Chemical processes on dust-grain surfaces and within their ice mantles are critical to the chemical evolution of the interstellar medium. Interstellar dust grains are expected to be amorphous, and laboratory experiments suggest significant porosities in interstellar ice analogs. However, until now, most chemical models have used only simple approximations to simulate surface chemistry, ignoring the macro- and microscopic morphologies of both the ices and the underlying grains. Kinetic Monte Carlo methods have so far used only on-lattice approaches.I will present an entirely new modeling technique that uses a fully 3-D, off-lattice Monte Carlo treatment of the chemical kinetics of dust grains and their ice mantles. This method allows every surface particle to be traced explicitly over time, with positions based on surface-binding potential minima, so that structure and porosity in the ices occurs naturally.The resultant ice structure is chemically segregated and somewhat porous, showing 'veins' or fissures filled with H2. Porosity is strongly dependent on accretion rate. The model results are visualized using ray-traced videos, so that the formation and structure of the ice may be observed directly. These simulations open up a new parameter space that may allow more direct comparisons to be drawn between experimental and interstellar ices, whose structures may be considerably different. Figure: Simulated dust-grain ice mantle. Molecular hydrogen is shown in blue for ease of identification.

3D off-lattice Monte Carlo simulations of interstellar grain chemistry and ice structure

247th National Meeting (2014)

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