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. Plain Language Summary: The surface a solar system body that does not have an atmosphere can be directly bombarded and processed by the small positively charged particles ejected by the Sun, known as solar wind ions. At the Earth Moon, previous work discovered that 0.1%–1% of the solar wind protons hitting the lunar surface are backscattered in space as charged particles instead of traveling through the planetary surface. The only other place where this fundamental process may have been detected so far is Phobos, a small rocky moon that orbits the red planet Mars. Indeed, ESA's satellite Mars Express (MEX) may have previously observed such protons at Phobos, but these detections remain uncertain. To advance on the question of solar wind proton backscattering at Phobos, we analyze in this article the proton measurements gathered by NASA's Mars Atmosphere and Volatile EvolutioN mission (MAVEN) mission close to Phobos. Models are employed to identify the origin of the protons observed by MAVEN and we find no evidence of protons backscattered by the Martian moon. This result puts into new context the past MEX detections and the future observation attempts of the upcoming JAXA Martian Moons Exploration mission. Key Points: Mars Atmosphere and Volatile EvolutioN mission (MAVEN) flew by Phobos 15 times at a distance lower than 370 kmWe find no evidence of solar wind protons backscattered by the surface of Phobos in the MAVEN proton measurementsForeshock—shock foot protons and exospheric pickup protons are often detected and can be misidentified as solar wind backscattered protons [ABSTRACT FROM AUTHOR]