George.Keighton

George Keighton CHEM 367: Chemical Information Retrieval Dr. Jean-Claude Bradley Fall 2013

__**Chemical Processes of Saturn’s Moon Titan**__

Saturn’s largest moon, Titan, is one of the most unique moons in the solar system. Its thick, hazy atmosphere has intrigued scientists and a wealth of information has been gathered from spacecraft sent to the moon. The chemistry of its surface has also been of great interest. This paper will discuss what has been learned about the environment of Titan and the implications on our understanding of what early Earth may have been like.
 * Introduction:**

According to Mousis et al, deuterium-to-hydrogen ratios offer important clues to planetary history. Titan has a particularly high D/H ratio of 1.32x10^-4. Previous attempts to explain this have been unsatisfactory. The traditional model for methanogenesis on Titan is serpentinization, driven by the hydration and metamorphosis of peridotite. In this process, hydrogen gas produced reacts with carbon grains or carbon dioxide to form methane. The values produced using this model doesn’t fully account for the observed D/H ratio. The authors believe direct capture of methane during the moon’s formation may be more plausible. Analysis of the vapor plumes from Enceladus have indicated a D/H ratio of 2.9x10^-4, similar to the observed value in comets. This suggests that material Enceladus accreted early on most likely originated in the outer solar nebula. Likewise, if Titan had accreted planetesimal matter in the same way, one would expect to see a similar D/H ratio in the water there. Over time, isotopic gas exchange may have occurred between infalling methane and hydrogen, eventually reducing the deuterium fraction of the methane. The D/H value then would have stabilized as the methane became trapped in ice crystals that would contribute to Titan’s formation [Mousis 2009]. Glein et al performed similar research on CH4, but also investigated the origin of Titan’s atmospheric nitrogen. If oxidized hydrothermal systems are present underneath Titan’s surface, ammonia could be oxidized to produce molecular nitrogen. If a similar hydrothermal system producing CO2 were present, this would strongly imply the existence of endogenic N2. Observations of Enceladus have shown a presence of N2 in outgassings. Further support comes from laboratory modeling which suggests that interstellar NH3 ice has a high fraction of the N15 isotope. Therefore, the high N15/N14 ratio of Titan’s atmosphere could be a result of the hydrothermal oxidation of NH3. Hydrothermal modeling turned out to be consistent with the concentration of N2 in Titan’s atmosphere. The estimates indicated that only 2% of the primordial NH3 would have had to be converted to N2. An alternative hypothesis is that NH3 was converted to N2 through photochemistry. More observations of plumes on Enceladus need to be performed. This would be greatly helpful due to the fact that Enceladus lacks an atmosphere- observations of isotopic composition would depend wholly on geochemical processes [Glein 2009].
 * Titan’s present atmosphere and its origin:**

//Figure 1: Schematic of tholin formation in Titan's atmosphere. [Wikipedia]//
 * Organic compounds in Titan’s atmosphere:**

In the late 1970s, Carl Sagan and Bishun Khare coined a new term, “tholins”, to categorize the unusual brown, sticky residue generated by the interaction of cosmically abundant gases with UV light or spark discharges. Tholins are believed to be a major constituent of interstellar grains, and experimental data suggests they play an important role in Titan’s upper atmosphere. Simple organic compounds were generated when a matrix of HCHO, H2O, NH3 and C2H6 were irradiated with near-UV light at a temperature of 77K. Based on the Frank-Rabinowitch principle, in-situ organic synthesis on ice grains would most commonly produce polycyclic aromatics, complexly branched aliphatics, and matrices of fixed rings and aliphatics. Observations of galactic IR sources showed evidence of long-chain alkanes and alkenes. This corresponds with the results seen in early tholin experiments. Organic molecules seen in microwave line spectroscopy most likely originated from the spallation, spluttering or photodissociation of tholins. Sagan and Khare studied data on carbonaceous chondrite meteorites and found evidence that tholins were present in the Mokoia meteorite, but obtained inconclusive results for other samples. They propose sending spacecraft to perform in situ studies of comets, since nitriles and aldehydes have been observed in cometary spectra and could have been produced by tholins. [Sagan 1979]. A scheme representing tholin formation via interaction with UV light is represented in Figure 1. Simulations of chemical reactions in Titan’s stratosphere were carried out by Somogyi et al. The generated tholins were analyzed using an array of analytical methods. Overall, the FT-ICR results confirmed that tholin compounds generally follow a CxHyNz molecular formula. As molecular weights increase, so does the number of nitrogens, as well as the degree of unsaturation [Somogyi 2005].

//Figure 2: Titan's Methane cycle. [Raulin]// Creating an atmospheric model for Earth before the onset of oxygen has been a challenge. If Earth’s early atmosphere had only consisted of N2 and CO2, one must question how biologically significant organic molecules came about. There must have also been other greenhouse gases present to account for the immature sun’s weaker output, since geologic evidence indicates CO2 levels alone could not have been high enough to sustain liquid water. A plausible solution is that the atmosphere contained a significant amount of methane, which could have given rise to similar photochemistry as what is currently found on Titan, albeit with some differences due to the presence of CO2 [Trainer 2006]. Trainer et al. conducted analysis of CH4/CO2/N2 photolysis using an aerosol mass spectrometer (AMS), scanning mobility particle sizer (SMPS) and a transmission electron microscope (TEM). To simulate Titan’s atmosphere, photolysis was performed with a UV lamp on varying concentrations of CH4 in N2, although changing the CH4 concentration made little difference in the chemical composition of aerosols. However, it was noted that CH4 concentration did have an effect on the amount of aerosols produced, up to 0.02% CH4. It is believed that at higher CH4 concentrations, excess CH4 inhibits the mechanisms that produce aerosols from the intermediates. Mass spectra results showed long, stable hydrocarbon chains and peaks indicative of benzene and toluene fragment ions. Data from the AMS and SMPS indicated an average particle size of 50nm diamater, in agreement with TEM imaging. Based on the observation of small, spherical particles, it is believed that the aerosols generated in the laboratory were spherical monomers, rather than fractal aggregates [Trainer 2006]. Simulations of early Earth were carried out by holding CH4 concentration constant while varying the concentration of CO2. Results showed that as CO2 increased, fewer aromatic structures were formed. The O radicals produced by photodissociation of CO2 may create oxygenated functional groups in hydrocarbon reactions. Mass spectra results support this notion- oxygenated organic fragments such as CH2O+ and COO+ were observed when the highest amount of CO2 (0.5%) was mixed. Similar results were obtained when the experiment was performed in N2, using the same proportions of CH4 to CO2, indicating that the absolute concentrations play less of a role than the C/O ratio. Trainer et al. go on to say that there is an optimum C/O ratio- if CO2 levels become too high, O can inhibit long-chain growth and hinder aerosol formation. As with the Titan simulations, particles were determined to be spherical with an average diameter of about 50nm [Trainer 2006]. Trainer’s simulations have significant implications on our understanding of early Earth. The degree of light scattering and UV shielding caused by the atmospheric haze would have been strongly dependent on particle size. The 50nm diameter observed in the laboratory models would create an atmosphere thick in the UV, but thin in visible light. The ability of spherical particles to form fractal aggregates would also have an effect on UV shielding. The data also suggests that the presence of CO2 in equal concentration to CH4 results in about 1.5 times more aerosol generation than CH4 alone. Other theories regarding the introduction of organic molecules have primarily focused on hydrothermal vents and extraterrestrial origins. Based on the haze calculations, the atmosphere of early Earth may have been capable of generating more organic material than either of the aforementioned mechanisms [Trainer 2006].
 * Titan’s Methane Cycle:**
 * Implications for early Earth:**

The similarity between Titan’s present-day atmosphere and the early Earth atmosphere leads one to ask whether Titan itself might be capable of supporting life one day. This possibility was explored in a paper by Lorenz et al. As the sun evolves into a red giant star, bodies in the solar system will experience significant heating. While Earth will no longer be capable of supporting life, the currently icy bodies of the outer solar system may reach temperatures where they may become viable. Titan’s surface temperature is presently about 94K, which takes into account a 21K greenhouse effect and a 9K antigreenhouse effect. Using a model that includes an IR-opaque, sunlight transparent troposphere and an IR-transparent, sunlight-absorbing stratosphere, Lorenz and his colleagues found a 90K increase in surface temperature for a tenfold increase in luminosity when haze levels were held constant. If the haze absorption optical depth was increased in proportion to temperature, the response to an increase in insolation was much less noticeable. In other words, increasing insolation will initially not have much effect on the temperature of Titan, because the atmosphere will expand, reducing the percentage of sunlight that penetrates through to the lower atmosphere. However, under a red giant sun, the UV flux will drop dramatically. Since aerosols in the atmosphere are driven primarily by UV photolysis, it is expected that this will lead to a substantial drop in haze production. With more visible light reaching the surface, Titan should become much warmer. The drop in UV output will be by far the most significant factor in determining Titan’s future surface temperature [Lorenz 1997]. The combination of decreased haze levels and abundant greenhouse gases like methane may cause Titan to warm enough to allow aqueous liquid to exist at the surface. Water-ammonia mixtures can exist in the liquid phase down to 176K. Such a solvent would be excellent for developing the necessary prebiotic and protobiotic molecules for life to develop. Appropriate conditions would be expected to exist on Titan for approximately 500 million years. This is longer than the amount of time it took for terrestrial life to develop, so it is plausible that Titan could become a habitable world in this timeframe [Lorenz 1997].
 * Future Titan:**

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