THE JUPITER HELIUM INTERFEROMETER EXPERIMENT ON THE GALILEO ENTRY PROBE U. VON ZAHN University of Bonn, Bonn, Germany and D.M. HUNTEN University of Arizona, Tucson, AZ, U.S.A. Abstract. We discuss the scientific objective, instrument design, and calibration of a miniaturized Jamin-Mascart interferometer which is to perform an accurate measurement of the refractive index of the Jovian atmosphere in the pressure range 2.5 to 10 bar. The instrument is to perform this measurement in December 1995 aboard the entry probe of the NASA Galileo spacecraft. From the data obtained the mole fraction of helium in the atmosphere of Jupiter is to be calculated with an estimated uncertainty of +- 0.0015. The instrument has a total mass of 1.4 kg and consumes 0.9 W of electrical power. Table of Contents Abstract 1. Introduction 2. Scientific Objective 2.1. Background 2.2. Objective 3. Instrument Description 3.1. Principle of 0peration 3.2. Instrument Design 3.3. Sensitivity and Accuracy 4. Tests and Calibrations 4.1. Tests 4.2. Calibrations 1. Introduction Hydrogen and helium are by far the most abundant elements in the Universe, the stars, the Sun, and the outer planets. If known, the mixing ratios of these elements in stellar and planetary bodies provide important constraints for theories about their origin and evolution. Yet, considerable uncertainty exists for these mixing ratios because they are difficult to measure. 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. TABLE I 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 ------------------------------------------------------------------------------ In astrophysics, element abundances are commonly quoted as mass fractions, whereas in planetary atmosphere studies these abundances are usually given as mole fractions (equivalent to mixing ratios). As we cannot avoid using both terminologies, we provide in Table I rigorous definitions of the more relevant abundance measures. 2. Scientific Objective 2.1. BACKGROUND Current theory holds that almost all of the hydrogen and helium now residing in the Universe formed billions of years ago in a cosmic fireball, called the Big Bang. It is of obvious scientific interest to find out what mass fractions these two elements had attained after the initial cool-down of the fireball and before additional, though limited, synthesis of helium in stars began. If known, the primordial mass fractions of H and He provide important constraints for theories trying to explain the formation and evolution of the Universe. The subject of the primordial helium abundance was treated extensively at an ESO workshop (Shaver et al., 1983). Recent reviews of He observations and of the standard Big-Bang theory put the primordial helium mass fraction Y_p into the range 0.23 <= Y_p <= 0.24 (e.g., Yang et al., 1984; Pagel, 1989). The quoted range of uncertainty can be reduced by accepting the latest correction to the half-life of the neutron (Paul et al., 1989). After synthesis of the primordial helium in the Big Bang some additional helium has been produced by nuclear burning of hydrogen inside stars. Until the time of formation of the solar system 4.5 billion years ago, this process increased the helium mass ratio in the Universe by DeltaY. Hence, one would assume the solar system to be formed from a pre-solar nebula having a helium mass ratio Y_s = Y_p + DeltaY. The value of Y_s has turned out to be notoriously difficult to establish. Gough (1983) estimates 0.23 <= Y_s <= 0.27. Stix (1989) writes: "...we see that presently the most accurate method to determine the solar abundance is the computation of solar models, and the adjustment of these models to the present Sun by a proper choice of the mass fraction Y_s of He. Virtually all standard solar models calculated so far yield Y_s values between 0.23 and 0.28." In view of the astrophysical importance of the value of Y_s alternate approaches to just studying the Sun seem justified to arrive at a determination of Y_s. One alternative is to assume that the giant planets, in particular Jupiter and Saturn, have also been formed with the pre-solar helium mass fraction Y_s and that they have preserved it until today. Indeed, when the preparations for the HAD experiment began during 1976, many scientists assumed that the relative abundances of helium were equal in Jupiter, Saturn, and the Sun (see Lewis, 1974; Podolak and Cameron, 1975). At that time, the helium abundance in the atmosphere of Jupiter was, however, only poorly known. Orton and Ingersoll (1976) summarized the then available remote sensing measurements as indicating a Jovian He mole fraction q_He = 0.12 +- 0.06, which for a H2 + He atmosphere is equivalent to Y_j = 0.214 +- 0.11. It does not appear to be very difficult to reduce the large uncertainty of either value significantly by a simple in-situ experiment aboard the Galileo entry probe. So far we have implicitly made the assumption that the giant planets have a uniform composition throughout. Only then would a He mole fraction measured in the atmosphere of one of these planets resemble the bulk composition of that planet. As early as 1967 doubt was expressed about the validity of this assumption for different reasons (e.g., Smoluchowski, 1967; Salpeter, 1973). In addition, the processes, which were postulated to lead to an inhomogeneous composition of the giant planets, are of varying effectiveness in the different planets. Hence, the notion of similar He abundances in the atmospheres of Jupiter, Saturn, and the Sun also needed reconsideration. However, it was only after 1979 that those doubts could be verified experimentally through data obtained by the two Voyager spacecrafts. Today it appears established that (a) the atmospheres of Jupiter, Saturn, and Uranus each accommodate a different He mole fraction q_He, and that (b) the atmosphere of Jupiter contains somewhat less He than that of the Sun. This follows from the Voyager data concerning the helium abundances in the atmospheres of the giant planets (Table II). TABLE II Helium abundances in the giant planets and the Sun ------------------------------------------------------------------------------ q_He Y Reference Jupiter 0.101 +- 0.0206 0.18 +- 0.04 Conrath et al. (1984) Saturn 0.032 +- 0.027 0.06 +- 0.05 Conrath et al. (1984) Uranus 0.152 +- 0.033 0.262 +- 0.048 Conrath et al. (1987) Sun 0.25 +- 0.02 Gough (1983) ------------------------------------------------------------------------------ The helium mass fractions Y for Uranus and the Sun, though uncertain, seem to agree; for Saturn, and perhaps Jupiter, helium may have been gravitationally separated in the form of 'raindrops' which may form at the high interior pressures (for a current review see Hubbard, 1989). Thus, it has become apparent that there is no simple relation between the current helium abundance in the atmospheres of the giant planets and that of the pre-solar nebula. Yet, the unexpectedly large differences in the He abundances of the giant planet atmospheres are themselves of eminent scientific interest and provide major inputs for our understanding of the origin and evolution of these atmospheres, as well as their energy balance. We do not expect to observe any change of the He mole fraction over the (very limited) pressure range that we sample in the Jovian atmosphere. This is because the time scale for vertical mixing by turbulence is almost certainly much shorter than that for any process causing gravitational separation of He and H2. 2.2. SCIENTIFIC OBJECTIVE 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. 3. Instrument Description 3.1. PRINCIPLE OF OPERATION More than 99.5 percent of the Jovian atmosphere consists of hydrogen and helium. Hence, to a first approximation, we can consider this atmosphere to be a binary gas mixture, for which the mole fraction q_He of helium can be derived from the ratio of refractive indices n_H2 - n_j q_He = --------------- , n_H2 - n_He with n_He the refractive index of helium; n_H2, refractive index of hydrogen; n_j, refractive index of the H2 + He mixture (= Jovian gas). The refractive index of gases is usually measured interferometrically and any two-beam interferometer can be used for this purpose. We have chosen a Jamin interferometer (see Jenkins and White, 1976), a schematic of which is shown in Figure 1. Let us assume that beam 1 passes through a sample gas cell CS filled with the Jovian gas and beam 2 passes through another gas cell CR filled with a reference gas of refractive Fig. 1. Schematic of a Jamin interferometer. LED = light emitting diode; F = interference filter; JP = (two) Jamin plates; CS = sample gas cell; CR = reference gas cell; L = objective; PDA = photodiode array. index n_ref. Both cells are of length L. The optical path difference, OPD, between the beams then becomes OPD = (n_j - n_ref)L and the order m of the interference pattern at the detector is m = OPD/lambda, hence, n_j = n_ref + lambda/L * m In order to account for variations of the pressure p and gas temperature T in the gas cells we assume that refractive indices n vary with p and T as follows: pT_0 n(p, T) - 1 = {n(p_0, T_0) - 1} ------ , P_0T with p_0, T_0 are the standard pressure and temperature (=STP); n(p_0, T_0) = n_0, refractive index at STP. This yields p_jT_0 p_refT_0 lambda (n_j,0 - 1) * --------- = (n_ref,0 - 1) * ------------ + -------- * m , p_0T_j p_0T_ref L with p_j, T_j are the pressure and temperature in the Jovian gas cell; P_ref, T_ref, pressure and temperature in the reference gas cell. Usually, the absolute value of m is not known. We can, however, measure the number of interference fringes Delta m while going from an initial state (i) to an end state (e) in pressures and temperatures. Let us assume that we start our measurement with evacuated gas cells (p_j,i = p_ref,i = 0). We then obtain for the end of the measurement p_j,eT_0 p_ref,eT_0 lambda (n_j,0 - 1) * ---------- = (n_ref,0 - 1) * ------------ + -------- * Delta m, p_pT_j,e p_0T_ref,e L with P_j,e, T_j,e are the pressure and temperature in the Jovian gas cell at the end of the measurement; P_ref,e, T_ref,e, pressure and temperature in the reference gas cell at the end of the measurement; Delta m = m_e - m_i. We now introduce Delta p_e = p_j,e - p_ref,e, the pressure difference between the two gas cells at the end of the measurement; Delta T_e = T_j,e - T_ref,e, the difference in temperature of the gases in two gas cells at the end of the measurement which allows us to develop (5) in combination with (1) into our final equation for determining the helium mole fraction q_He n_H2,0 - n_ref,0 1 p_0T_j,e lambda q_He = ------------------- + ----------------- * ------------------ Delta m + n_H2,0 - n_He,0 n_H2,0 - n_He,0 p_j,eT_0L n_ref,0 - 1 + ----------------- * ( Deltap_e/p_j,e - DeltaT_e/T_j,e ). n_H2,0 - n_He,0 Before evaluating (6) quantitatively we would like to note the following two items: First, at high pressures it is the so-called Lorentz-Lorenz function LL which is independent of temperature, pressure, and density rho (Lorentz, 1909). LL is given by n^2 - 1 1 LL = --------- * ----- . n^2 + 2 rho For n ~ 1 the Lorentz-Lorenz function can be approximated to LL = 2/3 * (n-1)/rho and, hence, yields (3). The application of (7) instead of (3) to our HAD experiment and inclusion of the compressibility of real gases has been studied in detail by Schulte (1983). Second, so far we have neglected the small amount of trace constituents in the Jovian atmosphere. In light of the desired accuracy of our helium measurement, this neglect needs further consideration. In fact, we have either to remove the trace gases from the Jovian gas sample before the refractive index is measured or we have to apply numerical corrections to the result obtained from (6). These corrections would depend on information from sources other than the HAD about the ambient abundances of NH3, H2O, and CH4. We have elected to remove these trace gases before measuring the refractive index (Schutze, 1986), because in the alternative approach it appears difficult to establish how much of the ambient NH3 and H2O in fact enters the interferometer gas cells. 3.2. INSTRUMENT DESIGN The Galileo Helium Abundance Detector uses a two-arm, double-pathlength interferometer or Jamin-Mascart interferometer (Mascart, 1874). This type of interferometer allows for a particularly compact and simple design (Figure 2). The light source (1) is a light emitting diode (LED) operating at a wavelength of 900 nm. An interference filter (2) with a 15 nm passband aids in producing near-monochromatic light. A Jamin plate (4) produces two parallel and coherent light beams (5 and 6). Four cells, each of length l = 100 mm, house the Jovian gas (7) and the reference gas (8). Additional optical elements are the collimator (3), the inversion prism (9), and the objective (10). 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 (12). 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 (11) 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 (10) and the Fig. 2. Optical layout of HAD interferometer (to scale). The length of one gas cell is l = 10 cm. 1 = light emitting diode; 2 = interference filter; 3 = collimator; 4 = (single) Jamin plate; 5, 6 = parallel light beams reflected from the Jamin plate; 7 = Jovian gas cells; 8 = reference gas cells; 9 = inversion prism; 10 = objective; 11 = test device; 12 = photodetector array. detector array (12). 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. Figure 3 shows a schematic of the gas flow system of the HAD instrument. After entering the Instrument at I, the Jovian gas is passed through a two-stage chemical absorber (A1 and A2) to be scrubbed first of traces of NH3 and H2O and then of CH4. In addition, immediately before entering the gas cells (CS, CR) the Jovian and the reference gases are each passed through heat exchangers (H) 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 (R) 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 (CR) by means of a membrane valve (V). It keeps the differential pressure between the Jovian gas cell (CS) and the reference gas cell (CR) 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 (PD) within Fig. 3. Schematic of gas flow system, with Jovian gas on the left side, the reference gas on the right side. I = inlet for sample gas; B1, B2, D = burst diaphragms; A1, A2 = absorbers No. 1 and No. 2: C = capillaries; M = release mechanism for reference gas; L = microleaks; H = heat exchangers; CS = interferometer cell(s) for sample gas; CR = interferometer cell(s) for reference gas; R = storage volume for reference gas; V = membrane regulated valve; TS, TC, TR = temperature sensors; PT, PS, PR, PD = pressure sensors; P = pinch-off tubes; VS, VR = valves for laboratory use. 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 (B1). This diaphragm is designed to burst upon reaching an outside pressure of 2.5 bar. Subsequently, the ambient pressure actuates a needle device (M) which pinches a hole in a second diaphragm (D) which previously had closed off the reference gas in its storage volume (R). Both gases are then passed into the interferometer through capillaries (C) 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 Fig. 4. The HAD flight instrument (without thermal cover). The right unit constitutes the interferometer and gas flow system; the box on the left contains the electronics. The small box on the top left side of the interferometer structure contains the photodetector array and preamplifiers; 3 small cylinders on top of the gas cells are pressure transducers. 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. A photograph of the fully assembled flight instrument is shown in Figure 4. A concise list of its important parameters is given in Table III. 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. TABLE III 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 ------------------------------------------------------------------------------ 3.3. SENSITIVITY AND ACCURACY Equation (6) can be used to quantitatively evaluate the sensitivity and accuracy of the HAD instrument for determining the helium mole fraction q_He. These evaluations are based on the input parameters as given in Table IV. TABLE IV 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 ------------------------------------------------------------------------------ The sensitivity S of the HAD instrument is given by the change Delta m of the order of the interference pattern for a given change Delta q_He of the He mole fraction: S = Delta m/Delta q_He. S is obtained directly from the second term of (6) p_j,eT_0L S = (n_H2,0 - n_He,0) * ----------------- . p_0T_j,e lambda Inserting the parameters as given in Table IV yields S = 222. Hence, if the genuine helium abundance deviates by 1 percent (Delta q_He = 0.01) from the value simulated by the reference gas (q_He = 0.111), the fringe counter would register a change Delta m of 2.22 fringes between 0 and 10 bar ambient pressure. For any value of q_He within the error bar of the q_He measurement of Conrath et al. (1984) we would obtain Delta m <= 7. As will be shown in the section on HAD calibrations, the coherence length of the LED/filter combination makes the HAD instrument capable of producing more than 50 interference fringes. The accuracy expected from the HAD experiment for determining the helium mole fraction is evaluated using (6). The error bar on the measured q_He depends to some measure on the uncertainty of each of the 10 variables entering (6). However, different variables affect the final result of q_He in quite different ways. Here it should suffice to point out only the most critical ones. We will denote with 1q_He the first term on the right-hand side of Equation (6) n_H2,0 - n_ref,0 1q_He = -------------------- . n_H2,0 - n_He,0 Substituting n_ref,0 from our standard reference gas we obtain 1q_He = 0.110883 . An error delta 1q_He calculated by using the uncertainties listed in Table IV has no direct relevance for the HAD experiment, because the value of 1q_He was verified by laboratory calibrations of the HAD. Each calibration consisted of a complete simulation of the descent of the HAD flight instrument into a hydrogen + helium atmosphere to pressure levels of at least 10 bar. All of these calibrations employed the same standard reference gas mixture in the HAD and hydrogen + helium mixtures surrounding the HAD instrument. Three hydrogen + helium mixtures were commercially procured and employed in the calibrations, each of them having a helium mole fraction of about 0.11, known with an accuracy of delta q_He <= 6 x 10^-5. In many dozens of such calibration runs, the HAD instrument measured with great precision the correct q_He of the commercial H2 + He mixtures with an accuracy delta q_He better than 1 x 10^-3, where 1q_He = 0.110883 was used to calculate the final result. This proves that when we use a value of 0.110883 for this ratio (in combination with our standard reference gas) its uncertainty has no significant effect on our q_He determination. We will denote with 2q_He the second term on the right-hand side of Equation (6). Again using the parameters of Table IV, one obtains 2q_He = 4.511 x 10^-3 Delta m. The error delta 2q_He vanishes entirely in the case that Delta m goes to zero. In the idealized case of Delta p_e = Delta T_e = 0, this happens when n_j = n_ref. Under these circumstances the uncertainties of T_j,0, P_j,0, lambda, L, and the two refractive indices do not enter at all, which is the basic advantage and incentive for performing a 'differential' measurement. Of all the parameters entering 2q_He it is the error delta p_j,e which has the largest effect on delta 2q_He. In Table IV, delta p_j,e/p_j,e is (conservatively) given as +- 0.01. Hence, considering (11), delta p_j,e alone causes an error delta 2q_He = +- 4.5 x 10^-5 Delta m. Taking the other parameters in 2q_He into account as well, this error rises slightly to delta 2q_He = +- 5.1 x 10^-5 Delta m . As outlined above, Delta m will be always smaller than 7 for 0.08 <= q_He <= 0.14. Hence, in this range of q_He we find absolute value[delta 2q_He] <= 3.5 x 10^-4 . We will denote with 3q_He the third term on the right-hand side of Equation (6). Again using the parameters of Table IV, one obtains delta 3q_He = +- 5.7 x 10^-4 . In this case, both delta Delta p_e and delta Delta T_e contribute about equally to the error sum. Unfortunately, it is not possible to actually measure the gas temperatures in the gas cells with the required accuracy (though the wall temperatures can be). We have to rely on the effectiveness of the heat exchangers and continued intensive contact of the gas with the walls of the gas cells to maintain the required thermal equilibrium between the two gases. As discussed in Section 4.2, we also have to include in the final estimate of the overall error delta q_He due to the effects of the chemical absorbers which introduce an uncertainty delta aq_He = +- 5 x 10^-4 . Adding quadratically the terms delta 2q_He, delta 3q_He, and delta aq_He we obtain the calculated overall error of the measured He mole fraction delta q_He = +- 8 X 10^-4 . This error still meets with our originally announced goal of an error smaller than +- 0.001 (von Zahn and Hoffmann, 1976), but it comes uncomfortably close to the latter. This is because during the instrument development phase the effects of the required chemical absorbers turned out to be more significant than estimated. To be conservative we now prefer to state that our overall error on the helium mole fraction will eventually turn out to be smaller than +- 0.0015. 4. Tests and Calibrations 4.1. TESTS The instrument subsystems and critical components underwent an intensive series of developmental tests. The major objectives addressed in these tests were: - radiation hardening of selected components, in particular the IR emitting diode and the photodiode array detector (Mett, 1980); - overpressure testing of electronics components; - temperature testing of critical electronics circuits; - testing of the entrance burst diaphragm, opening device for the reference gas, and of the membrane valve controlling the pressure differential between the Jovian and reference gas cells; - characterizing and quantifying the effectiveness of the absorbers employed to remove H2O, NH3, and CH4 from the measured Jovian gas sample; - verification of a sufficient coherence length of the IR light source. Fig. 5. Interference fringes caused by first filling the Jovian gas cell with N2 and then pumping it down, while the reference gas cell stays evacuated. Along the ordinate is shown the analog signal of the diode in the detector array, along the abscissa time (=No. of major frames of the telemetry). More than 50 fringes can be counted. From the large number of individual test results we show in Figure 5 an example of a test for sufficient coherence length of the LED. This test is started by evacuating all gas cells of the interferometer. Then nitrogen is slowly introduced in the 'Jovian' gas cell(s) which causes a shift of the interference fringes at the detector. This shift is registered by the photodiodes. The analog signal from one of the 9 diodes is shown in Figure 5 (along the ordinate). Along the abscissa, time is increasing (represented by the number of telemetry major frames). After counting 55 fringes the pressure rise is stopped and the nitrogen pumped out of the gas cell (with the final level of the diode signal matching nicely its initial level). As was noted above the maximum number of interference fringes to be counted during the descent into the Jovian atmosphere is 7. It follows that the bandwidth of the IF filter, and hence the coherence length of the LED assembly is chosen very conservatively and does not pose any limitations on the instrument performance. The entire HAD instruments underwent the following tests according to specifications laid down in the document NASA JP-512.03 (1979): - temperature tests (in 1 bar of dry nitrogen); - thermal-vacuum tests; - high pressure tests; - vibration tests (sine and random accelerations); - acoustics tests; - steady-state deceleration tests; - shock tests; - electromagnetic interference tests. The HAD engineering instrument was tested to design qualification levels, the HAD flight instrument to the expected flight levels. During these tests the performance of the HAD optics gave, in general, only pleasant surprises. Before the design of the instrument could be successfully qualified, however, we were required to spend a considerable effort isolating the instrument from thermal and mechanical stresses induced by the mounting platform of the Galileo Probe on the interferometer. At the same time, it is obviously necessary to provide a rigid mounting for the HAD instrument inside the Probe in order for it to survive without harm the extreme mechanical loads experienced during its entry into the Jovian atmosphere. We might add that after all this elaborate testing and only a few weeks after 'final' delivery of the flight instrument to NASA for integration into the Galileo entry probe, the sensor PT, which measured the pressure of the reference gas in its storage vessel, started to leak. This event raised serious concerns about the reliability of this particular type of pressure sensor. Since there was no time to qualify and integrate a new type of pressure sensor, the leaky sensor was substituted by a dummy. This, in fact, ended our ability to monitor the pressure p_r in the storage vessel for the reference gas. As this quantity does not enter in the evaluation of the Jovian helium abundance, no loss in accuracy of our results are expected from this mishaps. 4.2. CALIBRATIONS 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. We tested a large number of absorber materials 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. We measured this effect for many types of absorbers in a large number of descent simulation tests (see below). These were performed with the HAD instrument and covered the temperature range from -15 degrees C to +25 degrees C. We finally selected 1.3 g of silica gel 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 Section 3.3. 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. Fig. 6. Laboratory simulation of descent into the Jovian atmosphere (test No. 131): Flight instrument equipped with burst membranes; Jovian gas simulated by binary gas mixture of 11.08 percent He + the rest H2. The solid line gives helium mole fraction (V_He, in percent) calculated from measured fringe motion. Double lines near 10 bar indicate range of helium mole fraction 0.1108 +-0.001. The dotted line and right ordinate scale gives temperature T_WT of the simulated Jovian gas. Figure 6 shows a typical result of a laboratory simulation of descent into the Jovian atmosphere (test No. 131), performed with the HAD flight instrument equipped with burst membranes. The Jovian gas is simulated by a binary gas mixture of 11.08 percent He + rest H2. The solid line gives the helium mole fraction (V_He, in percent) calculated from the measured fringe motion. The abscissa gives the pressure p_s measured in the Jovian gas cell. After bursting the entrance diaphragm it rises comparatively rapidly. This is the reason why the data points, which are sampled at fixed time intervals, stretch out for p_s <= 2.5 bar. The double lines near 10 bar indicate the exact helium mole fraction (=0.1108) of the sampled gas with a 'permitted' measurement error of +-0.001. It is evident that the measured helium mole fraction (solid line) gives the correct result and does so well within the predicted uncertainty. The dotted line and right ordinate scale gives the temperature T_WT of simulated Jovian gas. Figure 7 shows a second example of such a descent simulation (test No. 341). Here the HAD engineering instrument No. 2 is tested (without inlet burst membranes). The Jovian gas is simulated by a mixture of 10.92 percent He, 0.198 percent CH4, and the rest H2. Again, the solid line gives the helium mole fraction (V_He, in percent) calculated from the measured fringe motion and applying the correction for hydrogen absorption in the chemical absorbers. The double lines near 10 bar indicate the exact helium mole fraction (=0.1092) of the sampled gas mixture with the 'permitted' measurement error of Fig. 7. Laboratory simulation of descent into the Jovian atmosphere (test No. 341): Engineering instrument No. 2 (without inlet burst membranes); Jovian gas simulated by mixture of 10.92 percent He, 0.198 percent CH4, and rest H2. The solid line gives helium mole fraction (V_He, in percent) calculated from measured fringe motion. Double lines near 10 bar indicate range of helium mole fraction 0.1092 +- 0.001. The dotted line and right ordinate scale gives temperature T_WT of simulated Jovian gas. +- 0.001. It is evident that the measured helium mole fraction (solid line) again gives the correct result and does so well within the predicted uncertainty. As in Figure 6, the dotted line and right ordinate scale gives the temperature T_WT of simulated Jovian gas. Acknowledgements We are most grateful to many of our colleagues and students for their diligent and untiring efforts on behalf of the HAD experiment. Over the years H. J. Hoffmann contributed heavily to the instrument development effort, W. Mett was responsible for radiation hardening of all the instrument subsystems, W. Schulte developed the apparatus for laboratory simulations of the instrument descent into the Jovian atmosphere, and H. Schutze performed the calibration and environmental tests of the various units of the HAD instrument. Integration of the instrument into the spacecraft and systems tests were supported by H. Schutze and G. Lehmacher. K. Pelka assisted during various phases of the project through software development. The interferometer part of the HAD instrument was developed by Carl Zeiss (Oberkochen), the other portion of the HAD by Messerschmitt - Bolkow - Blohm (Ottobrunn). 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