The role of water in the chemistry of the interstellar medium

The role of water in the chemistry of interstellar ices
H. Carrascosa1, G. M. Muñoz Caro1, C. Del Burgo Olivares1, Y. –J. Chen2
Centro de Astrobiología (CAB, CSIC-INTA), Ctra de Ajalvir, km 4, Torrejón de Ardoz, 28850, Madrid, Spain.
Department of physics, National Central University, Zhongli District, Taoyuan City, Taiwan.

Experiments performed under simulated interstellar conditions have shown the formation of a full variety of complex organic molecules (such as sugars, heterocycles, amides, etc.) from irradiation of ice samples. As water is, by far, the most abundant molecules in ice mantles in the interstellar medium, most of the species present in ice mantles will be surrounded by water molecules. Understanding the role of water is therefore essential to predict the conditions and environmental situations where certain species will be favoured.

It is well known that the presence of water determines the chemistry in specific directions, which can be drastically different from the chemistry without water. For example, methanol ice forms formaldehyde readily under UV radiation. Formaldehyde is a very reactive species, which polymerises, forming a polymer called polyoxymethylene (POM). However, in the presence of water, formaldehyde molecules are solvated by water molecules, and the formation of POM is highly inhibited [1].

We have experimentally studied the role of water in two different ice mixtures: 1) H2O:NH3:CH3OH and 2) H2O:H2S. Experiments were carried out using a high vacuum chamber, cooled down with liquid nitrogen and submitted to UV radiation with a deuterium lamp. After simultaneous deposition and irradiation, ice samples were warmed up to room temperature. A methanol extract of the organic residue at room temperature, was analysed by gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (HPLC-MS) equipments, to identify and quantify the molecules remaining in the organic residues. The composition and abundances of the detected molecules were compared in experiments with different water ratios [2].

Several works have already studied the formation of organic molecules in H2O:NH3:CH3OH ice mixtures ([3] and references therein). There is a full variety of organic species detected from UV irradiation of this ice samples. The experimental procedure prior to chromatographic analyses is designed in each work depending on the family of species of interest. Sugars, amino acids, or heterocycles are among the species which have been detected. We have not carried out any pretreatment of the samples, to avoid any induced chemistry after the extraction of the sample from the simulation chamber. GC-MS analyses have revealed that N-heterocycles are favoured over O-heterocycles, and we have investigated the role of water toward this chemistry [4].

Figure 1: chromatogram of a H2O:CH3OH:NH3 ice mixture. Gaussian profiles for each of the identified species are shown. Note that there are no O-heterocycles, as a consequence of the chemistry induced by the presence of ammonia, and the chemical properties of the O-H groups provided by photodissociation of water and methanol molecules.

The fate of sulphur in the interstellar medium is still unclear. Several authors have pointed out to the formation of long sulphur chains as sulphur reservoirs in the interstellar medium [5, 6, 7]. However, H2S has not been detected in ice mantles, suggesting that, if present, its abundance will be low compared to water. Consequently, H2S molecules will not be in close contact in ice mantles. Experiments were made with H2O:H2S ice samples in different ratios. If sulphur ends up forming chains, there must be an efficient mechanism which brings together sulphur atoms, making it possible to react and producing covalent S-S bonds. This mechanism has been elucidated [2] and will be presented here. In our experiments, water enhances the formation of octaedric sulphur by a factor of ~100. Water molecules play a key role, forming covalent bonds with sulphur intermediates that favours subsequent S-addition reactions. Some of these intermediates have been detected in the organic residue, which served to complete the chemical pathway to the formation of sulphur chains in a water environment.

In brief, we show strong evidence for the need of a water-rich ice environment in the photosynthesis of sulphur allotropes up to S8, and provide a reaction scheme that also requires the presence of water for the N-heterocycles formation.

Figure 2: mechanism of sulphur formation in two steps. First, water molecules favour the formation of S-SO3 species, which is required to elongate sulphur chains. 2) when sulphur chains are long enough, they will make an intramolecular reaction producing sulphur cycles. Alternatively, A’ shows a chemical pathway by which molecules could not elongate more, producing different species that were also detected in the chromatographic analyses.

[1] Schutte, W., Allamandola, L. J., and Sandford, S. A., 1993, Icarus, 104, 118-137.
[2] Del Burgo Olivares, C., Carrascosa, H., Muñoz Caro, et al., 2026, submitted to A&A.
[3] Muñoz Caro, G. M., Carrascosa, H., Martín-Doménech, R., 2025, Nat. Rev. Chem, 9, 537-552.
[4] Del Burgo Olivares, C., Carrascosa, H., Muñoz Caro, G. M., et al., 2026, submitted.
[5] Cazauz, S., Carrascosa, H., Muñoz Caro, G. M., et al. 2022 A&A 657, 1-12.

[6] Carrascosa, H., Muñoz Caro, G. M., Martín-Doménech, R. et al. 2024, MNRAS, 533, 1, 967-978
[7] Herath, A., McAnally, M., Turner, A. M., et al. 2025, Nat. Comm. 16, 5571.