This paper describes the results of a program of observations of the polarized intensity vector of the night sky in the anti-solar hemisphere obtained on 72 nights between May 1973 and November 1974. The observations were made with the night sky polarimeter at Mt. Haleakala, Maui, Hawaii, using a 5300A interference filter, 62A wide, to define the passband. They were made at fixed altitudes from 35° to 90° along the north-south meridian and repeated in sequence throughout the night. The diameter of the field of view was 6.0° and the integration time per point was either 30 or 120 seconds. Polarization was detected using a rotating polaroid (11 1/4 revs sec−1); the signal from the polaroid drive generator provided a phase trigger allowing the photo-multiplier signal to accumulate and be digitized for each of the four 90° phase intervals of the polarization modulation cycle. This process continued for the entire integration period. The digitization and data handling and recording (paper tape and teletype) was carried out by a PDP-81 computer, and calibrated Stokes parameters (I, Q, U) and the polarized intensity pI were calculated later. Intensity calibration in S10 (V, G2V) units was obtained using a C14 low brightness source, and the amplitude and phase of the polarization modulation were checked each night.

The results of detailed calculations on first order Rayleigh- and Mie-scattering in the Earth’s atmosphere illuminated by the Milky Way and the Zodiacal Light are presented. The influence of various independent parameters, as optical depth of the Rayleigh- and Mie-component and position of Milky Way and Zodiacal Light, is discussed. Linear and circular polarization of the scattered light are considered. The results are compared uith the procedures commonly applied in reducing photometric observations of extended sources of the night sky. It is shown that a substantial part of the discrepancies between the brightness distributions of Zodiacal Light resp. Milky Way given by different authors is due to an inaccurate treatment of the atmospheric scattered light.

The upper atmosphere acts as a giant collector of cosmic dust swept by the Earth on its travel through space. The transient presence of cosmic dust in the upper atmosphere was detected by the rise of the luminance of the twilight sky during the activity of several meteoric showers either by balloon photometers launched by C.N.E.S. at Aire sur l’Adour (Fehrenbach et al., 1972 a) or by the similar photometer at the Pic du Midi Observatory (Link, 1973).

A fundamental step towards the knowledge of interplanetary matter from zodiacal light photometry is to eliminate the integral along the line of sight – an intrinsic, cumbersome feature of all z.l. observations – so as to reach, by inversion, the local optioal properties of elementary volumes of space.

The measurements of the zodiacal light at elongations ε = 15° to 30° performed by the ESRO rocket experiment R – 214 mere discussed on the basis of model calculations.

Assuming that the decrease in the density of the interplanetary dust follows an exponential distribution both in the transverse and radial direction, we can write n = no Exp(-(h/H)-(r-l/R)), where h is the distance from the ecliptic plane and r the heliocentric distance both expressed in astronomical units (a.u.); then we show that the modulation of the radiance B(90, 0) of the zodiacal light observed at the ecliptic pole defines the parameter H as a function of the inclination angle B between the zodiacal cloud and the ecliptic plane; moreover, the experimental value of the ratio B(90, 0)/B(90, 90) defines the parameter R. It can be deduced that the flatness of the zodiacal cloud, expressed by R/H, is < 5 and that the plane of symmetry of the zodiacal cloud is very close to that of the invariant plane of the solar system (B<2°).

The knowledge of the functions which give the intensity of the scattered sunlight is important in order to establish the models of the zodiacal cloud.

In recent years satellite observations of the region of the Earth-Moon libration points L4 and L5 became available, which showed different results. Roach (1975) has measured perturbations of the Zodiacal Light intensity near the libration points L4 and L5 with a photometer on board the OSO-6 satellite. He interpreted the results as counterglow of a cloud of particles, having an angular diameter of 6 degrees and an average brightness of 20 S10v, whereas observations by Burnett et al. (1974) set an upper limit of 10 S10 blue. Earth bound observations made by Bruman (1969) with the 48-inch Palomar Schmidt telescope gave no indication for discrete objects nor for clouds.

Most model computations for the analysis of interplanetary dust are based on scattering functions of homogeneous spheres (Mie-theory). To investigate the influence of this restriction, scattering properties of nonspherical particles were studied by microwave analog experiments. The results of earlier investigations on dielectric rough spheres, cubes and octahedrons (Zerull, 1973; Zerull and Giese, 1974) may be shortly summarized as follows: Roughness (~ wavelength) does not significantly change the scattering behaviour of spheres, whereas cubes and octahedrons scatter evidently different than equivalent spheres. The dominant differences are increased intensity at medium scattering angles but no further increase towards backscattering, and nearly neutral polarization. These results encouraged to continue the investigations with really irregular particles of dielectric as well as absorbing material.

Increased sophistication in both, direct impact detectors and zodiacal light measurements encourages to discuss the compatibility of the results obtained by these quite different methods of investigating interplanetary dust. Taking recent measurements of particle fluxes and velocities obtained by the space missions of Pioneer 8/9 (Berg and Grün 1973), Heos 2 (Hoffmann et al. 1975), and comparing them with submicron-sized craters on lunar surface samples (Schneider et al. 1973, Fechtig et al. 1974) there seem to be two types of interplanetary dust populations: larger (>10−12 g) micrometeorites orbiting around the sun as the classical zodiacal dust cloud and a second component of very small (<10−12 g) particles coming radially from the direction of the sun with high velocities (>50 km/s). On the basis of the flux data referred to above and adopting for both components velocities of 10 or 50 km/s relative to the detector, respectively, a differential distribution function n(a) · da was found for the particle radii (a) as shown at a logarithmic scale in fig. 1. A density of 3 g/cm3 was adopted in order to convert particle masses into radii. The regions A, B, C (see Table 1) correspond approximately to the regimes of “submicron particles”, the classical zodiacal cloud particles, and the meteoritic component of the interplanetary dust complex. From this information the brightness I(ε) of the zodiacal light in the ecliptic plane can be computed as a function of elongation by approximating the distribution function n(a) in the different regions by simple power laws a−k ·da and by adopting a resonable scattering function σ(θ) for the average scattering behaviour of one particle of the mixture depending on the scattering angle θ. By use of an inverse (v = 1) decrease of particle number densities n = no · r−v with solar distance r(AU), where no is the number density at r=1 AU, one obtains with a particle size distribution law n(a)da ~ a−k da in the different intervals of sizes (Table 1) the intensity of the zodiacal light (in stars of 10th magnitude per square degree, S10) as shown in fig. 2. The two models (Maximum, Minimum) correspond to an upper and to a lower limit of particle number densities compatible with the in situ measurements, respectively.

A review is given on the techniques used to record and to quantitatively measure data of individual interplanetary dust particles. New developments in detection techniques are briefly discussed.

The main results from recent space missions at about 1 AU and in the earth-moon neighborhood are discussed and compared with the flux results from lunar microcrater studies. Spatial anisotropies and time fluctuations are found indicating that the earth is exposed to two main micrometeoroid dust populations: the “apex”-population and the B-meteoroids. The near planet-dust enrichments measured by HEOS 2 near the earth and by the Pioneer 10/11 near Jupiter are emphasized. The experimental data strongly suggest a fragmentation process associated with the earth. The role of the moon as a dust source is discussed. The important problems in the dust field for future space missions are summarized.

For the first time in situ measurements of interplanetary dust have been performed between 0.3 AU and 1 AU from the sun by the micrometeoroid experiment on board Helios A. The measured particle masses are between 10−15 g and 10−8 g and their measured speeds are between 2 km/sec and 20 km/sec. Particle impacts are identified by the time-of-flight spectra of the ions released upon impact. 15 large particles (m ≥ 10−12 g) were detected from Dec. 15, 1974 to Sept. 5, 1975. They show a strong increase of the impact rate (appr. a factor of 10) between 1 AU and 0.3 AU. The directions from which they impacted the sensor are concentrated between the solar direction and the apex direction of the Helios spacecraft.

The composition of the impact-plasma, produced by dust particles hitting an Au-target was measured, using a model of the HELIOS-mlcrometeoroid-detector. The 2 MV dust accelerators of the MPI für Kernphysik, Heidelberg, and the NASA Ames Research Center were used to accelerate particles consisting of Al, Al2O3, SiO2, Soda-Lime-Glass, Polystyrene and Kaolin to velocities between 2 km/sec and 15 km/sec. Fe-projectiles could be accelerated up to 40 km/sec. The masses of the dust grains were between 10−15 g and 3 × 10−10 g. The experiments showed, that because of the characteristic features of the measured spectra it is possible to separate noise events from impacts even at a high noise background. The smallest particles (m 10−15 g) triggering the experiment produce spectra well above the noise level (more than a factor 10) because of the high sensitivity of the ion-detector (multiplier).

The instrument measuring particle impacts on PIONEERS 8 and 9 has been extensively described in the literature (Reference 1) and will not be repeated here. This paper concerns itself with the analysis of the data obtained.

The production of hyperbolic meteoroids by inelastic collisions between meteoroids is estimated. It is found that, under reasonable assumptions, the calculated flux of hyperbolic meteoroids agrees with satellite data and with lunar microcrater distributions.

We have therefore obtained independent theoretical support for Zook and Berg’s (1975) ß-meteoroid hypothesis and for Fechtig et al. (1974) suggestion that submicron lunar microcraters are produced by ß-meteoroids.

Langley Research Center cosmic dust experiments aboard the Earth orbiting Meteoroid Technology Satellite (MTS) and the Pioneer 10 and 11 Jupiter fly-by spacecraft are described and discussed.

The Pioneer 10/11 spacecraft, launched in 1972 and 1973, carried three experiments to measure cosmic dust. A comparison of these first direct measurements of dust in the outer solar system indicates that the sizes, optical properties, and spatial distribution are more complex than previously supposed.

The asteroid belt is examined as a potential source of interplanetary dust. Using results from the Pioneer-10 experiments the relative contribution of asteroidal and cometary particles to the Zodiacal cloud is estimated using methods developed in earlier studies of meteoroidal collisions (collisional model). It is found that the contribution of asteroidal particles to dust in the asteroidal belt is small compared with the number density of cometary type particles. Similar conclusions apply to the Zodiacal cloud between the sun and the asteroid belt. When definitive criteria for differentiating between comets and asteroids become available, a reexamination of some of our conclusions may become necessary.

The distribution of asteroidal rotations is analyzed; it is found that the gross features of the distribution can be reproduced using the collisional model.

Conflicting data for depth-to-diameter ratios for lunar microcrater pits do not permit firm conclusions for distribution of meteoroid densities. The majority of meteoroids have equidimensional shapes. Meteoritic metal spherules have been detected in a small fraction of impact pit glasses, but contribution of meteoroidal material to most pit glasses is small to negligible. Impact pits less than 0.1 microns in diameter (impacting particle mass ~ 10−16 grams) have been observed. Size distributions -9 for microcrater pits less than 50 microns in diameter (particle mass < 10−9 grams) measured on different, samples differ significantly. An inflection in the cumulative size distribution curve at a diameter between 1 and 10 microns (particle masses between 10−14 and 10−11 grams) appears real, supporting the idea of a two-component model for the interplanetary dust. Data for the arrival direction of meteoroids at the moon are inconclusive.

Crater size frequency distributions vary to a degree which probably cannot be explained by variations in lunar surface orientation of the crater detectors or changes in micrometeoroid flux. Questions of sample representativity suggest that high ratios of small to large craters of micrometeoroids (e.g., a million 1.0 micron craters for each 500 micron crater) should be the most reliable. We obtain a flux for particles producing 0.1 micron diameter craters of approximately 300 per cm2 per steradian per year. We observe no anisotropy in the submicron particle flux between the plane of the ecliptic and the normal in the direction of lunar north. No change in flux over a 106 year period is indicated by our data.