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Atmospheric and water loss from early Venus

机译：金星早期的大气和水分流失

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Previous interpretations of the Pioneer Venus mass spectrometer data of the deuterium to hydrogen (D/H) ratio of 1.9 x 10~(-2) or 120 ± 40 times the terrestrial value indicate that Venus may have had at least an H_2O content of the order of about 0.3% of a terrestrial ocean (TO), and even much more during and shortly after the accretion period of ≤ 300 Myr, depending on the unknown ratio of a continuous supply of H_2O by comets to a hydrogen blow-off loss and impact erosion of the early atmosphere. In view of the low H_2O abundance in the present atmosphere, several studies suggest that the planet should have lost most of its H_2O during the early high X-ray, EUV and solar wind period of the active young Sun. Because oxygen did not accumulate in Venus' atmosphere it is commonly believed that a part of the oxygen from dissociated H_2O vapor was dragged off to space along with the escaping hydrogen during a blow-off period, or could have oxidized the surface minerals to produce FeO and Fe_2O_3 to the depths of a few kilometers to tens of kilometers depending on the initial amount of H_2O. We use in the present study, for the first time, multi-wavelength X-ray and EUV (XUV) observations by the ASCA, ROSAT, EUVE, FUSE and IUE satellites and stellar winds inferred from mass loss observations by the Hubble Space Telescope of solar proxies with ages < 4.6Gyr for the investigation of how efficiently the radiation and particle environment of the young Sun could have influenced the evolution of the early Venusian atmosphere and its H_2O inventory due to the removal of oxygen picked up by the solar wind. For modelling the Venusian thermosphere over the planetary history we apply a diffusive-gravitational equilibrium and thermal balance model and investigate the heating of the early thermosphere by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by CO_2 IR emission in the 15 μm band. Our model simulations result in expanded thermospheres with exobase levels between about 200 km at present and about 2200 km 4.5 Gyr ago. Moreover, our results yield high exospheric temperatures during the active phase of the young Sun even if we assume a "dry" CO_2 atmosphere with similar composition that is observed on present Venus of more than 8000 K after the Sun arrived at the zero-age-main-sequence (ZAMS). Exospheric temperatures above about 4000 K lead to diffusion-limited escape and high loss rates for atomic hydrogen. The duration of this blow-off phase for atomic hydrogen essentially depends on the mixing ratios of CO_2, N_2 and H_2O in the early Venusian atmosphere and could last between about 150 to several hundred Myr, which could result in a large thermal loss of hydrogen from Venus. For studying how much of the H_2O-related oxygen could have been lost to space by the ion pick up process due to the stronger solar wind and higher XUV fluxes of the young Sun we used our modelled atmospheric density profiles and studied the loss of O~+ ion pick up from the upper atmosphere of Venus over the planet's history by applying a numerical test particle model. Depending on the used solar wind parameters, our model simulations show that ion pick up by a strong early solar wind on a non-magnetized Venus could erode during 4.6 Gyr more than about 250 bar of O~+ ions, that corresponds to an equivalent amount of one terrestrial ocean. Finally, we discuss the implications of our findings for the formation of the Venusian atmosphere and discuss our results in the frame of previous studies.
机译：先锋金星质谱仪数据的氘氢（D / H）比为1.9 x 10〜（-2）或地面值的120±40倍的先前解释表明，金星可能至少具有H_2O含量约占陆地海洋（TO）的0.3％，在≤300 Myr吸积期期间和之后不久甚至更多，这取决于未知的彗星不断供应H_2O与氢吹出损失和影响早期大气的侵蚀。鉴于目前大气中的H_2O丰度低，一些研究表明，在活跃的年轻太阳的早期高X射线，EUV和太阳风时期，该行星应该损失了大部分H_2O。由于氧气没有在金星的大气中积聚，通常认为，在吹气期间，来自分解的H_2O蒸气的一部分氧气与逸出的氢气一起被拖到太空中，或者可能氧化了表面矿物而生成FeO。 Fe_2O_3的深度取决于H_2O的初始含量，可达到几公里至几十公里。在本研究中，我们首次使用了ASCA，ROSAT，EVE，FUSE和IUE卫星进行的多波长X射线和EUV（XUV）观测，以及哈勃太空望远镜从质量损失观测中推断出的恒星风。年龄小于4.6Gyr的太阳能代理，用于调查年轻太阳的辐射和粒子环境由于去除了太阳风吸收的氧气而可能如何有效地影响了早期金星大气及其H_2O存量的变化。为了对整个行星历史上的维纳斯热层建模，我们应用了扩散引力平衡和热平衡模型，并研究了由于热化学反应和15μm波段的CO_2 IR发射引起的冷却，通过光离解和电离过程对早期热层的加热。 。我们的模型仿真结果表明，外地基水平目前在大约200 km至4.5 Gyr之前的大约2200 km之间扩展了热圈。此外，即使我们假设“干燥的” CO_2大气具有相似的组成，在太阳到达零年龄点之后，目前的金星上观测到的成分超过8000 K，我们的结果仍会在年轻的太阳活跃期产生较高的大气温度。主序列（ZAMS）。高于4000 K的大气温度导致扩散受限的逸出和原子氢的高损失率。原子氢的这种喷出阶段的持续时间主要取决于金星早期大气中CO_2，N_2和H_2O的混合比，持续时间可能在150至数百Myr之间，这可能导致氢从氢中大量流失金星。为了研究由于年轻太阳的较强太阳风和较高的XUV通量，离子吸收过程可能会通过离子吸收过程将多少与H_2O有关的氧气损失到太空中，我们使用了模拟的大气密度剖面并研究了O〜的损失通过应用数值测试粒子模型，从金星的高层大气中提取+离子，这是整个地球的历史。根据使用的太阳风参数，我们的模型仿真表明，在4.6 Gyr的时间内，强磁过早的太阳风在未磁化的金星上吸收的离子会腐蚀超过250 bar的O〜+离子，这相当于当量一个陆地海洋。最后，我们讨论了我们的发现对金星大气层形成的影响，并在以前的研究框架中讨论了我们的结果。

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