ORIGINAL RESEARCH published: 20 May 2021 doi: 10.3389/fspas.2021.676548 Does Processing or Formation of Water Ice Mantles Affect the Capacity of Nanosilicates to Be the Source of Anomalous Microwave Emission? Joan Mariñoso Guiu 1, Stefano Ferrero 2, Antonio Macià Escatllar 1, Albert Rimola 2* and Stefan T. Bromley 1,3* 1 Departament de Ciencia de Materials i Química Física & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, Barcelona, Spain, 2Departament de Química, Universitat Autònoma de Barcelona, Bellaterra, Spain, 3Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain Anomalous microwave emission (AME) is detected in many astrophysical environments as a foreground feature typically peaking between 20–30 GHz and extending over a Edited by: 10–60 GHz range. One of the leading candidates for the source of AME is small Luca Bizzocchi, Max Planck Institute for Extraterrestrial spinning dust grains. Such grains should be very small (approx. ≤1 nm diameter) in Physics, Germany order for the rotational emission to fall within the observed frequency range. In addition, Reviewed by: these nanosized grains should possess a significant dipole moment to account for the Martin Robert Stewart McCoustra, Heriot-Watt University, observed emissivities. These constraints have been shown to be compatible with spinning United Kingdom bare nanosilicate clusters, assuming that ∼1% of the total Si mass budget is held in these Sergio Ioppolo, ultrasmall grains. Silicate dust can be hydroxylated by processing in the interstellar medium Queen Mary University of London, United Kingdom and is generally known to provide seeds for molecular water ice nucleation in denser *Correspondence: regions. Herein, we use quantum chemical calculations to investigate how the dipole Albert Rimola moment of Mg-rich pyroxenic (MgSiO3) nanoclusters is affected by both accretion of albert.rimola@uab.cat Stefan T. Bromley molecular water and dissociative hydration. Our work thus provides an indication of how s.bromley@ub.edu the formation of water ice mantles is likely to affect the capacity of nanosilicates to generate AME. Specialty section: This article was submitted to Keywords: anomalous microwave emission, dust grains, nanosilicates, water ices, interstellar medium, grain Astrochemistry, processing, hydroxylation, density functional theory a section of the journal Frontiers in Astronomy and Space Sciences INTRODUCTION Received: 05 March 2021 Accepted: 30 April 2021 Silicate dust is ubiquitously found in a wide range of astrophysical environments (Henning, 2010). Published: 20 May 2021 Chemical nucleation and growth in the circumstellar envelopes of oxygen-rich evolved stars is an Citation: important source of silicate dust (Goumans and Bromley, 2012; Gobrecht et al., 2016). From infrared Mariñoso Guiu J, Ferrero S, (IR) emission observations, it is inferred that most of these newly formed silicate grains are Mg-rich Macià Escatllar A, Rimola A and and have sizes of the order 0.1 μm (diameter) (Norris et al., 2012). Interactions between the dust and Bromley ST (2021) Does Processing or the stellar radiation field cause the grains to be ejected into the interstellar medium (ISM). Supernova Formation of Water Ice Mantles Affect shocks in the ISM induce grain–grain collisions which shatter the dust into smaller components the Capacity of Nanosilicates to Be the Source of Anomalous Microwave (Jones et al., 1996). A typical shock is thought to result in 5–15% of the original grain mass ending up Emission?. in the form of ultrasmall nanosilicates (<3 nm diameter). Nanograin production from larger grains is Front. Astron. Space Sci. 8:676548. also predicted to occur close to sources of intense radiation (e.g., massive stars and supernovae) via doi: 10.3389/fspas.2021.676548 rotational disruption of dust grains by radiative torques (Hoang et al., 2019). These models of dust Frontiers in Astronomy and Space Sciences | www.frontiersin.org 1 May 2021 | Volume 8 | Article 676548 Mariñoso Guiu et al. AME and Hydrated Nanosilicates FIGURE 1 | Bare (MgSiO3)6 nanosilicate structures: (A) low dipole (global minimum structure) and (B) high dipole (+1.11 eV relative to the global minimum). Dipole moments for each nanocluster are given in Debyes. Atom color key: silicon–yellow, magnesium–blue, and oxygen—red. FIGURE 2 | Lowest energy hydroxylated (MgSiO3)6 nanosilicate structures found with (A) one, (B) two, and (C) three water molecules. Dipole moments for each nanocluster are given in Debyes. Atom color key: silicon–yellow, magnesium–blue, and oxygen—red. processing are compatible with estimates that approximately 10% (Ali-Haïmoud, 2013; Dickinson et al., 2018). Observational of the Si in the ISM could be held in ≤3 nm diameter nanosilicate evidence has tended to rule out small spinning organic dust grains (Li and Draine, 2001) without violating observational species (i.e., polycyclic aromatic hydrocarbons—PAHs) as the constraints. most important source of AME (Hensley et al., 2016). The Via gas–grain collisions and interactions with radiation, small atomistically detailed structures and physical properties of nanosilicate grains will be rotationally excited. Such spinning nanosilicates with Mg-rich olivinic (Mg2SiO4)N and pyroxenic nanosilicate grains have been proposed as the main carrier of (MgSiO3)N compositions with diameters ≤1 nm have recently anomalous microwave emission (AME) – a microwave been determined by quantum chemical modeling (Macià foreground feature which typically peaks between 20–30 GHz Escatllar et al., 2019). These accurate calculations clearly show and extends over a 10–60 GHz range (Macià Escatllar and how the ionic chemical bonding in nanosilicates intrinsically Bromley, 2020; Hoang et al., 2016; Hensley and Draine, 2017). tends to lead to grains with significant dipoles. Based on those AME is observed in a number of astrophysical environments structures, it has been shown that the AME can be explained if including molecular clouds (Watson et al., 2005) and the diffuse only 1% of the Si in the ISM were held in ultrasmall (i.e., ≤1 nm ISM (Hildebrandt et al., 2007). As smaller grains can be spun up diameter) nanosilicate grains (Macià Escatllar and Bromley, to larger frequencies than larger grains in any given astronomical 2020). Although the dipoles possessed by these bare environment, proportionally they emit more power, and thus the nanosilicates are more than sufficient to account for the smallest grains are expected to dominate the spinning dust observed AME, it is not clear if these results can be extended microwave spectrum. For a grain to emit in the microwave to nanosilicates, which become hydroxylated by processing and/ region associated with AME via rotational emission, its size or upon which water ice forms. should be of the order of ∼1 nm in diameter (Draine and Silicate grains emerging from the outflows of evolved stars are Lazarian, 1998a; Draine and Lazarian, 1998b). In principle, processed from ion irradiation (e.g., protons, He+) from contributions to the AME from the spinning of such supernovae shocks in the ISM. From laboratory studies of the nanograins could occur from any type of nanograin if it is effect of the solar wind on lunar surface silicate material, proton sufficiently abundant and has a large enough dipole moment irradiation was found to cause two important intertwined Frontiers in Astronomy and Space Sciences | www.frontiersin.org 2 May 2021 | Volume 8 | Article 676548 Mariñoso Guiu et al. AME and Hydrated Nanosilicates In denser regions of the ISM such as molecular clouds, the ultraviolet radiation field is shielded and molecular water can accrete on silicate grain surfaces. This marks the beginning of the formation of water ice–dominated mantles on silicate grains. Water is one of the most abundant molecules in the universe in the solid phase (Gibb et al., 2004; Boogert et al., 2008). Interstellar water ices are thought to present an amorphous structural state because of the similarities of their IR features with those of amorphous solid water measured in terrestrial laboratories (Watanabe and Kouchi, 2008). Gas-phase reactions based on low-temperature ion-neutral and high-temperature neutral- neutral processes allow for the explaining of the abundance of water as a gaseous component, but they cannot justify the large abundance of water ice in interstellar and protostellar clouds (Hollenbach and McKee, 1989; van Dishoeck et al., 2013). Accordingly, reaction routes based on surface chemical processes have long been recognized to be the major channel for the formation of water ice (Tielens and Hagen, 1982; van Dishoeck et al., 2013). Therefore, water ice, rather than being formed by successive adsorption of water molecules coming from the gas phase, is formed in situ on the surfaces of dust grains. Different reactive channels leading to the formation of interstellar water on the surfaces of grains are hydrogenation of atomic O (e.g., Dulieu et al., 2010), of molecular O2 (e.g., Ioppolo et al., 2008), and even of O3 (e.g., Mokrane et al., 2009), among others. We refer to the work of Molpeceres et al. (2019) for a complete list of postulated reactions leading to water formation on interstellar grains. Herein, we perform a systematic quantum chemical study of the effect of both hydroxylation and molecular water interactions on ultrasmall (∼1 nm diameter) Mg-rich pyroxenic nanosilicates with a (MgSiO3)6 composition. We specifically focus on how the dipole moments of the original bare P-6 nanosilicates are affected by these chemical changes. If we focus on the dipole moments of the most energetically stable grains in a population, we find that both processes can significantly affect the dipole moment of these selected grains. However, when considering a more realistic population of nanosilicate grains, our results suggest that neither hydroxylation nor molecular hydration has a significant effect on the average magnitude of the dipole moment. Our results thus suggest that the capacity of nanosilicates to be the source of AME should not be significantly diminished by processing and/or initial water ice FIGURE 3 | Plot of relative energy vs. dipole moment for hydroxylated (MgSiO3)6 nanosilicates for (A) one, (B) two, and (C) three water molecules. mantle formation. The vertical blue line denotes the average dipole moment of the set. METHODS chemical processes: 1) hydroxyl formation (Zeller et al., 1966) and 2) Mg cation depletion by preferential sputtering (Keller and The initial bare (MgSiO3)6 isomer structures were obtained in a Mckay, 1997). Analysis of interplanetary dust has provided direct previous work (Macià Escatllar et al., 2019). We chose to compare evidence of such grain processing (Bradley, 1994). Such processes two distinct isomers: 1) the lowest energy (global minimum) would tend to transform Mg-rich nano-olivine dust grains into cluster structure with a very low dipole and 2) a slightly hydroxylated nano-pyroxene grains. Relatively elevated levels of energetically metastable cluster structure with a high dipole. pyroxene (with respect to the initial dominance of olivine) are These bare (MgSiO3)6 structures are shown in Figure 1. consistent with the pyroxene-rich environments of protostars In the case of hydroxylated bare nanosilicate clusters, we (Demyk et al., 1999) and the estimated fraction of amorphous carried out global optimization calculations to find the 50 pyroxene grains in the ISM (Kemper et al., 2004). lower energy isomers of (MgSiO3)6 after dissociative Frontiers in Astronomy and Space Sciences | www.frontiersin.org 3 May 2021 | Volume 8 | Article 676548 Mariñoso Guiu et al. AME and Hydrated Nanosilicates TABLE 1 | Evolution of total dipole moments (μ) in Debyes of (MgSiO3)6 nanosilicates with increasing hydration via hydroxylation (chemisorption) and molecular water adsorption (physisorption). The average dipoles of the hydrated nanosilicates are based on the data obtained herein and shown in Figures 2–4. Values of β are calculated as β  μ /√Natoms . Water addition process Dipole moment of Average dipole moment/β Average dipole moment/β Average dipole moment/β initial bare nanosilicates/β (+ 1 × H2O) (+ 2 × H2O) (+ 3 × H2O) Hydroxylation (chemisorption) 7.7a/1.41 6.71/1.17 6.60/1.1 6.22/1.04 Molecular adsorption (physisorption) 0.01b/0.002 2.87/0.50 3.32/0.55 3.30/0.55 Molecular adsorption (physisorption) 13.67c/2.50 14.40/2.51 11.97/2.00 12.65/2.12 a Average value of dipole moment calculated from a set of 180 (MgSiO3)6 structures—see Macià Escatllar and Bromley (2020). b Dipole moment of low dipole global minimum (MgSiO3)6 structure—see Macià Escatllar et al. (2019) and Figure 1A. c Dipole moment of high dipole (MgSiO3)6 structure (+1.1 eV with respect to the global minimum)—see Figure 1B. FIGURE 4 | Lowest energy molecularly hydrated (MgSiO3)6 nanosilicate structures starting from the low dipole (see Figure 1A) bare structure with (A) one, (B) two, and (C) three water molecules. Dipole moments for each nanocluster are given in Debyes. Atom color key: silicon–yellow, magnesium–blue, and oxygen—red. interaction of one, two, and three water molecules, respectively. ensure that our searches were not biased, for each addition of The global optimization calculations were split into two steps. water, we ran the MCBH searches a total of 20 times with First, we used the Monte Carlo basin hopping (MCBH) algorithm different starting structures. Each MCBH run consisted of (Wales and Doye, 1997), employing previously developed 10,000 steps. polarizable classical interatomic potentials (IPs) (Macià After this global optimization procedure, we selected 100 Escatllar et al., 2019), to explore the potential energy surface distinct lowest energy nanocluster structures for each degree of (PES), followed by the structural configurations of the hydroxylation for further refinement. The selected cluster hydroxylated clusters. Previous studies have shown that the structures were optimized using density functional theory MCBH method is an efficient method for exploring the low (DFT)–based calculations using the PBE functional (Perdew energy PES of nanoclusters and for finding the global et al., 1996) with a light-tier one numerical atom-centered minimum (Wales and Doye, 1997). The standard MCBH orbital (NAO) basis set. Previous studies on oxide algorithm uses random atomic displacements at each step to nanoparticles have shown that this basis set has a similar explore the PES. In addition, we also allowed that 0.5% of the quality to valence triple-ζ plus polarization Gaussian-type steps attempt an Mg ↔ Si cation exchange move assist in this Orbital basis sets (Lamiel-Garcia et al., 2017). These DFT- exploration. To run the MCBH calculations, we employed a based calculations employed the Fritz Haber Institute Ab previously developed cascade MCBH code (Cuko et al., 2017), Initio Molecular Simulation package (FHI-AIMS) (Blum et al., which uses the General Utility Lattice Package (GULP) (Gale 2009). and Rohl, 2003) to locally optimize the cluster structures at In the case of the molecularly hydrated (MgSiO3)6 clusters, the each step. To avoid problems with the local optimization of initial positions of the physisorbed water molecules on both bare distorted structures containing polarized ions, we preoptimize cluster isomers (Figure 1) were prepared using an electrostatic each cluster structure with an IP with polarizable shells compatibility criterion. Specifically, the negatively charged removed before optimizing using the IP with shells. We oxygen atom of each water molecule was placed at a distance note that, in principle, an MCBH global optimization search 2 Å above each positively charged magnesium cation of the only depends on the chemical composition of the system and silicate cluster. Subsequently, for each water + cluster should be independent of the initial hydroxylated cluster combination, the whole system structure was locally structure. In practice, the exploration of the PES can be optimized. All optimizations were carried out using DFT- somewhat biased by the initial structure and thus, to help based calculations using the PBE functional and a def2- Frontiers in Astronomy and Space Sciences | www.frontiersin.org 4 May 2021 | Volume 8 | Article 676548 Mariñoso Guiu et al. AME and Hydrated Nanosilicates RESULTS AND DISCUSSION In the following subsections, we describe our findings for both hydroxylation and molecular interaction with water for bare (MgSiO3)6 nanosilicate clusters (see Figure 1). Note that these bare nanosilicates contain 30 atoms and have a typical maximum diameter of ∼1 nm. The hydrated (MgSiO3)6(H2O)n nanosilicates we consider have up to 39 atoms, which induces only a minor increase in the average diameter. Although the chemical compositions of the hydrated nanosilicate systems considered in each case (i.e., hydroxylation and molecular interactions) are identical, astronomically each relate to distinct processes and regions of the ISM. As such, we discuss the two considered modes of nanosilicate hydration separately with relation to their astronomical context. Dissociative Hydration As discussed above, hydroxylation of silicate dust can occur through high energy processing by supernovae shocks in the diffuse ISM. Hydroxylation is also the natural chemical result of dissociative chemisorption of water molecules on dust grain surfaces. However, as water molecules are quickly photodissociated by high-energy ultraviolet (UV) photons in the diffuse ISM, chemical routes to hydroxylated nanosilicate dust would more likely occur via reactions with other species (e.g., O and H atoms). We note that the explicit modeling of these reactions and how the resulting hydroxylation affects subsequent H2 formation reactions were investigated in previous works on a (MgSiO3)4 nanopyroxene cluster (Goumans and Bromley, 2011; Kerkeni et al., 2017; Kerkeni et al., 2019). Generally, hydroxylation of silicate grain surfaces is the most plausible mechanism by which hydration of silicates can occur in the diffuse ISM. We consider hydroxylated nanopyroxene clusters with chemical compositions (MgSiO3)6(H2O)n, with n  1–3. After finding the low-energy minima of these systems using the MCBH global optimization method, although some of the higher energy isomers displayed physisorbed water molecules, most clusters exhibited 2n hydroxyls. First, this shows that hydroxylation is energetically more favorable than molecular water adsorption. The lowest energy (i.e., candidate global minima) hydroxylated isomers found for each n are shown in Figure 2. By only considering the lowest energy isomers, we can follow the evolution of the dipole moment from the bare (MgSiO3)6 with FIGURE 5 | Plot of relative energy vs. dipole moment for molecularly increasing hydroxylation (see Figure 2). Second, we note that the hydrated (MgSiO3)6 nanosilicates based on the low dipole bare nanosilicate global minimum bare (MgSiO3)6 nanosilicate isomer has a (Figure 1A) for (A) one, (B) two, and (C) three water molecules. The vertical negligible dipole moment, due to its high symmetry. However, blue line denotes the average dipole moment of each set. In each plot, all lowest energy hydroxylated nanosilicates have a significant data from approximately 100 isomers are shown. dipole moment and correspondingly low symmetry structures. Based on this relatively small change in the degree of hydroxylation for the lowest energy isomers, the initial relatively large increase in the dipole moment going from a TZVPP (i.e., valence triple-ζ plus polarization) basis set (Weigend bare cluster to one with two hydroxyls (+5.1 Debye) is then and Ahlrichs, 2005). This process was repeated for sampling all dampened with further hydroxylation increase. the Mg sites and all distinct combinations of adsorption sites In the ISM, due to energetic processing, the population of when using two and three water molecules. The ORCA code nanosilicates is likely to exhibit a range of isomeric structures and (Neese et al., 2020) was used for these calculations. compositions with a corresponding range of energetic stabilities. Frontiers in Astronomy and Space Sciences | www.frontiersin.org 5 May 2021 | Volume 8 | Article 676548 Mariñoso Guiu et al. AME and Hydrated Nanosilicates FIGURE 6 | Lowest energy molecularly hydrated (MgSiO3)6 nanosilicate structures starting from the high dipole (see Figure 1B) bare structure with (A) one, (B) two, and (C) three water molecules. Dipole moments for each nanocluster are given in Debyes. Atom color key: silicon–yellow, magnesium–blue, and oxygen—red. From an astronomical perspective, it is thus more relevant to Following our results for hydroxylation, we also consider the extract average properties of grain populations. From this more initial stages of molecular hydration in (MgSiO3)6(H2O)n species for general perspective, it has previously been shown that the high n  1–3. Note that unlike in the energetic process of hydroxylation in symmetry of the global minimum bare (MgSiO3)6 nanosilicate is the ISM, in protected molecular cloud environments, each molecular not typical of the majority of isomers. In fact, the average dipole hydration is a low-energy event in which we assume that the bare moment of (MgSiO3)6 isomers, considering over 100 lowest energy nanocluster structure is unaffected. We take the two nanocluster structures, is found to be 7.7 Debyes (Macià Escatllar and Bromley, structures in Figure 1 as seeds for studying the initial stages of 2020). In the case of the hydroxylated nanosilicates considered molecular physisorption. First, we consider the global minimum low herein, our global optimization searches also produce a large dipole nanocluster (see Figure 1A) which has a negligible dipole due number of low-energy isomer structures from which to obtain to its very symmetric structure. In Figure 4, we show the lowest the average dipole moments. In Figure 3, we show the relative energy (MgSiO3)6(H2O)n structures that we obtained for the energetic stability and corresponding dipole moments for addition of 1–3 water molecules. Upon nucleation of the first approximately 100 distinct nanocluster isomers for each degree of water molecule, the symmetry of the cluster is broken, resulting hydroxylation (i.e., n  1–3) considered. In Table 1, we report the in a dramatic increase in the dipole moment. Conversely, with two average dipole moments of all hydroxylated (MgSiO3)6(H2O)n water molecules, the lowest energy structure is highly symmetric, nanoclusters based on these data. It has been shown that for a leading to a very low dipole moment. Finally, with three water dipolar nanocluster species to contribute to AME, it should possess a molecules, the symmetry is once again broken and the lowest energy sufficiently large size-normalized dipole moment (β  μ /√Natoms ) system is found to exhibit a high dipole moment. As in the case of such that β > 0.3 (Hoang et al., 2016, Hensley and Draine, 2017). In the hydroxylated clusters, in astronomical environments there is Table 1, we also reported values of β based on the average dipole likely to be a range of isomers for each degree of hydration, and the moments on our sets of hydroxylated nanosilicate clusters. We note average effect of molecular hydration is of more interest. In this that although there is a very slight decrease in the average dipole case, due to the limited number of distinct binding sites, we have moment with increasing hydroxylation, the value of β is always over fewer possibilities for each water addition than in the case of three times the size required according to the above AME constraint. hydroxylated nanocluster isomers. Specifically, for the nucleation These data clearly show that in all cases, a population of of one water molecule, we find six distinct adsorption sites, and for hydroxylated (MgSiO3)6(H2O)n nanoclusters could significantly two and three water molecules, we consider 15 distinct adsorption contribute to the observed AME. sites. The relative energies and dipole moments of all these systems are shown in Figure 5. The corresponding average dipole moments Molecular Hydration (see Table 1) are all close to 3 Debyes, showing that molecular Hydroxylation via shock-induced processing or suitable chemical water adsorption can induce an average increase in the dipole reactions is initially very energetically favorable for bare grains, but, with moment, even for bare nanoclusters with negligible dipole increasing hydroxylation, the surface sites for the hydroxylation to take moments. The corresponding values of β are all close to 0.5, place quickly become occupied. As such, nanosized grains can only which is more than sufficient for such hydrated nanoclusters to maintain a limited number of hydroxyls on their surfaces before they contribute to AME. become too energetically favorable for any formed water to interact with In the case of molecular nucleation of a bare cluster with an the hydroxylated grain in a molecular physisorbed manner (Kerkeni initial large dipole moment (see Figure 1B), we show the lowest and Bromley, 2013). As mentioned above, the main mechanism energy (MgSiO3)6(H2O)n structures that we obtained in our through which water ice forms and grows on the core grains (e.g., searches in Figure 6. For these specific low-energy structures, silicates) is by cumulative water formation on the same grain surface, we see that the dipole moment stays high (>9 Debyes) for each where it remains firmly attached. Here, to check how the presence of water addition. More generally, we consider a similar range of interacting water molecules affects the dipole moment of the bare hydrated structures as in the low dipole moment case to obtain nanograin, we assume that water formation has already taken place on averages. In Figure 7, we show the relative energies and dipole the binding sites where the water molecules are molecularly adsorbed. moments of all considered systems. Here, we also see that the Frontiers in Astronomy and Space Sciences | www.frontiersin.org 6 May 2021 | Volume 8 | Article 676548 Mariñoso Guiu et al. AME and Hydrated Nanosilicates CONCLUSION We use searches based on global optimization and electrostatic considerations to obtain a range of structures for hydroxylated and molecularly hydrated nanoclusters, respectively, with a (MgSiO3)6(H2O)n composition (n  1–3). These hydrated nanopyroxenes could be relevant for nanosilicate grain processing in the ISM or initial water ice mantle formation in molecular clouds. We have used accurate quantum chemical DFT calculations to calculate the dipole moments of all hydrated pyroxene nanoclusters. We find that both hydroxylation and molecular hydration, irrespective of the initial dipole moment of the bare nanocluster, induce and/or maintain systems with dipole moments that are sufficiently high to significantly contribute to AME. These results thus strongly indicate that the validity of previous reports showing that nanosilicates are very plausible carriers of AME is not affected by a modest degree of nanosilicate hydration. Overall, this study broadens the range of potential nanosilicate species and types of astrophysical environments that could produce AME signatures. DATA AVAILABILITY STATEMENT The data supporting the findings of this study are available on reasonable request from the corresponding authors. AUTHOR CONTRIBUTIONS The original idea for the study was proposed by SB. All authors participated in the initial conception and planning of the study and in the discussion of the results. JMG and SF performed the calculations. SB and AR participated in the writing and editing of the manuscript and coordinated the project. FUNDING This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 811312 for the project “Astro- Chemical Origins” (ACO) and from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program grant agreement No. 865657 for the project “Quantum Chemistry on Interstellar Grains” (QUANTUMGRAIN). We acknowledge financial support from the Spanish Ministerio de FIGURE 7 | Plot of relative energy vs. dipole moment for molecularly Ciencia, Innovación y Universidades (projects CTQ 2017-89132-P, hydrated (MgSiO3)6 nanosilicates based on the high dipole bare nanosilicate RTI 2018-095460-B-100, and MDM-2017-0767 via the Spanish (Figure 1B) for (A) one, (B) two, and (C) three water molecules. The vertical Structures of Excellence María de Maeztu program) and the blue line denotes the average dipole moment of each set. Generalitat de Catalunya (projects 2017SGR1323 and 2017SGR13). ACKNOWLEDGMENTS average dipole moments (see also Table 1) for each degree of hydration also stay high. Correspondingly, the values of β AR is indebted to the “Ramón y Cajal” program. The Red obtained from these average dipole moment values are also Española de Supercomputación (RES) and Consorci de Serveis high (>2 Debyes), meaning that they would be strong emitters Universitaris de Catalunya (CSUC) are also acknowledged for the of microwave radiation. provision of supercomputing time. Frontiers in Astronomy and Space Sciences | www.frontiersin.org 7 May 2021 | Volume 8 | Article 676548 Mariñoso Guiu et al. AME and Hydrated Nanosilicates REFERENCES Hollenbach, D., and McKee, C. F. (1989). Molecule Formation and Infrared Emission in Fast Interstellar Shocks. III - Results for J Shocks in Molecular Clouds. ApJ 342, 306. doi:10.1086/167595 Ali-Haïmoud, Y. (2013). Spinning Dust Radiation: A Review of the Theory. Adv. Ioppolo, S., Cuppen, H. M., Romanzin, C., van Dishoeck, E. F., and Linnartz, H. Astron. 2013, 462697. doi:10.1155/2013/462697 (2008). Laboratory Evidence for Efficient Water Formation in Interstellar Ices. 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Balanced Basis Sets of Split Valence, Triple Copyright © 2021 Mariñoso Guiu, Ferrero, Macià Escatllar, Rimola and Bromley. Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and This is an open-access article distributed under the terms of the Creative Commons Assessment of Accuracy. Phys. Chem. Chem. Phys. 7, 3297. doi:10.1039/ Attribution License (CC BY). The use, distribution or reproduction in other forums is b508541a permitted, provided the original author(s) and the copyright owner(s) are credited Zeller, E. J., Ronca, L. B., and Levy, P. W. (1966). Proton-induced Hydroxyl and that the original publication in this journal is cited, in accordance with accepted Formation on the Lunar Surface. J. Geophys. Res. 71, 4855–4860. doi:10.1029/ academic practice. No use, distribution or reproduction is permitted which does not jz071i020p04855 comply with these terms. Frontiers in Astronomy and Space Sciences | www.frontiersin.org 9 May 2021 | Volume 8 | Article 676548