Comment on “Is the D/H ratio in the comet coma equal to the D/H ratio in the comet nucleus?” by M. Podolak, Y. Mekler, and D. Prialnik

In a recent Icarus note, Podolak et al. (2002, Icarus 160, 208–211) proposed a model, which results in a significant enrichment in the D/H ratio in the coma relative to that in the comet nucleus. This conclusion contradicts to the typical HDO/H2O ice-vapor fractionation, with a smaller or equal D/H in the vapor. It is based on an error in the relationship for sublimation flux, which adversely affects all conclusions of Podolak et al. (2002, Icarus 160, 208–211). Due to the very low diffusion in the H2O ice, the HDO/H2O ratio in the coma should closely match this ratio in the nucleus.