Phobos Surface Sputtering as Inferred From MAVEN Ion Observations

Abstract

Phobos is bombarded by both protons and alpha particles from the solar wind and by Martian atomic and molecular oxygen ions. A numerical model of the distribution of planetary ions has previously proposed that these ions may dominate solar wind ions in sputtering the surface of Phobos when the moon is located downstream of Mars. This conclusion suggests a unique link between planetary atmospheric escape at Mars and the surface processing of its moon yet, remains to be confirmed with in situ ion measurements. In this article, a 4‐year‐long average of the ion environment that Phobos is exposed to is constructed from in situ ion observations conducted by the Mars Atmosphere and Volatile Evolution (MAVEN) mission. In turn, the flux of material sputtered from the surface of Phobos by this environment is computed. We confirm that planetary atomic oxygen ions dominate over solar wind ions in sputtering the surface of Phobos downstream of Mars during 20% of the moon's orbit. We also reveal that molecular oxygen ions sputter the surface of Phobos as much as or more than atomic oxygen ions in the Martian magnetotail. In addition to the long‐term average picture, the time variation of Phobos surface sputtered fluxes is investigated during the series of solar wind events that hit the Martian system in March 2015. We find that the flux of material liberated from Phobos' surface increased by a factor of 50 during this period.

Experimental Insights Into Space Weathering of Phobos: Laboratory Investigation of Sputtering by Atomic and Molecular Planetary Ions

“…With a rescaling based on the presented experimental findings, the contribution by planetary O ions is still dominant in the tail by a factor of up to 10 and the longitudes where the ratio is above 1 are barely affected (red line). Averaged over a full Phobos orbit, the contribution from O sputtering to the total sputtering of Phobos' surface decreases from 22% to 13% compared to the calculation results by Nenon et al (2019). This value was calculated for the whole surface of Phobos and does not take into account its tidal locking.…”

“…This high-energy effect should not play a significant role for Phobos as the ion fluxes in the Martian plasma environment at these high energies do not significantly contribute to the sputtering of its surface. Sputtering is instead dominated by ions below 10 keV, as it was calculated by Nenon et al (2019).…”

“…Significant increases can also be observed for CO + and CO 2 + ions. Nevertheless, Nenon et al (2019) mention that MAVEN measurements of CO 2 + ion fluxes were found to be significantly lower than those of O + and O 2…”

“…Recently, the Mars Atmosphere and Volatile Evolution (MAVEN) mission has provided a more detailed look into Mars' ion environment (Jakosky et al, 2015;Ramstad et al, 2018). Based on these observations, Nenon et al (2019) modeled the effect of different sputtering sources on Phobos. Using measurements from the MAVEN instruments Supra thermal And Thermal Ion Composition and Solar Wind Ion Analyzer, the fluxes and energies of different ion species reaching the surface of Phobos were assessed.…”

In Situ Observations of Ions and Magnetic Field Around Phobos: The Mass Spectrum Analyzer (MSA) for the Martian Moons eXploration (MMX) Mission

“…The Martian moons would contain substantial materials of the Martian origin which have carried since their birth via ejecta (Hyodo et al, 2019) and escaping ions (e.g., Inui et al, 2018, Nénon, et al, 2019. The heavy isotope enrichment in major elements (C, O, etc.…”