Schematics of the predicted ice structure in the prestellar core stage (right) and the observed water deuterium fractionation toward embedded low-mass protostars (left). The outer layer of the ice mantle, which has high water D/H ratio, could sublimate in the cold envelope, while the whole mantle is sublimated in the hot corino. The schematical structure is proposed by Furuya et al. (2016). Observational values are from Persson et al. (2014); Coutens et al. (2013, 2014); Jensen et al. (2019).