Description of the Mass Spectrometer for the Jupiter Icy Moons Explorer Mission

The JUpiter ICy moons Explorer (JUICE) of the European Space Agency (ESA) will investigate Jupiter and its icy moons Europa, Ganymede and Callisto, with the aim to better understand the origin and evolution of our Solar System and the emergence of life. The Neutral gas and Ion Mass spectrometer (NIM) is one of six instruments of the Particle Environment Package (PEP) on board the JUICE spacecraft. PEP will measure neutral atoms and molecules, the ion population, and the electron population over an energy range covering from meV to MeV. The NIM instrument is designed to measure the chemical and isotope composition of the exospheres of three of Jupiter's satellites, the icy moons, both, during several flybys and during its final destination in Ganymede orbit. From measurements of the exosphere, we will derive the chemical composition of the surface, which will allow us a better understanding of the icy moons formation processes, interaction processes with the magnetospheric plasma and energetic particles of Jupiter's magnetospheric system. The NIM instrument is a compact time-of-flight mass spectrometer allowing measurements of thermal neutral molecules and ionospheric ions. To minimize the background radiation on the detector and protect electronics against the harsh radiation environment around Jupiter, elaborated radiation shielding was designed. NIM consists of two major subunits, namely, the ion-optical system and the electronics. This study presents details on the technical design and the results obtained from the calibration campaigns of different subsystems of the flight instrument including a mass range of m/z 1 to 650, a mass resolution m/Δm of at least 750 (FWHM), and an instantaneous dynamic range of almost 6 decades. These results are discussed in detail with respect to the scientific requirements. This performance in combination with its radiation tolerance allows for both a detailed analysis of the chemical composition of Jupiter's icy moons' exospheres and ionospheres, and to explore environments, where formation of life might be possible.