The Garrod group's MIMICK model (Model for Interstellar Monte Carlo Ice Chemical Kinetics) uses an off-lattice, microscopic Monte Carlo kinetics approach to simulate the formation and porous structure of amorphous ices, such as may be found in the interstellar medium. MIMICK can simulate ice formation both through direct deposition and surface chemistry of deposited atoms and molecules.

We recently used MIMICK to investigate the variation in density - and thereby, porosity - of laboratory water ice deposited at various temperatures (Clements, Berk, Cooke & Garrod 2018).

While measured ice densities (Brown et al. 1996) could be matched by the model at high temperatures (~100 K), the ice produced by the models at low temperatures was too dense, under the assumption of purely thermal diffusion (left panel, below, for the curve corresponding to deposition of 10¹³ molecules cm^-2 s^-1).

We introduced a non-thermal diffusion mechanism to the model (right panel), whereby the energy gained by a water molecule adsorbing into a surface binding site allows the molecule to undergo several hops between binding sites (losing energy each time) before settling into a final position. The addition of this mechanism was sufficient to reproduce the laboratory densities over the ~20 - 130 K temperature range tested. Around 70% (alpha=0.7) of the energy initially gained by the adsorbing molecule is required for further surface hops in order to to reproduce measured densities. This corresponds to ~1-2 surface hops per deposited water molecule, prior to relaxation into a thermalized state on the surface.

The success of the model in reproducing the laboratory data then allowed us to apply MIMICK to interstellar conditions, in which deposition rates are much lower and timescales much longer. Ices deposited under interstellar conditions are found to be of significanty lower porosity than laboratory-grown ices. This suggests a lesser ability for interstellar water ice to trap other molecules within such pores.

Further information can be found in our publication (link)