When a planet has an orbiting moon, atoms and molecules that escape the planetary atmosphere as ions and are accelerated into space may be implanted and preserved inside the moon’s surface. Here, we determine the long-term averaged anisotropy of ions escaping the atmosphere of Mars and impacting its moon Phobos from more than four years of in situ ion observations. These measurements are used to quantify an estimate of the average flux of ions that has been impacting each location on Phobos over geologic timescales. We find that the flux of bombarding Martian ions is highly asymmetric on the moon’s surface, as the nearside of Phobos sees a flux higher by a factor of 15 to 100 than its farside. We show that a first consequence of this is that Martian atmospheric oxygen, carbon, nitrogen and argon atoms are implanted and may be preserved inside the uppermost hundreds of nanometres of Phobos’s nearside regolith grains, which may be brought back to Earth by future sample return missions. The second effect is that alteration of the regolith properties is asymmetric on Phobos’s surface, as Martian ions accelerate weathering of the nearside by a factor of ~2.

当一颗行星有一个绕轨道运行的卫星时，以离子的形式逃离行星大气层并加速进入太空的原子和分子可能会被植入并保存在月球表面内。在这里，我们通过四年多的原位离子观测确定了逃离火星大气层并撞击其卫星火卫一的离子的长期平均各向异性。这些测量值用于量化在地质时间尺度上影响火卫一上每个位置的平均离子通量的估计值。我们发现轰击火星离子的通量在月球表面是高度不对称的，因为火卫一的近侧看到的通量比其背面高出 15 到 100 倍。我们表明，这样做的第一个后果是火星大气中的氧、碳、氮和氩原子被植入并可能保存在火卫一近侧风化层颗粒的最上部数百纳米内，这些颗粒可能会通过未来的样品返回任务带回地球。第二个影响是风化层性质的改变在火卫一表面上是不对称的，因为火星离子将近侧的风化加速了约 2 倍。