| DESCRIPTION |
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
----------------------------------------------------------------------
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