Sulphur isotope mass-independent fractionation observed in comet 67P/Churyumov–Gerasimenko by Rosetta/ROSINA

Abstract

The knowledge about sulphur isotopic fractionation in volatile cometary species is limited as only measurements in five comets have been done and only for 34 S/ 32 S. The lack of information about the fractionation of 33 S/ 32 S makes it impossible to compare them with what is known from refractories. We present results of 34 S/ 32 S and for the first time 33 S/ 32 S isotopic ratio in H 2 S, OCS, and CS 2 in the coma of comet 67P/Churyumov-Gerasimenko. Observations used for this study were performed with Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Double Focusing Mass Spectrometer during 2014 October and 2016 May. Bulk isotopic 33 S/ 32 S and 34 S/ 32 S ratios derived from these three species yield δ 33 S = (−302 ± 29) and δ 34 S = (−41 ± 17) , respectively. The observed isotopic fractionation is significantly different from the assumed Solar system standard [Vienna-Canyon Diablo Troilite (V-CDT)] and all other reported values for Solar system objects, except other comets. Furthermore, we show that neither mass-dependent nor mass-independent fractionation due to photodissociation as it has been observed in recent laboratory studies can be the cause of the significant depletion compared to Solar system standard. In addition, we conclude that there seems to be an intrinsic difference in sulphur isotopic fractionation in cometary volatiles and refractories while the difference between molecules is most likely due to different chemical pathways. The significant fractionation of sulphur isotopes together with a high D 2 O/HDO versus HDO/H 2 O and non-solar isotopic ratio for xenon as well as for Si point towards a non-homogeneously mixed protosolar nebula.Key words: comets: general -comets: individual: 67P/Churyumov-Gerasimenko. I N T RO D U C T I O NRosetta is a mission of the European Space Agency (ESA) with the aim to characterize Jupiter-family comet 67P/ChuryumovGerasimenko (hereafter 67P) in situ. Rosetta followed the comet from a heliocentric distance of more than 3.6 au to perihelion and finally away again from the Sun to almost 3.8 au. One of the science goals of Rosetta was the determination of the elemental, isotopic, E-mail: ursina.calmonte@space.unibe.ch (UC); kathrin.altwegg@space. unibe.ch (KA); martin.rubin@space.unibe.ch (MR) and molecular composition of the cometary material. Most of the composition measurements during the mission, especially isotopic measurements, were done from the Rosetta orbiter in the coma, by the instrument ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis; Balsiger et al. 2007) Sensor (COPS). The composition of the coma, including dust and gas, depends on the actual composition of the nucleus and the physical and chemical processes involved in the desorption, sublimation and transport of the material into the coma. ROSINA has been sampling the volatiles in the coma throughout the mission. ROSINA observations revealed some remarkable links to pre-solar ices, including elevated D/H ratios in singly and doubly deuterated water (Altwegg ...

On the Origin and Evolution of the Material in 67P/Churyumov-Gerasimenko

“…Sulfur is an abundant species found in many comets (Altwegg 1995;Biver et al 2016;Calmonte et al 2016;Jewitt et al 1997). Correspondingly, the isotopes of sulfur have also been observed in the volatiles of several comets, from in situ atomic S at comet 1P/Halley (Altwegg 1995) by the Giotto mission and in H 2 S, CS, and OCS at comet 67P/C-G (Calmonte et al 2017), as well as by remote sensing observations of H 2 S and CS in Hale-Bopp by Jewitt et al (1997) and , respectively, and CS in both comets C/2013 R1 (Lovejoy) and C/2012 F6 (Lemmon) (Biver et al 2016). Furthermore, sulfur isotopes have also been measured in the refractory material of comet Wild 2 returned by the Stardust mission (Heck et al 2012) and at 67P/C-G by the COSIMA dust mass spectrometer .…”

Elemental and molecular abundances in comet 67P/Churyumov-Gerasimenko

“…While some of the volatile species have indeed been shown to be associated with grains (De Keyser et al 2017, Altwegg et al 2016, no clear indication for distributed sources was found for water, and Biver et al (2019) estimated an upper limit of 50% for contributions from grains. For our analysis, we assumed that grains moving at ~1 m/s would have lost their volatiles by the time they reached Rosetta (Lien 1990), which was more than 100 km from Figure 3: Measured abundances of a suite of volatile species with respect to water from the pre-perihelion period at the end of May 2015, suitable for deriving bulk abundances in the ices of comet 67P/Churyumov-Gerasimenko (Calmonte et al 2016).…”

“…Details on the ROSINA calibrated sensitivities % and fragmentation patterns %→A + can be obtained from PSA, along with the Level-3 ROSINA datasets. For species that were not calibrated due to their toxicity or corrosiveness, we have applied the empirical formula for the sensitivity derived by Calmonte (2015) and included fragmentation patterns obtained from reference spectra published by NIST, the National Institute of Standards and Technology (Kim et al 2005). Sensitivity errors for the individual species are on the order of 15 -20%.…”

The ALMA-PILS survey: the sulphur connection between protostars and comets: IRAS 16293–2422 B and 67P/Churyumov–Gerasimenko

“…With the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA; Balsiger et al 2007) aboard the orbiter, the coma has been shown to contain H 2 S, atomic S, SO 2 , SO, OCS, H 2 CS, CS 2 , and S 2 (and tentatively CS, as the mass spectrometer cannot distinguish it from CO 2 ) gases (Le Roy et al 2015). Furthermore, S 3 , S 4 , CH 3 SH, and C 2 H 6 S have now been detected (Calmonte et al 2016) and information on isotopologues is available (Calmonte et al 2017). It seems that ∼80 per cent of sulphur is in refractories (dust) with only ∼20 per cent hidden in volatiles (ice; see Appendix A for the details).…”