large-scale global transport of water, as it affects the total amount of water in the upper branch of the
Hadley cell. The main source of the discrepancy is the ability (or inability) to resolve large terrain
features such as Olympus Mons, the other Tharsis Montes, and Elysium Mons. These mountains
greatly affect the mean circulation by introducing mountain waves and transporting copious amounts of
water into the upper troposphere. We conclude that the mean circulation defined by the conventional
Hadley cell transport is not sufficient to model all of the vertical transport of water, and that mountain-
induced waves, which only manifest as strong influences at higher resolutions, are just as important, if
not more. While there was a noticeable increase in the height of the water in the transition to 2°x3°, the
model results still do not match the observations. Additional testing is required to determine if the
cloud placement will improve further at even higher resolutions, or if other parameterizations are
necessary.
Investigating Titan’s Atmospheric Chemistry with the Titan Haze Simulation Experiment Ella Sciamma-O’Brien*1,2 and Farid Salama1 (1NASA ARC, Moffett Field, CA, 2BAERI, Petaluma, CA)
In Titan’s atmosphere, a complex organic chemistry induced by UV radiation and electron bombardment occurs between the main constituents, N2 and CH4, and leads to the production of larger
molecules and solid aerosols. Because the reactive carbon and nitrogen species present in Titan’s aerosols could meet the functionality requirements for precursors to prebiotics, the study of Titan’s aerosol has become a topic of extensive research in the fields of astrobiology and astrochemistry in recent years. Since 2004, the Cassini-Huygens mission has shed light over Titan’s atmospheric chemistry only to uncover a more complex system than previously suspected, in particular with the detection of benzene and toluene, known precursors of polycyclic aromatic hydrocarbons (PAHs)[1], and an unidentified spectral emission at
3.28 µm that could be explained by the presence of aromatic molecules such as PAHs[2].
Here we will present the Titan Haze Simulation (THS) experiment, which was developed at the NASA Ames COSmIC facility to investigate the formation process of large hydrocarbon aerosols, by studying the chemical pathways that link the simple precursor molecules resulting from the first steps of the N2-CH4
chemistry (C2H2, C2H4, HCN…) to benzene, and to polycyclic aromatic hydrocarbons (PAHs) and nitrogen
containing PAHs (or PANHs) as precursors to solid aerosols. In the COSmIC/THS, the chemistry is simulated by plasma in the stream of a supersonic expansion. With this unique design, the gas is jet-cooled to Titan-like temperature (~150K) before inducing the chemistry by plasma[3], and remains at low temperature in the plasma discharge (~200K). The residence time of the gas in the pulsed plasma discharge is on the order of 3 µs, resulting in a truncated chemistry that allows us to probe the first and intermediate steps of the chemistry, and hence monitor the evolution of the chemical growth, by injecting different N2-
CH4-based gas mixtures in the plasma, with or without the addition of heavier precursors present as trace
elements on Titan (C2H2, C2H4, C2H6, C6H6…).
We will discuss the results of two complementary studies of the gas phase[4] and solid phase[5] products that have been performed in 4 different gas mixtures: N2-CH4, N2-CH4-C2H2, N2-CH4-C6H6 and N2-CH4-
C2H2-C6H6 using a combination of in situ and ex situ diagnostics. The mass spectrometry analysis of the
gas phase was the first to demonstrate that the THS is a unique tool to monitor the first and intermediate steps of Titan’s atmospheric chemistry at Titan-like temperature, as well as investigate specific chemical pathways. In particular, the mass spectra obtained in a N2-CH4-C2H2-C6H6 mixture showed promising
results[4] when compared to observational data from the Cassini Plasma Spectrometer – Ion Beam
Spectrometer (CAPS-IBS). The solid phase is in the form of grains and aggregates that are formed in volume in the gas expansion, and can be deposited on different substrates. Differences in the morphology of these grains and aggregates, depending on the initial mixture, have been observed by scanning electron microscopy, and could be representative of potential differences in growth processes depending on the composition of the atmosphere at different altitudes on Titan. Mixtures containing acetylene (C2H2) appear
to produce more spherical grains, and the aggregates produced in mixtures containing benzene (C6H6)
appear to be submitted to additional growth after grain aggregation. X-ray Absorption Near Edge Structure measurements of the solid phase show the presence of nitrogen chemistry (nitrile and imine have been detected) and differences in the level of nitrogen incorporation and how it is integrated in the solid phase depending on the initial gas mixture. Mid-infrared spectroscopic analyses highlight a change in the nitrogen chemistry and an increase in aromatic abundance when benzene is present. A comparison of THS mid-IR spectra to Cassini Visible Infrared Mapping Spectrometer (VIMS) data[5] has shown that the THS aerosols
produced in simpler mixtures (without benzene) i.e., samples that contain more nitrogen and incorporate this nitrogen in isocyanide-type molecules instead of nitriles, are more representative of Titan’s aerosols.
References:
[1]Waite, J.H. Jr. et al. Science 316, 870-875 (2007).
[2] Dinelli, B.M. et al. Geophys. Res. Letters 40, 1489-1493 (2011). [3] Biennier, L. et al. Chem. Phys. 326, 445-457 (2006).
[4] Sciamma-O’Brien, E. et al. Icarus 243, 325 (2014). [5] Sciamma-O’Brien, E. et al., Icarus (under review).
Acknowledgements: The authors acknowledge the support of NASA SMD. The SEM images were obtained at the
UCSC MACS Facility at NASA Ames Research Center (NASA grant NNX09AQ44A to UCSC). The authors acknowledge the outstanding technical support of R. Walker and E. Quigley.