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-mantle formation. CO and other heavy species are generally expected to accrete efficiently, and astrochemical models commonly assume near-unity sticking probabilities at low temperatures. However, recent laboratory measurements on realistic grain analogs show that sticking can remain far below unity even at 10 K, challenging the conventional hit-and-stick picture. Using CO as a prototype for low-temperature accretion of heavy species, we aim to identify the origin of reduced sticking and determine how nanoscale surface morphology controls gas freeze-out and ice growth. CO adsorption near 10 K was investigated on highly oriented pyrolytic graphite (used as a flat reference) and carbon soot (used as a rough cosmic-dust analog). X-ray photoelectron spectroscopy (XPS), low-temperature scanning tunneling microscopy (STM), kinetic Monte Carlo simulations (KMC), and a thermodynamic description were combined to relate molecular retention to local surface morphology. X-ray photoelectron spectroscopy shows that CO does not adsorb with unit sticking on graphite. Adsorption proceeds through initial monolayer growth, a reduced-retention crossover near monolayer completion, and delayed multilayer growth. STM reveals high CO mobility on graphite terraces and shows that CO is effectively retained only after reaching stabilizing configurations, including island edges and terrace steps. On soot, the same sequence occurs at much higher exposures because of lower sticking; this is consistent with KMC simulations showing preferential retention in concave regions and poor wetting of convex asperities. Low-temperature sticking is controlled by post-impact dynamics: mobile species transiently explore the surface and desorb unless they reach a stabilizing site. Nanoscale morphology amplifies this mechanism, reducing effective sticking probabilities, delaying gas freeze-out, and limiting the accretion of CO and---more generally---heavy species on realistic dust grains.