Thermal and energetic processing of astrophysical ice analogues rich in SO₂

Abstract

Context. Sulfur is an abundant element in the cosmos and it is thus an important contributor to astrochemistry in the interstellar medium and in the solar system. Astronomical observations of the gas and of the solid phases in the dense interstellar/circumstellar regions have evidenced that sulfur is underabundant. The hypothesis to explain such a circumstance is that it is incorporated in some species in the solid phase (i.e. as frozen gases and/or refractory solids) and/or in the gas phase, which for different reasons have not been observed so far. Aims. Here we wish to give a contribution to the field by studying the chemistry induced by thermal and energetic processing of frozen mixtures of sulfur dioxide (one of the most abundant sulfur-bearing molecules observed so far) and water. Methods. We present the results of a series of laboratory experiments concerning thermal processing of different H 2 O:SO 2 mixtures and ion bombardment (30 keV He + ) of the same mixtures. We used in situ Fourier transform infrared (FTIR) spectroscopy to investigate the induced effects. Results. The results indicate that ionic species such as HSO − 3 , HSO − 4 , and S 2 O 2− 5 are easily produced. Energetic processing also produces SO 3 polymers and a sulfurous refractory residue. Conclusions. The produced ionic species exhibit spectral features in a region that, in astronomical spectra of dense molecular clouds, is dominated by strong silicate absorption. However, such a dominant feature is associated with some spectral features, some of which have not yet been identified. We suggest adding the sulfur-bearing ionic species to the list of candidates to help explain some of those features. In addition, we suggest that once expelled in the gas phase by sublimation, due to the temperature increase, and/or by non-thermal erosion those species would constitute a class of molecular ions not detected so far. We also suggest that molecular sulfur-bearing ions could be present on the surfaces and/or in the atmospheres of several objects in the solar system, for example icy satellites of the giant planets and comets.

Sulfur Ice Astrochemistry: A Review of Laboratory Studies

“…Molecular dynamics simulations of a 20 MeV S + ion impacting a complex multi-component ice whose composition is relevant to the Europan surface revealed that this collision results in a net loss of SO 2 molecules (which were initially present in the ice mixture) due to their oxidation to HSO − 3 and SO − 3 (Anders and Urbassek 2019a). These results therefore parallel the laboratory findings of Boduch et al (2016) and their sulfur ion radiolysis of simpler ices, as well as those of Kaňuchová et al (2017) who considered the ion irradiation and thermal processing of SO 2 :H 2 O mixed ices. However, this computational simulation did not consider the implantation of the sulfur ion into the ice (Anders and Urbassek 2019a), and so any chemistry resulting from this implantation which could potentially lead to the formation of novel sulfur-bearing compounds was not considered.…”

High-energy ion impacts into the sulfur-bearing ice surface of Europa: an atomistic study of chemical transformations

“…The numbers of SO 2 initially increases -because it is a dissociation product of SO 4 -but then decreases again, as some molecules are oxidized to sulfite, SO 3 2− . This production of sulfite, including the hydrogenated species, HSO 3 − , is interesting since it parallels experimental findings of S ion bombardment of oxygen-rich targets (O 2 and CO 2 ) (Boduch et al 2016) and of ion-irradiation and thermal processing of H 2 O:SO 2 mixtures (Kanuchová et al 2017). In both experiments sulfite production was observed.…”

“…Interest in the sulfur content stems not only from such astrobiological implications, but has also another fundamental astrochemical origin: observed sulfur abundances in the dense interstellar medium are smaller than the cosmic abundance by two orders of magnitude (Jiménez-Escobar & Muñoz Caro 2011). To clarify this "problem of missing sulfur", studies of the thermal and irradiation chemistry of solid-phase sulfur compounds need to be undertaken (Kanuchová et al 2017).…”

“…Lv et al (2014) perform sulfur ion impacts into CO and CO 2 ices and Boduch et al (2016) into O 2 and CO 2 ices, studying the formation of compounds with the implanted sulfur ion. Kanuchová et al (2017) directly irradiate ice analogs rich in SO 2 to investigate the induced chemistry.…”

Negative Ion Formation by Thermal Surface Ionization of Sulfur Dioxide

“…The SO 2 in solid phase has been detected in icy grain mantles in the neighborhood of the high mass protostars. [12] Sulfur dioxide was also identified by Voyager 1 mission in gas and condensed phase on the Io and Europa -satellites of the Jupiter. [13,14] The interaction of the identified in the ISM molecules with electrons, atoms and ions may leads to the formation of positive or negative ions as well as to the fragmentation of molecules.…”