Analysis of Titan's neutral upper atmosphere from Cassini Ion Neutral Mass Spectrometer measurements
Résumé
In this paper we present an in-depth study of the distributions of various neutral species in Titan's upper atmosphere, between 950 and 1500 km for abundant species (N2, CH4, H2) and between 950 and 1200 km for other minor species. Our analysis is based on a large sample of Cassini/INMS (Ion Neutral Mass Spectrometer) measurements in the CSN (Closed Source Neutral) mode, obtained during 15 close flybys of Titan. To untangle the overlapping cracking patterns, we adopt Singular Value Decomposition (SVD) to determine simultaneously the densities of different species. Except for N2, CH4, H2 and 40Ar (as well as their isotopes), all species present density enhancements measured during the outbound legs. This can be interpreted as a result of wall effects, which could be either adsorption/desorption of these molecules or heterogeneous surface chemistry of the associated radicals on the chamber walls. In this paper, we provide both direct inbound measurements assuming ram pressure enhancement only and abundances corrected for wall adsorption/desorption based on a simple model to reproduce the observed time behavior. Among all minor species of photochemical interest, we have firm detections of C2H2, C2H4, C2H6, CH3C2H, C4H2, C6H6, CH3CN, HC3N, C2N2 and NH3 in Titan's upper atmosphere. Upper limits are given for other minor species. The globally averaged distributions of N2, CH4 and H2 are each modeled with the diffusion approximation. The N2 profile suggests an average thermospheric temperature of 151 K. The CH4 and H2 profiles constrain their fluxes to be View the MathML source and View the MathML source, referred to Titan's surface. Both fluxes are significantly higher than the Jeans escape values. The INMS data also suggest horizontal/diurnal variations of temperature and neutral gas distribution in Titan's thermosphere. The equatorial region, the ramside, as well as the nightside hemisphere of Titan appear to be warmer and present some evidence for the depletion of light species such as CH4. Meridional variations of some heavy species are also observed, with a trend of depletion toward the north pole. Though some of the above variations might be interpreted by either the solar-driven models or auroral-driven models, a physical scenario that reconciles all the observed horizontal/diurnal variations in a consistent way is still missing. With a careful evaluation of the effect of restricted sampling, some of the features shown in the INMS data are more likely to be observational biases.