Sulfur Isotopic Ratios at 67P/Churyumov-Gerasimenko and Characterization of ROSINA-DFMS FM & FS

Calmonte, Ursina Maria (2015). Sulfur Isotopic Ratios at 67P/Churyumov-Gerasimenko and Characterization of ROSINA-DFMS FM & FS. (Dissertation, Universität Bern, Philosophisch-naturwissenschaftliche Fakultät, Physikalisches Institut)

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Comets are thought to be the most pristine bodies present in the Solar System. In
consequence of spending the majority of their existence beyond 30 AU, their composition
can give insights on the physical and chemical conditions during their formation. Since
August 2014 the European Space Agency spacecraft Rosetta accompanies the Jupiter
family comet 67P/Churyumov-Gerasimenko on its way to perihelion and beyond. In this
study the isotope fractionation of 34S are reported in H2S, OCS, SO2, S2, and CS2 at 67P.
In addition for the first time the isotope fractionation for 33S is presented for cometary
volatiles. The ratio 32S/33S is given for H2S, SO2 and a tentative value is given for CS2.
With a mean value of -50 ± 22‰ and -306 ± 31‰ for δ34S and δ33S respectively, H2S
shows a significant depletion in both 34S and 33S. For SO2 the depletion is less distinct with
δ34S and δ33S being -67 ± 40‰ and -130 ± 53‰, respectively. The strongest depletion is
present for CS2 with -114 ± 21‰and -276 ± 55‰, respectively. For OCS and S2 only δ34S
could be determined which is -252 ± 77‰ and -357 ± 145‰, respectively. A comparison
with sulfur isotopic ratios measured in SiC grains revealed that both SiC grains and the
five volatile species have similar sulfur isotopic ratios. However, it is beyond the scope
of this work to investigate the possibility of a link between SiC grains and cometary ices.
Nevertheless, mass-dependent or mass-independent fractionation due to photo dissociation
can be ruled out as sole cause of the seen depletion of 33S and 34S. Furthermore, an upper
limit of (9.64 ± 0.19)·10.4 for D/H in HDS has been determined. This value is about a
factor two higher than D/H in H2O for the same comet reported by (Altwegg et al., 2015).
Besides the investigation concerning isotopic ratios of sulfur bearing species in this work
the calibration and characterization of ROSINA/DFMS has been continued. Here it is
reported about the deviation of the mass scale for MCP/LEDA low resolution spectra and
the calibration measurements performed in the laboratory. Furthermore the outcome of the
attempt to describe the sensitivity of DFMS with an empirical function will be discussed.
The last part of the characterization of DFMS is dedicated to determine the so-called
individual pixel gain for the laboratory and the flight model. Moreover, correlation between
the depletion’s manifestation of the MCP with respect to the applied voltages has been
investigated for both models. It has been found that further measurements are needed
to understand the manifestation of depletion at the laboratory model. For the model on
board of Rosetta it could be shown that most of the present feature are due to the usage
of the MCP and suggestions have been made in order to answer the remaining question
considering the depletion of the MCP.

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