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Titan's atmospheric composition from Cassini Ultraviolet Imaging Spectrograph data analysis
2010 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

Life and astrobiology related topics form a very wide field of study. One interesting subtopic is the identification of organic compounds in places other than Earth, their interrelation and the conditions they are subject to. One particularly interesting object from this point of view is Titan, Saturn's largest moon and the only one in the Solar System known to have a thick N2/CH4, planet-like atmosphere. This atmosphere is long ago known to be rich in organic compounds of high interest for astrobiology. The NASA-ESA-ASI mission Cassini-Huygens, designed to explore the Saturnian system, gives an excellent opportunity to study Titan. The Ultraviolet Imaging Spectrograph (UVIS) onboard Cassini is a valuable tool that, among other capabilities, allows to study composition, elements distribution and temperatures in Titan's upper atmosphere. Star occultation by Titan's atmosphere is a convenient technique to retrieve altitude profiles of absorbing compounds. Cassini fly-bys provide excellent opportunities for this. The quality of the result is highly dependent on the resolution of the measured transmission spectra. It also depends on the available molecular absorption cross section data for the atmosphere's component species used in the retrieval (Ferradaz et al., 2009). In this work, transmission spectra for different altitudes in Titan's atmosphere are obtained from UVIS data. The data, in the form of spatial spectral cubes, correspond to star occultation events. The data products include those of the December 2004 Titan fly-by (Tb). The transmission spectra are used together with recent experimental measurements of absorption cross sections of the relevant molecules to determine the presence and distribution of carbonaceous compounds in Titan's atmosphere. UVIS and absorption cross section spectral resolution are improved with respect to previous instruments and experimental data respectively (for example Vervack et al. (2004)). The modeled atmosphere used for the retrieval includes CH4, C2H2, HCN, C2H4, C2H6, C4H2, HC3N and C2N2. Some of the results are in agreement with previous works and photochemical models but in some cases the densities presented are a few orders of magnitude bigger than the references. The presented combined results for C2N2 and HC3N was not found in the bibliography consulted. The profiles retrieved from two star occultations are coincident for most of the species and altitudes. The densities retrieved with absorption cross section measured at temperatures consistent with the ones in the upper atmosphere of Titan do not differ significantly from those retrieved using absorption cross sections measured at room temperature. The analysis of the results includes a discussion of the effect of each species in different regions of the transmission spectra. This permits to asses the deficiencies in the procedure employed. The comparison of the results with several available photochemical models will help to improve the comprehension of Titan's atmospheric organic chemistry.

Place, publisher, year, edition, pages
Keyword [en]
Physics Chemistry Maths, Life, astrobiology, Titan, atmospheres, physics, chemistry, organic compounds, UVIS, Cassini, Cassini-Huygens, data, analysis, star occultations, radiative transfer, absorption, cross sections, CH4, C2H2, HCN, C2H4, C2H6, C4H2, HC3N, C2N2, retrieval techniques, number densities, mixing ratios, SPICE, optical depths, spectra, PDS, density profiles
Keyword [sv]
Fysik, Kemi, Matematik
URN: urn:nbn:se:ltu:diva-56171ISRN: LTU-PB-EX--10/058--SELocal ID: cf5ae8bf-9a2f-469a-a0ed-23dc1c52339aOAI: diva2:1029558
Subject / course
Student thesis, at least 30 credits
Educational program
Space Engineering, master's level
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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