The objective of this investigation was the precise (0.1%) determination of the helium abundance in the Jovian atmosphere from 3-8 bars. A two-arm, double-pathlength optical interferometer that includes an infrared LED light source operating at a wavelength of 900 nm, an interference filter, and a photodetector array, is used to measure the refractive index difference between an atmospheric sample and a reference gas mixture. It was mounted on the probe with an inlet pipe to the ambient atmosphere.

Von Zahn, U. and D.M. Hunten, The Jupiter Helium Interferometer Experiment on
                   the Galileo Entry Probe, Space Sci. Rev. 60, pp. 263-281, 1992.

Instrument Overview
  ===================
    The NASA Galileo mission to the planet Jupiter provides us with
    the first opportunity to measure the He/H2 abundance ratio inside
    a heavenly body and with high accuracy.  To this end, the Galileo
    entry probe carries a miniaturized interferometer which is to
    perform an accurate measurement of the refractive index of the
    Jovian atmosphere, after removal of NH3, H2O, and CH4, in the
    pressure range from 2.5 to 10 bar.  From these data the helium
    mole fraction (about 0.10) can be calculated with an estimated
    accuracy of +- 0.0015.  The instrument is called the Helium
    Abundance Detector, or HAD for short.  It has a mass of 1.4 kg
    and an electrical power consumption of less than 1 W.  Before it
    can perform its measurements within the Jovian atmosphere, the
    HAD instrument has to survive an in-space-storage period of more
    than 6 years, a radiation dose of 75 krad and a deceleration
    during entry into the Jovian atmosphere of approximately 300 g.
 
    The Galileo Helium Abundance Detector uses a two-arm,
    double-pathlength interferometer or Jamin-Mascart interferometer.
    This type of interferometer allows for a particularly compact and
    simple design.  The light source is a light emitting diode (LED)
    operating at a wavelength of 900 nm.  An interference filter with
    a 15 nm passband aids in producing near-monochromatic light.  A
    Jamin plate produces two parallel and coherent light beams.  Four
    cells, each of length l = 100 mm, house the Jovian gas and the
    reference gas.  Additional optical elements are the collimator,
    the inversion prism, and the objective.  The inversion prim is
    very slightly tilted about an axis parallel to the incoming light
    beams.  This feature, in combination with the objective produces
    a well-defined interference pattern of consecutive equidistant
    bright and dark fringes at a linear array of nine photodetectors.
    This pattern does not change if both cells are filled with gas
    mixtures having the same refractive index.  However, any
    differences between the refractive indices of the Jovian and the
    reference gas causes a continuous shifting of the pattern with
    increasing pressure (that is, as the entry probe penetrates
    deeper into the Jovian atmosphere).  The detector allows
    measurement of the position and motion of the interference
    fringes in multiples of 1/8 of the fringe separation.
 
    The instrument carries a simple optical test device which allows
    a measurement of the contrast of the interference fringes and a
    verification of the operation of the fringe counter during
    Earth-based tests and the interplanetary cruise of the Galileo
    spacecraft.  It consists of a plane-parallel glass plate mounted
    between the objective and the detector array.  By telemetry
    command this plate can be slowly tilted up to about 30 degrees
    about an axis parallel to the interference fringes at the
    detector.  This causes a lateral shift of the interference
    pattern across the detector array.
 
    After entering the Instrument, the Jovian gas is passed through a
    two-stage chemical absorber to be scrubbed first of traces of NH3
    and H2O and then of CH4.  In addition, immediately before
    entering the gas cells the Jovian and the reference gases are
    each passed through heat exchangers made of stainless steel wool
    to fully accommodate the gas temperatures to that of the
    surrounding metal structure.
 
    The reference gas consists of a mixture of argon and neon having
    the same refractive index as a mixture of 11.1 percent He and
    88.9 percent H2.  The reference gas is carried within the
    instrument in a storage volume of about 20 cm^3 at a pressure of
    25 bar.  During the descent into the Jovian atmosphere the
    reference gas is released into its interferometer cells by means
    of a membrane valve.  It keeps the differential pressure between
    the Jovian gas cell and the reference gas cell near 75 mbar.  The
    latter value is nearly independent of the total pressure because
    opening of the valve is largely determined by the pressure
    difference across the membrane (and the elastic constant of the
    membrane).  This pressure difference is measured by a pressure
    sensor within a few millibar to fully account for the influence
    of this pressure differential on the observed fringe motion.
 
    During launch and cruise of the Galileo spacecraft towards
    Jupiter the entrance orifice of the instrument for Jovian gas is
    closed by a thin metal diaphragm.  This diaphragm is designed to
    burst upon reaching an outside pressure of 2.5 bar.
    Subsequently, the ambient pressure actuates a needle device which
    pinches a hole in a second diaphragm which previously had closed
    off the reference gas in its storage volume.  Both gases are then
    passed into the interferometer through capillaries which limit
    the initial rate of pressure increase inside the interferometer
    cells to 50 mbar s^-1.  Measurements are to continue until the
    reference gas is expanded to the local ambient pressure which
    should occur near 12 bar ambient pressure.  Recent calculations
    of the descent profile of the entry probe predict that it will
    take the entry probe about 28 min to descend from 2.5 to 12 bar
    ambient pressure.  The fringe counter measures the motion of the
    interference pattern starting from vacuum conditions through the
    'in-rush' period near 2.5 bar ambient pressure and up to 12 bar.
 
    The structure carrying the optical elements of the
    interferometer, the gas flow elements, the storage volume for the
    reference gas, 3 pressure sensors, and 4 temperature sensors is
    machined from beryllium.  This material was chosen for its high
    mechanical rigidity, low specific mass, and good thermal
    conductivity.
 
    To save energy, the LED is powered only for 0.5 ms at 64 Hz.
    Also, the fringe position is measured 64 times per second which
    allows the fringe counter to follow the fringe motions for
    pressure surges of up to 750 mbar s^-1.  The average power
    consumption of the HAD instrument is 0.9 W.
 
    One telemetry data frame of the HAD instrument consists of 256
    bits and is transmitted every 64 s.  It contains the content of
    the fringe counter, the readings from 3 precision pressure
    sensors, 4 precision temperature sensors, a number of
    housekeeping channels and the analog signal of one of the
    photodetectors.  The latter should enable us to obtain a
    reasonable result from the HAD experiment even if the logic of
    the fringe counter fails.
 
                       Parameters of the helium interferometer
 
----------------------------------------------------------------------
Length l of individual gas cell               100 mm
 
Pathlength L of light beams in gas cells      200 mm
 
Wavelength lambda                             900 nm
 
Range of Jovian pressure p_j                  up to 12 bar
 
Reference gas                                 27.64 percent Ar, rest Ne
 
Interferometer structure                      beryllium
 
Mass of instrument                            1.4 kg
 
Internal measuring speed                      64 fringe positions per s
 
Telemetry data rate                           1 sample per 64 s
                                              (= 4 bit per s)
 
Power consumption                             0.9 W
 
Sensitivity Delta m = 1/8 corresponds to      Delta q_He = 0.0006
 
Accuracy                                      delta q_He = +- 0.0015
----------------------------------------------------------------------
 
 
  Principal Investigator
  ======================
    The Principal Investigators for the HAD instrument were Ulf Von
    Zahn and Donald Hunten.
 
 
  Scientific Objectives
  =====================
    The foremost scientific aim of the HAD experiment is to obtain an
    accurate measurement of the He abundance in the Jovian
    atmosphere.  This datum will more accurately define the small
    difference between the helium mass fractions of Jupiter and the
    Sun and the large differences in the He mass fractions among the
    atmospheres of the giant planets.  Beyond that, the helium mass
    fraction in the Jovian atmosphere represents an important lower
    boundary for the helium abundance in the pre-solar nebula.  As
    such, it also impacts on theories about the origin of the solar
    system as a whole.
 
 
  Calibration
  ===========
    The HAD instrument carries 4 precision thermistors TS, TR, TC,
    and TF for temperature measurements in the range from -25 degrees
    C to +40 degrees C.  With the help of the following electronics
    the read-out from the sensors is made nearly linear between -10
    degrees C and +15 degrees C, the range in which the telemetry
    resolution is 0.2 degrees C (except for TF which measures the IR
    filter temperature with a resolution of 0.8 degrees C).  The
    absolute calibration of all four sensors is performed jointly to
    an accuracy of +-0.5 degrees C.  During the first checkout of the
    HAD instrument on its cruise towards Jupiter each of the TS, TR,
    and TC sensors read a temperature within the range of +7.20
    degrees C +-0.12 degrees C.  The spread of values is fully
    accounted for by the telemetry resolution.
 
    The sensors PS and PR measure the pressures in the gas cells in
    the range from 0 to 20 bar, while the sensor PD has 400 mbar full
    range.  The absolute sensitivity of each sensor was calibrated
    against a rotating piston gage to within 0.1 percent of its
    reading, as was the temperature dependence of these
    sensitivities.  It turned out that the temperature dependencies
    of the PS and PR sensors are negligible for our experiment.  The
    temperature dependence of the PD sensor, however, needs to be
    taken into account in deriving the helium mole fraction.  At an
    indicated pressure of 100 mbar it amounts to change of about 3
    mbar in the temperature range from -30 degrees C to +35 degrees
    C.  Accounting for this temperature dependence, the
    reproducibility of the pressure readings are within 1 mbar
    throughout the expected operating range of the instrument.
 
    The spectral intensity distribution of the LED/filter combination
    was measured and the temperature shift of its centroid wavelength
    determined.  The latter is
 
         lambda (T) = (900.5 + 0.041 T) nm ,
 
    with T in degrees C.  The temperature of the IR filter is
    measured by the sensor TF, but its value is obviously not
    critical.
 
    A large number of absorber materials were tested but unexpectedly
    could not find one which absorbed methane efficiently, but did
    not absorb hydrogen.  The effect of hydrogen absorption is not
    large, but measurable with the accuracy of our instrument.  This
    effect was measured for many types of absorbers in a large number
    of descent simulation tests.  These were performed with the HAD
    instrument and covered the temperature range from -15 degrees C
    to +25 degrees C.  1.3 g of silica gel was finally selected as
    absorber for water and ammonia (absorber No.  1) and 1.15 g of
    activated carbon as absorber for methane (absorber No. 2).
    Additional tests using this absorber combination quantified the
    required correction Delta aq_He of the measured helium mole
    fraction for gas mixtures having q_He in the range between 0.008
    and 0.014.  For a true He mole fraction q_He = 0.11 the
    correction is
 
         Delta aq_He = 4.0 x 10^-3 - 7.9 x 10^-5 T_a +- 5 x 10^-4  ,
 
    with T_a being the absorber temperature, again measured in
    degrees C.  The quoted uncertainty of this correction term is to
    be taken independent of temperature and helium mole fraction in
    the tested ranges of those parameters.
 
    Verification of the value of 1q_He, the ratio of refractive index
    differences, has been discussed in [VONZAHN&HUNTEN1992].  To this
    end a great number of laboratory simulations of the descent of
    the instrument into the Jovian atmosphere have been performed.
    These consisted of mounting the HAD instrument in a high-pressure
    chamber in which the chamber pressure, the temperature of the
    mounting platform, and the temperature of the chamber gas could
    be programmed to follow the values expected during the actual
    Jovian descent.  Many of these tests were performed with all of
    the burst diaphragms in place inside the HAD instrument, but in
    the majority of the tests no burst diaphragms were installed.
    For the development and calibration of HAD instruments, more than
    500 such descent simulations were performed and evaluated.
 
 
  Operational Considerations
  ==========================
 
                         HAD parameters and uncertainties
 
----------------------------------------------------------------------
Parameter                 Value             Uncertainty     Unit
----------------------------------------------------------------------
 
Center wavelength             900.5         +- 3            nm
 
Length of gas cells       2 x 100.0         +- 0.05         mm
 
(n_H2,0 - 1)  x 10^6          137.026       +- 0.04
 
(n_He,0 - 1)  x 10^6           34.7196      +- 0.04
 
(n_ref,0 - 1) x 10^6          125.682       +- 0.05
 
Delta m                       <= 6          +- 0.0625
 
T_j,e                        ~280           +- 0.5          K
 
Delta T_e                       0           +- 0.1          K
 
P_j,e                         ~10000        +- 100          mbar
 
Delta P_e                     ~75           +- 3            mbar
----------------------------------------------------------------------
 
 
Measured Parameters
===================
 
                        Definitions for abundance measures
----------------------------------------------------------------------
 
'Mass fraction'                                    N_Hm_H + 2N_H2m_H
Hydrogen mass fraction  X is equivalent to  ---------------------------
                                            SUM of (N_jm_j) for all j's
 
                                                        N_Hem_He
Helium mass fraction    Y is equivalent to  ---------------------------
                                            SUM of (N_jm_j) for all j's
 
Mass fraction of all other elements   Z is equivalent to 1 - X - Y
 
with N_i the number density of particles of type i;
m_i mass of a particle of type i
 
'Mole fraction'                                         N_i
(mixing ratio)          q_i is equivalent to   ------------------------
                                               SUM of (N_j) for all j's
 
with    SUM of (q_j) for all j's = 1
 
                                                            N_He
In particular at Jupiter  q_He is approximately   --------------------
                                                        N_H2 + N_He
 
'Abundance ratio' of                                        N_He
 helium/hydrogen          R_He is equivalent to   --------------------
                                                            N_H2
 
----------------------------------------------------------------------

extracted metadata from PDS3 catalog and
                    modified to comply with PDS4 Information Model

Galileo Probe Helium Abundance Detector

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