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.
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MAVEN/STATIC Level 2 data are publicly available on the NASA Planetary Data System website (https://pds-ppi.igpp.ucla.edu/) in the volume named ‘MAVEN-Mars-STATIC’. The long-term average of STATIC directionally resolved fluxes at the orbit of Phobos computed in this article, the ion precipitation maps on the surface of Phobos and the computed implantation rate and deposited collisional energy depth profiles have been delivered to the published Figshare repository: https://doi.org/10.6084/m9.figshare.12939872.
The SRIM software is accessible online at http://www.srim.org/.
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Q.N. and A.R.P. acknowledge funding from the NASA Solar System Research Virtual Institute via both the DREAM2 Team, grant #NNX14AG16A, and the LEADER team, grant #80NSSC20M0060. A.R. and J.P.M. acknowledge funding by NASA through MAVEN Project subcontracts managed by LASP, University of Colorado under the direction of the MAVEN principal investigator, B. M. Jakosky. The MAVEN mission has been made possible through NASA sponsorship and the dedicated efforts of NASA Goddard Space Flight Center, LASP, Lockheed project management and the MAVEN Technical and Science Teams. We thank A. Quillen (University of Rochester) for discussions on the tumbling of Phobos and H. Hofsäss (University of Göttingen) and W. Weber (University of Tennessee) for discussions on the use of the SRIM simulation tool.
The authors declare no competing interests.
Peer review information Nature Geoscience thanks Roy Christoffersen and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Tamara Goldin; Stefan Lachowycz.
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Nénon, Q., Poppe, A.R., Rahmati, A. et al. Implantation of Martian atmospheric ions within the regolith of Phobos. Nat. Geosci. 14, 61–66 (2021). https://doi.org/10.1038/s41561-020-00682-0