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Reactivity in interstellar ice analogs : role of the structural evolution

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Authors : P. Ghesquière, A. Ivlev, J. A. Noble, and P. Theulé
Abstract :
Context. The synthesis of interstellar complex organic molecules in ice
involves several types of reactions between molecules and/or radicals
that are usually considered to be diffusion controlled.

Aims. We aim to understand the coupling between diffusion and
reactivity in the interstellar ice mantle using a model binary reaction
in the diffusion-limited regime.

Methods. We performed isothermal kinetic laboratory experiments on
interstellar ice analogs at low temperatures, using the NH3:CO2:H2O
model system where reactants NH3 and CO2 have a low reaction barrier
and are diluted in a water-dominated ice.

Results. We found that in the diffusion-limited regime, the reaction
kinetics is not determined by the intrinsic bulk diffusivity of
reactants. Instead, reactions are driven by structural changes evolving
in amorphous water ice, such as pore collapse and
crystallization. Diffusion of reactants in this case likely occurs
along the surface of (tiny) cracks generated by the structural changes.

Conclusions. The reactivity driven by the structural changes breaks the
conventional picture of reactant molecules/radicals diffusing in a bulk
water ice. This phenomenon is expected to lead to a dramatic increase
in production rates of interstellar complex organic molecules in
star-forming regions.
Journal : Astronomy & Astrophysics A&A