Eric Hezekiah Wilson

Current State of Modeling the Photochemistry of Titan's Mutually Dependent Atmosphere and Ionosphere

[1] In the context of recent observations, microphysical models, and laboratory data, a photochemical model of Titan’s atmosphere, including updated chemistry focusing on rate coefficients and cross sections measured under appropriate conditions, has been developed to increase understanding of these processes and improve upon previous Titan photochemical models. The model employs a two-stream discrete ordinates method to characterize the transfer of solar radiation, and the effects of electron-impact, cosmic-ray deposition, and aerosol opacities from fractal and Mie particles are analyzed. Sensitivity studies demonstrate that an eddy diffusion profile with a homopause level of 850 km and a methane stratospheric mole fraction of 2.2% provides the best fit of stratospheric and upper atmosphere observations and an improved fit over previous Titan photochemical models. Lack of fits for C3H8 ,H C3N, and possibly C2H3CN can be resolved with adjustments in aerosol opacity. The model presents a benzene profile consistent with its detection in Titan’s stratosphere [Coustenis et al., 2003], which may play an important role in the formation of Titan hazes. An electron peak concentration of 4200 cm � 3 is

Titan’s Carbon Budget and the Case of the Missing Ethane†

The retrieval of data from the Cassini-Huygens mission has revealed much about Titans atmospheric-surface system and has precipitated more questions. One of these questions involves the lack of large reservoirs of ethane that were predicted by a variety of studies prior to the arrival of the Cassini-Huygens spacecraft. Using an updated and comprehensive photochemical model, we examine the nature of Titans carbon budget, initiated by the destruction of methane, and the role that ethane condensation plays in this budget. Model results show that 40% of methane destruction results in ethane formation, with a net production rate of 2.7 x 10(9) molecules cm(-2) s(-1), due primarily to acetylenic catalysis in Titans stratosphere. This corresponds to a liquid ethane layer of several hundred meters over geologic time. However, episodic methane outgassing, subsurface sequestration, and chemical processing of Titans surface are likely responsible for the limiting of ethane condensate on Titans surface to less than 10 m globally averaged.

Sensitivity studies of methane photolysis and its impact on hydrocarbon chemistry in the atmosphere of Titan

The photodissociation of methane at Lyman α (1216 A) has been the focus of much scrutiny over the past few years. Methane photolysis leads to the formation of H2 molecules as well as H, CH, 1CH2, 3CH2, and CH3 radicals, which promote the propagation of hydrocarbon chemistry. However, recent studies [Mordaunt et al., 1993; Romani, 1996; Smith and Raulin, 1999] have not fully resolved the issue of methane photolytic product yields at this wavelength. We use a one-dimensional photochemical model with updated chemistry to investigate the significance of these quantum yield schemes on the hydrocarbon chemistry of Titans atmosphere, where Lyman α radiation accounts for 75% of methane photolysis longward of 1000 A. Sensitivity studies show that while simple hydrocarbons like C2H2 (acetylene) and C2H4 (ethylene), which serve as important intermediates to the formation of more complex hydrocarbons, show virtually no variation in abundance, minor C3 molecules do show substantial sensitivity to choice of quantum yield scheme. We find that the C3H4 isomers (methylacetylene, allene) and C3H6 (propylene) display major variation in atmospheric mixing ratios under the implementation of these schemes, with a maximum variation of approximately a factor of 5 in C3H4 abundance and approximately a factor of 4 for C3H6. In these cases our nominal scheme, recommended by Romani [1996], offers an intermediate result in comparison with the other schemes. We also find that choice of pathway for non-Lyman α methane absorption does affect hydrocarbon chemistry in the atmosphere of Titan, but this effect is minimal. A 65% variation in C2H6 (ethane) abundance, a value within observational uncertainty, is the largest divergence found for a wide range of possible non-Lyman α photofragment quantum yields. These results will have significance in future modeling and interpretation of observations of the atmosphere of Titan.

Perchlorate Formation on Mars Through Surface Radiolysis-Initiated Atmospheric Chemistry: A Potential Mechanism

Abstract Recent observations of the Martian surface by the Phoenix lander and the Sample Analysis at Mars indicate the presence of perchlorate (ClO4 –). The abundance and isotopic composition of these perchlorates suggest that the mechanisms responsible for their formation in the Martian environment may be unique in our solar system. With this in mind, we propose a potential mechanism for the production of Martian perchlorate: the radiolysis of the Martian surface by galactic cosmic rays, followed by the sublimation of chlorine oxides into the atmosphere and their subsequent synthesis to form perchloric acid (HClO4) in the atmosphere, and the surface deposition and subsequent mineralization of HClO4 in the regolith to form surface perchlorates. To evaluate the viability of this mechanism, we employ a one‐dimensional chemical model, examining chlorine chemistry in the context of Martian atmospheric chemistry. Considering the chlorine oxide, OClO, we find that an OClO flux as low as 3.2 × 107 molecules cm–2 s–1 sublimated into the atmosphere from the surface could produce sufficient HClO4 to explain the perchlorate concentration on Mars, assuming an accumulation depth of 30 cm and integrated over the Amazonian period. Radiolysis provides an efficient pathway for the oxidation of chlorine, bypassing the efficient Cl/HCl recycling mechanism that characterizes HClO4 formation mechanisms proposed for the Earth but not Mars.