Date of Award
Doctor of Philosophy (PhD)
Chemistry & Biochemistry
Margaret A. Tolbert
Jose L. Jimenez
Robert P. Parson
Douglas A. Day
Carol E. Cleland
Planetary atmospheres can be thought of as global-scale reactors capable of synthesizing large, complex molecules from small gases such as methane (CH4), carbon dioxide (CO2), and nitrogen (N2). The atmosphere of Titan, the largest moon of Saturn covered by a thick organic haze, contains trace amounts (2%) of CH4 in an atmosphere of N2 at a surface pressure of 1.5 bar. This is similar to the Earth's Archaean atmosphere, which possibly contained trace amounts of CH4 and CO2 (1,000 ppmv each) in an N2-dominant atmosphere before the rise of biogenic oxygen. Laboratory simulations of the atmospheric chemistry on Titan and the early Earth have shown that these atmospheres are capable of generating biologically-relevant molecules that condense to form particles which can then settle to the surface of the planetary body, possibly providing the molecules required for the emergence of life. The work presented here examines the mechanisms by which FUV photochemistry initiates incorporation of N atoms into Titan aerosol analogs, and C atoms into early Earth aerosol analogs.
Results from the Aerosol Collector and Pyrolyser onboard the Huygens lander reveal the presence of nitrogen in Titan's aerosols. This nitrogen incorporation is thought to occur primarily by extreme-UV photons or energetic electrons. However, recent results from our laboratory indicate a surprising amount of nitrogen incorporation- up to 16% by mass- in Titan aerosol analogs produced by photochemistry initiated by FUV irradiation of CH4/N2 mixtures. The termolecular reaction
CH+N2 +M --> HCN2
has been proposed to account for this observation. Here, we test this hypothesis by using a high- resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) to measure the mass loading and chemical composition of aerosol produced at a range of pressures from roughly 0.1 to 1 atm. We report a 10-fold increase in aerosol mass loading across the range of pressures studied, indicating that the mechanism controlling the total mass produced depends on pressure. We also report an overall increase with pressure in the N/C ratio, which supports the importance of a pressure- dependent mechanism for nitrogen incorporation.
In order to understand carbon incorporation into early Earth aerosols, we devised an analysis technique that allows retrieval of the elemental analysis from unit-mass resolution (UMR) mass spectra of isotopically-labeled data. A quadrupole aerosol mass spectrometer (Q-AMS) was used to obtain UMR data of 13C-labeled and unlabeled aerosol generated by FUV photochemistry of gas mixtures containing 0.1% of either CH4 or 13CH4 in N2. In this method, the differences in the positions of ion groups in the resulting spectra are used to estimate the mass fraction of carbon in the aerosol, and estimation of the remaining elements follows. Analysis of the UMR data yields an elemental composition of 63±7% C, 8±1% H, and 29±7% N by mass. Unlabeled aerosols formed under the same conditions are found by the HR-ToF-AMS to have an elemental composition of 63±3% C, 8±1% H, 20±4% N, and 9±3% O by mass, in good agreement with the UMR method. This favorable comparison verifies the method, which expands the UMR mass spectrometry toolkit.
Chemical mechanisms posited to explain the aerosol-forming chemistry treat CH4 as carbon source in these hazes and treat CO2 as a source of oxygen only. We have generated early Earth aerosol analogs in the laboratory by FUV irradiation of gas mixtures containing isotopically-labeled permutations of 1,000 ppmv unlabeled and 13C-labeled CH4 and CO2 in N2. Products in the particle phase were analyzed by the Q-AMS and the HR-ToF-AMS. Results indicate that CH4 can account for 100% of the total carbon contained in the hazes. These results have implications for the geochemical interpretations of inclusions found in Archaean rocks on Earth, and for the astrobiological potential of other planetary atmospheres.
Hicks, Raea Kay, "Mechanistic Studies of Planetary Haze Formation" (2015). Chemistry & Biochemistry Graduate Theses & Dissertations. 156.