PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = " 2004-11-29 David Gell (U. Mich.), initial;" RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = CO INSTRUMENT_ID = INMS OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "ION AND NEUTRAL MASS SPECTROMETER" INSTRUMENT_TYPE = "QUADRAPOLE MASS SPECTROMETER" INSTRUMENT_DESC = " 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 [WAITEETAL2004]. 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 D5 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 -- a closed source and an open source -- rather 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 deg. 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 Cassini's Fields and Particles Pallet (FPP). The out-ward normal to both the open and closed source INMS apertures lies in the spacecraft X direction. The open source geometric field of view is about 8.6 deg. 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 2^1 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 X direction. Venting occurs at right angles to the X axis. ELECTRONICS =========== The INMS electronics system is based on designs used for the Huygen's 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 extra 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 3 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." 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