The Introduction and Appendix i-A outline briefly the history of Titan exploration since its discovery by Christiaan Huygens in 1675 through the recent International Mission of Cassini- Huygens. It discusses the roles of some of the ten most abundant elements in the Universe (H, He, C, N, O, Ne, Mg, Si, S, and Fe) in the formation of methane (CH4), water (H2O), and carbon dioxide (CO2). As far as the element abundances are concerned, all of carbon could have combined with hydrogen to make CH4. Alternatively, all of oxygen could have combined with C, Si, S, Fe, and H, making silicates, oxides, CO2 and H2O. The relatively high freezing temperatures of H2O (273 K) and CO2 (220 K) make them less suitable as atmospheric components on the outlying planets and satellites, where CH4 (91 K) and N2 (77 K) are more likely to exist as gases. The Introduction also discusses several possible mechanisms of cooling of Titan and its silicate core after accretion, and the uncertainties in the estimates of the possible radioactive heat generation in Titan’s interior. Chapter 1: This chapter discusses two possible pathways of loss of the two main gases from Titan’s post-accretional atmosphere, methane (CH4) and ammonia (NH3), by the mechanisms of thermal escape and emission from the interior coupled with thermal escape. An accretion temperature of 300 to 355 K is calculated, and an atmospheric composition of 19.6 bar CH4 and 5.8 bar NH3, which declines to its present-day levels of 0.1 bar CH4 and 1.4 bar N2 (or equivalent 1.7 bar NH3, as a precursor of N2). In the first 0.5 – 0.6 Myr after accretion, Titan’s surface cools to 150 K and it takes about 5 Myr to cool to near its present temperature of 94 K. Using an accretion temperature of 355 K, emission of CH4 and NH3 from the interior in combination with thermal escape is needed to produce near-steady-state CH4 and NH3 atmospheric masses, as they are at the present. At the lower accretion temperature of 300 K, thermal escape of gases alone allows their atmospheric masses to decrease from the primordial to the present day levels in 50,000 – 70,000 years. Chapter 2: In this chapter, a simple photolysis model is created, where the second most abundant component of the present-day Titan atmosphere, methane (CH4), can either escape the atmosphere or undergo photolytic conversion to ethane (C2H6). Using this model, up to 8.46×1017 kg or 1.37×106 km3 of liquid ethane might have been produced since Titan’s accretion. This amount is 104 times larger than the present-day atmospheric ethane mass of 9.24×1013 kg, suggesting that most of the remaining ethane resides in liquid form on or within Titan. The estimate for the amount of liquid ethane storage potential on Titan’s surface is 50,000 km3 in lakes and seas and an additional 61,000 km3 in craters. As these are much smaller than the total volume of liquid ethane produced in the course of Titan’s history, the excess may be stored in the subsurface of the crust, made primarily of water ice. The minimum porosity of the crust needed to accommodate all the liquid ethane would be only 0.9% of the uppermost 2 km of the crust. Chapter 3: This chapter examines different fluvial features on Titan, identified by the Cassini spacecraft, and evaluates the possibilities of channel formation by two mechanisms: dissolution of ice by a concentrated solution of ammonium sulfate, and by mechanical erosion by flow of liquid ammonia and liquid ethane. It concludes that chemical erosion of Titan’s channels could be completed in 280 to 1100 years, much shorter than the period of about 84,000 years that a concentrated (NH4)2SO4-H2O solution could exist as a liquid on the Titan surface. Mechanical erosion of Titan’s channels is generally a much slower process, on the order of 102 to 105 years. The erosional sequence of the channels may have starter after the formation of water-ice on the surface by the process of chemical dissolution by (NH4)2SO4-H2O, overlapping, or followed by, a period of mechanical erosion by liquid NH3. A final stage on the cooling surface might have been characterized by liquid C2H6 as an agent of mechanical erosion. Chapter 4: Three chemical reactions can represent, as a shorthand summarizing many intermediate processes, the transformation of methane to ethane and other hydrocarbons in Titan’s atmosphere: CH4 (1st order, k12 yr-1) → CH3 (2nd order, k23 cm3 molecule-1 yr-1) → C2H6 (1st order, k3 yr-1) → Other products. This chapter presents: (1) new explicit mathematical solutions of mixed 1st and 2nd order chemical reactions, represented by ordinary differential first-degree and Riccati equations; (2) the computed present-day concentrations of the three gases in Titan’s scale atmosphere, treated as at near-steady state; and (3) an analysis of the reported and computed atmospheric concentrations of CH4, CH3, and C2H6 on Titan, based on the reaction rate parameters of the species, the rate parameters taken as constants representative of their mean values. Chapter 5: This chapter examines the possible reactions of methane formation in terms of the thermodynamic relationships of the reactions that include pure carbon as graphite, the gases H2, CO2, H2O, and serpentinization and magnetite formation from olivine fayalite. The reactions are analyzed for the conditions on Titan and the Terrestrial Planets Mars, Earth, Venus, and Mercury, at the range of their temperatures, atmospheric pressures, and composition. The partial pressures of CH4, calculated from the composition of planetary atmospheres, are compared to the reported values of CH4 on some of the planets. The equilibrium pCH4 values depend on the nature of the reactants and other products, as represented by the five solid-gas and gas-gas CH4-forming reactions. On present-day and primordial-Titan, and on Mercury, methane could have escaped by the Maxwell-Boltzmann mechanism. On Mars, Venus, and on primordial Earth, the high values of escape velocity would have assured retention of methane in the planetary atmosphere, if it formed

Last modified

• 02/19/2018

Creator

• Ashley Middle Gilliam

DOI

• https://doi.org/10.21985/N23X2H

Subject

• Earth and Planetary Sciences

Keyword

• evolution
• Hydrocarbon cycle
• chemistry
• Photochemistry
• Liquid methane and ethane
• Titan
• Atmospheres

Date created

• 2016-01-01

Resource type

• Dissertation

Rights statement

• In Copyright