Neutral species densities in Titan's upper atmosphere from Bayesian inversion of CassiniINMS ion mass spectra

Details Neutral species densities in Titan's upper atmosphere from Bayesian inversion of CassiniINMS ion mass spectra Neutral species densities in Titan's upper atmosphere from Bayesian inversion of CassiniINMS ion mass spectra

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..213p&link_type=abstract

European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a

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We optimized densities of neutral species in Titan's ionosphere by fitting INMS ion mass spectra with an equilibrium ionospheric model [1]. This inversion is challenging because of the numerous sources of uncertainty (data, chemical model structure, chemical parameters). Concerning the uncertainty on the mass spectra, we considered counting statistics and calibration [2]. We acknowledged the following pitfalls in model uncertainties: - model incompleteness as a source of prediction bias: the chemical model is strikingly sparse for the reactions involving species with masses above m/z=50 amu [3]. Consequently, these mass peaks were not considered in our inversion; - parametric uncertainties: the rate constants and branching ratios are taken from measurements at 300 K, which adds an extrapolation component to their measurement uncertainty [1]; - uncertainty representation: correlations between parameters have to be taken into account in the optimization process. We showed that significant errors can result from neglecting them [1]. Bayesian inversion methods, enabling a consistent treatment of all these sources of uncertainty, were implemented and benchmarked. The recovered neutral mole fractions at 1100 km are shown in Fig.1, and compared with those manually optimized by Vuitton et al. [4], and those predicted by a photochemical model [5]. Similarities and discrepancies will be further discussed later. For the present abstract, we want to emphasize the following points: - in the present status of ionospheric chemistry modelling, ion mass spectrum inversion is not systematically more precise than direct photochemical modelling; - our systematic exploration of parameter space enables to characterize the precision of recovered neutral mole fractions, reducing the influence of a priori hypotheses.

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