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."
END_OBJECT = INSTRUMENT_INFORMATION
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "WAITEETAL2004"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "FOX&YELLE1997"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "STROBELETAL1992"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "KELLERETAL1998"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "KASPRZAKETAL1996"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "MAHAFFY&LAI1990"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
END_OBJECT = INSTRUMENT
END
|