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 (>10−12g) micrometeorites orbiting around the sun as the classical zodiacal dust cloud and a second component of very small (<10−12g) 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/cm3was 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−vwith solar distance r(AU), where nois the number density at r=1 AU, one obtains with a particle size distribution law n(a)da ~ a−kda 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.
6.1 The Zodiacal Light
