Venus water loss is dominated by HCO+ dissociative recombination

• 成果类型: Article
• 署名作者: Chaffin, M. S.; Cangi, E. M.; Gregory, B. S.; Yelle, R. V.; Deighan, J.; Elliott, R. D.; Groller, H.
• 署名单位: University of Colorado System; University of Colorado Boulder; University of Arizona
• 刊物名称: Nature
• ISSN/ISSBN: 0028-5267
• DOI: 10.1038/s41586-024-07261-y
• 发表日期: 2024-05-09
• 关键词: hydrodynamic escape; upper-atmosphere; d/h ratio; hydrogen; deuterium; thermosphere; ionosphere; exosphere; ion; spectrometers

摘要

Despite its Earth-like size and source material(1,2), Venus is extremely dry(3,4), indicating near-total water loss to space by means of hydrogen outflow from an ancient, steam-dominated atmosphere(5,6). Such hydrodynamic escape likely removed most of an initial Earth-like 3-km global equivalent layer (GEL) of water but cannot deplete the atmosphere to the observed 3-cm GEL because it shuts down below about 10-100 m GEL(5,7). To complete Venus water loss, and to produce the observed bulk atmospheric enrichment in deuterium of about 120 times Earth(8,9), nonthermal H escape mechanisms still operating today are required(10,11). Early studies identified these as resonant charge exchange(12-14), hot oxygen impact(15,16) and ion outflow(17,18), establishing a consensus view of H escape(10,19) that has since received only minimal updates(20). Here we show that this consensus omits the most important present-day H loss process, HCO+ dissociative recombination. This process nearly doubles the Venus H escape rate and, consequently, doubles the amount of present-day volcanic water outgassing and/or impactor infall required to maintain a steady-state atmospheric water abundance. These higher loss rates resolve long-standing difficulties in simultaneously explaining the measured abundance and isotope ratio of Venusian water(21,22) and would enable faster desiccation in the wake of speculative late ocean scenarios(23). Design limitations prevented past Venus missions from measuring both HCO+ and the escaping hydrogen produced by its recombination; future spacecraft measurements are imperative.