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.     

The H− equilibrium using coupled rate equations for H−, H,H+, H2, and H2 +

We formulate rate equations for the reaction network coupling H, H^–, H⁺, H₂, and H₂ ⁺. We attempt to systematize the notation, and to write the equations in a form suitable for modern computational methods of handling the coupled rate equations and radiative transfer equations, for both dynamical and static atmospheres. We have accounted for more processes than are generally considered in most current work; some of these may have an impact on the equilibrium of H^– (hence its opacity) and on charge conservation (hence the proton density) in the atmospheres of solar-type stars.Operated by the Association of Universities for Research in Astronomy, Inc. under Contract AST 78-17292 with the National Science Foundation.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

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—rebuttal

We defend the position taken in our earlier note that under certain conditions the D/H ratio measured in the coma of a comet can be much higher that the D/H ratio in to cometary ice itself.