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23 April 2024 |
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Retrieval of Chemical Abundances in Titan's Upper Atmosphere from Cassini UVIS Observations with Pointing Motion | | Siteng Fan
; Donald E. Shemansky
; Cheng Li
; Peter Gao
; Linfeng Wan
; Yuk L. Yung
; | | Date: |
5 Jun 2019 | | Abstract: | Cassini/UVIS FUV observations of stellar occultations at Titan are well
suited for probing its atmospheric composition and structure. However, due to
instrument pointing motion, only five out of tens of observations have been
analyzed. We present an innovative retrieval method that corrects for the
effect of pointing motion by forward modeling the Cassini/UVIS instrument
response function with the pointing motion value obtained from the SPICE
C-kernel along the spectral dimension. To illustrate the methodology, an
occultation observation made during flyby T52 is analyzed, when the Cassini
spacecraft had insufficient attitude control. A high-resolution stellar model
and an instrument response simulator that includes the position of the point
source on the detector are used for the analysis of the pointing motion. The
Markov Chain Monte-Carlo method is used to retrieve the line-of-sight abundance
profiles of eleven species (CH4, C2H2, C2H4, C2H6, C4H2, C6H6, HCN, C2N2, HC3N,
C6N2 and haze particles) in the spectral vector fitting process. We obtain
tight constraints on all of the species aside from C2H6, C2N2 and C6N2, for
which we only retrieved upper limits. This is the first time that the T52
occultation was used to derive abundances of major hydrocarbon and nitrile
species in Titan’s upper and middle atmosphere, as pointing motion prohibited
prior analysis. With this new method, nearly all of the occultations obtained
over the entire Cassini mission could yield reliable profiles of atmospheric
composition, allowing exploration of Titan’s upper atmosphere over seasons,
latitudes, and longitudes. | | Source: | arXiv, 1906.2230 | | Services: | Forum | Review | PDF | Favorites | |
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