GALILEO PROBE NEPHELOMETER EXPERIMENT B.RAGENT San Jose State University Foundation, San Jose, CA 95192-0139, U.S.A. C. A. PRIVETTE NASA Ames Research Center, Moffett Field, CA 94035, U.S.A. and P. AVRIN, J.G. WARING, C.E. CARLSTON, T.C.D. KNIGHT, and J.P. MARTIN Martin Marietta Astronautics Group, Denver, CO 80201, U.S.A. Abstract. The objective of the Nephelometer Experiment aboard the Probe of the Galileo mission is to explore the vertical structure and microphysical properties of the clouds and hazes in the atmosphere of Jupiter along the descent trajectory of the Probe (nominally from 0.1 to > 10 bars). The measurements, to be obtained at least every kilometer of the Probe descent, will provide the bases for inferences of mean particle sizes, particle number densities (and hence, opacities, mass densities, and columnar mass loading) and, for non-highly absorbing particles, for distinguishing between solid and liquid particles. These quantities, especially the location of the cloud bases, together with other quantities derived from this and other experiments aboard the Probe, will not only yield strong evidence for the composition of the particles, but, using thermochemical models, for species abundances as well. The measurements in the upper troposphere will provide 'ground truth' data for correlation with remote sensing instruments aboard the Galileo Orbiter vehicle. The instrument is carefully designed and calibrated to measure the light scattering properties of the particulate clouds and hazes at scattering angles of 5.8, 16, 40, 70, and 178 degrees. The measurement sensitivity and accuracy is such that useful estimates of mean particle radii in the range from about 0.2 to 20 microns can be inferred. The instrument will detect the presence of typical cloud particles with radii of about 1.0 microns, or larger, at concentrations of less than 1 cm^3. 1. Introduction The Galileo Jupiter mission atmospheric Probe (O'Neil and Mitchell, 1983; Givens et al., 1983) includes a Nephelometer as part of the Probe instrument complement. The scientific objective of the Nephelometer experiment* is to explore the vertical structure and microphysical properties of the clouds and hazes in the Jovian atmosphere. The experiment will provide evidence for the existence or absence of particulates along the descent trajectory of the Probe (nominally from altitudes corresponding to ambient pressures of 0.1 bars to greater than 10 bars). It will yield data that will be used to infer the properties of these particulates, including mean size and particle number density, and to differentiate between liquid and solid particles (i.e., spherical and non-spherical). * The nephelometer experiment team includes Boris Ragent, San Jose State University Foundation, Principal Investigator, and Co-Investigators Philip Avrin, Martin Marietta Astronautics Group, Jacques E. Blamont, Service d'Aeronomie du C.N.R.S., David Colburn, San Jose State University Foundation, Gerald Grams, Georgia Institute of Technology, and James Pollack, NASA/Ames Research Center. In conjunction with data from other Galileo Jupiter Probe and Orbiter experiments, the Nephelometer measurements will be used to confirm the consistency of predictions of the effective particle indices of refraction, including absorption. Approximate cloud opacities at a wavelength of 0.9 microns, a measure of the local cloud mass density along the descent trajectory, and the total columnar mass loading in each cloud will be obtained. All of the above results, together with the cloud base locations, that will be well documented by this experiment, will not only yield strong evidence for inferring the gross composition of the particles, but, by comparison with thermochemical models, will constrain the particles' species abundances. Finally. the data will provide in situ information about particles in the upper levels of the troposphere at the Probe entry site to provide 'ground truth' data for correlation with remote sensing experiments aboard the Galileo Jupiter Orbiter vehicle. The launch of the Galileo spacecraft took place on October 18, 1989, and the Probe will enter the Jovian atmosphere at a latitude of 6.5 N at about local Jovian sunset (nominally at 22:11 UT) on December 7, 1995. It is expected that, after deployment, Nephelometer measurements will be obtained, at a minimum, every kilometer for at least 100 km or more during the course of the Probe descent. In the following sections we summarize the presently held views of the clouds and hazes of Jupiter and what we may expect to learn from our experiment, describe the design of the instrument, and discuss its calibration, testing, and expected performance. 2. Background Many of the aspects of the nature, structure, and variability of the clouds and hazes in the troposphere of Jupiter at pressures less than 5 bars are subjects of very active ongoing discussion in the literature. (See, for example, West et al., 1986, who have recently provided an excellent review of the status of knowledge of the Jovian clouds and aerosols; Carlson et al., 1987, 1988, 1990; Cunningham et al., 1988; West, 1988: West et al., 1989; Del Genio and McGrattan, 1990). The foundation for ideas of the aerosol and cloud structure comes from equilibrium thermochemical models that depend on the abundances of relevant species and the atmospheric structure functions (atmospheric temperature versus pressure curves). By assuming that pertinent species (N, O, and S) are present with solar abundances, the presence of three cloud layers in the upper troposphere are predicted by such models (cf. Lewis, 1969a, b; Weidenschilling and Lewis, 1973). NH3 condenses at the altitude corresponding to a pressure near 0.7 bars, mixed NH3 and H2S and/or ammonia polysulfides are condensed at about 2 bars, and H2O condenses at about 5 bars. The precise condensed particle compositions and pressure levels of these cloud bases are dependent on the actual local species abundances and temperature structure. Model calculations based on microwave wavelength measurements have determined that NH3 is undersaturated above the NH3 cloud base predicted for solar abundance and at high levels (0.3-0.5 bars), depleted relative to solar abundance at 0.5 to 2 bars, and superabundant at higher pressures (de Pater, 1986). The depletions appear to be due to chemistry, cloud physics, and atmospheric dynamics. Taking these into account, some investigators currently believe that the NH3 abundance above the clouds may be essentially solar (Carlson, 1990, private communication), so that the abundance at lower levels in the clouds is solar or larger, although there is some question about the very deep NH3 mixing ratio. Most investigators agree that the current experimental data do not yield reliable information on sulfur abundances and that, therefore, they do not provide constraints on the NH4SH cloud, even on the question of its existence. From the analysis of airborne telescope and Voyager IRIS spectra Bjoraker et al. (1986), inferred an H2O abundance approximately 50 times less than solar abundance in the 2 to 6 bar region of Jupiter. Such an abundance would shift the predicted water cloud to a level much higher than that predicted for a solar abundance atmosphere, perhaps up to the 2 bar level. However, Carlson et al. (1990) have reanalyzed the same experimental data (including cloud scattering that had not been considered in the earlier analyses) and conclude instead that the H2O abundance is solar or greater, which would place the cloud at a level close to the original level discussed above. The equilibrium thermochemical models alone cannot yield realistic descriptions of the vertical or horizontal cloud structures, or predict details such as particle sizes and number densities. They cannot give the details of particle formation or destruction, or discuss the apparently large spatial and temporal variations. These characteristics are intimately related to the local and regional atmospheric dynamics, precipitation and the concentration of condensation nuclei, and involve the cloud microphysics. For example, suggestions have been made that molecular weight differentiation and latent heat release in condensible species may control the dynamics in the cloud regions (Barcilon and Gierasch, 1970; Gierasch, 1976; Ingersoll, 1976; Gierasch and Conrath, 1985, 1987), and that moist convection processes are important (Stoker, 1986; Lunine and Hunten, 1987; Del Genio and McGrattan, 1990). Due to the large range of length scales involved and the lack of fundamental information such as the concentration of condensation nuclei, the prediction from first principles of the detailed cloud properties appears to be well beyond the capability of existing dynamical models in the absence of in situ measurements (Glerasch, 1988). The comparison of remote sensing observations of scattered sunlight and thermal radiation with radiative transfer calculations provide several constraints on the cloud structures in Jupiter's troposphere. The complexity of the cloud structure, along with significant infrared opacity over most wavelengths, has, however, made the unique inversion of the remote sensing observations difficult. This uncertainty is evident in the continuing discussion in the literature among different investigators using the same and different data sets. The following summary of the clouds of Jupiter in the upper troposphere is based on presently available data and analyses (West et al., 1986; Tomasko, 1989, private conversation; Carlson, 1990, private conversation). Haze is present in the stratosphere above, and perhaps persists down into the tropopause. This haze consists of meteoritic particles and/or energetic particle impact-generated or photochemically-produced aerosols with a mean radius of about 0.1 microns and a total optical depth of about 0.1 to 0.3 at visible wavelengths. There are more particles and greater optical depths at higher latitudes, presumably due to the more intense radiation belts and larger number of energetic particle impacts (Tomasko, 1989, private communication). The pressure range from 200 to 700 mb contains the clouds and hazes that form the layer observed at visible wavelengths, although the precise level of the visible clouds within this range is a subject of some discussion (West et al., 1986). Also, a comparison of longitudinal structures and zonal motions of features in infrared maps and visible images suggests that the visible features have tops throughout this pressure range (cf. Magalhaes et al., 1990). Essentially no holes permitting direct viewing to deeper levels at visible wavelengths exist in this layer down to very small spatial scales (West et al., 1986). Two particle populations are believed to exist in this pressure range. A diffuse ubiquitous layer of particles with mean radii of about 1-2 microns is present and may extend up to about 200-300 mb at low latitudes; this layer is required by the results of studies of scattered visible sunlight. The optical depth of this layer of particles at visible wavelengths is between 2 and 10, and particle densities (based on opacity arguments) are estimated to be of the order of 100-1000 cm^-3. The absence of identifiable spectral features of NH3 ice particles implies that these aerosols may not be composed completely of NH3 (Orton et al., 1982; West et al., 1989). However, the particles may consist of NH3 with large occlusions, mixtures, or coatings, consisting, for example, largely of photochemical products of NH3 such as N2H4, or of various proposed chromophores or other species. A layer of larger particles (with radii of 3-100 microns that is horizontally patchy and confined to a fraction of the gas scale height above the NH3 condensation level at approximately 700 mb is required by observations in the thermal infrared (cf. Gierasch et al., 1986). The zonally-averaged latitudinal variation of the optical cloud depth of this cloud layer correlates well with the zonally averaged visible reflectivity ('the belt-zone structure of the planet') and the zonally-averaged NH3 abundance at the 700 mb level (Glerasch et al., 1986). The composition of the particles is presumably NH3 and the optical depth at visible wavelengths is comparable to or less than that of the diffuse layer. Particle number densities are estimated to be of the order of 1-10 cm^-3. The characterization of clouds at deeper levels is poorly constrained by remote sensing observations of cloud and aerosol structure. An inhomogeneous time-variable cloud is thought to be located in the vicinity of the altitude corresponding to a pressure of about 2 bars. Its composition is not well defined by observations. If the abundance of H2S is solar, then this cloud contains NH4SH and/or polysulfides particles. In addition, if the H2O abundance is appreciably depleted from solar, then, as mentioned above, this cloud may contain appreciable amounts of H2O. The cloud is patchy, may also contain chromophores, and also contributes to the appearance of the 5 micron images. The vertical extent of this cloud is ill-defined, and there are no estimates of particle sizes. An H2O or H2O-NH3 cloud may exist in the 4 to 6 bar region. If the mixing ratio of O is equal to or greater than corresponding solar mixing ratios, then this cloud may be quite massive, containing ice crystals of the order of a micron in size near its top and large droplets of the order of 100 microns or greater in size precipitating, perhaps, from the cloud base. This cloud, if it exists, is also spatially inhomogeneous, perhaps almost disappearing in the regions of the 5 micron features (West et al., 1986). It should be emphasized that there is only weak experimental evidence for the existence of this cloud, evidence for which other explanations may be made without invoking its existence. From the above discussion it is clear that there are many uncertainties about the clouds of Jupiter. By making in-situ measurements the Nephelometer experiment will help to resolve a number of the outstanding questions. The experiment is expected to yield information on the number of clouds, their location, especially of their bases, the amount of material in these clouds and its vertical distribution, the particle sizes and number densities (and hence opacities) in the clouds, and indications of the physical form of the particles. These results can then be used in models to attempt to obtain species abundances, to explain radiation balances, and to investigate atmospheric dynamics. 3. Instrument Design 3.1. GENERAL The Nephelometer is designed to achieve the desired objectives by comparing simultaneous measurements of the light scattered at five angles from a well-defined volume of atmosphere in the vicinity of the Probe with theoretical models of light scattering from particulate matter. A similar approach was successfully used by Marov et al. (1980), for measurements made from the Venera Probes in the Venus atmosphere. A cloud or haze is characterized by the way in which it scatters light. In particular, each unit of volume illuminated by a beam of light will scatter the light at a given angle, theta, in proportion to the product of the particle number density, n, and the probability of the particles in that volume to scatter light into a unit solid angle at that angle, the differential scattering cross section, [d sigma/d Omega]_theta. The Nephelometer measured this quantity, at five angles. Measurements are then compared with calculations of the same quantities for model aerosols to obtain the best agreement with the experimental data. Results of such comparisons yield mean particle sizes, particle number densities, and indications of non-sphericity of the particles and/or absorption in the particles. The accuracy with which these quantities can be determined depends on the accuracy of the experimental data and, to a small extent, on the availability of subsidiary information, for example hints or particle composition from other experiments on the Probe. A description of one method of performing such comparisons to obtain the best fit to the data is given by Marov et al. (1980). The instrument contains the following components: (1) pulsed solid state laser light sources, (2) solid state scattered light detectors, (3) collimating, defining, collecting optics, including a deployable axicon (axially-symmetric conical) mirror system, and spectral filters, (4) optical alignment, surface condensation and source output monitors, (5) other housekeeping measurement systems to monitor instrument operation and performance, and (6) analog and digital electronics circuitry and power supplies. The mechanical structure, deployment system, and thermal design assure that the instrument will survive the severe launch, cruise phase. atmospheric entry, and descent environments. A number of complicating factors must be considered in the design of the instrument. For example, the required high sensitivity to small scattered light signals and the relatively large background light levels (up to 10^6 times as large as the minimum signal levels), as well as the large dynamic range of expected signals (of the order of 10^5 to 10^6), necessitate very careful signal processing. An irradiating light beam collimated highly enough for the measurement of small angle scattering in the forward direction, yet powerful enough to provide sufficient scattered light for measurement of the relatively small scattering at wide angles is required. This requirement is further complicated by the need to reduce instrumentally scattered light, the severely limited space, and the need for reliable source operation after an extended multiple-year cruise phase. In addition, large zero-signal baseline effects may be caused by electrical signals induced by the operation of high power pulsed sources near very sensitive detector circuitry. There is a need to survive not only the severe launch, cruise phase, and atmospheric entry environments, but also the intense high-energy radiation in passing through the Jovian Fig. 1. Galileo Probe Nephelometer instrument. The instrument is positioned on a holding fixture. The forward scatter unit, shown with its mirror arm assembly in the deployed position, is directly mounted on the back scatter unit. For reference, the cylindrical electronics unit is 18.8 cm in diameter and 16.5 cm high. radiation belts. The effects of this radiation on the reliability and stability of electronic components and circuitry need to be carefully considered in the instrument design. Finally, there are the requirements of relatively low allowable weight, space, power, and data rate. Physically, the instrument is constructed in three parts, a vented sensor head containing the forward scatter unit, a vented sensor head containing the backward scatter configuration, and a pressure-tight electronics unit containing the bulk of the electronics. A photograph is shown in Figure 1. The scaled unit is capable of withstanding pressures of greater than 20 bars with negligible leakage. The vented sensor heads, containing components also capable of withstanding pressures greater than 20 bars, are connected to the electronics unit with cables terminating in pressure-tight connectors sealed into the wall of the electronics unit. Both units are mounted onto the aft side of the instrument shelf of the Probe. The faces of the sensor units are flush with the Probe skin, and the TABLE I Instrument characteristics. The dynamic range for all channels is approximately 10^6, and the mean source wavelength for both forward and backscatter sources is approximately 904 nm. The effective sampling volume decreases for strong signals as the number of sampled pulses is reduced. ------------------------------------------------------------------------------ Performance Scatter channels 5 16 40 70 180 (Bkwd) Sensitivity, m^-1 sr^-1 cnt^-1 9.3x10^-7 5.1x10^-7 1.3x10^-7 1.5x10^-7 1.1x10^-8 Mean scattering angle, degrees 5.82 16.01 40.01 70.00 178.1 Angular resolution, FWHM, degrees 0.64 1.08 1.72 1.76 4.0 Effective sampling volume, 1 1.25 0.63 0.65 0.40 16.4 Physical description Mechanical Weight, kg Sensor assembly 1.4 Electronics 3.0 Total 4.4 Dimensions, cm Sensor assembly 50.8 x 8.9 x 12.7 Electronics 18.8 dia x 16.5 Electrical Power, W Instrument 4.8 Heater 6.5 Total 11.33 average Data rate 10 bps Data storage on Probe 800 bits Data output a digital, 2 bilevel Timing signals minor frame Commands 3 stored, 4 real time ------------------------------------------------------------------------------ instrument is oriented on the Probe so that sampled volumes extend out of the Probe essentially radially. A 'closeout' structure is used to seal the edges of the sensor faces to the Probe skin. A deployable arm containing the axicon mirror segments, as well as the pyrotechnic pin puller that activates the deployment mechanism, extends from the upper corner of the top of the sensor unit out through the Probe skin. This assembly allows forward scattering sample volumes to be situated in relatively undisturbed air, outboard of flow regimes near the skin of the Probe in which aerodynamic effects may severely modify the particle size distributions with respect to the true ambient free-stream distributions. Calculation of these effects for the present case have been performed using modified methods similar to those described by Chow (1979). The detector external windows and the axicon mirror assembly are electrically heated continuously during Probe descent to prevent condensation of atmospheric vapors. During transit to Jupiter and the period of high heating on entry into the Jovian atmosphere, the Probe is immersed in the heat shield with the axicon mirror arm stowed in its undeployed position. Targets are mounted on the inner surface of the heat shield, scattering fixed amounts of light from the forward and backward irradiating sources. This scattered light is measured by the instrument, permitting checks of calibration stability during the long test and cruise phases of the mission, and shortly before entry into the Jovian atmosphere. Initiation of the Nephelometer experiment begins after entry and deployment of the Probe parachute, removal of the Probe from the heat shield, and deployment of the axicon mirror arm. The instrument characteristics are summarized in Table I. A preliminary description of the instrument has been published earlier (Yeates et al, 1985). 3.2. OPTICAL DESIGN A schematic of the optical system is shown in Figure 2, which illustrates the scattering configuration for each of the four forward scattering channels, Figure 2(a), and the backward scatter channel, Figure 2(b). For each channel the effective scattering volume is defined by the intersection of the source beam with the field of view of the detector. In the case of the forward scattering channels, light scattered in the forward direction impinges on a portion of an axially-symmetric conical mirror (axicon) and is reflected backward through a window onto the collecting lens of the detector assembly. The light is focussed at the plane of a field stop aperture through which it passes onto a spectral filter and into the detector. For the measurement of back-scattered light no mirror is necessary. The source beam and collecting lens optical axes are parallel and displaced from each other so that only light scattered from the source beam at very wide angles, close to 180 degrees, is collected. A slightly displaced off-axis aperture is used. The scattering configuration and the effect of the aperture are shown schematically in Figure 2(b). Rays a are characteristic of light scattered at 180 degrees (from an infinite distance along the irradiating beam), focussed onto and passing through the aperture onto the spectral filter and detector. Ray c represents an extremal ray, back-scattered at a minimal angle less than 180 degrees, that, after passing through the lens, just grazes the edge of the aperture. Rays b, back-scattered at an angle between the minimal angle of ray c and 180 degrees, are Fig. 2. Schematics of scattering configurations. (a) Forward scatter configuration. The mean scattering angle, theta, is defined by the conical mirror angle, theta/2, and the angular acceptance angle, alpha, by the aperture that determines the angular limits at which scattered light rays may enter the detector. (b) Backward scatter configuration. Ray a corresponds to light back-scattered from the source beam at theta_a = 180 degrees. Ray c is an extremal ray, back-scattered from the source beam at an angle, theta_c, defined by the off-axis aperture, and ray b is for back-scattering at an intermediate angle theta_b. The mean scattering angle and the angular acceptance function are determined from ray tracing calculations, or empirically, from target scans of the scatter volume. characteristic of all other rays scattered at intermediate angles passing through the aperture. The configuration of the components, the aperture, lens characteristics, masking, and source beam dimensions determine the angular acceptance range, +- alpha, for light scattered at the angle theta (fixed by the mirror angle theta/2, for forward scattered light). Ray-tracing calculations, verified by measurements performed by passing diffusely scattering targets through the sample volumes, have established that the angular acceptance functions for the four forward scattering channels are essentially symmetrical with mean values of 5.82, 16.01, 40.01, and 70.00 degrees. Full widths at half maximum for the corresponding angular acceptance functions are 0.64, 1.08, 1.72. and 1.76 degrees, respectively. The effective angular acceptance function of the back-scatter unit, determined by scanning the scattering volume with a diffusely reflecting target, is slightly asymmetric with a mean value of 178.1 degrees and a full width at half maximum of 4.0 degrees (cf. Ragent and Blamont, 1980). A cutaway view of the vented optical sensor head and electronic unit is shown in Figure 3. The forward scatter assembly is mounted above the backward scatter assembly. A solid state laser light source was used in each assembly. This type of source was especially chosen to provide the small effective source size necessary to produce the low divergence light beam required for small angle forward scattering measurements. This source, a gallium arsenide laser injection diode (LID), Laser Diode Lab. Model LID-60, was selected because of its small active area (2.03 x 10^-4 by 7.62 x 10^-3 cm), its high optical power output (approximately 2 W peak when driven by a 200 ns wide, 9 A current pulse), and a favorable match between its spectral output (approximately 904 +- 5.0 nm) and the spectral response of the detectors. The output of the light source is collected by an anti-reflection coated lens corrected for spherical aberration. For the forward scattering unit the light output is further collimated by a 40 cm long baffled tube to reduce scattered light and to keep effective beam divergence to acceptable levels. The backscatter unit source-lens combination is recessed 10.2 cm into the backscatter housing so that no portion of the forward scatter mirror assembly or its mounting configuration is in the field of view of the backscatter unit. The reflecting axicon mirror assembly, mounted on the end of the deployable arm, consists of four coaxial 90 degree segments of the frustums of gold-coated conical mirrors, each reflecting scattered light at a designated angle to one of the forward scatter channels. A hole through the center of the assembly allows unscattered light in the source beam to pass to the atmosphere outside of the collection volume. As mentioned above, the axicon mirrors are electrically heated during Probe descent to reduce the risk of condensation of atmospheric vapors onto their surfaces. A grid-heated glass window on each of the forward and backward-scattering units helps to protect the detector assemblies from thermal effects, surface condensation, and electrostatic charging. These assemblies consist of light collecting lenses, lens masks, field stops, filters, and detectors. The lenses are light-weight, moderate resolution, acrylic Fresnel lenses, each cut to the same shape and size as the projection of the corresponding axicon element in the exterior plane of the detector module. Lens masks are used to further define the shape of the scattering volume. Field stops are placed at the paraxial image points of the collector lenses. In the case of the backscatter channel the field stop is slightly displaced from the lens axis so as to admit light scattered from angles of about 176 to 180 degrees. The detectors are silicon p-i-n detectors (E. G. and G. Electrooptics Division Model 200), back-biased at 90 V, with a responsivity of about 0.64 A W^-1 at 900 nm, and a rise time of 10 ns. Interference filters that reject light other Fig. 3. Cutaway drawing of the vented sensor head and pressure-tight electronics unit. than at wavelengths near that of the laser diode are used as cover glasses for the detectors. The bandpass curve for these filters is essentially flat with a full width at half maximum of about 66 nm, centered on 904 nm at a temperature of about 20 degrees C. The width is sufficient to make allowance for non-perpendicular rays impinging on the filter, as well as wavelength shifts due to instrument temperature variations during the mission. Several 'housekeeping' measurements documenting the instrument status are included in the design. These are a contamination monitor, an alignment monitor, output monitors of the sources, three temperature monitors, and one voltage monitor. The contamination/alignment monitor uses a low power light-emitting-diode (LED) source (Texas Instrument, Inc. Model TIES 35) located in the back-scatter unit to produce a reference light beam. This beam is reflected by a flat mirror on the axicon assembly onto the contamination/alignment detector, a quadrature detector (E. G. and G. Electrooptics Division Model SGD 444-4), in the back-scatter unit. Any contamination on the mirror and on the window in front of the detector will reduce the total signal produced by summing the signals from the four segments of the detector, indicating contamination in the scattering channels. Alignment changes are manifested by motion of the reference beam on the four segments of the detector, changing the magnitude of the signals measured by each segment. 3.3. ELECTRONIC DESIGN The Nephelometer electronic system acquires and processes all of the data from the forward- and backward-scatter channels and, also, from the 'housekeeping' sensors, formatting these data for transmission to the Probe telemetry system. It provides the drives for the laser injection diode (LID) and light emitting diode (LED) light sources, appropriate timing signals, and conditions the power received from the spacecraft 28-VDC power bus to provide regulated supplies to the instrument. An overall block diagram of the analog circuitry is shown in Figure 4 and a very simplified block diagram of the digital signal processing is shown in Figure 5. 3.3.1. Scatter Data Collection and Processing Considerations The predicted Probe descent trajectory and range of ambient cloud parameters give rise to requirements that lead to a fairly complex data collection and processing system. The acceptable dynamic range of the measurements is to exceed 10^5. Scatter measurements are to be made at least once during each kilometer of Probe descent even though the descent velocity of the Probe varies greatly during descent. Because of the limitation on data transmission rate allocated to the Nephelometer, this means that the data collection rate exceeds the data transmission rate during the early part of the mission. These considerations, as well as good design practice to maximize signal-to-noise ratios, lead to the following data collection and reduction scheme. The LIDs are continuously pulsed at 2000 pps with the forward-scatter LID pulse leading the backscatter LID pulse by 250 micro seconds. The data sampling sequence is controlled by the digital timing circuits. During a sampling period a measurement involves the detection, amplification, digitization, and digital integration of the detected scatter Fig. 4. Block diagram of the Nephelometer analog signal processing electronics. Fig. 5. Block diagram of the Nephelometer digital signal processing electronics. signals in each scatter channel from 64 bursts, each burst consisting of 64 light pulses emitted at equal time intervals. The scatter channel detectors produce signals for each light pulse. However, for each burst of 64 pulses, only 1, 8, or 64 of the signals are digitally integrated, as controlled by the digital circuitry from a test of the output of the A/D converter. This scheme provides a digital gain change factor of 64. An additional gain change factor of 8 is provided by adjusting the gain of an analog amplifier in the signal processing circuitry for each scatter channel. The dynamic range of > 10^5 is achieved by using an 8-bit A/D converter along with the combined digital and analog gain change factors. The A/D converter used here allows for some possible negative baseline drift in the most sensitive ranges of each channel. The interval between the samples is varied so that at the beginning of the descent, data from 64 bursts are collected every 3 s (corresponding to predictions of less than 1 km of descent) for each of the scatter channels and this 3-s sampling period is repeated 10 times. The interval is then increased to 4 s for the next 10 records, then to 5 s for 10 records, etc., until the interval is 12 s for 10 records. At this time, 750 s after the start of data acquisition, the interval is reduced to a constant 8 s so that the data acquisition rate and data transmission rate are equal for the rest of the mission. A Nephelometer data frame consists of the synchronization word, engineering data, and data from 10 scattering data records for a total of 800 bits. The data transmission rate to the spacecraft telemetry system is 128 bps, of which 10 bps is allocated to the Nephelometer. Since initially at deployment the data for the first sample are collected at 26.7 bps (corresponding to an 800 bit frame in 30 s), a buffer memory is provided in the form of a digital, first-in-first-out (FIFO), random-access-memory (RAM). The data stored in this FIFO RAM are transferred to it from an integrating RAM configuration in the form of a compressed 10-bit word using a sign bit, 3 bits for a power of 2 exponent, and 6 bits for the 6 active most significant bits (MSBS) in the 24-bit register of the integrating RAM. The gain changing technique involves monitoring the A/D converter output word for each scatter channel conversion. If the four most significant bits (MSBS) of the word are all ones the channel gain is decreased by a factor of eight. If the 4 MSBs are all zeros, the gain is increased by a factor of 8. The digital gain at which the data are collected is taken into account in the integrating RAM by the proper shifting of the new data from the A/D converter output before it is added to the register contents. The gain (512, 64, 8, or 1) of the 16 degree forward scatter channel is also reported in the output data stream four times during each measurement interval, as a rough measure of the variability of the scatter signal (or cloud variability). 3.3.2. Analog Electronics An overall block diagram for the analog circuitry is given in Figure 4. The high power LID light sources, driven by a train of 9 A, 200 ns pulses at 2000 pps from an SCR-controlled capacitor discharge, illuminate the scattering volume. The cloud-scattered light for each of the four forward and single backward-scatter channels is sensed by silicon p-i-n diodes operated in the photoconductive mode. Each photodiode signal is ac-coupled to its charge sensitive preamplifier to prevent the high background illumination signal from saturating the channel. The preamplifier, identical in each channel except for gain, is a wide bandwidth, low noise configuration with a very high dynamic range and slew rate. The output of each preamplifier is then fed to an active bandpass filter to optimize the signal to noise ratio and provide some gain before being multiplexed and sent from the sensor head to the electronics unit. An 8:1 gain change amplifier and a balanced integrate and hold circuit complete the analog processing of the signal prior to A/D conversion. Considerable care was taken in the design of all of these elements because of the low level, high-speed processing required, the constraints on power and size, and the need to survive and function properly in the high-energy radiation environment. The alignment/contamination LED is driven by a train of 300 micro second pulses at 1000 pps. The LED light reflected from the flat mirror mounted on the axicon base is detected by a quad photodiode each of whose diodes is operated in the photoconductive mode. The output signal from each section of the detector is amplified by a transimpedance amplifier and multiplexed to the A/D converter. For each fifth light pulse from the LED the four amplified photodiode signals are added in a summing amplifier, and the result multiplexed to the A/D converter. The A/D converter is an 8-bit, bipolar. dual slope, integrating type with auto-zeroing. It contains integrated circuit operational amplifiers and comparators and CMOS central logic operating with a 2 MHz clock. Source light output monitors for the LIDs are provided. Each monitor consists of a silicon p-i-n diode (E. G. and G. Electro-optics Division Model SGD 040B) operated in the photovoltaic mode, and is illuminated by a portion of its LID's light output. A charge sensitive preamplifier and a sample and hold circuit process the sampled signal from one light pulse for A/D conversion and inclusion in the output instrument condition data. A photodiode/preamplifier combination unit is used to monitor the output of the LED, and its output signal is also multiplexed to the A/D converter. The 5.5-V reference and signals from three temperature sensors, one near each of the LIDs in the sensor head and one near the A/D converter in the electronics unit, are separately multiplexed in one of the four special multiplexers used for instrument condition signals, then amplified and multiplexed to the A/D converter. 3.3.3. Digital electronics A simplified block diagram of the digital electronics is shown in Figure 5. The Nephelometer digital electronics provides all system timing, A/D converter control, data formatting and integration, and telemetry interface control. No microprocessor is used. Nephelometer operation requires three sequential operational modes, the gathering of scatter data 64 times and transfer to integrating memory, transfer of scatter data integrations into another memory awaiting transmission to telemetry, and preparation of preface information, comprising a sync word, frame count, and instrument status data. The oscillator-clock generator generates all of the system timing signals required for all of the subsystems, such as the power supply, and, especially, for commanding the system into one of the three modes of operation. The sequencer logic provides the LID and LED firing pulses, the integrate, sample, and reset commands, and commands to transfer data to the spacecraft telemetry system. The integrating RAM section performs the digital integration on 64 analog data measurements and provides signals to the analog gain changing circuitry. Transfer circuitry compresses the data in the integrating RAM and transfers it to the FIFO RAM for storage. During the preface mode of operation the frame and sync word generator and digitized instrument status data are transferred into the FIFO RAM storage. 3.4. MECHANICAL DESIGN The Nephelometer instrument consists of three separate physical units, the forward scatter sensor unit on which the deployment mechanism is mounted, the backward scatter sensor unit, and the electronics enclosure. Figure 3 is a cutaway view of the Nephelometer with key components labeled. The two sensor units, vented to the ambient atmosphere to eliminate optical distortion or mechanical fracture, are fastened to each other and mounted to the Probe equipment mounting plate. The combined sensor units are slightly tilted on the mounting plate to allow for clearance with other equipment. Shims are used to align the optical systems except for a lateral adjustment of the laser diode sources. The electronics enclosure is sealed with one Earth atmosphere inside. A pressure sensor inside the sealed unit was used in laboratory tests in vacuum and under pressure to verify, within the test experimental limits, the integrity of the seal both for the long cruise phase and entry to at least the 20 bar level. The electronic boards inside are mounted in a manner to minimize the effects of being subjected to the 400 g entry deceleration pulse, as well as launch vibration. For example, the relatively heavy power supply is mounted near the bottom of the enclosure. The deployment mechanism consists of a folded, hinged, spring-loaded arm on which the axicon mirror assembly is mounted, and a positive latching mechanism. The arm is folded through launch, cruise, and initial entry, and retained in position by a pin mounted in a pyrotechnically-initiated pin puller. At a pressure of approximately 0.07 bar the aeroshell is jettisoned, the pin puller fired, and the axicon assembly deployed under the action of a helical spring. The inboard section of the arm is a perforated rectangular tube and the outboard section a perforated channel to which the axicon assembly is mounted. The arm is downstream and off to the side of the centerline of the axicon assembly, to reduce possible flow field interference with the measurements, and is perforated to mitigate aerodynamic lift effects (that might affect Probe rotation) and sympathetic vibrations due to von Karman vortex shedding. The end cap of the axicon assembly is convex and is vented to produce a pressure differential, helping to prevent particle condensation on mirror surfaces. The positive latching mechanism consists of the deployment spring, a counter-action leaf spring, and two overlapping spring plates, the ends of which are wedge-shaped and pass over each other, latching on the back ends of the wedges. The leaf spring forces the contact between these surfaces. The spring constants are chosen to assure deployment even at a zero Probe rotation rate, to absorb enough energy in latching to prevent arm distortion due to deformation during deployment at a possible 80 RPM Probe rotation rate, and to force close contact between the backs of the wedges. Materials are selected to minimize distortion by differential thermal expansion. Calculations, using a Monte-Carlo program, have indicated that, for the worst case (involving offsets in pitch angle creating opposite sensitivity shifts in the 5.8 and 16.0 degree channels), alignment offsets of less than 0.06 degrees will produce sensitivity shifts of less than one digitization step. Acceptable performance has been verified from tests that have demonstrated alignment reproducibility to less than 0.05 degrees on repeated deployment, and misalignment of less than 0.05 degrees due to liquid nitrogen induced thermal shock. Calculations of the predicted thermal behavior (temperatures, thermal contraction/expansion, stresses) of the Nephelometer during the course of the Probe descent in the Jovian atmosphere have been performed using the MITA (Martin Integrated Thermal Analysis) program alone with a 118 node thermal model of the instrument. Input quantities are derived from sample entry trajectories, atmospheric structure models, transport and thermodynamic properties of the atmospheric gases, internal Probe temperatures and initial conditions, and free heat transfer coefficients. The results of these calculations for the final design of the Nephelometer are such that no serious thermal effects on instrument performance are predicted to occur during the course of the Probe mission. 4. Calibration, Tests, and Performance 4.1. GENERAL In order to derive particle sizes, number densities and other properties. the measured relative magnitudes of the signals in each channel with respect to the other channels are compared for best fit with those obtained from Mie-scattering calculations. Model particle size distributions (normalized to one particle per cm^3) are used, with mean size and distribution width as parameters. Particle indices of refraction must, in general, be assumed or inferred from other sources for these calculations, although the calculated results are not strongly sensitive to the choice of index, especially in the forward direction. Conversely, if the particles are assumed to be only weakly absorbing and roughly spherical, the measured data, especially from the backward scatter channel, will provide constraints on allowable indices. Also, if the particles are not strongly absorbing, deviation of the measured scattering at wide angles from that calculated for spherical particle distributions is a strong hint that the particles may not be spherical. The calculated differential scattering cross section for each channel for the particle size distribution with these best fit parameters is then compared with the absolute value measured in each channel n[dsigma/dOmega]_theta, yielding the particle number density, n. Inference of opacities and mass densities follows directly from the calculated total cross sections and reasonable assumptions about particle mass density. 4.2. CALIBRATION Two methods were used to calibrate the Nephelometer. The first is similar to the method described by Pritchard and Elliott (1960), as modified for application to the present case. This technique involves recording the response of each of the scattering channels to the scattered light produced by a diffusely scattering target positioned perpendicular to the source beam optical axis, as the target is stepped along the source beam until the sensitive volume for each channel has been traversed. For the forward-scattering channels a carefully documented diffusely transmitting screen mounted into the end of a set of telescoping tubes is used. The transmitting screen transmittance is carefully measured using a standard integrating sphere and the screen's angular response and polarization characteristics are documented with a specially constructed goniometer. Similar procedures are used to verify the characteristics of a large specially constructed Lambertian reflector that was used to calibrate the backward scattering channel. Calibrated neutral density attenuating filters are used in front of the collecting optics for the detectors in each channel to maintain the signals within the dynamic range of the instrument. The manner of relating the readings obtained using this scanning method to the calibration constants to be used in measuring actual aerosols is described below. The Nephelometer instrument produces counts, C, in proportion to the product of particle differential scattering cross section, at angle theta, [dsigma/dOmega]_theta (with units of m^2 sr^-1), and particle number density, n (with units of m^-3) with combined units for this product, n[dsigma/dOmega]_theta of m^-1 sr^-1. The proportionality constant is the product of source intensity I_s, effective sampling volume V_eff, and detector/electronics/optics gain constant K. The instrument count output C can be written as follows: C = (KI_sV_eff)n[dsigma/dOmega]_theta = (1/E)n[dsigma/dOmega]_theta and, the desired measured value, n[dsigma/dOmega]_theta = CE = C/(KI_sV_eff) . In response to a diffuse calibration target normal to the source beam at position x, filling an effective area A_eff(x), and having reflectivity (or transmission) at angle of T cos theta, the instrument count output will be given by C(x) = t(x) = [KI_sA_eff(x)] (T cos theta)/pi . Because the normal calibration target is so bright, it is necessary to reduce the amount of scattered radiation reaching the detector with an attenuator of attenuation factor F. By moving this calibration target along the beam over all x at which response is obtained, and integrating the response over all x, we obtain integral [C(x) dx] = integral [t(x) dx] = K(I_s/piF)(T cos theta) integral [A_eff(x) dx] = K(I_s/piF)(T cos theta)V_eff . Thus, the proportionality constant E, in units of m^-1 sr^-1 count^-1, can then be evaluated from E = (KI_sV_eff)^-1 = T cos theta{(piF)(integral[t(x) dx])}^-1 . In practice it is also necessary to make small corrections to account for the deviation of the reflection or transmission screens from true diffuse behavior, and polarization characteristics of the sources, screens, and detection system. The accuracy of this calibration procedure is a function of the accuracy of our knowledge of the reflection (or transmission) of the screen used to calibrate the Nephelometer and its simulation of diffuse reflection (or transmission), the accuracy of the measurement of the attenuation factor of the attenuator, the accuracy of the data taken at each target position, and the accuracy of the integration yielding the calibration factor. Estimates of the overall accuracy range from less than +- 5 percent for the 5, 15, and 180 degree channels to less than +- 10 percent for the 40 and 70 degree channels. The second type of calibration method involves obtaining the response of the instrument to a well-documented 'standard' aerosol environment. These tests were performed in a large test chamber at Particle Measuring Systems, Inc. (PMS) of Boulder, Colorado. An aerosol with a very narrowly dispersed size distribution was produced by atomizing a suspension of spherical polystyrene or polyvinyl toluene particles into a large spherical chamber. The particle sizes were measured using standard electron microscope sizing techniques developed for aerosol research at Ames Research Center. The density of particles and the proportion of single particles to 'doublets','triplets', etc., in the actual aerosol was documented using standard particle sizing instrumentation manufactured and calibrated by PMS. Nephelometer responses were recorded for a variety of particle sizes, particle densities and particle composition. The calibration for each of the scatter channels was then determined, using Mie-scattering cross sections calculated for the PMS-documented aerosol distributions. In general, the results obtained were within 30 to 50 percent (often within 10 percent) of those measured using the first method. However, the variations in the results of repeated experiments in the particle chamber indicated that the results were less reliable than those of the target scanning technique. Closer investigation indicated a number of variables in the test conditions that were apparently difficult to control. For example, small persistent air currents in the test chamber were present, produced during aerosol injection, by thermal gradients, by the sampling of the PMS instrumentation, or by other causes. These currents introduced inhomogeneities and differences in the particle densities as measured by the test instrumentation and the Nephelometer. In addition, it proved to be difficult to produce an aerosol with a low enough content of aggregate particles, such that these larger particles did not appreciably affect the measured scattering cross sections. It was suspected that some of the particles might also have been electrically charged and that electrical effects, for example, at the chamber walls, may have produced differences between the aerosol sampled by the PMS instruments and the Nephelometer. 4.3. Tests A number of tests have been performed to characterize and validate the performance of the Nephelometer. Environmental tests have included vibration and acceleration tests, steady state and transient thermal tests, and vacuum and pressure tests (of the electronics unit). Particle flow experiments in a wind tunnel were performed on a simulated Probe configuration to validate theoretical particle flow calculations and the effectiveness of the design of the deployed arm in obviating distortions in the particle size distribution functions in the sample volumes (Ragent and Snyder, 1992). The arm-deployment mechanism was tested for its ability to repeatedly deploy the arm and mirror assembly accurately. Tests of the effects of possible thermal distortion or thermal shock on the scattering angle during deployment were also performed. 4.4 Performance Instrument characteristics are summarized in Table I. The calibration constants obtained at a fixed instrument temperature of approximately 293 K for each of the five scatter channels are listed. As mentioned above, the absolute uncertainty of the calibration for each channel is estimated to be less than 10 percent. For actual measurements in the Jovian atmosphere we must include errors arising from possible telemetry effects, granulation uncertainties in the data extending from about + 1.7 percent to + 6.7 percent inherent in the data compression scheme used here, variations in the measured values due to noise in each channel, and particle sampling statistical fluctuations, which will be small for all reasonable signals (cf. Ragent and Blamont, 1980). Multiplying the calibration constants by the adjusted number of counts attributable to scattering from particles in the channel sample volume yields the desired measured quantity n[dsigma/dOmega]_theta for each channel. The adjusted value is obtained by correcting the raw counts for temperature dependences of channel sensitivity and baseline values in each channel. Curves of these temperature corrections were obtained in the test programs mentioned above. Although these corrections vary nonlinearly with temperature and differ for each channel, a rough value of the magnitude of baseline shift with temperature is about 50 counts per 10 degrees change from the nominal calibration temperature, and about a 10 percent change per 10 degrees for the channel sensitivity change. Calculations of the thermal variation of the instrument during Probe descent predict that sensitive components will not vary by more than 10 percent and that, therefore temperature corrections of the adjusted data will be possible to within a few percent. Noise values in each channel also vary with temperature and are greater in the more sensitive channels, the 40, 70, and 180 degree channels. The values vary from about +- 5 counts in the 5 degree channel to about +- 30 counts in the 180 degree channel at about 293 K, and increase by about a factor of 1.5 for a temperature increase of 10 degrees above the nominal calibration temperature. Our knowledge and uncertainties about the kinds and densities of particles we may expect to encounter during Probe descent in the Jovian atmosphere were summarized above. Although it is quite likely that we will encounter small haze or chromophore particles with sizes of the order of 0.1 microns, as well as large particles of NH3 ice, H2O ice and liquid, or other large particles, the only fairly well identified particles at the present time are the approximately 1.0 - 2.0 micron size particles (particle densities of the order of a few hundred per cm^-3) in the upper cloud. For this Jovian cloud, that may be roughly analogous to some optically thin Earth-type light clouds (cf., for example, Tables T.60 and T.105 of Deirmendjian, 1969), we may expect angular scattering cross sections varying from about 10^-2 to 10^-3 m^-1 sr^-1 at 6 degrees from the forward direction to about 10^-4 to 10^-5 m^-1 sr^-1 in the backward direction, and a volume extinction coefficient of about 3 x 10^-3 m^-1. The instrument can be expected to read at least several thousand counts in all channels, except perhaps for the 70 degree channel, which would be expected to read somewhat less than 1000 counts. We may thus expect to begin to characterize this cloud at particle concentrations greater than about 5 to 10 cm^-3, and to detect it at considerably lower concentrations. We have attempted to investigate the accuracy of recovery of cloud parameters from our measured data be the following procedure. We have assumed that our particles can be modeled by spherical particles using two parameters to characterize the particle size distribution, r_m, the mean particle radius, as weighted by the geometric cross section, and sigma_m, the variance of the distribution, again weighted by the geometric cross section. We generated 'measured' values (labeled r_m^* and sigma_m^*) by selecting values for r_m and sigma_m, and used the Mie-scattering theory to find the scattering phase function P(theta_j) for a range of values of r_m and sigma_m. From these calculated phase functions as a function of r_m and sigma_m, we then computed the fractional deviation, FDV, defined as FDV = {N^-1[w_jP*(theta_j) - w_jP(theta_j)]^2}^1/2, where P*(theta_j) is the phase function at angle theta_j corresponding to r_m^* and sigma_m^*, N = 4 for the current case, and w_j is a weighting function that depends on the accuracy of the measurements. For the case where the expected error in the measurement is independent of the value of the measured quantity ('constant error'), w_j = (P_max^*)^-1, where P_max^* is the largest value of P*(theta_j). Alternatively, if the measurement error is proportional to the value of the measured quantity ('constant relative error'), w_j = [P*(theta_j)]^-1. For our instrument, the errors lie between these two cases, but are closer to the latter. Examination of the results of calculations of the FDV over the ranges of the parameters r_m and sigma_m for various assumed forms of the particle size distribution function (e.g., log-normal and modified gamma function) and particle indices of refraction have indicated that, for the measurement errors discussed above, it should be possible to obtain useful estimates of the mean particle radius within less than a factor of 2 over the entire mean radius range of about 0.2 to 20 microns, and to within a factor of less than 1.4 over the range from about 0.3 to 6.0 microns. The loss of sensitivity for small values of r_m is due to the particles becoming Rayleigh scatterers, whereas for large r_m the diffraction peak lies entirely at angles smaller than the smallest angle of observation. The calculations have also shown that the sensitivity of the FDV to changes in sigma_m is low, and consequently, only modest bounds may be placed on the width of the particle size distribution. 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