The Cassini Ion and Neutral Mass Spectrometer (INMS) investigation

J. H. Waite, W. S. Lewis, W. T. Kasprzak, V. G. Anicich, B. P. Block, T. E. Cravens, G. G. Fletcher, W. H. Ip, J. G. Luhmann, R. L. Mcnutt, H. B. Niemann, J. K. Parejko, J. E. Richards, R. L. Thorpe, E. M. Walter, R. V. Yelle

Research output: Contribution to journalArticlepeer-review

190 Scopus citations


The Cassini Ion and Neutral Mass Spectrometer (INMS) investigation will determine 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 positive ions and neutral species in Saturn's inner magnetosphere. Measurement of material sputtered from the satellites and the rings by magnetospheric charged particle and micrometeorite bombardment is expected to provide information about the formation of the giant neutral cloud of water molecules and water products that surrounds Saturn out to a distance of ∼12 planetary radii and about the genesis and evolution of the rings. 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 will be 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. Instrument sensitivity is greatest in the first mode, because the ram pressure of the inflowing gas can be used to enhance the density of the sampled non-reactive neutrals in the closed source antechamber. In this mode, neutral species with concentrations on the order of ≥104 cm -3 will be detected (compared with ≥105 cm-3 in the open source neutral mode). For ions the detection threshold is on the order of 10-2 cm-3 at Titan relative velocity (6 km sec-1). The INMS instrument has a mass range of 1-99 Daltons and a mass resolutionM/ΔM of 100 at 10% of the mass peak height, which will allow detection of heavier hydrocarbon species and of possible cyclic hydrocarbons such as C6H6. The INMS instrument was built by a team of engineers and scientists working at NASA's Goddard Space Flight Center (Planetary Atmospheres Laboratory) and the University of Michigan (Space Physics Research Laboratory). INMS development and fabrication were directed by Dr. Hasso B. Niemann (Goddard Space Flight Center). The instrument is operated by a Science Team, which is also responsible for data analysis and distribution. The INMS Science Team is led by Dr. J. Hunter Waite, Jr. (University of Michigan).

Original languageEnglish (US)
Pages (from-to)113-231
Number of pages119
JournalSpace Science Reviews
Issue number1-4
StatePublished - Dec 2004


  • Cassini
  • Huygens
  • Mass Spectrometry
  • Saturn
  • Titan

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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