Laboratory-based sticking coefficients for ices on a variety of small-grain analogues

Abundances and the partitioning between ices and gases in gas–grain chemistry are governed by adsorption and desorption on grains. Understanding of astrophysical observations relies on laboratory measurements of adsorption and desorption rates on dust grains analogues. On flat surfaces, gas adsorption probabilities (or sticking coefficients) have been found to be close to unity for most gases1–3. Here we report a strong decrease in the sticking coefficients of H2O and CO2 on substrates more akin to cosmic dust, such as submicrometre-sized particles of carbon and olivine, bare or covered with ice. This effect results from the local curvature of the grains, and then extends to larger grains made of aggregated small particles, such as fluffy or porous dust in more evolved media (for example, circumstellar disks). The main astrophysical implication is that accretion rates of gases are reduced accordingly, slowing the growth of cosmic ices. Furthermore, volatile species that are not adsorbed on a grain at their freeze-out temperature will persist in the gas phase, which will impact gas–ice partitions. We also found that thermal desorption of H2O is not modified by grain size, and thus the temperature of snowlines should be independent of the dust size distribution. Sticking coefficients quantify the readiness of gas molecules to freeze onto the surface of dust grains. Here, laboratory measurements of the sticking coefficients of water and carbon dioxide onto realistic bare and icy dust grain analogues provide key data for the study of snowlines in protoplanetary disks, for example.