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Figure 5. The Cassini Ion and Neutral Mass Spectrometer (INMS). The INMS is a quadrupole mass analyzer fed by two ion sources. The closed source is designed to sample inert species such as N₂, CH₄, C₂H₂, C₂H₆, and Ar. In the closed source mode, the ram pressure of the inflowing gas produces a 50-fold enhancement in INMS sensitivity. The open source mode is used to measure ions and reactive neutral species such as atomic nitrogen. INMS is less sensitive in this mode because there is no ram density enhancement.