Silverberg (1970) has explained the “dust storms” observed by the early satellite-borne microphone detectors by postulating that the orbital planes of short-period, low-inclination comets are filled with micrometeoroids. We report here on three separate approaches to test the validity of this hypothesis.

1. (1) Optical scans of the Gegenschein brightness can yield no useful information on the nearly isotropic sheets of dust predicted by Silverberg.

2. (2) An attempt to directly collect dust particles during a predicted high flux period by means of a sounding rocket yielded negative results.

3. (3) Over three years of particle impact data from extremely sensitive detectors flown aboard Pioneers 8 and 9 show no observable dust storms.

Hence Silverberg’s hypothesis appears untenable.

However, we should not rule out the possibility that observable showers of very small particles can be blown directly off the nuclei of some comets passing between the Earth and the Sun.

About 5000 microcraters on seven lunar rocks recovered during the Apollo 12 mission have been systematically studied using a stereomicroscope. Based on comparisons with laboratory cratering experiments, at least 95 percent of all millimeter-sized craters observed were formed by impacts in which the impact velocity exceeded 10 km/s. The dynamics of particle motion near the Moon and the distribution of microcraters on the rocks require an extralunar origin for these impacting particles.

The microcrater population on at least one side of all rocks studied was in equilibrium for millimeter-sized craters; i.e., statistically, craters a few millimeters in diameter and smaller were being removed by the superposition of new craters at the same rate new craters were being formed. Selected surfaces of some rocks, particularly those with glass coatings, are not in equilibrium. For every particle incident upon these “production” surfaces, there remains for observation a corresponding crater; thus the population of craters on such a surface is directly related to the total population of particles impacting that surface.

Crater size-distribution data from production surfaces, together with an experimentally determined relationship between the crater size and the physical parameters of the impacting particle, yield the mass distribution of the interplanetary dust at 1 AU. Based on assumptions corresponding to an impact velocity of about 20 km/s and a particle density of 3 g/cm3, the cumulative particle flux versus mass distribution relationship is

where N is the number of particles of mass m in grams, and larger, and C depends on the time-area product, which is, for the present, unknown. For particles smaller than 10-8 g, our observations indicate a sharper decrease in the absolute value of the slope of the flux versus mass curve than is indicated by satellite-borne-experiment data. This result may be due to a genuine relative decrease in the number or kinetic energy of smaller particles, or it may be due to our inability to observe quantitatively the smallest microcraters. For particles larger than 10-6 g, the slope of the flux versus mass curve increases smoothly to an absolute value greater than one.

Artificial meteor ablation was performed on natural minerals, composed predominately of magnetite and hematite, using an arc-heated plasma stream of air. Analysis of the ablated debris indicates most was composed of two or more minerals. Wustite, a metastable mineral, was found to occur as a common product. The “magnetite” model, whose content was 80 percent magnetite, 14 percent hematite, 4 percent apatite, and 2 percent quartz, yielded ablated products consisting of over 12 different minerals. Magnetite occurred in 91 percent of all specimens examined, hematite in 16 percent, and wustite in 39 percent. The “hematite” model, whose content was 96 percent hematite and 3 percent quartz, yielded ablated products consisting of over 13 different minerals. Hematite occurred in 47 percent of all specimens examined, magnetite in 60 percent, and wustite in 28 percent. The more volatile elements (Si, P, and Cl) were depleted by a reduction of about 50 percent in the amounts present. Also, the relative abundance of Fe increased as a result of both volatile depletion (loss of Si, P, Cl, and Ca) and a reduction in its oxidation state. Hematite was converted to magnetite in the ablation zone along the model’s front face. Also, quartz and apatite minerals were converted to an Fe-rich glass consisting of varying amounts of Si, P, Cl, and Ca, depending upon the accessory minerals available at the time of melting. These glass phases occurred as unusual myrmekiticlike intergrowths, which are unique textural indicators of the environment through which the material has survived. The chemistry and mineralogy of these phases remains the only trace of the original minerals. This study has shown that artificially created ablation products from iron oxides exhibit unique properties that can be used for their identification. These properties depend on the composition of the original material and the environmental conditions of formation. In addition to the accepted elemental criteria, these properties are morphologic characteristics, textural parameters, and the existence of metastable minerals.

Öpik (1937) and Whipple (1950) have predicted that small dust particles should be able to enter the Earth’s atmosphere without reaching their melting temperature and thus retain their original shape prior to entry. They have shown that the critical size is a strong function of velocity as well as the physical parameters of the dust particle. A number of investigators have collected particles at high altitude believed to be of this type (Hemenway and Soberman, 1962; Wright and Hodge, 1964; Thomsen, 1963). This brief paper presents possible evidence of the existence of small bodies collected with our Sesame collector (Hemenway et al., 1967; Hemenway et al., 1971) which apparently were sufficiently large to experience surface ablation and melting.

Photographic and Radar Observations of meteors reveal essential discrepancies from the simplest physical theory of meteors. The simplest theory (Whipple, 1943; Herlofson, 1948; Kascheev et al., 1967) proceeds from the following suppositions: (1) the meteoroid is a dense non-fragmenting body; (2) the sole ablation mechanism is evaporation; and (3) the whole energy transferred to a body by colliding air molecules is spent on evaporation. In addition, the simplest theory of radiowave reflection from meteor trails that does not take into account diffusive and thermodiffusive expansion of a meteor trail and change of the electron line density along the trail is used for the interpretation of the results of radar observations.

By application of Lyttleton’s theory for the formation of comets, it is shown that a possible mechanism for the origin and formation of a concentration of cosmic particles around the Earth and the other planets of the solar system exists.

In the vicinity of the neutral point, where the velocity of colliding particles is not greater than 6 km/s, it is found that if the solid particles after collision must remain in a solid state, there can be no possibility of accretion for Mercury, Mars, and the Moon, where the maximum value of the “closing-in parameter” p (distance of the center of the planet to the asymptotic trajectory) is less than the radius of the planet.

On the other hand, the capture radii of microparticles in solid form varies from a minimum of 2.95 planetary radii for Venus and 3.47 for the Earth, to about 986 for Jupiter.

Three-color observations of the nightglow brightness and polarization were made at the Majaky Observatory in October 1969 to determine the brightness of zodiacal light at the ecliptic pole. On the assumption that the degree of polarization of zodiacal light is 0.20 at the ecliptic pole, the corresponding brightnesses are found to be 68, 70, and 96 G2 stars of the 10th magnitude per square deg. at 0.37, 0.53, and 0.58μ.

The large diurnal variations in observed intensity and direction of polarization are attributed to the effects of dust in the Earth’s atmosphere.

Three methods of approximation are described and are used to separate the primary twilight brightness from the observed brightness. Photoelectric observations obtained at the Majaky Observatory in 1968-69 (Zaginayilo) are combined with observations from a balloon and from Pic-du-Midi Observatory (Link et al., 1967) to derive the atmospheric scattering phase functions at 0.37 μ and 0.58 μ as a function of height.

Comparison of these data with data for a Rayleigh atmosphere provide information on the optical properties of dust in the upper atmosphere.

On August 8, 1970, two Pandora sounding rocket payloads were launched from the ESRO range in Kiruna, Sweden during a noctilucent cloud display. Large numbers of sub-micron particles were collected, most of which appear to be made up of a high-density material coated with a low-density material. Typical electron micrographs are shown. Particle chemical compositions have been measured by use of dispersive x-ray analysis equipment attached to a Philips EM 300 electron microscope and have revealed that most of the high-density particle nuclei have atomic weights greater than iron.

Particles were collected at an altitude of 35 km by two flights of a volume-sampling micrometeorite collector. The collection scheme is very sensitive and is capable of collecting a significant number of particles. Many of the particles collected have chemical compositions similar to solar or to iron meteorites. Morphology of collected particles indicates that both true micrometeorites and ablation products were collected.

Cosmic flakes belong to the category of cosmic microparticles. They occur together with cosmic spherules in sediments (Utech, 1965). These flakes appear to represent ablation products of iron meteorites burnt away during the passage of a meteorite through the atmosphere by friction against air molecules. Microprobe analyses offtakes from deep-sea deposits proved the particles to consist of magnetite which in most cases had been altered into maghemite in the outer zones. This is in agreement with the assumption that they represent fragments of fusion crust of iron meteorites. Most of the flakes, however, contained a small percentage of manganese. This element is not a common constituent of iron meteorites. The author attempts to explain the presence of manganese in flakes and its absence in cosmic spherules.

Determination of the elemental composition of cosmic dust particles by means of an impact ionization time-of-flight mass spectrometer has been investigated at several institutions. In most configurations, the instrument supplies the identity of ion groups of both target and particle materials extracted from the impact plasma and the number of ions contained in each group. Experiments have shown that the fractional ionization of a given species is not constant with impact velocity nor is the fractional ionization the same for different kinds of atoms. A model of the impact ionization effect developed at TRW involves an equilibrium plasma condition with the consequence that the fractional ionization for an arbitrary atomic species can be specified by the Saha equation if the plasma volume (V) and temperature (T) are known. It follows that T can be determined by taking the ratio of the Saha equations for two elements present in the target in known concentration. (Taking the ratio negates the requirement of knowing V.) Given T, the procedure can be reversed to yield the relative abundance of elements contained in the impacting particle. To test the model, a PbZrO3-PbTiO3 target was bombarded with high velocity Fe, MoB, and NiAl particles and the number of Pb, Ti, and Zr ions was determined in a time-of-flight mass spectrometer. For each event, the relative abundance of Ti to Pb was taken as known (from electron microprobe analysis) and T was determined from the Ti-Pb measurement. The Zr to Pb ratio was found to be in good agreement with the microprobe analysis (0.38 calculated mean value compared to 0.34 actual). The result was valid for all particle materials and for a velocity range 17<v<47 km/s. T ranged from 3300 to 11 500° K and was only mildly velocity dependent.

Attention is drawn to the fact that neither astronomical observations, nor laboratory data can, as yet, sufficiently constrain models of the origin and evolution of the solar system. But, if correctly approached and interpreted, the magnetic remanence of meteorites could help in constructing a self-consistent model.

In the context of various models for the early evolution of a solar nebula, the possible roles assigned to ambient magnetic fields and the paleointensities required to establish the stable natural remanent magnetization (NRM in range 10-4 to 10-1 cgsm) observed in meteorites, are discussed. It is suggested that the record of paleofields present during condensation, growth, and accumulation of grains is likely to have been preserved as chemical (CRM) or thermochemical (TCRM) remanence in unaltered meteoritic material. This interpretation of the meteoritic NRM is made plausible by experimental and theoretical results from the contiguous fields of rock magnetism, magnetic materials, interstellar grains, etc. Several arguments (such as the anisotropy of susceptibility in chondrites, suggesting alignment of elongated ferromagnetic grains, or the characteristic sizes and morphology of carrier phases of remanence, etc.) as well as general evidence from meteoritics (cooling rates, chemical and mineralogical data) can be used to challenge the interpretation of NRM as thermo-remanence (TRM) acquired on a “planetary” parent body during cooling of magnetic mineral phases through the Curie point in fields of 0.2 to 0.9 Oe.

Fine-particle theories appear adequate for treating meteoritic remanence, if models based on corresponding types of permanent magnet materials, e.g., powder-ferrites for chondrites; diffusion hardened alloys for iron meteorites, are adopted, as suggested here.

Finally, a potentially fruitful sequence of experiments is suggested for separating the useful component of NRM in determining the paleofield intensity and its time evolution.

A quantitative examination of the gravitational and nongravitational changes of orbits shows that for larger interplanetary bodies the perturbations by Jupiter strongly predominate over all other effects, which include perturbations by other planets, splitting of comet nuclei and jet effects of cometary ejections. In an approximation to the restricted three-body problem, Sun-Jupiter-comet/asteroid, the value of the Jacobian integral represents a parameter of conspicuous stability which can be applied to delineate the evolutionary paths of the potential parent bodies of the meteoroids in the system of conventional orbital elements. Earth-crossing orbits can be reached along three main paths by the comets, and along two by the asteroids.

The structure of meteor streams, however, indicates that the mutual compensation of the changes in individual elements entering the Jacobian integral, which is characteristic for the comets, does not work among the meteoroids. It appears that additional forces of a different kind must exert appreciable influence on the motion of interplanetary particles of meteoroid size. Nevertheless, the distribution of the Jacobian constant in various samples of meteor orbits, from those of faint Super-Schmidt meteors up to those of meteorite-dropping fireballs, furnishes some information on the type of their parent bodies and on the relative contribution of individual sources.

Monte Carlo calculations indicate excellent agreement between observed and predicted orbits of Prairie network fireballs, if it is assumed that fireballs are derived from remnants of short period comets of Jupiter’s family. No such satisfactory agreement has been found for any other proposed source. The distribution of radiants and time of fall observed for chondrites will also be reproduced by this source, provided that consideration is given to the fact that the Earth’s atmosphere will permit low velocity bodies to survive but will destroy high velocity bodies. Again, no other proposed source has been found to be adequate.

It now appears likely that the mean lifetime of chondrites is limited to ˜107 yr by the high probability of complete fragmentation following impact by smaller bodies. This improves the agreement between the observed cosmic ray exposure ages and those predicted for a cometary source. This also requires some modification of the earlier discussions of alternative sources, but does not result in them becoming more satisfactory.

It is shown that aerodynamic forces could have played a significant role in the selective accumulation of chondrules on asteroids that are moving with respect to the gas in a primeval solar nebula. Particles smaller than millimeter chondrules would sweep by an asteroid moving more slowly than a critical velocity, while larger particles could be accumulated by impact. Theory and calculation cover the case of subsonic velocity and asteroidal diameter up to 50 km independent of the nebular density from 10-8 to 10-6 g cm-3 or higher for smaller asteroids, as illustrated in table 1.

The calculations are based on a solar mixture of materials at T = 550° K and on Stokes’ law of aerodynamic drag, which fails for nebular densities below about 10-8 g cm-3 for millimeter sized spheres. Below this limit the limiting velocity falls off proportionally to gas density. Note that the accretion rates for small particles can be severely restricted because of aerodynamic flow at small asteroidal velocities with respect to the gas. Near and above the critical velocity the asteroidal gravity does not materially add to the accretion rate regardless of asteroidal mass.

The problems of relating collapse conditions in an interstellar cloud to a model of the primitive solar nebula are discussed. In such a nebula there is a radial force balance between gravity, the pressure gradient, and centrifugal forces due to the rotation. Approximate values are given for the combinations of temperature and density throughout the nebula, from a maximum of about 2000° K near the center to less than 200° K in the outer portion. These conditions are based upon the compression adiabats in the terminal stages of the collapse of an interstellar cloud. One general conclusion, of great importance for accumulation of bodies within the solar system, is that interstellar grains should not be completely evaporated at distances in the nebula beyond about one or two astronomical units.

In the classical rotating Laplacian-type nebula, pressure gradients can develop radially to the protosun because of central radiation, particle ejection, and magnetic-field expansion or because of radial temperature or total gas density gradients. Except for the last two effects, the acting central acceleration for the gas is reduced from the gravitational value; the pressure gradient in the gas caused by temperature or density gradients may either add to or subtract from the gravitational acceleration, depending on the sense of the pressure gradient. Planetesimals in the nebula may thus experience tangential accelerations (+ or —) with respect to the gas because of the differential radial accelerations acting on the particles and the gas. As a consequence, the planetesimals may spiral outward or inward with respect to the protosun. The present paper deals with growing planetesimals and a range of drag laws depending on the Reynolds number and on the ratio of particle size to mean free path.

Particles spiral in the direction of positive pressure gradient, thus being concentrated toward toroidal concentrations of gas. The effect increases with decreasing rates of particle growth, i.e., with increasing time scales of planet formation by accretion. In the outer regions, where evidence suggests that comets were formed and Uranus and Neptune were so accumulated, the effect of the pressure gradient is to clear the forming comets from those regions. The large mass of Neptune may have developed because of this effect, perhaps Neptune’s solar distance was reduced from Bode’s “law,” and perhaps no comet belt exists beyond Neptune. In the asteroid belt, on a slow time scale, the effect may have spiraled planetesimals toward Mars and Jupiter, thus contributing to the lack of planet formation in this region.

The observed mass distribution of meteoroids at 1 AU from the Sun is briefly reviewed in a survey that ranges over the bulk of the mass spectrum from micrometeoroids to meteorite parent objects. The evolution of meteoroids under the influence of collisions, planetary perturbations, the Poynting-Robertson effect and radiation pressure is then discussed.

Most micrometeoroids are expelled from the solar system by radiation pressure shortly after their production as secondary ejecta during impact by larger objects or as dust ejected by comets. Particles that survive will eventually be swept out by the Poynting-Robertson effect.

Meteoroids in the radio and photographic ranges are destroyed in collisions faster than they can be replaced by the production of secondary fragments during collisions between larger objects. The source of new particles needed to maintain the population of these meteoroids in a stationary distribution may be material expelled by comets.

The survival of large objects is limited by gravitational scattering during close planetary encounters and by collisions as well, if they spend sufficient time in the asteroid belt. The observed radiation-exposure ages of chondrites are shown to be consistent with this model.

The possibility for detection of interstellar particles in the Earth’s environment is considered on the basis of the passage of the solar system through the interstellar medium. Among the forces which inhibit interstellar particle penetration, the deflection by the solar magnetic field and the repulsive force due to the radiation from the Sun are by far the most important.