SM049-03
Asymmetric ion bombardment and weathering of the Martian Moon Phobos

Tuesday, 15 December 2020: 17:38
Virtual
Quentin Nenon, Space Sciences Laboratory, University of California at Berkeley, Berkeley, United States, Andrew R Poppe, Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA, United States, Ali Rahmati, University of California Berkeley, Space Sciences Laboratory, Berkeley, CA, United States and James P McFadden, Univ California Berkeley, Berkeley, CA, United States
Recorded Presentation
Abstract:
Phobos, a small airless moon of Mars, is not only bombarded by solar wind ions (mainly H+ and He++ ions), but also by ions escaping the atmosphere of the red planet: “Martian ions” (mainly O+, O2+, CO2+ ions). In this presentation, we investigate the long-term weathering and contamination of Phobos’ surface by ions. To do so, in-situ ion observations conducted near the orbit of Phobos by the NASA MAVEN mission are combined with ion transport and interaction simulations (SRIM software).

MAVEN has repeatedly crossed the orbit of Phobos from 2015 to 2019, which enabled the mission to gather a unique long-term dataset of ion observations at Phobos’ altitude at all longitudes along the moon’s orbit, i.e. in the upstream solar wind and inside the Martian induced magnetosphere. Measurements from three ion instruments are combined in this presentation, including specie-resolved observations from the MAVEN STATIC instrument which separates light solar wind from heavy planetary ions.

MAVEN STATIC direction-resolved flux measurements reveal that the long-term bombardment of Phobos by ions is highly inhomogeneous on the moon’s surface, as solar wind ions mainly impact Phobos on its far side (which always points away from Mars), whereas the flux of Martian ions is 100x more intense on the near side of Phobos than on the far side.

Martian ions are found to significantly weather the near side of Phobos, as they contribute to 50% of its total sputtering and accelerate by a factor of 2 the creation of amorphous rims atop regolith grains. In addition, oxygen, carbon, and noble gas atoms transported by ions escaping the atmosphere of Mars are implanted in the top tens of nanometers of Phobos’ near-side regolith. This regolith is therefore contaminated by atmospheric ions, but also preserves a unique archive of the ancient atmosphere of Mars. This should be considered when interpreting the isotopic composition of the samples that will be brought back to Earth by the JAXA MMX mission.

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