A simulation model has been developed to study the results of the Poynting-Robertson (PR) effect and collisions on the dynamical evolution of an interplanetary dust cloud. Fragmentational and accretional effects are neglected. With a mean free collision time of the order of the PR lifetime collisional effects become of importance. As the individual grains still spiral inwards collisions act to make the mean eccentricity and inclination of the grain orbits both decrease at comparable rates, giving rise to an expanding fan shaped dust cloud.

A new method is presented for calculating the electrostatic potential of cosmic grains taking into account the plasma sheath and its bulk velocity. Results for various solar wind situations are given.

The problem of electromagnetic perturbations of charged dust particle orbits in interplanetary space has been re-examined in the light of our better understanding of the large scale spatial and temporal interplanetary plasma and field topology. In the equatorial plane, the magnetic sectors, caused by the warped current sheet, produce stochastic orbit perturbations. From this a diffusive description of particle motion can be derived, provided the dust particles are sufficiently small. The effects of large unipolar magnetic field regions at high heliographic latitudes will be briefly discussed.

Electromagnetic effects on charged zodiacal dust particles were investigated. It can be shown that: 1) stochastic variations induced by electromagnetic forces are unimportant for the zodiacal dust cloud except for the lowest masses, 2) systematic variations in orbit inclinations are unimportant if orbital radii are larger than 10 A.U. This is due to the solar cycle variation in magnetic polarity which tends to cancel out systematic effects, 3) systematic variations in orbital parameters (inclination, longitude of ascending node, longitude of perihelion) induced by electromagnetic forces inside 1 A.U. tend to shift the plane of symmetry of the zodiacal dust cloud somewhat towards the solar magnetic equatorial plane, 4) inside 0.3 A.U. there is a possibility that dust particles may enter a region of “magnetically resonant” orbits for some time. Changes in orbit parameters are then correspondingly enhanced, 5) the observed similarity of the plane of symmetry of zodiacal light with the solar equatorial plane may be the effect of the interaction of charged interplanetary dust particles with the interplanetary magnetic field. Numerical orbit calculation of dust particles show that one of the results of this interaction is the rotation of the orbit plane about the solar rotational axis.

The effects of electromagnetic forces on charged interstellar grains and β-meteoroids - both moving on hyperbolic orbits - are investigated. It is shown that the unipolar field regimes at high latitudes lead either to a “focussing” or a “defocussing” of interstellar dust and β-meteoroids with respect to the solar magnetic equator. This should lead to a solar cycle variation. The stochastic magnetic fluctuations in the equatorial region, caused by the warping of the current sheet which separates the polar fields, lead to a diffusive description of particle transport at the low mass end. Consequences for the ability of interstellar dust particles to penetrate the inner heliosphere are discussed. It is concluded that dust particles with radii s > 10−5cm can penetrate deeply into the heliosphere if their incidence direction at the heliopause is almost radially inward and close to the solar magnetic equatorial plane, whereas dust particles with radii s < 10−5 cm are prevented from reaching the inner heliosphere. The relationship between interstellar dust and β-meteoroids is discussed.

We study the conditions for the onset of two-stream instabilities in interacting streams of charged grains. These conditions are such that instabilities cannot be important in the solar system of today and have probably been of very limited importance in the protoplanetary cloud.

The equation of motion for interplanetary dust particles close to the Sun has been solved numerically taking into consideration the interaction with the radiation field of the Sun and the temperature distribution as a function of grain size and heliocentric distance for different materials.

The Helios Spacecraft is able to detect micrometeoroids along its orbit in the inner Solar System between 0.3 AU and 1 AU distance from the sun (Grün et al. 1977, Grün et al. 1979). Data of the first 6 orbits are presented here. 168 particles have been identified in such a way that their osculating orbital elements (here especially semimajor axis a, eccentricity e and inclination i) can be calculated. Detailed information on impact speed is available only up to about 50 km s−1 which corresponds to a lower mass limit of 10−11g (Dietzel et al. 1973). So the detected micrometeoroids are divided into two groups below and above 10−11g for each sensor. Table 1 gives the absolute number of dust particles in the different mass intervals.

This paper investigates the way in which the mass distribution index, s, of the particles in a dust cloud varies as a function of time and particle mass, assuming that accretion, and only accretion, takes place after interparticle collisions. The approach is an extension of one first put forward by Napier and Dodd (1974). Two cases are considered. In the first the cloud starts with 500 particles of unit mass and as accretion takes place, more unit mass particles are added to keep that number constant at 500. This simulates a constant source feeding the low mass end of the distribution. In the second case the cloud starts with 5000 particles of unit mass and no additions are made. This corresponds to accretion in a closed system.

Submicron particles (mass<10−15g) were observed in certain meteor streams by impact detectors on satellite Explorer 46. This unexpected finding leads to conclusions about the nature and origin of the particles and about the importance of nongravitational forces.

Interplanetary dust particles collected in the form of micrometeorites in the stratosphere and meteor ablation spherules in deep sea sediments are possibly a relatively unbiased sample of the micrometeoroid complex near 1 AU. Detailed laboratory analysis of the particles has provided information on physical properties which may be useful in modeling a variety of aspects of interplanetary dust.

We consider the consequences of the assumption that the interplanetary particles which produce the zodiacal light have evolved from interstellar dust via comets. The chemical evolution of interstellar dust followed by the process of aggregation into the cometary nucleus and the subsequent ejection of cometary debris provide the basis for a model for the interplanetary particles. The scattering properties of these particles are reasonably consistent with current observations of the variation with elongation angle of the brightness and polarization of the zodiacal light. The major chemical constituents of the model are in the form of a matrix of volatile ices and complex nonvolatile molecules containing C, N and O in which are imbedded silicate and metallic inclusions.

Thin layers of an amorphous silicate coating were obtained by ion sputtering of crystalline olivine on KBr substrates. The dielectric functions of the coating were determined from reflection and transmission measurements in the wavelength range between 5 and 25 μ. The small particle extinction of the amorphous silicate shows a strong feature at 9.7 μ and a weaker one at 17.5 μ. In wavelength position both absorption bands agree with astronomically observed features.

A variety of complex dark brown organic solids called tholins have been produced from a mixture of cosmically abundant gases upon irradiation with ultraviolet light or electrical discharges (Sagan and Khare 1979). Such tholins were probably produced in the primitive solar nebula that contributed to the present composition of carbonaceous chondrites, comets, interstellar grains and gas. Spark tholins produced from approximately equimolar mixtures of CH4 and NH3, with 2.6% H20 exhibit 50% thermal dissociation temperature of about 900°C. Sequential and non-sequential pyrolysis followed by gas chromatography/mass spectrometry are employed to study both UV (Khare et al. 1978) and spark tholins. Typical pyrolyzates of spark tholins include alkanes, alkenes, aromatic hydrocarbon, abundant nitriles, pyrroles, pyrazines and alkylbenzenes. Organic molecules uncovered by microwave line spectroscopy of the interstellar gas are plausibly derived as spallation products of interstellar tholins comparable to the origin we propose for cometary nitriles and aldehydes as the spallation products of the cometary tholins. Spark tholins are highly insulating. Their measured resistivity is 71.4 × 108 ohms-cm and does not change up to a pressure of 105 bars.

Cratering experiments for μm- and mm-sized iron- and steel-projectiles on various target materials show that crater depths and the ratios of crater diameter to crater depth D/T depend on the densities of the projectile- and target-material and on the ductility of the target material. Cratering experiments into low density material (Saffile ρ = 0.28 g/cm3) have produced elongated impact “craters”. For low target densities the “crater” depth is up to 100 times the projectile diameter, depending on its impact speed. This impact process leads to a complete accretion of the projectile mass within the target.

Thin films are used in micrometeoroid experiments to shield the sensors from external noise sources. Because of the spin of the Helios spacecraft the ecliptic sensor is exposed to the Sun. Therefore a 3000 Å parylene film coated on one side with 750 Å aluminium protects the experiment against UV-radiation and solar wind particles. The south sensor is shielded from solar emissions by the spacecraft rim and therefore has an open aperture.

Thermomagnetic and saturation magnetization curves were obtained for separate chondrules and fine matrix grains from one H- (Allegan) and four L-type (Bjurböle, Elenovka, Saratov, Nikolskoe) chondrites. Mineralogical and petrological data were recovered from chondrule thin sections. The differences observed in the obtained curves seem to suggest episodes in the story of chondrules predating their incorporation into matrices.

Humes et al. (1974) deduced from the P-10 (Pioneer 10) meteoroid penetration data that the spatial density of 2 × 10−9 g and larger meteoroids was nearly constant (or possibly increasing) with increasing heliocentric distances between 2 and 5 AU from the sun. With an assumed mass density of 0.5 g/cm3, P-10 particles (particles whose mass is ⪞ 2 × 10−9 g) would have a particle radius in excess of about 10 μm. The observation of a constant, or increasing, spatial density leads to some interesting conclusions regarding the processes that control the population of P-10 particles between 2 and 5 AU from the sun. We shall explore some of these processes below and shall obtain the result that the P-10 meteoroid data can be best understood if many of the penetrating meteoroids are made of ice.

Consideration of the nucleation and growth processes and observational arguments suggest that besides crystalline grains in the solar system and interplanetary space there are also amorphous H20-ice, Si02 and C grains. Of particular importance are effects of the bombardment of these grains by protons in the solar wind and in the planetary radiation belts and the ensuing various erosion processes. Differences in the physical properties of crystalline and amorphous grains lead to interesting astrophysical consequences.

The catastrophic disruption of interplanetary dust grains, including water-ice, obsidian and magnetite, by impinging solar cosmic rays is investigated. The disruption is caused by the stress wave emanating from the heated lattice atoms along the path of an impinging particle. We find that the disruption plays an important role in the mass loss rate of grains compared with that due to sublimation and sputtering by solar particles.