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