MAVEN Proton Observations Near the Martian Moon Phobos: Does Phobos Backscatter Solar Wind Protons?

ESA's Mars Express (MEX) may have observed twice solar wind protons backscattered by Phobos. However, these detections remain uncertain and call for an independent confirmation. Here, we analyze the proton measurements collected by the Suprathermal And Thermal Ion Composition experiment onboard NASA's Mars Atmosphere and Volatile EvolutioN mission (MAVEN) during the 15 closest encounters of MAVEN with Phobos (closest approach between 80 and 370 km). We use a model of magnetic connectivity to the Martian bow shock and an exospheric pickup ion model to identify the origin of the protons observed close to the Martian moon. We find no evidence of protons backscattered by Phobos. Instead, foreshock-shock foot protons and Martian exospheric pickup protons are often observed near Phobos. These results put into new context not only the past MEX detections, but also the future observation attempts of the upcoming JAXA Martian Moons Exploration mission.