ABSTRACT
  ========
 
    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 text of this instrument description has been abstracted from
    the instrument paper [WAITEETTAL2004].
 
    Waite, Jr J.H, 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,
    R. L. Thorpe, E. M. Walter, R. V. Yelle, The Cassini Ion and
    Neutral Mass Spectrometer (INMS) Investigation, Space Sci. Rev.,
    in press, 2004.
 
 
  INSTRUMENT OVERVIEW
  ===========================
 
    The INMS instrument [KASPRZAKETAL1996] 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
    ante-chamber. In this mode, neutral species with concentrations on
    greater than approximately 1.0E04 per cubic centimeter will be
    detected (compared with approximately 1.0E05 per cubic centimeter
    in the open source neutral mode). For ions the detection threshold
    is on the order of 1.0E-02 per cubic centimeter at Titan relative
    velocity of (6 kps). The INMS instrument has a mass range of 1 to
    99 Daltons and a mass resolution M/(deltaM) 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.
 
 
  SCIENTIFIC OBJECTIVES
  =====================
 
    The primary objectives of the Cassini Ion and Neutral Mass
    Spectrometer investigation are to study composition and structure
    of Titan's upper atmosphere and the neutral and plasma
    environments of Saturn's ring system, icy moons and inner
    magnetosphere.
 
    -Objectives concerning the upper atmosphere of Titan include:
 
      Determine the thermal structure of Titan's upper atmosphere.
 
      Determine the bulk composition of Titan's upper atmosphere and
      the key chemical processes which determine it.
 
      Investigate the interaction between Titan's and Saturn's
      magnetosphere and between Titan and the solar wind.
 
      Determine if Titan's ionosphere is magnetized and the source of
      the magnetization.
 
      Investigate the interactions at the upper ionosphere boundary.
 
      Determine the relative contributions of various loss processes.
 
      Determine the contribution of neutrals and ions to Saturn's
      magnetosphere by Titan
 
    -Objectives concerning Saturn's inner magnetosphere, rings and icy
      satellites include:
 
      Determine the composition and density of the ring system neutral
      atmosphere and ionosphere.
 
      Investigate the interactions between the icy satellites and the
      magnetospheric plasma.
 
 
  CALIBRATION
  ===========
 
    The characterization of the INMS flight unit was performed at
    Goddard Space Flight Center using a high-vacuum test station with
    both thermal neutral and ion sources.  A neutral beam system was
    not available at the time of the INMS testing. Thus the ion beam
    was also used to characterize instrument performance in the open
    source neutral mode; for these tests, however, the INMS entrance
    lens (OL4) potential was set at Ð5 V, as required for the
    neutral beaming mode.  The test station was designed so that all
    the INMS operational modes could be characterized without breaking
    the vacuum and thus necessitating re-baking the sensor. Neutral
    gases and ions used for characterization testing were introduced
    into the main vacuum chamber, to which the INMS was attached by a
    flexible bellows with two degrees of rotational freedom for angles
    up to about 5 degrees.  The instrument could be translated to
    allow appropriate positioning of the source being tested (i. e.,
    of the open source with respect to the ion beam).  Pressures
    inside the main vacuum chamber were kept below ~10^-6 hPa in order
    to prevent possible damage to the secondary electron multipliers.
    Thus the operation of the instrument at higher pressures, i. e, up
    to mid-10^-5 hPa, the estimated ram pressure at Titan closest
    approach, was not tested.  Laboratory support electronics were
    used for early testing; flight electronics were used for the final
    characterization.  Characterization of INMS performance will
    continue during the post- launch period with testing of the
    engineering unit.
 
 
  OPERATIONAL CONSIDERATIONS
  ===========================
 
    [to be supplied]
 
  DETECTORS
  =========
 
    The INMS instrument [KASPRZAKETAL1996] is a modification of the
    Neutral Gas and Ion Mass Spectrometer instrument designed for the
    Comet Rendezvous Asteroid Flyby Mission.  The Cassini instrument
    consists of two separate ion sources for sampling ambient neutrals
    and ions, an ion deflector/trap, four hot-filament electron guns,
    an electrostatic quadrupole switching lens that selects between
    the sources, various focusing lenses, a quadrupole mass analyzer,
    and two secondary electron multiplier (SEM) detectors. Instrument
    control is provided by the Flight Computer, according to the
    values entered in various software tables.
 
 
    The gas densities at Titan and other INMS targets are nearly
    optimal for direct sampling without ambient pressure reduction.
    Two separate ion sources Ñ a closed source and an open
    sourceÑrather than a single combined quasi-open ion source are
    used in the INMS instrument in order to optimize interpretation of
    the neutral species.  In the closed source mode, the ram pressure
    of the inflowing gas creates a density enhancement in the source
    antechamber, allowing the sampled species to be measured with
    relatively high precision and sensitivity.  This mode will be used
    to measure species, such as N2 and CH4, which do not react with
    the antechamber surfaces. The open source has the advantage that
    it can measure reactive neutral radicals, such as atomic nitrogen,
    and ions.  In this mode, the ambient neutral gas density is
    sampled directly with no stagnation enhancement and no collisions
    with the surfaces of the instrument.  For open source ion
    measurements, the INMS angular response can be increased beyond
    the geometric view cone (8.6¡ cone half angle) by adjusting the
    voltages on the plates in the ion deflector/trap and the exit
    aperture lens (top plate lens).  For neutral sampling in the open
    source mode, the ion trap removes incoming ions and electrons,
    which could cause spurious ionization of neutral species, and
    allows only neutrals to pass into the ionization region.  In both
    the closed and open source modes, impacting electrons emitted from
    the hot- filament electron guns ionize the sampled neutrals.
 
    Electrostatic lenses are used to focus the ambient ions and those
    created from ambient neutrals by electron impact into the
    quadrupole switching lens [MAHAFFY&LAI1990], an electrostatic
    device that steers ions from either the closed or open source
    through a system of focusing lenses into a dual radio frequency
    (RF) quadrupole mass analyzer.  The mass analyzer selectively
    filters the ions according to their mass-to-charge ratio.  Two
    secondary electron multipliers operating in pulse-counting mode
    cover the dynamic range required.  The INMS mass range was
    increased from its initial value of 1-66 to 1-99 Daltons (atomic
    mass units) to allow detection of heavier hydrocarbon species and
    possible pre-biotic cyclic hydrocarbons such as C6H6.  Using two
    different radio frequencies and scanning the mass to charge ratios
    from 1 to 8 and then from 12 to 99 Daltons accomplish this.
 
    The INMS instrument is mounted on the CassiniÕs Fields and
    Particles Pallet (FPP).  The out-ward normal to both the open and
    closed source INMS apertures lies in the spacecraft ÐX
    direction.  The open source geometric field of view is about 8.6¡
    cone half angle.  This limits the angular response for neutral and
    ions measured in the open source mode, although, as noted above,
    the angular response for the measurement of ambient ions can be
    improved by adjusting the voltage applied to the open source ion
    deflectors.  In contrast, the closed source has a much wider
    geometric field of view of approximately 2¹ steradians.  The
    open source is vented to lower the ion source and analyzer
    pressures (increasing the ion mean free path) during a Titan pass
    when the spacecraft ram is approximately along the ÐX
    direction.  Venting occurs at right angles to the ÐX axis.
 
 
  ELECTRONICS
  ===========
 
    The INMS electronics system is based on designs used for the
    HuygenÕs Probe GCMS instrument.  A low-voltage (LV) power
    supply converts spacecraft power to well-regulated DC voltages
    that are supplied to the instrument electronics.  A
    pulse-width-modulated converter allows efficient generation of
    multiple secondary voltages while providing secondary-to-primary
    isolation.  A large number of voltages are required to bias the
    various focus electrodes as well as to supply DC voltages for the
    secondary electron multipliers.  Analog modules are used for
    regulating the emission of the electron guns, for providing fixed
    and programmable voltages to set lens potentials, for supplying RF
    and DC for the quadrupole mass analyzer, for supplying high
    voltages for the detectors, and for the pulse-counting circuits.
    The digital electronics includes a single micro-processor, a
    spacecraft bus interface circuit, and an interface between the CPU
    and the analog modules.  Major portions of the electronics are
    packaged in hybrid circuits to save weight and space.
 
    A radio frequency generator drives the quadrupole at two resonant
    frequencies in order to reduce the need for large amplitude
    potential for the required mass range (1-99 Daltons).  A
    solid-state switched bandpass filter performs frequency selection.
    The DC voltage is created by high-voltage operational amplifiers
    and is superimposed on the RF amplitude.  Digital-to-analog
    converters program both the RF and DC amplitudes.
 
    Charge pulses at the anode of the electron multiplier are
    converted by a pulse amplifier into voltage pulses that are
    counted if they are above a pre-set threshold.  Analog measurement
    of the multiplier current is used to determine the in-flight
    multiplier gain.
 
    The Flight Computer uses a 16-bit Marconi MA31750 microprocessor
    running at 10 MHz, with 64 K primary RAM, 64 K ROM, and 32 K
    extraÓ RAM (used only for data storage, not for execution of
    flight software).  The computer controls the INMS measurement
    sequence, counts the detector pulses, provides analog-to-digital
    conversion of the detector current, and monitors instrument
    housekeeping parameters.  The computer is programmed in Ada as the
    target language with some use of assembly language to handle
    time-critical functions, input/output, and interrupts.  The
    instrument ROM/RAM contains the default measurement and test
    sequences without requiring memory upload.
 
 
  OPERATIONAL MODES
  ===========================
 
    INMS measurement strategies and sampling methods are determined by
    the investigation's science objectives and must take into account
    the region and species being sampled.  The basic sampling sequence
    is the 'scan' which is a series of 68 mass/charge measurements;
    the mass numbers to be sampled in each scan are specified by a
    particular 'Mass Table'.  Each measurement period or 'integration
    period' (IP) lasts 34 ms (a 31-ms counting period plus ~3 ms for
    set up and read out).  Each scan therefore requires 2.3 s (= 34 ms
    per sample x 68 IPs or samples).  A scan or series of scans to be
    repeated constitutes a 'cycle' The operation of the instrument for
    each scan in a cycle is defined by a 'Cycle Table' which indicates
    the mass table and other control tables to be used for a
    particular scan.  One or more cycles make up a 'science sequence'.
    A science sequence is initiated by receipt of a time-tagged
    'trigger' command from the Orbiter. Trigger commands will be sent
    from the ground to the Orbiter and stored in the Solid State
    Recorder (SSR) for later execution; under some circumstances, it
    may be possible to command an orbital sequence from the ground in
    real time.  The cycles to be performed during the sequence are
    identified in a 'Sequence Table', which also specifies a velocity
    constant used to modify the quad lens voltages for velocity
    compensation in the open source mode.  Several science sequences
    were defined prior to SOI.  Additional sequences are expected to
    be designed and uploaded to the INMS flight computer once
    exploration of the Saturn system is under way.
 
    Default Science Sequence
    ------------------------
 
      The 'Default Science - 1498bps' sequence is the basic sequence
      executed by the INMS unless another orbital sequence has been
      commanded.  This sequence comprises two cycles.  In Cycle 1, the
      instrument performs two unitary survey scans from 1 to 8 and 12
      to 70 Daltons (Mass Table 1 for CSN and 26 for OSI), the first
      in the closed source mode and the second in the open source ion
      mode.  Cycle 1 is performed in 4.6 s and repeated for ~30
      minutes (389 scans in each mode). Cycle 2 consists of
      alternating survey scans in the open source ion mode and in the
      closed source mode.  Mass Tables 2-13 for CSN and 27-38 for OSI,
      covering the mass ranges 0.5-8.5 and 11.5-99.5, are used for
      both surveys in Cycle 2.  Cycle 2 takes 55.2 s to execute.  The
      sequence is looped until a different sequence is commanded.
 
      There are three other Default Science sequences, each tailored
      to a specific data rate: 100, 50 and 6.2 bits per second (bps).
      These rates are designed to make use of the co-adding function
      of the INMS, while keeping a very similar measurement order and
      timing to the full rate Default Science mode.
 
    Titan Exploratory Sequence
    --------------------------
      The 'Titan Exploratory - TA' Sequence will be executed during
      the Orbiter's first two flybys of Titan (Ta: Oct. 2004, Tb: Dec.
      2004) and will occur at an altitude of approximately 1250 km.
      The initial flybys will take place prior to the descent of the
      Hugyens Probe (Tc: Jan. 2005).  Execution of this sequence will
      initiate the INMS investigation of Titan's thermosphere and
      ionosphere, which is the primary science objective of the INMS
      experiment. In addition the INMS measurements of atmospheric
      density made during the initial flybys will be operationally as
      well as scientifically important because they will allow
      assessment of atmospheric drag effects on the Orbiter during
      subsequent flybys at lower altitudes.
 
      The Titan Exploratory Sequence is composed of five cycles and is
      designed to characterize the major neutral species in Titan's
      upper atmosphere.  The INMS will execute Cycle 1 from an
      altitude of ~10,000 km until ~180 seconds before Titan closest
      approach.  Two scans will be performed in sequence, first in the
      closed source mode (using mass tables 16 and 17) and then in the
      open source neutral mode (using mass tables 54 and 55).  As
      specified by these two tables, the INMS will alternate sampling
      of masses 2, 16, 17, 28, and 29 with mass surveys in 1-Dalton
      increments until the entire mass range of 1-99 Daltons
      (excluding 9-11 Daltons) has been covered.  Repeated measurement
      of masses 2, 16, 17, 28, and 29 during the two scans will
      provide high-temporal-resolution data on the density profiles of
      the principal neutral and ion species known or expected to be
      present in Titan's atmosphere: H2 (2), CH4 (16), N2 (28), H2CN+
      (28), CH5+ (17), and C2H5+ (29).  With these data, scale heights
      can be calculated with a resolution of ²3 km, thus allowing
      the detailed structure of Titan's upper atmosphere to be
      determined.  After ~1435 s, Cycle 2 will start, performing the
      same mass scans as Cycle 1 but with a slightly different
      velocity compensation value to reflect the changing
      Titan-relative radial velocity of Cassini.  Cycle 3 will start
      ~18 s before closest approach.  In Cycle 3 the INMS will perform
      an alternating sequence of adaptive/unitary scans (Mass Tables
      16 and 17 for CSN) and adaptive/fractional scans (Mass Tables
      18/19 for CSN and 56/57 for OSNB).  Throughout all of these
      scans, masses 2, 16, 17, 28, and 29 will be sampled at the same
      rate, to keep a consistent measurement of the primary
      constituents of Titan's atmosphere.  At ~18 seconds after
      closest approach the INMS will perform Cycle 2 followed by Cycle
      1 ~160 seconds later in an exact mirror of the beginning of the
      sequence.
 
    Titan High-Altitude Neutral Atmosphere and Ionosphere Sequence
    --------------------------------------------------------------
      The 'Titan High-Altitude Ionosphere Flyby' Sequence will be used
      during Titan flybys at altitudes above 1500 km, i.e., above the
      exobase (~1425 km).  This sequence consists of a single repeated
      cycle identical to Cycle 1 in the 'Titan Exploratory - TA'
      Sequence with OSI replacing OSNB mode.  It thus will provide
      both the survey data needed to characterize the composition of
      Titan's exosphere and ionosphere and the
      high-temporal-resolution data on the expected major constituents
      (masses 2, 16, 17, 28, and 29) needed to establish the structure
      of the upper atmosphere.
 
    Titan Low-Altitude Aeronomy Sequence
    ------------------------------------
      The 'Titan Low-Altitude 006TI_T5' Sequence is designed for
      composition measurements at altitudes as low as is consistent
      with Orbiter safety (this version is specifically tailored to the
      5th Titan Pass).   Several such low-altitude passes, with
      spacecraft orientation optimized to point the open source
      aperture into the spacecraft ram direction, are required for
      successful completion of the INMS science investigation.  A
      minimum flyby altitude of 950 km has been selected for these
      passes, based on densities predicted by theoretical models.
      Flybys at this altitude will allow for data acquisition well
      below the ionospheric peak and the homopause - both of which are
      predicted to occur at ~1050 km [STROBELETAL1992] [FOX&YELLE1997]
      [KELLERETAL1998] - and well into the region where the
      photochemical production of complex hydrocarbons and nitriles is
      initiated.  At this altitude, the INMS will be able to measure
      with maximum sensitivity minor species, including short-lived
      chemically active neutral and ion species that play an important
      role in titan's photochemistry and ion-neutral chemistry.
 
    Outer Magnetosphere Sequence
    -----------------------------
      There are two different versions of the Outer Magnetosphere
      Sequence, one for purely neutral measurements (used during the
      inbound leg of the orbit) and one for ion and neutral
      measurements (used during the outbound leg of the orbit).  Each
      of these measures the same mass values in the same order, but
      uses OSNB or OSI mode, respectively, for the second set of
      measurements.  Ve-locity compensation values were selected to
      account for expected spacecraft-relative velocities of particles
      in Keplarian, corotating or magnetic-field-aligned orbits.
      There are also 4 different data rate modes currently available,
      just as there are for Default Science: 1498, 100, 50, and 6.2
      bps, with 1, 15, 30, and 240 co-added scans, respectively.
 
      Because densities are expected to be low, long accumulation
      periods will be used and the mass scans co-added to improve
      counting statistics.  Mass Tables 25 (CSN) and 44 (OSNB) are
      used for exclusively neutral measurements and 25 (CSN) and 63
      (OSI) are used to sample ions; masses of particular interest are
      14 (N, N+), 16 (O, O+), 17 (OH, OH+), 18 (H2O, H2O+), and 28
      (N2/H2CN, N2+/H2CN+).
 
    Inner Magnetosphere Sequence
    ----------------------------
      The organization of these sequences is very similar to that of the
      'Outer Magnetosphere' sequences: a single mass range sampled
      alternately in CSN and OSNB or CSN and OSI modes.  The choice of
      neutral or ion and neutral is the same as used for the outer
      magnetosphere: neutral for inbound and ion and neutral for
      outbound. The four data rates are organized in the same way.
      Neutral particles in Keplerian orbits closer to Saturn will move
      faster, and the velocity compensation values were increased
      accordingly. The Mass Tables used – 14 for CSN, 39
      for OSI and 53 for OSNB – involve repeated
      measurement of masses 12-19 interleaved with measurements of the
      mass ranges 1-8, 20-27, 28-35, and 26-47 Daltons.  This provides
      for the repeated sampling during each scan of the water group
      neutrals O (16), OH (17), and H2O (18), and ions O+ (16), OH+
      (17), H2O+ (18), and H3O+.  Although the densities of these
      species are expected to be at a maximum near the predicted
      source regions, they will still be at the lower end of the INMS
      sensitivity.
 
    Ring Overflight for SOI
    -----------------------
      The 'Ring Overflight for SOI' Sequence is designed to sample the
      neutral and plasma environments of the rings and icy satellites
      in Saturn's inner magnetosphere and will be executed during the
      overflight of the rings following SOI.  A modified version of
      this sequence could also be used during the planned flybys of
      Iapatus, Enceladus, Dione, and Rhea.  Because the INMS team has
      primary spacecraft axis control during a period after SOI, a
      specific sequence was designed to cover this period.  The Mass
      Tables used are the same as those used in the 'Inner
      Magnetosphere' sequences, but the timing is different.  The
      first ~700 s of the measurement period centers on
      magnetic-field-aligned and corotating ions, while the next ~480
      s will measure neutrals corotating and in Keplerian orbits.
      Velocity compensation values were chosen to match the Cassini-
      relative velocities of particles in each type of orbit at that
      time period, based on estimated particle masses and energies.