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Loss of hydrogen and oxygen from the upper atmosphere of Venus

机译：金星上层大气中氢和氧的损失

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Atmospheric escape from the upper atmosphere of Venus is mainly influenced by the loss of hydrogen and oxygen caused by the interaction of solar radiation and particle flux with the unprotected planetary environment. Because one main aim of the ASPERA-4 particle/plasma and VEX-MAG magnetic field experiments on board of ESA's forthcoming Venus Express mission is the investigation of atmospheric erosion processes from the planet's ionosphere-exosphere environment, we study the total loss of hydrogen and oxygen and identified the efficiency of several escape mechanisms involved. For the estimation of pick up loss rates we use a gas dynamic test particle model and obtained average loss rates for H~+, and O~+ pick up ions of about 1 x 10~(25) s~(-1) and about 1.6 x 10~(25) s~(-1), respectively. Further, we estimate ion loss rates due to detached plasma clouds, which were observed by the pioneer Venus orbiter and may be triggered by the Kelvin-Helmholtz instability of about 0.5-1 x 10~(25)s~(-1). Thermal atmospheric escape processes and atmospheric loss by photo-chemically produced oxygen atoms yield negligible loss rates. Sputtering by incident pick up O~+ ions give O atom loss rates in the order of about 6 x 10~(24)s~(-1). On the other hand, photo-chemically produced hot hydrogen atoms are a very efficient loss mechanism for hydrogen on Venus with a global average total loss rate of about 3.8 x 10~(25) s~(-1), which is in agreement with Donahue and Hartle [1992. Solar cycle variations in H~+ and D~+ densities in the Venus ionosphere: implications for escape. Geophys. Res. Lett. 12, 2449-2452] and of the same order but less than the estimated H~+ ion outflow on the Venus nightside of about 7.0 x 10~(25) s~(-1) due to acceleration by an outward electric polarization force related to ionospheric holes by Hartle and Grebowsky [1993. Light ion flow in the nightside ionosphere of Venus. J. Geophys. Res. 98, 7437-7445]. Our study indicates that on Venus, due to its larger mass and size compared to Mars, the most relevant atmospheric escape processes of oxygen involve ions and are caused by the interaction with the solar wind. The obtained results indicate that the ratio between H/O escape to space from the Venusian upper atmosphere is about 4, and is in a much better agreement with the stoichiometrically H/O escape ratio of 2:1, which is not the case on Mars. However, a detailed analysis of the outflow of ions from the Venus upper atmosphere by the ASPERA-4 and VEX-MAG instruments aboard Venus Express will lead to more accurate atmospheric loss estimations and a better understanding of the planet's water inventory.
机译：从金星的高层大气逃逸主要受到太阳辐射和粒子通量与不受保护的行星环境的相互作用所引起的氢和氧的损失的影响。由于ESA即将执行的Venus Express任务上的ASPERA-4粒子/等离子体和VEX-MAG磁场实验的主要目的是研究地球电离层-外层大气环境的大气侵蚀过程，因此我们研究了氢和氢的总损耗氧气并确定了涉及的几种逃逸机制的效率。为了估算吸收损失率，我们使用气体动力学测试粒子模型，获得了H〜+和O〜+吸收离子的平均损失率，约为1 x 10〜（25）s〜（-1）且约为1.6 x 10〜（25）s〜（-1）。此外，我们估计了由于分离的等离子云造成的离子损失率，这是由先驱金星轨道器观测到的，并且可能是由开尔文-亥姆霍兹不稳定性约0.5-1 x 10〜（25）s〜（-1）触发的。热大气逸出过程和光化学产生的氧原子造成的大气损失产生的损失率可忽略不计。 O〜+离子的入射溅射使O原子的损失速率约为6 x 10〜（24）s〜（-1）。另一方面，光化学产生的热氢原子是金星上氢的非常有效的损失机制，全球平均总损失率约为3.8 x 10〜（25）s〜（-1），与Donahue和Hartle [1992年。金星电离层中H〜+和D〜+密度的太阳周期变化：对逃逸的影响。地理学。 Res。来吧12，12，2449-2452]和相同的数量级，但比由于向外极化极化力引起的加速而估计的金星夜间的H〜+离子流出量少约7.0 x 10〜（25）s〜（-1）。 Hartle和Grebowsky [1993年。轻离子流在金星的夜间电离层中。 J.地球物理。 Res。 98，7437-7445]。我们的研究表明，在金星上，由于其质量和大小都比火星大，因此与氧气最相关的大气逸出过程涉及离子，并且是由与太阳风的相互作用引起的。获得的结果表明，金星上方高层大气的H / O逸出与空间之比约为4，并且与化学计量的H / O逸出比为2：1更好地吻合，而火星则不是这样。但是，通过金星快车上的ASPERA-4和VEX-MAG仪器对金星从高空大气中流出的离子进行的详细分析，将可以更准确地估算大气损失，并更好地了解地球的水量。

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