Top: a H₂O molecule (with H atoms in yellow) surrounded by water molecules absorbs an UV photon and dissociates into H and OH. H and OH recombine to form H₂O that eventually desorbs from the ice surface via the direct mechanism. Bottom: a D₂O molecule (with D atoms in black) surrounded by water molecules absorbs an UV photon and dissociates into D and OD. The heavier D atom transfers its momentum to one of the surrounding H₂O molecules that desorbs from the ice via the “kick-out” mechanism.