by CH4 -rich or CO2 -rich satellitesimals. These satellitesimals are thought to form Titan and other icy satellites by collisional accretion. Although the formation of Titan has not been understood completely, theoretical studies suggest that Titan had a thick proto-atmosphere during its accretion stage (e.g., Lunine and Stevenson, 1982; Kuramoto and Matsui, 1994). Kuramoto and Matsui (1994) indicate that when the accretion time is less than 105 years, the surface temperature becomes higher than 500 K and that a thick proto-atmosphere is formed by the evaporation of icy satellitesimals. Recent theoretical study on the evolution of subnebula and on the formation of satellites around gas planets indicates that the accretion time of Titan is about 104 –105 years (Mosqueira and Estrada, 2003). In this study, we assume that Titan had a hot and thick proto-atmosphere during its accretion stage. When the entire subnebula was rich in CO and CO2 but poor in CH4 as the solar nebula was, the CO2 -rich satellitesimals must have formed Titan. If this is the case, a large
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amount of CO2 would have been degassed during the accretion stage of Titan. As the atmospheric temperature decreases with time, CO2 condenses and may form a thick CO2 ice crust over the surface of Titan. Coradini et al. (1989) suggest that Titan may be formed from the satellitesimals not only in the inner region but also the outer region of the subnebula by migration of the satellitesimals. Actually, the mass of Titan is more than 90% of total mass of saturnian satellite system. It may have collected solid matter from the entire subnebula region. Our results suggest that CH4 -rich satellitesimals are formed in the inner region and CO2 -rich satellitesimals are formed in the outer region of subnebula. If CO2 -rich satellitesimals in the outer region were accreted on Titan by migration, CO2 became one of the major components in the proto-atmosphere during accretion stage and then a thick CO2 icy crust was formed on the surface after the accretion stage. However, even if this is the case, CH4 -rich satellitesimals formed in the inner subnebula are important for the origin of CH4 on Titan because they would contain CH4 as much as 10–200 times that in the outer satellitesimals. Then, the surface composition of satellites may be affected by the subsequent processes after the accretion. UV reaction, for instance, may form organic haze and ices in the atmosphere, and they also deposit on the surface (e.g., Sagan and Thompson, 1984; Yung et al., 1984). Impact cratering may excavate the materials buried under the ground to the surface and expose them to the atmosphere. If Huygens probe in Cassini mission observe a thick CO2 icy crust under the organic deposits, it may indicate that the entire or a part of Titan is formed by CO2 -rich satellitesimals. When the circum-saturnian subnebula contained a significant amount of CH4 , Titan could have been formed by CH4 rich satellitesimals. Then, Titan surface should not have a thick CO2 icy crust. If our model of CH4 -rich satellitesimals is correct chemical/geologic evidence that supports CO2 rich subsurface of Iapetus and no thick CO2 layer over the surface of Titan would be observed by spacecrafts, such as Cassini and Huygens.
6. Conclusions The experimental data indicate that the conversion of CO into CH4 via Fischer–Tropsch catalysis is efficient only around 550 K at subnebular condition. At temperatures higher than 600 K, our experimental data indicate that the catalyst suffers from poisoning even at high H2 /CO ratio (i.e., 1000) and that the CH4 formation rate decreases as temperature rises. At temperatures lower than 550 K, catalytic efficiency is lower because of Arrhenius factor (i.e., exp(−E/kT )). At temperatures between 500 and 600 K, both CO and CO2 are converted into CH4 in a short period of time relative to the lifetime of the solar nebula. This result suggests that Fischer–Tropsch catalysis proceeds efficiently
within a narrow region of a subnebula and that an entire subnebula may not necessarily become rich in CH4 . Our calculation based on our experimental results suggests that the chemical compositions of volatile in satellitesimals are considerably different depending on the radial distance from the central gas planet. Outside the catalytically active inner region, we assume that the gas composition of a subnebula will be similar to that of the solar nebula. In such a region, our calculation indicates that CO2 -rich satellitesimals are formed. However, CH4 -rich satellitesimals are formed from the CH4 -rich nebula gas region, where Fischer– Tropsch catalysis proceeds efficiently. The results of the experiments and calculations in this study indicate that observation of the surface composition of Titan and the subsurface composition of Iapetus may provide key information on the chemical evolution of the circum-saturnian subnebula because these compositions are sensitive to the distribution of CH4 , CO, and CO2 in the subnebula. If the high-pressure model for subnebula evolution is correct, the subsurface CO2 on Iapetus should be observed, but thick CO2 ice layer over Titan’s icy crust should not be observed. In contrast, if the efficiency of radial mixing in a subnebula is high (Prinn, 1990), both the subsurface CO2 on Iapetus and thick CO2 ice layer on Titan should not be observed. If the gas pressure in a subnebula is low (Mousis et al., 2002), both the subsurface CO2 on Iapetus and the CO2 ice crust over Titan should be observed. Thus, observation of subsurface (e.g., crater floor) composition of Iapetus and surface composition of Titan by Cassini–Huygens mission should deserved particular attention in the light of subnebular evolution.
Acknowledgments One of the authors (Y.S.) thanks T. Sasaki and Y. Namai for useful discussions on the experiments and K. Kuramoto for fruitful discussions and suggestions. The authors also thank to O. Mousis and an anonymous reviewer for careful reviews and their valuable comments. This research is partially supported by Grant in Aid from Japan Society for the Promotion.
Appendix A The thermodynamic properties and the theory of formation and dissociation of the clathrate hydrate have been developed by van der Waals and Platteeuw (1959) and McKoy and Sinano˘glu (1963). Lunine and Stevenson (1985) applied these theories to solar nebula and circum-planetary subnebula conditions. Their assumptions are adopted in this study and are as follows: (a) The chemical potential of the lattice is independent of the guest occupation; (b) the encaged molecules are localized in the cavities, and a cavity cannot hold more than one guest molecule; (c) guest molecules rotate
Fischer–Tropsch catalysis for the origin of CH4 on Titan
freely within the cage; (d) guest molecules do not interact with each other; (e) classical statistical mechanics are valid. According to van der Waals and Platteeuw (1959), the chemical potential of the water-ice in clathrate hydrate is given by β
µH2 O = µH2 O
ysj + vl ln 1 − ylj , + kT vs ln 1 − j
j
(A.1) β µH2 O ,
where k, T , vs , vl , ysj , and ylj are chemical potential of the empty clathrate hydrate lattice, Boltzmann’s constant, temperature, the number of cage sites per water molecule for the small and large cages of the clathrate hydrate, and the probabilities of finding a molecule of j in the small and large cages, respectively. At the phase boundary (ice + gas = clathrate), the chemical potential of the water-ice in clathrate is required to be equal to that of ordinary ice (µiH2 O ). The probability of finding a molecule of j in the small and large cages is given by yij =
1+
Cij Pj , k Cik Pk
(A.2)
where Cij , and Pj are Langmuir constant and partial pressure of gas molecule j , respectively. Langmuir constants depend on the molecular properties through the spherically averaged potential energy of the guest molecule in clathrate cage. In this paper, we use the equations in McKoy and Sinano˘glu (1963) for the spherically averaged potential energy and fit the results of dissociation pressures of various gas species to existing laboratory data (Lunine and Stevenson, 1985).
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