Implantation of Martian atmospheric ions within the regolith of Phobos

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. Martian atmospheric atoms are implanted in and alter regolith grains on the nearside surface of Phobos, according to an analysis of observations of ion escape from Mars’s atmosphere.