Elemental and isotopic fractionation as fossils of water escape from Venus

• Published in: Geochimica et cosmochimica acta Vol. 361; pp. 228 - 244
• Main Authors: Zahnle, Kevin J., Kasting, James F.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15.11.2023
• Subjects: Atmospheric escape; Noble gases; Oxygen isotopes; Venus; Noble gases; Atmospheric escape; Oxygen isotopes; Venus
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2023.09.023

We develop a new model of diffusively modulated hydrodynamic escape to predict oxygen isotopic fractionations caused by the loss of water from a steam atmosphere of Venus. The chief technical advance over previous work is including CO2 as a major species. We find that ordinary (δ18O) and mass-independent (Δ17O) fractionations depend mostly on the extent of lithospheric buffering and the ferocity of EUV heating when escape took place, and relatively little on the size of the lost ocean(s). It is likely that Δ17O evolved significantly from its birth state, not only in the atmosphere but also in the silicates of the crust and upper mantle. If both δ18O and Δ17O of Venus are identical to Earth and Moon, we may conclude that Venus and Earth accreted from a common pool. But differences in δ18O and Δ17O can be attributed to escape rather than to genetics. If the differences are large enough, they can be used to constrain when escape took place and the extent of volatile exchange with the lithosphere. Neon and argon systematics are most consistent with minimal escape, especially if an Ar-rich source, possibly derived from comets, is added. However, we also find a novel class of solutions in which Ne and Ar of Venus, Earth, and Mars are evolved from a common source material subject to different vigors of hydrodynamic escape, least extreme for Earth and most extreme for Mars. These alternative models require that Venus was always rather dry (<10% of an Earth ocean) and its water lost very early (before <100 Myrs). The two styles of escape – minimal or extreme – should be readily distinguished by an unambiguous measurement of the Ar/Kr ratio. Finally, we find that predicted D/H enrichments are of order 100 for almost all model parameters. This result, a direct consequence of diffusion-limited escape of H and D, provides support for the overall scenario. •Oxygen isotopes can be strongly fractionated by hydrodynamic escape.•There can be no expectation that Venus retains an interpretable record of its birth composition in its oxygen isotopes.•Diffusion-limited hydrogen escape correctly predicts D/H fractionation on Venus.•We develop a novel explanation of Ne/Ar systematics that can encompass Venus, Earth, Mars, and chondrites.