The Cassini Ion and Neutral Mass Spectrometer (INMS) investigation measures the mass composition and number densities
of neutral species and low-energy ions in key regions of the Saturn system. The primary focus of the INMS
investigation is on the composition and structure of Titan’s upper atmosphere and its interaction with Saturn’s
magnetospheric plasma. Of particular interest is the high-altitude region, between 900 and 1000 km, where the methane
and nitrogen photochemistry is initiated that leads to the creation of complex hydrocarbons and nitriles that may
eventually precipitate onto the moon’s surface to form hydrocarbon–nitrile lakes or oceans. The investigation is
also focused on the neutral and plasma environments of Saturn’s ring system and icy moons and on the identification
of neutral species in the plume of Enceladus.
The INMS instrument consists of a closed ion source and an open ion source, various focusing lenses, an
electrostatic quadrupole switching lens, a radio frequency quadrupole mass analyzer, two secondary electron
multiplier detectors, and the associated supporting electronics and power supply systems. The INMS is operated
in three different modes: a closed source neutral mode, for the measurement of non-reactive neutrals such as
N2 and CH4; an open source neutral mode, for reactive neutrals such as atomic nitrogen;
and an open source ion mode, for positive ions with energies less than 100 eV. The INMS instrument has a mass
range of 1–99 Daltons and a mass resolution M/ΔM of 100 at 10% of the mass peak height, which allows
detection of heavier hydrocarbon species and of cyclic hydrocarbons such as C6H6.