The Neutral Gas and Ion Mass Spectrometer on CONTOUR
The Neutral Gas and Ion Mass Spectrometer will measure the abundance and isotope ratios for many neutral and ion species in the coma of each comet during the flyby. These measurements together with data from the dust experiment will contribute to the understanding of the chemical composition of the nucleus itself and allow differences between the comets to be studied. NIGMS is a quadrupole mass spectrometer that employs two ion sources each optimized for a specific set of measurements. Using these two sources, NGIMS will rapidly switch between measurements of the cometary neutral gas and ambient ions from the coma as the CONTOUR spacecraft flies by the nucleus. Understanding the chemical state of comets is expected to provide the best possible record of conditions present in the outer solar nebula early in its history, since the comet nuclei of comets likely contains primitive materials unaltered by the chemical and physical processes that have transformed other solar system objects.
Science Objectives of NGIMS
The chemical composition measurements of NGIMS are expected to provide an understanding of the relative abundances of simple species such as H2O, CH4, CO2, NH3, and H2S and considerably more complex molecules in the early stages of the protosolar nebula. Since comets have been in cold storage since their formation, this record is expected to be largely preserved. Cometary infall contributes volatile species to the planets including the atmosphere and oceans of the Earth and the chemical complexity of this material from space can be understood through the NGIMS data. Another scientific goal is to clarify the relationship of the solar nebula to the parent interstellar cloud. A key to each of these scientific objectives is the measurement of isotope ratios such as D/H and the abundance of "thermometer" gases in comets such as argon and other simple species. The history of the chemical and physical processing of the nucleus material can only be understood though such a detailed study.
NGIMS Functions
High Sensitivity Sampling in the Closed Ion Source
The closed ion source will use RAM density enhancement to provide measurements of higher accuracy and sensitivity for the more inert atomic and molecular species than provided by the open source. The sensitivity enhancement is achieved by sampling the ambient gas through an orifice into an enclosed antechamber. The density enhancement in the source compared to the ambient density can be as high as several hundred depending on the flyby velocity, the molecular weight of the sampled species, and the source temperature. The closed source works best with species that do not chemically transform on the source walls.
Open Source Sampling of Thermal and Medium Energy Ions
Ion species present in the coma within the mass range of the NGIMS will be sampled through the open source. These ions are created by photoionization, electron ionization, and other processes acting on gases in the coma.
Ion Source Selection in the Quadrupole Deflector
Ions are sequentially directed to the mass analyzer from the selected ion source by changing the potentials on the quadrupole deflector. This electrostatic device allows either source of ions to be focused into a common exit lens system.
The Quadrupole Mass Analyzer
The quadrupole mass analyzer consists of four precision ground hyperbolic rods mounted in a rigid mechanical assembly. The transmitted mass to charge value and the resolution are controlled by variations in RF and DC electric fields between adjacent rod pairs while opposite rod pairs are kept at the same potential. Three fixed frequencies (in the 1 to 5 MHz range) are selected to cover the mass range (1 to 300 amu).
The Dual Detector System
The detector system counts ions transmitted through the quadrupole analyzer to produce a mass spectrum. The system is redundant and configured for a wide dynamic range.
