Kinetic Monte Carlo simulations of ice processing: from the lab to interstellar environments

Dust grains in cold, dense interstellar clouds build up ice mantles through the accretion and subsequent surface chemistry of atoms and molecules from the gas. These mantles, of thicknesses on the order of 100 monolayers, are primarily composed of H2O, CO, and CO2. The formation of molecules on grain surfaces, and the subsequent production of complex organic molecules, has been demonstrated in laboratory experiments. However, interstellar physical conditions can never be exactly reproduced, and the timescales are vastly different (minutes in the lab versus many thousands of years in the ISM), making the correspondence of the two regimes highly uncertain. Laboratory experiments using interstellar ice analogs have shown that porosity could be present and can facilitate diffusion of molecules along the inner pore surfaces. Yet, there is considerable uncertainty about the structure of interstellar ices, their ability to store volatiles, and under what conditions. Further, the movement of molecules within and upon the ice is poorly described by current chemical kinetics models, making it difficult either to reproduce the formation of experimental porous ice structures or to extrapolate generalized laboratory results to interstellar conditions.
Here we use the off-lattice Monte Carlo kinetics model MIMICK to investigate the effects that various deposition parameters have on laboratory ice structures. We reproduce experimental trends in the density of amorphous solid water (ASW) for varied deposition angle, rate and surface temperature; ice density decreases when the incident angle or deposition rate is increased while increasing temperature results in more-compact water ice. The models indicate that the density behavior at higher temperatures (∼80 K) is dependent on molecular rearrangement resulting from thermal diffusion. To reproduce trends at lower temperatures, it is necessary to take account of non-thermal diffusion by newly-adsorbed molecules, which bring kinetic energy both from the gas phase and from their acceleration into a surface binding site. Extrapolation of the model to conditions appropriate to protoplanetary disks, in which direct accretion of water from the gas phase may be the dominant ice formation mechanism, indicate that these ices may be less porous than laboratory ices.
To further elucidate the porosity of ices, we used infrared absorption spectra of CO on the pore surface of porous amorphous solid water (ASW), and quantified the effective pore surface area of ASW. Additionally, we present results obtained from a Monte Carlo model of ASW in which the morphology of the ice is directly visualized and quantified. We found that 200 ML of ASW annealed to 20 K has a total pore surface area that is equivalent to 46 ML. This surface area decreases linearly with increasing temperatures. We also found that dangling OH bonds only exist on the surface of pores; almost all of the pores in the ASW are connected to the vacuum-ice interface and are accessible for adsorption of volatiles from the gas phase; there are few closed cavities inside ASW at least up to a thickness of 200 ML; the total pore surface area is proportional to the total three-coordinated water molecules in the ASW.
The presence of such pores could facilitate diffusion of molecules on the inner pore surfaces. However, the movement of molecules within the primary component of ices, H2O, is not fully understood, making it difficult either to reproduce the formation of experimental porous ice structures or to extrapolate generalized laboratory results to interstellar conditions. Previous work has established kinetic barriers for CO diffusion on and into H2O ice, but the microscopic details are still unknown, and the mixing barrier is not well determined. Here we provide insight into the diffusion of CO on amorphous water ices with various morphologies. A microscopically-detailed kinetic Monte Carlo model is used to deposit layered H2O-CO ices utilizing background and uniform-angle deposition, which produces H2O ices of various densities. A series of isothermal diffusion experiments are run to calculate mixing at each time step and combined to calculate barriers of diffusion. The density and, more importantly, the pore structure play a role in the ability for CO to mix, thus influencing its diffusion barriers. For example, the most porous ice, deposited using background deposition, produces the largest barrier. Utilizing uniform angle deposition, barriers are slightly reduced. Uniform angle deposition produces denser ices with column-like structures. The empty space between the columns appears to be narrower at lower angles of incidence, preventing volatiles from mixing. This result suggests that uniform-angled deposition yields ices that are unreliable as interstellar ice analogs.
We show that using an off-lattice Monte Carlo kinetics model can illustrate how CO first diffuses through and then desorbs from H2O ices. The strength of this model lies in the ability to monitor molecules desorbing from a surface, which produces spectra consistent with temperature-programmed desorption experiments. From the desorption profile, information about the binding energies and insight into the interacting structures can be extracted, providing direct comparisons to laboratory experiments. With this kinetics model, various stages of an experiment can be visualized, and precise structures can be explored. In addition, the model successfully predicts the characteristics of temperature-dependent structural changes phenomena that are observed experimentally. We find that the composition of the deposited ice is reflected in the desorption temperatures and profiles: layered ices have a two-peak structure at lower temperatures whereas mixed ices have a singular desorption peak. We also find efficient trapping in mixed ices where an ice 50 ML mixed was able to retain 68% of its original CO material. Layered porous ices also demonstrate trapping but with less efficiency. Ice trapping can play a significant role in protoplanetary disks where frozen volatiles provide material for the production of more complex species or be released at later times, changing the gas-phase chemistry.