Sources of 10 micron Interplanetary Dust: The Contribution from the Kuiper Belt

Two major sources are observed to contribute to the Zodiacal Cloud: main-belt asteroids and active comets. However, the discovery of 100 km size objects in trans-Neptunian orbits (Jewitt & Luu 1993) coupled with Pioneer 10 measurements showing an essentially constant flux of 10 μm dust from 4 to 18 AU with indications that dust may be in near-circular orbits (Humes 1980) suggests that collisions in the Kuiper Belt may contribute significantly to the Zodiacal Cloud and to the interplanetary dust collected from the Earth's stratosphere. Kuiper Belt dust collected at Earth could be identified by high densities of solar flare tracks, unusually thick amorphous rims, and high concentrations of spallogenic isotopes.

Comparative mineral chemistry of IDPs, micrometeorites and meteorite matrices

Chondritic porous aggregates are one subclass of Interplanetary Dust Particles (IDPs). The remaining classes presently identified are hydrated IDPs consisting mainly of serpentine or smectite. We have been investigating mineral compositions in these types of IDPs, in micrometeorites from Antarctica and compared them to mineral compositions in finegrained meteorite materials, like matrices and chondrule dust mantles from several meteorite classes. Based on our mineral analyses we subdivide anhydrous IDPs into three types. Type I contains olivines and/or pyroxenes having very variable iron contents from Fa 0 to Fa 35 and Fs 0 to Fs 30. Mineral phases in these particles are truly unequilibrated. Mineral grains in several particles of type I IDPs were found to contain solar flare tracks (pers. comm. John Bradley, McCrone Associates, Chicago). Almost all type I IDPs studied contain low-iron manganese enriched (LIME) olivines and/or pyroxenes.

Physical and Mineralogical Properties of Anhydrous Interplanetary Dust Particles in the Analytical Electron Microscope

The fine grained mineralogy and petrography of anhydrous “pyroxene” and “olivine” classes of chondritic interplanetary dust have been investigated by numerous electron microscopic studies. The “pyroxene” interplanetary dust particles (IDPs) are porous, unequilibrated assemblages of mineral grains, metal, glass, and carbonaceous material. They contain enstatite whiskers, FeNi carbides, and high-Mn olivines and pyroxenes, all of which are likely to be well preserved products of nebular gas reactions. Solar flare tracks are prominent in most “pyroxene” IDPs, indicating that they were not strongly heated during atmospheric entry. The “olivine” IDPs are coarse grained, equilibrated mineral assemblages that have probably experienced strong heating. Since most “olivine” IDPs do not contain tracks, it is possible that this heating occurred during atmospheric entry.

The Interplanetary Micrometeoroid Flux and Lunar Primary and Secondary Microcraters

We are gaining an increased awareness and understanding of Earth-orbiting space debris. Meteoroid experiments in near-Earth orbit must therefore now be able to differentiate between interplanetary meteoroids and space debris. Space debris impacts are not thought, however, to have significantly affected near-Earth meteoroid measurements carried out in the early 1960’s. New experimental evidence also makes it appear very probable that most impact pits on lunar rocks with pit diameters smaller than 7 micrometers have been generated by lunar secondary ejecta impacts, and not by primary meteoroid impacts. In addition, ages determined from solar flare tracks in lunar rocks are not considered secure. Lunar crater production rates are more reliably deduced from meteoroid space experiments and not from solar flare track ages. When all of the above qualifications are taken into account, however, a rather satisfactorily self-consistent meteoroid flux versus mass distribution is obtained.

Complementary Laboratory Measurements of Individual Interplanetary Dust Particles

Complementary analysis techniques including electron microscopy (SEM/EDX and TEM), molecular spectroscopy (FTIR and Raman), and secondary ion mass spectrometry (SIMS), are used to study individual dust particles collected in the stratosphere. Large deuterium enrichments and solar flare tracks show that most particles in the “chondritic” class are interplanetary dust particles (IDPs). Infrared transmission spectra of most IDPs fall into three major classes (layer-lattice silicates, pyroxenes and olivines). TEM and Raman measurements confirm this classification. The IR spectra show certain similarities to spectra observed in comets and protostars. In particular the 6.8 μm features observed in protostars and IDPs may have a common origin. Large D excesses are observed in IDPs of the first two IR classes. The correlation of D/H ratios with the C concentration indicates a carbonaceous carrier of the excess D. The D enrichments and IR spectra provide links to interstellar molecular cloud material.

Solar-flare exposure and thermoluminescence of Luna 24 core material

Mineral grains from three depths within the Luna 24 drill core ( ca . 90, 125 and 196 cm) have been examined for solar-flare tracks. Large proportions (55-100%) of grains from all three levels are found to be track-rich (with central track densities p e > 10 8 cm -2 ), and a substantial fraction ( ca . 25-50%) of all grains display trackdensity gradients. These observations indicate that most of the mineral grains have been cycled through the top ca . 1 mm of the lunar surface at some time in their history. Some degree of submaturity is observed towards the bottom of the core. The most likely depositional model envisages rapid infall of highly irradiated material into a less mature local component with rather little subsequent reworking. Thermoluminescence (t.l.) studies indicate a lower natural radiation dose in samples from the 196 cm level compared with those from the two upper levels. This can result either from random variations in the local internal radioactivity or from mixing properties of the pre-irradiated material over time scales of less than ca . 100 ka. Radiation sensitization of samples suggests a possible use of t.l. sensitivity for the interpretation of lunar radiation history.