Post-Cassini Investigations of Titan Atmospheric Chemistry

Abstract

The arrival of the Cassini-Huygens mission to the Saturn system ushered in a new era in the study of Titan. Armed with a variety of instruments capable of remote sensing and in situ investigations of Titan's atmosphere and surface, Cassini and Huygens have provided a wealth of new information about Titan and have finally allowed humankind to see its surface. This work focuses on two discoveries made by the Cassini Plasma Spectrometer (CAPS): the detection of oxygen ions (O<super>+</super>) precipitating into Titan's atmosphere (Hartle et al., 2006) and the discovery of very large positive (Waite et al., 2007; Crary et al., 2009) and negative ions (Coates et al., 2007, 2009) present in Titan's thermosphere.Through the use of a photochemical model, I demonstrate that the observed densities of CO, CO₂ and H₂O can be explained by a combination of O and OH or H₂O input to the upper atmosphere. Given the detection of O<super>+</super> precipitation into Titan's upper atmosphere, it is no longer necessary to invoke outgassing from Titan's interior as a source for atmospheric CO or to assume that the observed CO is the remnant of a larger primordial abundance in Titan's atmosphere. Instead, it is most likely that the oxygen bearing species in Titan's atmosphere are the result of external input, most likely from Enceladus.I have also used very high resolution mass spectrometry to investigate the com- position of Titan aerosol analogues, or "tholins". Although there are an enormous number of molecules present in tholin samples, they exhibit numerous patterns, in- cluding very regular spectral spacing. These patterns may help constrain the com- position of the very large ions observed in the CAPS spectra, since the resolution of the instrument makes identification of the molecules impossible. Additionally, tholins produced with CO possess molecules of prebiotic interest, including all 5 nucleotide bases and the 2 smallest amino acids (glycine and alanine). This indicates that chemistry occurring in Titan's upper atmosphere may be capable of forming incredibly complex organic molecules, which may have implications for the origin of life on Earth and elsewhere in the universe.