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Interplanetary dust particles (IDPs) collected from the stratosphere are grains of cosmic dust delivered from asteroids and comets. IDPs are composed of complex mixtures of carbon, silicate glass and submicron to nanometer-sized minerals including Fe-Mg silicates, Fe-Ni sulfides, oxides and metal. Bulk SEM and detailed TEM investigations of stratospheric IDPs in the size range of 5 - 15 μm have shown that many IDPs have remarkably high concentrations of carbon - essentially the highest carbon abundances of any known extraterrestrial materials - and often this carbon is intermixed at the nanometer scale with other constituent phases. in order to investigate the properties, occurrence and spatial distribution of carbon in IDPs, we have developed a unique aciddissolution technique using standard microtomed thin-sections to remove the silicate and oxide fractions, thereby leaving a residue primarily of carbon and Fe-Ni sulfides. This technique causes no damage to the delicate microtomed sections or TEM grid and dramatically minimizes the quantity of acids required for etching.
Cometary debris of all sizes impacts the Earth but it is likely that only particles the size of dust survive atmospheric entry and are collected as meteoritic samples. Conventional meteorites and a substantial fraction of collected interplanetary dust particles appear to be asteroidal debris. Nearly half of the collected interplanetary particles have properties consistent with cometary material and resemble Halley dust that has lost the maiority of its carbon and nitrogen. These particles might be aggregates of presolar grains ana they provide some insight into the properties of interstellar grains.
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.
Interplanetary dust probably has a cometary origin. Collectible samples of this material are likely to be more representative of the meteoroid complex than are meteorites. Since 1974, NASA U-2 spacecraft has been able to collect over 200 interplanetary particles in the stratosphere. Roughly 10% have spheroidal shapes indicating previous melting. The rest seems to come from the gentle fragmentation of a single type of parent-body material: a black aggregate of grains mainly 1000 A in size, whose properties are closely similar to C1 and C2 chondritic material.
This paper presents physical properties of interplanetary dust determined by in-situ techniques. It is probable that, like millimeter-sized meteoroids (Jacchia, et al. 1967), most interplanetary dust is cometary matter. Although a cometary origin for interplanetary dust is widely accepted (Whipple 1967) (Millman, 1972) there is currently no unambiguous proof of this hypothesis. The results presented here must be interpreted accordingly. It must also be remembered that even if interplanetary particles are cometary, they might possibly be altered in the interplanetary medium by collisions and by thermal effects during close perihelion passages, so the dust particles may not be representative of unaltered cometary material.
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