Phobos Surface Sputtering as inferred from MAVEN Ion Observations
Abstract
Sputtering of airless body surfaces by energetic ions (typically >100 eV) is a universal process across the Solar System. Its study is important as it is one of the major weathering processes and liberates surface material directly into space.
Phobos and Deimos, the two moons of Mars, are unique as they are exposed to not only solar wind ions (protons and alpha particles), but also to ions coming from the red planet itself (oxygen and dioxygen ions). The relative importance of solar wind and planetary ions in Phobos surface sputtering has only been inferred from advanced models so far [e.g., Poppe and Curry, 2014] and remains to be constrained from in-situ measurements. The MAVEN spacecraft has repeatedly crossed the orbit of Phobos from January 2015 to February 2019 and is fully equipped to observe ions with kinetic energies from 0.1 eV to more than 6 MeV with three instruments: the Solar Wind Ion Analyzer (SWIA, 30 eV to 30 keV), the SupraThermal And Thermal Ion Composition instrument (STATIC, 0.1 eV to 30 keV, designed to separate ion masses) and the Solar Energetic Particle detector (SEP, 20 keV to more than 6 MeV). In this presentation, we will first show the four-year average ion environment that Phobos is exposed to as observed by MAVEN in all the plasma regions encountered by the moon: solar wind, magnetosheath, and magnetotail. In turn, the flux of material sputtered from the surface of Phobos by this sputtering environment has then been computed. MAVEN in-situ measurements confirm that planetary oxygen ions are an important contributor to surface sputtering. Unexpectedly, dioxygen ions have been revealed to dominate sputtering in the martian magnetotail. In addition to the long-term average picture, the amplitude of the time variation of Phobos sputtered fluxes has been investigated during the two major solar wind events of March 2015 and September 2017. Finally, numerical models predict the existence of a putative Phobos neutral torus sourced by surface sputtered material [Cipriani et al., 2011; Poppe et al., 2016]. Heavy aluminum, iron, and silicon ions are unique to the torus and can thus be used to unambiguously detect the Phobos neutral torus. We will report on our attempt to detect these ions with the MAVEN STATIC instrument, which is able to identify ion masses.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.P31C3482N
- Keywords:
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- 6205 Asteroids;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 6230 Martian satellites;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 6250 Moon;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS