Mahieux Arnaud, Viscardy Sébastien, Yelle Roger Vincent, Karyu Hiroki, Chamberlain Sarah, Robert Séverine, Piccialli Arianna, Trompet Loïc, Erwin Justin Tyler, Ubukata Soma, Nakagawa Hiromu, Koyama Shungo, Maggiolo Romain, Pereira Nuno, Cessateur Gaël, Willame Yannick, Vandaele Ann Carine
Division of Planetary Atmosphere, Royal Belgian Institute for Space Aeronomy, Brussels 1180, Belgium.
Computational Flow Physics Laboratory, Department of Aerospace Engineering and Engineering Mechanics, The University of Texas at Austin, Austin, TX 78712.
Proc Natl Acad Sci U S A. 2024 Aug 20;121(34):e2401638121. doi: 10.1073/pnas.2401638121. Epub 2024 Aug 12.
This study analyzes HO and HDO vertical profiles in the Venus mesosphere using Venus Express/Solar Occultation in the InfraRed data. The findings show increasing HO and HDO volume mixing ratios with altitude, with the D/H ratio rising significantly from 0.025 at ~70 km to 0.24 at ~108 km. This indicates an increase from 162 to 1,519 times the Earth's ratio within 40 km. The study explores two hypotheses for these results: isotopic fractionation from photolysis of HO over HDO or from phase change processes. The latter, involving condensation and evaporation of sulfuric acid aerosols, as suggested by previous authors [X. Zhang , 3, 834-837 (2010)], aligns more closely with the rapid changes observed. Vertical transport computations for HO, HDO, and aerosols show water vapor downwelling and aerosols upwelling. We propose a mechanism where aerosols form in the lower mesosphere due to temperatures below the water condensation threshold, leading to deuterium-enriched aerosols. These aerosols ascend, evaporate at higher temperatures, and release more HDO than HO, which are then transported downward. Moreover, this cycle may explain the SO increase in the upper mesosphere observed above 80 km. The study highlights two crucial implications. First, altitude variation is critical to determining the Venus deuterium and hydrogen reservoirs. Second, the altitude-dependent increase of the D/H ratio affects H and D escape rates. The photolysis of HO and HDO at higher altitudes releases more D, influencing long-term D/H evolution. These findings suggest that evolutionary models should incorporate altitude-dependent processes for accurate D/H fractionation predictions.
本研究利用金星快车/红外太阳掩星数据，分析了金星中层大气中HO和HDO的垂直分布。研究结果表明，HO和HDO的体积混合比随高度增加，D/H比从约70公里处的0.025显著上升至约108公里处的0.24。这表明在40公里范围内，从地球比值的162倍增加到了1519倍。该研究探讨了导致这些结果的两种假设：HO相对于HDO光解的同位素分馏或相变过程。正如之前的作者[X. Zhang, 3, 834 - 837 (2010)]所指出的，后者涉及硫酸气溶胶的凝结和蒸发，与观测到的快速变化更为吻合。HO、HDO和气溶胶的垂直传输计算表明，水汽向下流动，气溶胶向上流动。我们提出了一种机制，即在中层大气下部，由于温度低于水的凝结阈值，形成了气溶胶，导致富含氘的气溶胶。这些气溶胶上升，在较高温度下蒸发，释放出的HDO比HO更多，然后向下传输。此外，这个循环可能解释了在80公里以上的中层大气上部观测到的SO增加。该研究突出了两个关键意义。第一，高度变化对于确定金星的氘和氢储量至关重要。第二，D/H比随高度的增加会影响H和D的逃逸率。在较高高度处HO和HDO的光解释放出更多的D，影响着D/H的长期演化。这些发现表明，演化模型应纳入依赖高度的过程，以准确预测D/H分馏。
