Icy dust grains form by accretion and surface chemistry in the transition region from the diffuse to dense interstellar medium (ISM). Using the theoretical methods and realistic dust grain surfaces developed in WP1-1 we will construct models of water ice cluster and few-layer ices on grain surfaces using established methods. The effect on cluster structures of the freeze out and formation of further molecular species – methanol, ammonia, CO etc. will be investigated. Effects such as poisoning of bare grain catalytic sites, reactivity at the interface and on/within the ices themselves, as well as the reservoir effect of having abundant reaction partners embedded in the ice will be gauged using the surrogate energy landscapes and the sampling techniques developed in the center.
Ice clusters and thin amorphous ice films will be grown on realistic dust grain analogues synthesized under theme 1 at temperatures down to 5 K. Ice cluster structures and possible surface wetting properties will be investigated using cryogenic STM, as previously demonstrated for water cluster growth on graphene and their catalytic effect will be investigated using XPS and mass spectrometry. STM will be used to monitor reaction induced ice-cluster annealing and desorption. Based on the progress in WP2-1 and WP2-2 we will investigate whether the presence of ice-layers on dust grain surfaces facilitates the catalytic formation of caproic acid and cytosine.
At UL co-deposition of molecules and atoms or small radicals has been shown to lead to the formation of larger complex molecules. Here we will revisit these effects using realistic substrate surfaces identified under theme 1. Photo-processing of ice layers with increasing thickness on realistic dust grain analogue surfaces will be performed with the aim of ascertaining at which stage in the cosmochemical cycle the catalytic role of the dust surface is taken over by that of the growing ice layers. Measurements of reaction yields will be performed using the methods of WP3-2 for thin ices and WP4-1 for thick ices. Measured catalytic processes and yields will be compared to JWST observations of simple and more complex organic molecules in ices as function of extinction under the IceAge project.
Initial key reaction: Ribose formation