An ENA Instrument for the Mercury-Exploring BepiColombo mission

The Mercury environment is a closely coupled system, in which three components, surface, exosphere, and magnetosphere interact with each other. In this environment low energy neutral atoms (LENA) with energies of few tens eV to few keV, which can be used for imaging, result from (1) charge-exchange of energetic ions in the near-planet environment with exospheric gasses, (2) sputtering from the planetary surface, and (3) backscattering of precipitating ions. Imaging these LENAs will visualize global distributions of magnetospheric ions, solar-wind ions, and ions of the planetary origin. It will also provide direct measurements to understand contribution of the sputtering sources to the Mercury exosphere. These global measurements are the key in understanding highly dynamic and compact magnetosphere. They will contribute to studies of morphology and dynamics of the precipitation zones, substorms at Mercury, magnetospheric structure and dynamics, and dynamics of planetary ions. We have proposed an ENA instrument (Energetic Neutral atom Analyzer) for Mercury Magnetospheric Orbiter (MMO) of the BepiColombo mission, which is a joint mission of Europe and Japan for Mercury exploration. The ENA instrument is based on surface ionization/reflection technique. A neutral particle is first ionized on a specially-developed surface and is selected in energy in an electrostatic analyzer. After energy analysis the particle hits a reflection surface and goes to detectors. Measuring time-of-flight from the reflection to the detection provides the velocity, which gives the mass of the particle by combining the energy analysis. The instrument is carefully designed to be light-weight (< 2.0 kg), with high photon suppression, and capable of resolving masses to meet the scientific objectives under tight designing limitations.