Chemical Kinetics Simulations of Ice Chemistry on Porous Versus Non-Porous Dust Grains

Drew A Christianson,Robin T Garrod

doi:10.3389/fspas.2021.643297

Drew A Christianson, Robin T Garrod

Open Access

https://doi.org/10.3389/fspas.2021.643297

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Abstract

The degree of porosity in interstellar dust-grain material is poorly defined, although recent work has suggested that the grains could be highly porous. Aside from influencing the optical properties of the dust, porosity has the potential to affect the chemistry occurring on dust-grain surfaces, via increased surface area, enhanced local binding energies, and the possibility of trapping of molecules within the pores as ice mantles build up on the grains. Through computational kinetics simulations, we investigate how interstellar grain-surface chemistry and ice composition are affected by the porosity of the underlying dust-grain material. Using a simple routine, idealized three-dimensional dust-grains are constructed, atom by atom, with varying degrees of porosity. Diffusive chemistry is then simulated on these surfaces using the off-lattice microscopic Monte Carlo chemical kinetics model, MIMICK, assuming physical conditions appropriate to dark interstellar clouds. On the porous grain surface, the build-up of ice mantles, mostly composed of water, leads to the covering over of the pores, leaving empty pockets. Once the pores are completely covered, the chemical and structural behavior is similar to non-porous grains of the same size. The most prominent chemical effect of the presence of grain porosity is the trapping of molecular hydrogen, formed on the grain surfaces, within the ices and voids inside the grain pores. Trapping of H2 in this way may indicate that other volatiles, such as inert gases not included in these models, could be trapped within dust-grain porous structures when ices begin to form.

Highlights

Dust grains are known to play a major role in the chemical evolution of interstellar clouds and starforming regions
Based on the results of the hydrogen/oxygen system modeled by Garrod (2013a), the ice produced on dust grains under darkcloud physical conditions should be non-porous, as heavy atoms accreted from the gas phase would have ample time to diffuse on the surface, finding and filling any nascent surface inhomogeneities, which tend to manifest initially as stronger binding sites due to their local curvature
We propose the following as the most likely: 1) the porous surface enhances the overall conversion of H into H2, with mobile H2 formed in the pores diffusing away to be incorporated into the general bulk ices; 2) the porous surface enhances production rates locally within the pores, leading to the build-up of H2 where it is formed; 3) H2 that is formed anywhere on the grain surface may diffuse into the pores, where it becomes trapped

Summary

Introduction

Dust grains are known to play a major role in the chemical evolution of interstellar clouds and starforming regions. Of critical importance is the production of molecular hydrogen on grain surfaces, through the reactions of hydrogen atoms adsorbed from the gas phase (e.g., Gould and Salpeter, 1963; Hollenbach and Salpeter, 1971); much of the H2 produced in this way desorbs again, to become the dominant component of the gas in dense, dark clouds. In clouds of sufficient visual extinction, the dust grains may build up ice mantles composed mostly of water, but with substantial amounts of CO, CO2, NH3 and several other simple species (Boogert et al, 2015). Much of the chemistry that produces these grain-surface molecules involves the surface diffusion of atomic hydrogen, which is understood to be mobile even under the cold (∼10 K) conditions of dark clouds (Senevirathne et al, 2017). The adsorption of atoms, as well as molecules such as CO, from the gas phase and their subsequent hydrogenation leads to the build up of ice mantles many monolayers in thickness (Cuppen et al, 2017)

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