As an Earth-like planet Venus probably had a primordial dipole field for several million years after formation of the planet. Since this dipole field eventually vanished the ionosphere of Venus has been exposed to the solar wind. The solar wind is shocked near Venus, and then scavenges the ionospheric particles through the magnetosheath and the magnetotail. The escape rate of oxygen ions (O⁺) estimated from spacecraft observations over the past several decades has manifested its importance for the evolution of planetary habitability, considering the accumulated effect over the history of Venus. However, all the previous observations were made in the shocked solar wind and/or inside the wake, though some simulations showed that unshocked solar wind can also ablate O⁺ ions. Here we report Venus Express observations of O⁺ ions in the unshocked solar wind during the solar minimum. The observations suggest that these O⁺ ions are accelerated by the unshocked solar wind through pickup processes. The estimated O⁺ escape rate, 2.1 × 10²⁴ ions/s, is comparable to those measured in the shocked solar wind and the wake. This escape rate could result in about 2 cm global water loss over 4.5 billion years. Our results suggest that the atmospheric loss at unmagnetized planets is significantly underestimated by previous observations, and thus we can emphasize the importance of an Earth-like dipole for planetary habitability.

作为一颗类地行星，金星可能在行星形成后的数百万年里都有一个原始偶极子场。由于这个偶极场最终消失，金星的电离层暴露在太阳风中。太阳风在金星附近受到冲击，然后通过磁鞘和磁尾清除电离层粒子。考虑到金星历史上的累积效应，根据过去几十年的航天器观测估计的氧离子 (O ⁺ )的逃逸率已经表明其对行星宜居性演变的重要性。然而，之前的所有观察都是在受激太阳风和/或尾流内部进行的，尽管一些模拟表明未受激太阳风也可以消融 O ⁺离子。在这里，我们报告了金星快车在太阳极小期期间对未受冲击的太阳风中O ^{+离子的观测。}观察结果表明，这些 O ⁺离子通过拾取过程被未受冲击的太阳风加速。估计的 O ⁺逃逸率 2.1 × 10 ²⁴ 离子/秒，与在冲击太阳风和尾流中测得的那些相当。这种逃逸率可能导致 45 亿年间全球水流失约 2 厘米。我们的结果表明，未磁化行星的大气损失被先前的观测显着低估，因此我们可以强调类地偶极子对行星宜居性的重要性。