How nanoscale physics shapes ice formation in the Universe: Rethinking gas freeze-out on dust grains
In cold molecular clouds, gas freeze-out onto dust grains initiates interstellar ice formation, yet sticking probabilities of heavy species are often assumed to be near unity at low temperature. Recent laboratory measurements on realistic grain analogues show that this assumption can fail. Using CO as a prototype, we investigate how nanoscale surface morphology controls adsorption and ice growth at 10 K on highly oriented pyrolytic graphite and carbon soot. X-ray photoelectron spectroscopy, low-temperature scanning tunneling microscopy, kinetic Monte Carlo simulations, and a thermodynamic description are combined to relate molecular retention to local surface structure. CO does not adsorb with unit sticking on graphite: adsorption proceeds through monolayer growth, a reduced-retention crossover near monolayer completion, and delayed multilayer growth. STM shows that CO remains highly mobile on graphite terraces and is stabilized mainly at island edges and terrace steps. On soot, the same sequence occurs at much higher exposures and with substantially lower sticking coefficients, while simulations show preferential retention in concave regions and poor wetting of convex asperities. These results indicate that low-temperature sticking is governed by post-impact exploration and competition between stabilization and escape. Nanoscale morphology amplifies this mechanism, reducing effective sticking probabilities and delaying gas freeze-out on realistic dust grains.