Instrument Overview
===================
 
See [NIEMANNETAL1997].
 
The Gas Chromatograph Mass Spectrometer (GCMS) on the Huygens Probe will
measure the chemical composition of Titan's atmosphere from 170 km altitude
(~1 hPa) to the surface (~1500 hPa) and determine the isotope ratios of the
major gaseous constituents. GCMS will also analyse gas samples from the
Aerosol Collector Pyrolyser (ACP) and will be able to investigate the
composition (including isotope ratios) of several candidate surface materials.
GCMS is a quadrupole mass filter with a secondary electron multiplier
detection system and a gas sampling system providing continuous direct
atmospheric composition measurements and batch sampling through three gas
chromatograph (GC) columns. The mass spectrometer employs five ion sources
sequentially feeding the mass analyser. Three ion sources serve as detectors
for the GC columns and two are dedicated to direct atmosphere sampling and ACP
gas sampling, respectively. The instrument is also equipped with a chemical
scrubber cell for noble gas analysis, and a sample enrichment cell for
selective measurement of high boiling-point carbon-containing constituents.
The mass range is 2-141 dalton and the nominal detection threshold is at a
mixing ratio of 10^-8.
The data rate available from the Probe system is 885 bit/s. GCMS mass is 17.3
kg and the energy required for warm-up and 150 min of operation is 110 Wh.
 
 
Scientific Objectives
=====================
 
Titan is unique in the solar system in several respects. The dense atmosphere
is still chemically reducing, even though Titan is small enough to allow
hydrogen to escape readily from its gravitational field. The major
constituents of the atmosphere, nitrogen and methane, are continuously broken
apart by a combination of solar UV, impinging electrons from Saturn's
magnetosphere, and a steady flux of cosmic rays. The resulting molecular
fragments recombine to form a variety of new species, many of which were
detected for the first time by Voyager 1 [BROADFOOTETAL1981]; [HANELETAL1981];
[KUNDEETAL1981]; [SAMUELSONETAL1981]; [SAMUELSONETAL1983]; [LUTZETAL1983];
[BEZARDETAL1993]. Other simple molecules must be present, while the
existence of still more complex compounds is manifested by the ubiquitous,
surface-hiding aerosol blanket. In addition to hydrocarbons and nitriles, the
atmosphere is known to contain CO and CO2 (see reviews by [HUNTENETAL1984];
[MORRISONETAL1986]; [LUNINEETAL1989]; [GAUTIER&RAULIN1997]).
Chemical reactions are continuously converting methane, nitrogen, carbon
monoxide and possibly externally-delivered H2O into more complex substances.
The origin of this atmosphere, the processes involved in its evolution, the
end products and their subsequent fate as they interact with the surface
remain to be elucidated. A particularly interesting aspect of this
investigation is the possible relevance of the chemical evolution currently
occurring on Titan to some of the prebiotic syntheses that took place on the
Earth. It is the purpose of GCMS to provide an accurate analysis of Titan's
atmospheric composition along Huygens' descent trajectory.
 
Atmospheric composition: argon, isotopes and organic compounds
--------------------------------------------------------------
Despite the great success of Voyager 1, the basic composition of Titan's
atmosphere is still poorly known (Table 1). The present uncertainty in the
methane mixing ratio and its variation with altitude can be resolved easily
from the continuous recording of mass spectra by GCMS during the Probe's
descent. The Voyager observations left open the possibility that several
percent of some heavy, spectroscopically-undetectable gas might be present
[SAMUELSONETAL1981]. Non-radiogenic argon (36Ar + 38Ar) is the most
likely candidate [OWEN1982]; the mass spectrometer can detect it down to
mixing ratios of 10-100 ppb. The amount of argon that could remain
undetectable in presently available observations has been steadily decreasing
with improved treatment of the data, first to <= 10% [STROBELETAL1993],
then to <= 6% [COURTINETAL1996], the same upper limit originally reported
by [BROADFOOTETAL1981]. In fact, several percent of argon would be
difficult to explain based on current models for the origin of Titan's
atmosphere [OWEN&BARNUN1995].
The full range of abundance and isotope data provided by GCMS will be employed
to study atmospheric origin and evolution. For example, the ratio 14N/15N will
permit an evaluation of atmospheric escape processes, as will the value of
12C/13C. Once escape and chemical exchange have been studied, it will be
possible to establish the original value of D/H in Titan's methane. For
example. if D/H ~ 2X10^-5, the value in Saturn's hydrogen, this might favour a
sub-nebula origin for most of the methane. A determination of 40Ar/36Ar could
be used to model escape of gas from Titan's rocky interior.
The oxidised compounds offer other opportunities and challenges. The
production of CO2 requires hydroxyl (OH) that may come from either outside the
atmosphere (e.g. by bombarding ice particles or micrometeorites) or from
internal sources, from CO plus CH4. The external source seems likely to be
more efficient, but we do not know the flux of incoming particles. If the
nitrogen we now see in Titan's atmosphere was originally incorporated as N2,
one would expect a comparable amount of primordial CO. In that case, a large
amount of CO2 would have been produced and several metres of 'dry ice' could
now be present on the surface [SAMUELSONETAL1983]; [OWEN&GAUTIER1989].
The CO chemistry on Titan will be tested by measuring the abundance of this
gas at the four altitudes at which atmospheric GC samples are taken.
The drivers for the chemistry on Titan are solar ultraviolet radiation, the
charged particle-induced chemistry when Titan is immersed in Saturn's
magnetosphere, and galactic cosmic rays, especially for lower atmospheric
chemistry. The chemistry of CH4 on Titan proceeds, to some extent, in a manner
similar to that proposed for Jupiter, with the exception that H2 is replaced
by N2 as the major gas. The stable hydrocarbons resulting from the CH4
photochemistry are C2H6, C2H2 and C2H4. Subsequent reactions involving C2H2
result in the formation of methylacetylene or allene (C3H4) and polynes
(C2nH2, n = 2, 3, 4). Propane C3H8, butane C4H10 and other heavier
hydrocarbons are expected to be formed following the reaction of the radical
CH3 with C2H5, C3H7, etc.
Lower mixing ratios are expected with an increasing number of C atoms, but
available observations paint a more complex picture. Careful re-analysis of
the Voyager data by Coustenis and colleagues has revealed striking variations
in abundances of minor constituents with latitude on Titan
([COUSTENIS&BEZARD1995] and references therein). At 50 deg N, they find the
following order of increasing abundances: C4H2. C2N2, HC3N, C3H4, C3H8, C2H8,
HCN, C2H2, C2H6. Abundances measured at this latitude can be more than 17
times the values at southern latitudes. The abundances measured by GCMS along
the Probe trajectory can be used to calibrate the remote measurements by the
Composite Infrared Spectrometer (CIRS) on the Orbiter, allowing analysis of
these variations over the globe during a season different from that sampled
by Voyager 1.
Unlike Jupiter, the CH4 chemistry on Titan is not isolated, as the above list
of constituents shows (cf. Table 1). Atomic nitrogen produced on dissociation
of N2 reacts with CH4 and products of its photochemistry, CH3 and CH2, to
produce HCN. Photolysis of HCN produces CN. The reactions of CN with CH4, CN
(or HCN), C2H2 and C2H4 yield, respectively, HCN, C2N2, HC3N and C2H3CN (not
yet detected). Reactions of CN with the other hydrocarbons result in the
formation of additional nitriles such as CH3CN. Other possible nitriles with
mixing ratios in the range 10^-8 to 10^-9 are: C2H5-N, CH2=CH-CN, CH3C=-C-CN
and HC4-CN. Lower mole fractions are expected for nitriles of higher molecular
weight.
GCMS will approach this analysis problem in two ways: by taking mass spectra
continuously, thereby measuring every m/e peak within 2-141 dalton; and by
making discrete GC analyses at various altitudes, including sample enrichment,
thereby permitting both greater sensitivity and specificity in identification
at those points. A third approach is possible in collaboration with the ACP
experiment (see [ISRAELETAL1997]).
 
Table 1. Atmospheric composition of Titan (50 deg N, Voyager 1)*
-----------------------------------------------------------------
Constituent                     Mixing Ratio
-----------------------------------------------------------------
Major Species (global values)
N2                               0.85-0.98        Hanel et al., 1981;
Ar                                <= 0.06         Broadfoot et al., 1981;
CH4                         <= 0.15 at surface    Courtin et al., 1995.
 
Minor Species
H Group
H2                               2-6x10^-3        De Bergh et al., 1988;
D/H                            0.7-2.3x10^-4 Coustenis et al.,1989;Orton,1992.
 
CN Group
C2N2                             ~2x10^-8         Coustenis & Bezard, 1995;
C4N2                          condensed phase     Bezard et al., 1993.
 
C-N-H Group
HCN                              ~ 10^-6          Coustenis & Bezard. 1995;
HC3N                            ~3x10^-8          Bezard et al., 1993.
CH3CH                       1-5x10^-9 (global)
 
C-O Group (global values)
CO                               4-6x10^-5  Samuelson et al.,1983;Lutz et al.,
CO2                              ~1x10^-8   1983; Gurwell & Muhleman, 1995.
 
C-H Group
C2H6                             1-2x10^-5        Coustenis & Bezard, 1995;
C3H8                               10^-6          Bezard et al., 1993.
C2H2                             2-6x10^-6
C2H4                               10^-6
CH3C2H                           ~3x10^-8
C4H2                             ~2x10^-8
-----------------------------------------------------------------
* values at other latitudes and seasons likely to be different.
 
Descent sequence
----------------
Direct measurement with the mass spectrometer will be made continuously from
initiation of experiments until landing. A minimum of five GC samples will be
taken, one immediately after the opening of the inlet valve, another in the
lower stratosphere, a third near 60 km (where concentrations of most complex
trace gases are thought to be the highest) and two more below 60 km altitude.
One of the latter will be devoted to analysis of the output from the aerosol
pyrolysis experiment and the other taken near the atmospheric temperature
minimum to provide the best CO/N2 separation. A sixth sample can be taken
close to the surface if the nominal descent time is maintained. Its purpose is
to characterise conditions near the ground, especially to search for evidence
of the vapour phases of possible surface condensates. The timing will be
adjusted to ensure GCMS readout before impact for the nominal descent
scenario.
 
Surface science
---------------
Titan has the largest unexplored surface in the solar system. This surface is
currently being studied at very low spatial resolution ( ~ 300 km) by
ground-based and Hubble Space Telescope observations through near-IR windows
([SMITHETAL1996] and references therein) and by means of radar
[MUHLEMANETAL1995]. Owing to the presence of Titan's thick, chemically active
atmosphere, the surface of this satellite must be one of the most unusual we
are ever likely to see. This surface must contain or conceal a reservoir for
atmospheric methane, since the present atmospheric abundance of this gas will
be destroyed by photochemistry in just 10^7 years. Unless we just happen to be
living at the time when Titan's original methane comes to an end, the surface
(or subsurface) must provide a means for replenishing this gas. The products
of atmospheric chemistry will accumulate on Titan's surface over geologic
time, with the potential of producing deposits with depths on the order of a
kilometre or more. If liquids are present, one can imagine their influence on
the landscape through erosion, and the possibility that further chemical
processing also occurs in them [RAULINETAL1995].
Three extreme models for Titan's surface have been proposed:
  - a global ocean of hydrocarbons dominated by ethane but containing methane,
    nitrogen, carbon monoxide and many other dissolved species;
  - a global covering of precipitated aerosols;
  - an icy landscape dominated by impact craters, perhaps including rocky
    debris.
It is now clear that the first of these models cannot be correct. A solid
surface with lakes or seas of liquid hydrocarbons and some areas dominated by
aerosol deposits is more consistent with existing data [LUNINE1993];
[LUNINE1994]; [LORENZ1993]; [SMITHETAL1996].
If Huygens lands in a liquid, a compositional analysis with GCMS is
straightforward. Mass spectra of evaporating liquid showing the relative
abundances of nitrogen, ethane, methane, argon and other noble gases, simple
hydrocarbons, nitriles and oxides would be an outstanding contribution to
understanding the origin and evolution of the atmosphere. If the Probe settles
into a deposit of aerosols, one needs to extrapolate the accumulated
information from the descent measurements to interpret the data. This would
offer an opportunity to determine the level of chemical complexity achieved by
chemical synthesis in the atmosphere, as even rare aerosols may accumulate in
measurable concentrations on the surface. Here the GCMS heated inlet will
ensure that the more volatile components of such aerosols reach the
instrument. Landing on exposed ice could still permit a measurement of H2O ice
'bedrock' and a search for condensed CO2, measurements of fundamental
importance to an understanding of atmospheric evolution. A determination of
D/H in H2O on the surface would be of great interest for comparison with
atmospheric values in CH4 and other species. It is recognised, however, that
this is the most challenging landing scenario, both for Probe survival and for
a good interface between the gas inlet and the surface.
 
 
Instrument Description
======================
 
Mass spectrometry as the principal chemical identification technique is
ideally suited for an exploratory mission such as Huygens. All atoms and
molecules within the mass and sensitivity range of the mass spectrometer will
be detected. No a priori knowledge of the composition is required. The
simultaneous occurrence of species of similar composition can sometimes lead
to difficulties in species identification. Multiple electron beam energies in
electron impact ionisation usually remove the difficulties by generating
energy-dependent fractionation patterns. Further improvement of species
separation and more accurate species identification is achieved with a gas
chromatographic system coupled with the mass spectrometer. Gas chromatograph
mass spectrometer systems are among the most powerful analytical tools for
chemical analysis of many types of compounds, and especially of gas mixtures.
The added complexity, compared to standalone gas chromatographs or mass
spectrometers, is recognised but the benefits resulting from a combined
instrument outweigh the possible disadvantage of increased instrument
complexity.
The main elements of the instrument are:
  - a mass spectrometer system consisting of ion sources, mass analyser and
    ion detector;
  - a gas sampling system consisting of a direct atmospheric sampling system
    to introduce atmospheric gas into the ion source and to enrich trace
    species and noble gases;
  - a gas chromatograph for batch sampling at specific altitudes in the
    atmosphere and subsequent time separation of species and identification by
    the mass spectrometer;
  - a sample transfer system for gas mixtures, generated by the aerosol
    pyrolyser, to the mass spectrometer sample inlet systems.
The mass spectrometer has five ion sources feeding a common mass analyser one
ion source at a time. The first source, IS1, samples the atmosphere
continuously. The second ion source, IS2, samples the ACP output, and the IS3,
IS4 and IS5 ion sources are detectors for three gas chromatographic columns
(GC columns). The choice is prescribed for the descent by a preprogrammed
sequence.
The source connected to the direct atmospheric sample, IS1, is selected during
the descent's first 30 min and at any time when no peaks are present at the
output of any of the GC columns. For the analysis of the gas mixture from the
ACP, the associated source will be selected and during the GC analysis of
these mixtures the sequence will be identical to that associated with the
atmospheric GC samples.
 
Gas sampling system
===================
The gas sampling system has three subsystems: direct atmospheric sampling, the
gas chromatograph and the ACP sample line. The direct atmospheric sampling and
the gas chromatograph are self-contained units sharing only the mass
spectrometer. The ACP sample line is connected from outside the instrument to
a Pyrolyzer Transfer Line (PTL) and interfaces with both the gas chromatograph
and the mass spectrometer. The direct atmospheric sampling and gas
chromatograph share gas flow lines with a gas inlet port in the fore dome at
the Probe apex near the stagnation point and an outlet port at the minimum
pressure point at the Probe rear. Inlet and outlet ports are sealed by metal
ceramic devices. All lines are evacuated after instrument calibration prior to
shipment. They will be opened in sequence by redundant pyrotechnic actuators
after Probe entry and ejection of the Probe Front Shield.
 
Direct atmospheric sampling
---------------------------
Most of the composition measurements will be obtained from direct atmospheric
sampling during descent. Ambient atmospheric gas is conducted through
pressure-reducing devices into the ion source, and a sample enrichment and
scrubber cell will enhance trace constituent detection and rare gas analysis.
At an altitude of approximately 170 km when the Probe is ready for instrument
deployment, i.e. all protective devices are jettisoned, the Atmospheric Inlet
and Outlet will be opened to the atmosphere. A dynamic pressure of about 70 Pa
before parachute jettison and 10 Pa after will force atmospheric gas to flow
close to the ion source through the sample system tubulation and manifolds.
Small quantities of atmospheric gas will be introduced from the sample system
into the ion source through fixed size leaks and removed at a constant rate by
chemical getters and a sputter ion pump. Noble gases will be pumped only by
the sputter ion pump and at a slower rate than the reactive gases, increasing
the noble gas mixing ratio in the ion source relative to the atmosphere.
Laboratory calibration will establish exactly the relationship between ambient
and ion source partial pressures.
The gas leaks are arrays of glass capillaries located in the ion source.
Typically, seven capillaries per leak are used, with inside diameters ranging
from 2 um for the lowest conductance leak to 20 um for the largest. Capillary
arrays instead of single capillaries were chosen in order to reduce the chance
of blocking by small aerosol particles. The gas conductances are selected so
that the pressure in the ion source does not exceed 10^-4 hPa in a nominal
descent.
The full dynamic range of the mass spectrometer is best used when the ion
source pressure is kept at the maximum operating value. Fixed size leaks do
not allow this because the ambient pressure increases during descent. To
prevent a large pressure change in the ion source and to accommodate a
purified noble gas and an enrichment cell measurement, the direct sampling is
divided into two sections. From time t0 to time t1, approximately 36 min, leak
L1 will be opened by switching the microvalves VL1 and VZ. While the ambient
atmosphere is sampled through leak L1, the enrichment cell EC will be charged
for a brief period. The enrichment cell adsorbs trace gases, e.g. high boiling
point hydrocarbons and nitriles, but no nitrogen or noble gases. By opening
valves VS7, VE and VV, gas flows through the cell until the evacuated volume
ECV is filled. All remaining reactive gases except methane will then be
removed by getter G3 after closing of valves VS7 and VV and opening VG.
The gas flow through leak L1 is discontinued after 36 min by closing valves
VL1 and VZ and remaining gas in the ion source is pumped out, leaving only
background pressure. Following the background pressure measurement, the
enrichment cell will be isolated by closing valve VE and heated to desorb the
collected trace gases. Simultaneously, the gas mixture residing between the
valves VV, VE, VG and VL3 will be introduced into the ion source IS1 through
L3 for noble gas analysis. The gas content of the enrichment cell will then be
added to the gas mixture for analysis by opening valve VE. When the enrichment
cell and noble gas analysis are complete, the subsystem will be isolated from
the ion source by closing valve VL3 and background pressure is observed again.
At time t5 direct leak L2 will be activated until the end of the mission.
Sampling for the first 36 min through leak L1 will be continuous and at a high
rate. The direct atmospheric sampling through leak L2 will be interrupted
repeatedly by sampling sequences for the analysis of elutents from the gas
chromatographic columns and ACP products.
 
Gas chromatographic analysis
----------------------------
Gas chromatography allows, under suitable conditions, the gas phase separation
of complex mixtures of hydrocarbons, nitriles and permanent gases, including
carbon monoxide.
A small amount of a mixture of gases is introduced into a carrier gas stream
that flows continuously through the column. Each component in the mixture, in
the ideal case, elutes from the column outlet at a different time. A detector
at the outlet gives a signal related to the quantity or concentration of the
components of the gas mixture.
Difficulties in data interpretation result when universal detectors are used
because the exact chemical composition of the elutent is not known. A mass
spectrometer eliminates most of the difficulties.
Advantages and disadvantages of combined GCMS instrument techniques have been
discussed in great detail in the technical literature and the arguments apply
to this application as well. One disadvantage is the increased complexity of
the instrument which must be of particular concern here because of the
specific mission environment. The short time available for sampling and
analysis, long time reliability and severe limits placed on mass and power
require special considerations.
The use of open capillary columns and of packed columns has been considered.
Best instrument performance and moderate instrument complexity have to be
balanced in this design. Three chromatographic columns with different
properties are used and operated in parallel to cover the range of expected
atmospheric species. One column will separate CO and N2 and other permanent
gases. A second column will separate nitriles and other organics with up to
three carbon atoms. A third column will provide the separation of C3 to C8
saturated and unsaturated hydrocarbons and nitriles of up to C4. A silica
steel micropacked GC column 2 m in length with 0.75 mm internal diameter (for
column 1) and silica steel wall coated open tubular (WCOT) GC capillary
columns 10 m and 14 m in length with 0.18 mm internal diameter (for columns 2
and 3), were found to be most suitable. The columns are wound in a 178 mm
diameter coil on high temperature foil heaters. A thermally isolated oven
will enclose each column to allow efficient heating. The columns will be
operated at 0.18 MPa inlet pressure and the outlet is vented through a flow
restrictor to the ambient atmosphere.
Hydrogen was selected as carrier gas because of efficient storage and pumping.
It is stored in a hydride metal alloy enclosed in a stainless steel housing.
The amount of hydrogen required for the GC operation, approximately 3 litres,
is stored in 100 g of hydride metal alloy, assuming 180 min descent time and a
50% reserve.
The hydrogen carrier gas reservoir CGR will be equipped with an injection
valve IV. The valve is solenoid-operated and similar in design to the
microvalves. The valve plunger punctures a diaphragm and initiates carrier
gas flow. A pressure regulator PR controls the flow through the flow
restrictors and columns. For safety, a burst diaphragm BP installed in the
CGR is set to burst at 3 MPa.
The Probe does not spend sufficient time in the altitude region between 176 km
and 60 km for repeated gas chromatographic analysis. To overcome this
difficulty, samples will be collected at specified times during the descent
through the upper region of the atmosphere in sample volumes SV1 to SV3 for
later analysis. Samples will also be injected directly from the atmosphere
into the carrier gas flow path of the GC near the surface.
At system initiation, hydrogen carrier gas flows from the hydrogen reservoir
CGR, through the injection valve IV, the pressure regulator PR, flow
restrictor FR2, valve VD4 past the sample injection valves VS5, VG 1, 2, 3,
then splits into the three GC columns. A fraction of the flow exiting the
columns will be split off and conducted through capillary leak arrays into the
ion sources.
The atmospheric samples collected in the sample volumes will be analysed one
at a time by first flushing the inlet manifold between valves VD1 and VD2 with
carrier gas and then operating valve pairs VS1, VG1, etc for several
milliseconds to allow the sample volume to be discharged into the carrier gas
stream. The GC analysis time allowed per sample is about 10 min. Direct sample
injection will be accomplished in a similar fashion by first closing VD1 and
VD2 and injecting part of the trapped sample gas through valve VS5 by
redirecting the carrier gas flow through VD3 for a short time interval.
Injection of the ACP sample from the sample transfer manifold into the GC
column will be accomplished through valve VS6 in a similar manner.
 
Aerosol Collector Pyrolyser analysis
====================================
 
ACP operation and details of the sample transfer are described in a separate
paper in this volume. The pyrolysis products will be provided through a sample
transfer line made of 0.5 mm internal diameter nickel tube connected to an
injection valve IVA at the centre of the instrument housing. Internally, a
feed tube connects to the GC inlet valve VS6, the direct inlet valve VL4 and
to the outlet via valves VAA, VAB and flow restrictor FRA.
The ACP line will be opened just prior to the first sample transfer. After gas
flow stabilisation, direct gas analysis begins using the dedicated ion source
IS2. After analysis is completed, sample gas in the ACP line is vented through
the outlet and the line is refilled with the next sample. The sample for the
gas chromatograph is injected by opening valve VS6 for a short time interval
to superimpose the sample on the carrier gas stream.
 
Post surface impact analysis
============================
 
It is likely that Huygens will continue to operate for a short time after
surface impact. As the nature of the surface is not known, it is hard to
predict the specific contact the Probe will make. The most probable landing
position is expected to be upright, which is also optimum for the instrument.
In case of a landing on a liquid surface, the heated inlet tube will be
submerged in the liquid, which will rapidly evaporate in the inlet tube and
the vapours will flow through the inlet lines. Rapid direct sampling through
leak L2 will provide composition measurements of the vapour. In case of a
landing on a solid surface, the surface will be heated locally by the inlet
tube and volatised gases will flow through the inlet lines and be available
for analysis.
GC analysis will be initiated through valve VS5 as described above if the
Probe survives for more than 2 min. The surface sampling mode will be
initiated by an altimeter signal shortly before surface impact.
 
Ion sources
===========
 
Electron impact ionisation is used in the miniaturised ion sources. A well
collimated electron beam is directed through the ionisation region into which
the gas stream is conducted by the capillary leaks. The flow paths are short
because the valves and capillary leaks are mounted directly on the ion
sources. The electron guns have heated filaments of 0.075 mm-diameter 97%
tungsten, 3% rhenium wire and require approximately 1 W of power. The electron
beam energies can be chosen from two preselected values (25 eV or 70 eV) to
permit species identification and discrimination by observing energy dependent
fractionation. A typical electron beam current is 80 uA. Ions are focused and
transmitted into a quadrupole switching lens assembly by multi-element ion
lenses of small aperture. Quadrupole switching lenses are operated as ion beam
deflectors. Any one of the five ion beams can be deflected by the switching
lens into the quadrupole mass analyser at any time. Switching is accomplished
in microseconds by changing the bias voltages to the appropriate values for
the ion source of choice.
Sample gas decoupling of the ionisation regions is achieved by differential
pumping. By designing the gas conductances C such that their ratios C1/C2 and
C1/C3 are approximately 10^3 and 10^2, respectively, the separation in partial
pressures between ionisation regions is 10^6. For example, gas in ion source
IS1 with a partial pressure of 10^-6 hPa will be seen by the other ion sources
as a partial pressure of 10^-12 hPa, and the pressure in the quadrupole mass
filter and detector region is below 10^-6 hPa to eliminate ion scattering. The
size of conductances C2 is determined by the ion lens apertures, which are
designed to be long and narrow. The pumping speed of the getters and the
sputter ion pumps determine the size of conductances C1 and C4.
 
Quadrupole mass filter and ion detector
=======================================
 
The quadrupole mass filter accepts the ion beam generated by the ion sources
transmitting only ions of a chosen charge to mass ratio. The selected ion
beams are focused on two secondary electron multiplier ion detectors. The
nominal mass range measured by the instrument is 2-141 dalton. The quadrupole
rods are excited by radio frequency (VAC) and direct current (VDC) potentials
which together create a dynamic electric field within the quadrupole region
that controls the transmitted mass (m/e value) and the resolution. A mass scan
is executed by varying the radio frequency potential VAC to satisfy the
relationship M=0.55 VAC/f^2 where VAC is in volts, f in MHz, and M in dalton.
The resolution will be controlled over the mass range by programming the ratio
of VDC to VAC to maintain the resolving power defined by a crosstalk criterion
appropriate for that mass range.
The transmission efficiency will be 100%, resulting in flat top mass peaks
over the mass range of interest. This allows a mass scan mode in which each
mass is monitored by a single step.
In another operating mode, the DC voltage will be reduced to zero, which
creates a high pass filter giving the sum of all masses greater than, for
example, 2 or 141 dalton. This feature allows the use of the mass spectrometer
as a non-specific GC-detector excluding the detection of the abundant hydrogen
ions produced by the GC carrier gas.
The ions passing the mass filter will be detected by a pair of continuous
dynode secondary electron multipliers with effective entrance aperture sizes
differing by a factor of 3x10^3. Charge pulses at the anodes of the secondary
electron multipliers are amplified and counted. The background noise of the
secondary electron multipliers is approximately one count per minute. The
upper count rate is limited to approximately 3x10^7 count/s by the pulse width
of the anode pulses of the secondary electron multipliers. The instrument
sensitivity for 100% ion transmission is approximately 1x10^14 count/s per
hPa. Secondary electron multiplier background count rates of one count per
minute or less yields a detection threshold of 1.7x10^-16 hPa partial pressure
in the ion source region for a signal to background count ratio of unity. The
maximum pressure level in the ionisation region is limited by free path
conditions to about 10^-3 hPa. In the low mass range ( <46 dalton), the lower
detection limit is often not realisable because of background gases emitted
from the surrounding surfaces or because of interference at some mass numbers
from other gases present in high concentrations. Typical background gases in
the ion source are H2, CH4, H2O, CO and CO2. The exact sensitivity is
established by calibration and varies with species because of differing
ionisation efficiencies for neutrals, and the conversion efficiency at the
secondary electron multiplier.
 
Pumping systems
===============
 
The pumps establish a flow of sample gas through the ion sources when a
sampling device is opened and they remove the gas from the ion source regions
after analysis and closure. Non-evaporable getters and sputter ion pumps are
used because of simple adaptation to space flight systems. The sputter ion
pumps depend only on electrical power for operation and work without moving
parts. Hydrogen is sorbed reversibly at a very high rate and in very large
quantities by the getter material, while nitrogen is efficiently pumped by
irreversible bonding. The getter material is sintered titanium and molybdenum
powder. The sorption capacity for hydrogen is ~ 20 hPa litre/g for an
equilibrium pressure of 10^-4 hPa at a worst-case temperature of 200 deg C.
More favourable conditions exist at the expected operating temperature of <100
deg C. The getters, after activation in a vacuum, will remain activated
indefinitely at room temperature.
Uncertainties exist about the concentration of argon and methane in Titan's
atmosphere. These gases will be pumped by sputter ion pumps. Synthesis of
hydrocarbons in the sputter ion pumps is eliminated by special processing of
the cathodes.
 
Electronic system
=================
 
The various subsystems required to control the sample flow, to power the
sensor and ion detectors and to process the output signal are under the
control of a microcomputer. Instrument potentials are produced by a number of
programmable, floating-secondary DC-DC converters that are configured for
each measurement.
Telemetry and command streams connect to the spacecraft through redundant
serial interfaces. The electronics system provides the flexibility to
accommodate the diverse measurement and testing requirements.
The pyrotechnic devices used to break the ceramic seal in the sample inlet and
outlet systems are fired by the Probe pyro bus. The sample-sequencing
microvalves are powered by switching circuits under microcomputer control.
Each of the five ion source supplies contains a filament emission regulator
and electrode supply to provide the required voltages and currents. Fast
switching of the ion beam deflectors is accomplished by high speed bipolar
electronic switches.
The quadrupole VAC and VDC potentials are generated in a dedicated supply.
Before each measurement, the microcomputer calculates the proper amplitudes
and frequency step for the VAC potential. An auto-tune algorithm is
incorporated into the control software to ensure that changes such as
component ageing in the supply do not affect mass tuning and resolution.
Current pulses from the secondary electron multiplier are amplified by a low
noise trans-resistance amplifier and counted. The counts are held in a 32-bit
register, which is then read by the microcomputer. A parallel electrometer
channel measures multiplier current when the maximum count rate is approached,
allowing an inflight gain check.
A 1750A microprocessor controls and sequences the instrument in accordance
with software instructions contained in programmable read-only memory (PROM).
Control of the many subsystems is accomplished by writing to a separate, high
speed sensor data bus. The instrument software was written in a high level
language (ADA) whenever possible and in assembly language when speed is
crucial. Code has been developed in a modular, top-down manner to increase
testability and improve maintainability. The instrument will receive commands
and transmit (science and housekeeping data) through the Probe Command and
Data Management System (CDMS) interfaces.
Power to the instrument is derived from the common +28 V spacecraft bus;
inrush current limiting and overload protection is provided in the main power
converter. The spacecraft bus voltage is transformed by a DC-DC converter to a
number of standard secondary potentials that power the subsystems. To minimise
size and mass of the electronics system, approximately 80% of the electronic
circuits are packaged in hybrids.
 
Mechanical configuration
========================
 
The ion source and mass analyser assembly constitute the main body of the
sensor system. Getter pumps and sputter ion pumps are directly mounted to the
ion sources at the upper part of the assembly for compactness. The GC columns
and the gas sampling system are concentrically arranged around the ion source
assembly. The gas inlet tube extends forward to penetrate the fore dome. The
electronics system is located below the gas sampling system. The structure is
made of aluminium alloy and is attached to the centre section of the
instrument housing.
The instrument housing is also made of aluminium alloy and consists of three
sections. The lower housing encloses the sampling system and the ion sources.
It also provides the inlet port interface with the fore dome. The upper
housing covers the electronics system. The centre housing provides the
mounting support for all instrument components and most external interfaces.
It is hermetically sealed by metal seals. The overall helium leak rates of
<10^-8 cm^3/s is sufficient to maintain the housing pressurised for more than
ten years. The housing is designed to withstand 0.15 MPa internal
pressurisation and an external pressure of 0.15 MPa above the internal
pressure. For flight, the housing will be filled with dry nitrogen to 0.12
MPa.
The instrument is mounted on the Experiment Platform of the probe via a flange
at the centre housing. The sample inlet line penetrates the fore dome near the
stagnation point outside of the boundary layer of the gas flow around the
Probe body. The sample outlet port is at the rear section of the housing.
ACP's mounting position on the Experiment Platform is adjacent to the GCMS for
efficient sample transfer.
 
Sampling strategy
=================
Throughout the entire descent of Huygens through Titan's atmosphere, GCMS will
perform a measurement each 5.008 ms and select from one of the five ion
sources and process that measurement for telemetry. The mass range measured
for each ion source is determined by a stored sequence. The sequence also
determines which ion sources are enabled for sampling during various stages of
the descent. During the first 36 min of the descent, the direct inlet into a
mass spectrometer ion source is analysed and atmospheric samples are collected
for subsequent analysis by GCMS. During the same interval, an atmospheric
sample is processed to enrich the noble gas content to extend the sensitivity
of the noble gas ratio analysis. During the descent interval at 40-95 min, the
collected samples are analysed sequentially by the GCMS. The ACP output is
analysed both directly by MS and by GCMS during intervening periods. In each
of these analyses, the characteristics of the mass scan can be programmed.
Each integral mass number from 2 to 141 dalton is sampled sequentially. During
all mass scans, the total output from each ion source (with carrier gas
rejected) will be measured both for the purpose of providing a continuous
record of the total density in each ion source and for the purpose of
selecting the ion source to be sampled when an unknown gas mixture is flowing
through the GC columns.
For the direct mass spectrometer measurements, 936.5 ms are required to
complete a Full Scan. This resolution is more than adequate to define the
atmospheric profile defined by the descent rate and will be used.
A diagnostic scan of the full mass range in 1/8 dalton increments will be made
at times during the descent when the rate of change of the atmospheric samples
is lowest. This diagnostic scan and others will be interleaved with the
atmospheric measurements.
The ACP output will be sampled both directly and through one of the GC columns
at the appropriate time in the descent. The same mass scan capabilities
available for GCMS measurements are available to the ACP instrument.
 
Data format
===========
 
GCMS is intrinsically capable of generating much more data than can be
transmitted within the bandwidth allocated to it. Counter data are produced as
two 16-bit words each integration period, one from the high sensitivity and
one from the low sensitivity secondary electron multiplier. Only data from one
counter are selected for telemetry. The data are compressed by taking the
square root above a pulse count rate of 2.56x10^4/s to yield 8 bits of counter
data per integration period, and one bit is added for counter identification.
Even with data compression, data are produced at approximately twice the
available rate of a single telemetry channel assigned to the instrument. For
this reason, the data will be sent alternately to the two (redundant)
channels. If both telemetry channels function, all data will be recovered. If
one channel fails, the effect will be to reduce the temporal resolution of the
science data.
The data are configured as subpackets within the standard Huygens Probe
telemetry packet. The GCMS is allotted 15 telemetry packets per 16 s cycle;
each packet is 126 octets in length, but seven of those are reserved for
packet header and error correction. This results in an actual data rate
available to the GCMS of 885 bit/s per channel.
  - Science Data Subpacket: data from one mass scan is packetised along with
    time tag information and is sent to telemetry once every 936.5 ms. This
    subpacket contains 1488 bits, for a data rate of 1589 bit/s.
  - Housekeeping Data Subpackets: three different subpackets are used for low,
    medium and high speed housekeeping data. The total of the three types is
    approximately 100 bit/s.
  - Acknowledge Subpackets: these are used to send confirmation of external
    events such as ACP sync pulses and telecommands. Each subpacket consists
    of 32 bits.
Subpacket synchronisation and descent sequence monitoring data consume an
additional 52 bit/s (for both channels). This results in a total GCMS data
production rate of approximately 1741 bit/s, versus the available telemetry of
1770 bit/s for two channels used alternately. If the Probe retrieves data
faster than science data are being produced, GCMS will insert idle subpackets
containing housekeeping data.
 
 
Calibration
===========
 
In order to determine the overall system transfer characteristics, the
instrument was calibrated on a dynamic flow system where the time, pressure
and temperature profile encountered during descent was simulated. Gas mixtures
containing known mixing ratios of gases were introduced into the high pressure
flow system of the sample inlet system of the flight instrument. The design of
the sample inlet system allows complete instrument calibration as it is used
in flight. Components with limited operational life time, i.e. getter
materials and the sputter ion pumps, were replaced after calibration. All
pumps are designed to operate in a conductance limited mode so that small
changes in pump performance will have a negligible effect on the instrument
transfer function.
The calibration system consists of two parts: the high pressure gas flow and
sample mixing system and the ultra high vacuum pumping stand. The flight
instrument inlet and outlet ports are connected directly with vacuum flanges
to the appropriate terminals of the high pressure gas flow system. All lines
are thermally isolated and the gas temperature is controlled by heaters and
heat exchangers. Absolute system pressure and the differential pressure across
the inlet are monitored with precision pressure gauges. The gas flow will be
adjusted so that the differential pressure is equal to the Probe stagnation
pressure expected in flight. Trace gas mixtures were added through the
calibration gas line. The exact quantity of added calibration gas is
determined by measuring the gas flow with flow meters and the pressure at the
injection port.
 
 
Instrument Summary
==================
 
Science objectives, instrument operating characteristics and the required
Probe resources are summarised in Tables 2-4.
 
Table 2. GCMS science objectives.
------------------------------------------------------------------------------
Objective                      Measurement
------------------------------------------------------------------------------
1. Atmospheric Composition     Abundances of all constituents within the
                               mass range of the instrument with mixing
                               ratios > 10^-8, selected species to 10^-10.
 
2. Atmospheric Origin.         Discrimination between primordial and
                               radiogenic argon. Surface composition. Other
                               noble gas abundances. Value of D/H.
 
3. Atmospheric and Interior    Major element isotopes. Surface composition.
   Evolution.                  Noble gas abundances. CO vertical distribution.
 
4. Chemical Evolution.         Abundances of organic compounds. Variations
                               with altitude. Surface composition.
------------------------------------------------------------------------------
 
 
Table 3. GCMS operating characteristics.
------------------------------------------------------------------------------
Gas Sampling:             1. Continuous direct atmospheric gas sampling.
                          2. Batch sampling for GCMS analysis, distributed
                             over descent altitude.
                          3. Sample enrichment for MS (100-1000X enrichment).
 
Ambient Pressure Range:   1-1500 hPa.
 
Altitude Above Surface:   176-0 km.
 
GC System:                3 parallel columns with H2 carrier gas.
                          Independent MS ion source detection.
 
Mass Analyzer:            Quadrupole Mass Filter.
 
Ion Source:               Five sources, electron impact ionisation. Maximum
                          operating pressure 1x10^-3 hPa H2, 5x10^-4 hPa N2.
 
Ion Detector:             Dual Secondary Electron Multipliers, pulse counting
                          and analogue current mode.
 
Background Noise:         1 count/min.
 
Mass Range:               2-141 dalton.
 
Sensitivity:              1x10^14 count/s/hPa source pressure.
 
Dynamic Range:            >= 1x10^8.
 
Resolution/Crosstalk:     1x10^-6 for adjacent half mass up to 60 dalton,
                          less for higher mass.
------------------------------------------------------------------------------
 
 
Table 4. Probe resources required for GCMS.
------------------------------------------------------------------------------
Data Rate:                15 packets per cycle.
 
Viewing Requirements:     1. Sample inlet near stagnation point.
                          2. Sample outlet near minimum pressure point
                             (e.g. inside Probe body).
 
Deployment Mechanisms:    1. Metal ceramic breakoff caps, pyrotechnically
                             operated.
                          2. Valves, solenoid operated.
 
Altitude Information:     Obtained from altimeter data provided by Probe or
                          ambient pressure desired near surface.
 
Temperature Range:        -20 deg C to +50 deg C operating, -20 deg C to +60
                          storage.
 
Power:                    41 W average. 71 W peak.
 
Energy:                   110 Wh.
 
I (Max):                  1.68 A (main), 1.32 A (protected).
 
Mass:                     17.3 kg.
 
Size:                     Cylindrical, 198 mm diameter. 470 mm high.
                          Mounting flange bolt circle 248 mm.
------------------------------------------------------------------------------