A small, idealized interstellar dust grain is generated, and hydrogen and oxygen atoms are allowed to accrete onto it from the gas phase, under typical dark-cloud conditions. On the grain surface, atoms and molecules may thermally diffuse; when reactive species meet they may react to form a new molecule.
A new off-lattice technique is used to calculate the positions of the potential minima on the grain/ice surface, and thus the positions of the atoms/molecules, as they are needed. Surface binding and diffusion barriers are dependent on the number and type of atoms/molecules to which the particle in question is bonded. The accretion, evaporation, diffusion and reaction processes of each atom or molecule are treated stochastically, using Gillespie’s method (1976). The simulations were rendered using the freeware ray-tracing software POV-Ray.
The current chemical network includes the following species: H, O, OH, H2, O2, H2O, H2O2.
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You can learn more about my Monte Carlo grain chemistry models in my research pages.
Each frame of the video shows a single hopping event. This visualization is not linear in time, since the hopping timescales are dependent on the local binding conditions.
A hydrogen atom (white) thermally diffuses around the grain surface until it meets an oxygen atom (red), which react together to form OH. Another hydrogen atom accretes onto the grain from the gas phase, diffusing around the grain surface until it meets the OH (red & white), forming water.
Full chemical simulation
Each frame of this video corresponds to the formation of a single water molecule. Thermal diffusion of reactants is not shown.
Most of the molecules formed on the grain/ice surface are water molecules formed as described above. However, molecular hydrogen (H2) and molecular oxygen (O2) are also formed, via the surface diffusion and addition of two hydrogen or oxygen atoms, respectively.
Because the newly-accreted oxygen atoms are sufficiently mobile to find the strongest binding sites, prior to reaction with hydrogen to form water, the resultant ice structure is fairly compact.
Simple accretion of gas-phase water
This simulation considers only the direct accretion of gas-phase water, with no surface chemistry. The trajectory of each in-coming water molecule is randomized, to mimic true gas-phase accretion (as was done for H and O in the previous models). The rate of water accretion is set equal to that of the atomic oxygen in the previous simulation.
Because the newly-accreted water molecules are essentially immobile (compared to the rate of accretion) they stick to the surface at the nearest binding site to their contact point. The resultant ice structure is therefore extremely porous.
True interstellar ices are understood to form via the chemical processes shown above, but laboratory ices are formed via deposition (i.e. accretion). It is likely, therefore, that experimental interstellar-ice analogs and real interstellar ices exhibit significant differences in porosity.
Computer-generated dust grain
This video shows a 360° view of a sample dust grain generated by randomized accretion of particles onto a spherical core. The grain is comprised of ~20,000 particles.
Irregularly-shaped grains of this kind may be used as surfaces for the simulation of dust-grain chemistry with the kinetic Monte Carlo model.