Three problems will be discussed: A) The relationship between mlcro-meteoroids and solar flare particles averaged over the recent geologic past (~1 my); B) the past record of this relationship as measured in lunar soils and lunar and meteoritic breccias; C) the determination of the time at which different extraterrestrial samples were exposed to free space. Data bearing on these points obtained from studies of special lunar rocks and from measurements on individual crystals removed from lunar cores will be presented. Progress in using ion-probe mass spectrometry to link measurements of micro-impact craters with the past properties of the solar wind will also be discussed. Comparing microcraters and solar flare tracks in individual crystals from lunar cores, we find no evidence of any extraordinary variations for a time span covering an interval of ~109 yrs. Crystals 100µ to 400µ in size in mature lunar soil samples appear to have been exposed 3 to free space at the top of the lunar regolith for times from 103 to 104 yrs.

In December, 1973, a Lunar Ejecta and Meteorites (LEAM) experiment was placed in the Taurus-Littrow area of the moon by the Apollo 17 Astronauts. Objectives of the experiment were centered around measurements of impact parameters of cosmic dust on the lunar surface. During preliminary attempts to analyze the data it became evident that the events registered by the sensors could not be attributed to cosmic dust but could only be identified with the lunar surface and the local sun angle. The nature of these data coupled with post-flight studies of instrument characteristics, have led to a conclusion that the LEAM experiment is responding primarily to a flux of highly charged, slowly moving lunar surface fines. Undoubtedly concealed in these data is the normal impact activity from cosmic dust and probably lunar ejecta, as well. This paper is based on the recognition that the bulk of events registered by the LEAM experiment are not signatures of hypervelocity cosmic dust particles, as expected, but are induced signatures of electrostatically charged and transported lunar fines.

An investigation has been made to study a possibility that dust particles might catastrophically explode on the lunar surface due to electrostatic charging. It is shown that for the dark side along the terminator zone, dust balls and compact stony particles of micron and submicron sizes will be blown up if their surface potential is as low as a kilovolt negative. This mechanism will not operate on the sunlit side because the potential is only 3.5 ~ 20 volts positive. Some of these fragments may possibly levitate in the vicinity of the terminator.

Using a Van de Graaff dust accelerator an experimental program has been carried out in order to study crater parameters as a function of projectile incidence angle. Iron particles were shot into quartz glass targets. The angle of incidence which is the angle between target normal and the impact direction of the projectile has been varied from 0° to 70° in steps of 10°. Projectile masses ranged from 10−11 to 10−13g with velocities between 3 and 20 km/sec and projectile masses at 10−2 g with 4 km/sec impact velocity using a light gas gun at the Ernst-Mach-Institut, Freiburg i.Br. The so called circularity index which is the ratio of the crater area to the area of the smallest circle around the crater is a measure of the asymmetry of a crater. The circularity index decreases linearly with increasing angle of incidence. Also a small increase of the circularity index with increasing projectile velocity has been found i.e. the craters have a rounder shape with increasing velocity at the same angle. The circularity index appears to be independent from the projectile mass in the mass range from 10−11 to 10−2 g for stainless steel targets.

Ejecta parameters such as masses and velocities as a function of ejection angle have been determined in impact experiments performed with a light gas gun. Impact plasma detectors and secondary targets mounted around the primary impact site served for the detection of secondary particles.

Impact experiments have been performed with the 2 MV dust accelerator; the dependence of the maximum light flash energy and intensity on the projectile mass and velocity has been determined experimentally. The temperature of the radiating gas and plasma was estimated to be in the range from 2500K to 5000K, depending on the impact velocity. The distribution of the maximum ejecta speed as well as the normalized distribution of ejected mass have been determined as a function of the ejection angle. A rough estimate of the degree of vaporization of the displaced mass was obtained.

A review is given which suggests that cosmic dust theoretical and experimental studies are still beset with uncertainty and inaccuracy. A significant body of interrelated evidence exists which indicates that the solar system has two populations of dust particles, a submicron population generated and emitted by the sun and a larger size population spiraling inward toward the sun. The submicron component may provide the missing coupling mechanism between solar sunspot activity and meteorological activity in the earth’s atmosphere.

Analysis of craters found on polished plates exposed during Skylab has provided data for a flux measurement over the mass range 10−15 to 10−7 gms. Chemical analysis of residues in the craters shows a high incidence of aluminium. A variety of morphological forms are described.

During the Skylab experiment S 149 three different sets of areas were exposed. 71.5 cm2 were facing the sun for 46 days, and 36 cm2 for 33 days, whereas 77.5 cm2 were exposed in anti-solar direction for 34 days. A fourth set is currently being exposed with the hope of future recovery. The exposed surfaces consisted of stainless steel, aluminium, platinum, glass, and pyroxene. The recovered targets have been investigated with a light microscope and a scanning electron microscope. We found two groups of possible impact structures:

1.) Five craters between 1 and 30 µm. These craters show clear signs of hypervelocity impact. Measurements yielded diameter to depth ratios between 2 and 3. Chemical investigations in the craters yielded an enhancement in aluminium in one case.

2.) 44 crater-like structures between 1 and 4 (µm in diameter. These features have been found on 4 cm2 of pyroxene exposed in solar direction. They show diameter to depth ratios between 5 and 8. Chemical measurements of the interior of these structures indicate the elements of the pyroxene composition.

The five impacts of the first group correspond to a cumulative flux of the order of 10−4 (m−2s−l) for masses of about 10−12 g. The second group may indicate a fragmentation process at altitudes around 450 km. Considering these 44 crater-like structures having been produced by fragments of one projectile, the impact rate could be comparable to that calculated for the first group. If individual projectiles had produced these structures, the corresponding flux could be 2 orders of magnitude higher.

Over the past several years we have collected 2µm to 30µm particles from the stratosphere using high volume air sampling techniques. In 1970 and 1971 we flew balloon experiments to 34 km, sampling particles from 1.1 × 104 m3 of ambient air. Beginning in March 1974 we have flown 100 hours of sampling time on a NASA U-2 aircraft yielding a sampling volume of 9.3 × 104 m3. In both programs particles are collected by inertial deposition from a 200 ms−1 airstream on to clean surfaces coated with thick films of 500,000 centistokes silicone oil.

A new balloon collection technique, Magellan, for obtaining large cosmic dust particles > 50µ is described. The technique utilized a 40 m2 aperture funnel suspended 300 m below a balloon at an altitude of 25 km. First results indicate that the collected particles are diverse both chemically and physically.

Sampling of magnetic particles in the 5 to 20 µm diameter size range on the earth’s surface at different latitudes was used to detect meteor streams composed of particles of masses too small to be detected by other means. Magnetic spheroid concentration peaks appeared simultaneously at different latitudes in the Northern and Southern Hemispheres, proving that the majority of spheroids is of extraterrestrial origin. Extraterrestrial particles consist of two major classes: (1) magnetic particles containing a high concentration of iron oxide in the form of magnetite; and (2) glassy particles containing a low concentration of iron and a large concentration of Si and Al; they contain Ni, Mn, Cr and Ti; the concentration of Ti is approximately one hundred times larger in glassy particles than in magnetic spheroids. Comparison of chemical composition of particles collected from different meteor streams must be very carefully evaluated; chemical composition of surfaces and interiors of magnetic particles is different due to selective fractionation of different chemical compounds between molten and solid phases and due to evaporation during entry of meteoritic particles into the atmosphere. The presence of H2O, CO2 and CnH2n+2 in internal cavities of a few spheroids was found to be difficult to explain; those gases were associated with meteoritic particles and consequently they existed in space. The yearly earth flux of extraterrestrial particles can only be determined by the summation of daily fluxes obtained at different latitudes; the daily flux changes from year to year. Fluxes of particles present in January and October were calculated. Particles collected in January seem to originate from sources present between November 6 and January 15; during this period of time S. and N. Taurid, Geminid, Ursid and Quadrantid meteor streams are active. Particles collected in October seem to originate from the unknown meteor stream or from the Giacobini-Zinner meteor stream providing it is preceded by micrometeoritic particles approximately two weeks prior to the visual display.

Cosmic dust fragmentation in near-earth space is strongly suggested by the high fluxes (~102/m2s) of submicron particles observed in the upper atmospheric collection experiments, the lower fluxes (~10-l/m2s) observed in near-earth space by the lower sensitivity detection experiments and the much lower fluxes (~10−4/m2s) observed in the deep-space experiments. Various possible fragmentation mechanisms are discussed.

Results of many current attemps to estimate the physical and chemical composition of dust particles in comets are reviewed and discussed. It is shown that even the most basic parameters, such as albedo of the cometary dust, are not properly known at the present time. The emission feature in the infrared spectra of comets, which resembles those observed in the interstellar (and circumstellar) clouds and which indicates a relation between the general composition of comets and interstellar matter, is widely ascribed to silicates. Similar features may, however, be also caused by polymerized molecules or hydrocarbons mantles of the dust grains.

The present set of short-period comets produces only 250 kg/sec of dust whereas the flux of long-period and “new” comets produces an average of 20 tons/sec.

The combined effects of the Poynting-Robertson drag, collisions and sputtering are destroying the interplanetary dust cloud. The mass-losses estimated by different authors reveal great discrepancies. The estimations range from 1 t sec−1 (purely Poynting-Robertson loss) over some 10 t sec−1 given by Whipple (1967) to a value of 100 t sec−1 which is propagated by Bandermann (1967). The latter derives this high value from the assumption that the main mass loss is due to self-collisions.

Optical features of comets suggest that the dust in the coma is composed of a micron size core and a mantle formed by an aggregate of submicron grains.

Analytical expressions for the dust entrainment in the limits of low and high gas production from cometary nuclei are derived. The resulting volume density is used to evaluate the dust production of Comets Encke and Kohoutek from IR-measurements. It is predicted, that the dust in solar direction was diffuse for Comet Kohoutek during approach to the Sun, but sharp for Comet Encke and nearly independent of heliocentric distance. The dust production rate of Encke ist not sufficient to sustain the Zodiacal cloud of micron sized particles. Particle impact probabilities and hazards are estimated for a space probe in a fly-by mission to Encke.

During a period of 60 days around the time when the HEOS 2 satellite penetrated the orbital plane of Comet Kohoutek (1973f) the micro-meteoroid experiment on board registered an excessive particle flux. These particles are identified with those originated in Comet Kohoutek. Orbit calculations show that their emission occurred outside 3.8 AU from the sun. The ratio of the force of radiation pressure to that of gravity of these particles was determined to ß =1 ± 0.1, their mass has been measured from the satellite data (10−13g to 10−11 g). The velocity and the rate of dust particles emitted from the comet is studied on the basis of the theory of dust comets formulated by Finson and Probstein. An emission rate of appr. 1.2 × 1018 particles per second in the size range corresponding to 0.9 ≤ ß < 1.1 and an emission velocity of appr. 0.5 km/sec match best the observed data.

It was shown elsewhere (Sekanina, 1974) that the observability from the earth of an anomalous tail (antitail) of a comet can be rather straightforwardly predicted from the dynamical and geometric conditions. The physical presence or absence of the antitail at a precalculated time is then a measure of the comet’s production rate, at the relevant emission times, of relatively heavy dust particles (mostly of submillimeter size) that constitute such an antitail. Because the large grains are e-mitted from the nucleus at very low velocities (typically meters or tens of meters per second), an antitail is essentially a two-dimensional formation in the orbit plane of the comet and can be recognized best when projected edge-on, i.e., when the earth crosses the nodal line of the comet’s orbit. In general, however, this condition is not essential for the recognition of antitails (cf., e.g., Comet Kohoutek 1973 XII).