Venus D and H Loss: An Overview of Measurements and Outstanding Questions

The highly elevated D/H ratio of Venus relative to the Earth indicates that the planet has lost most of its initial water to space, likely as an indirect result of runaway greenhouse warming. However, the large mass of the planet combined with its relatively cool thermosphere means that thermal loss is extremely inefficient at removing large quantities of H or D, and that escape must proceed by nonthermal mechanisms, including photochemical and ion loss. Significant outstanding questions remain about the absolute magnitude of D and H loss at present, the relative importance of ion and neutral loss for these species, and the drivers that control variations in loss both today and throughout time. We will present a review of Mariner, Pioneer Venus Orbiter, and Venus Express spacecraft observations of D and H loss as neutrals and ions, as well as prior data-driven modeling estimates of overall D and H escape, which have uncertainties of a factor of several in absolute escape rates, ion vs. neutral loss ratios, and fractionation factor. Each of these has a potentially significant impact on the total inventory and timing of water loss by Venus, as well as implications for the loss of water from Venus-like planets throughout the universe. New modeling and potentially new measurements are required to understand these processes more fully: as one example we will present new neutral modeling results relevant to the photochemical escape of Venus H. We emphasize that because thermal H and D loss is inefficient at Venus, a detailed understanding of the active loss processes of H and D from the atmosphere to space is required to understand Venus as an integrated evolving system.