A variety of particle types have been identified, but the most common are 

 particles that have chondritic (solar) abundances for all the major and minor ele- 

 ments found in meteorites. They are black, contain 2- to 5-wt% carbon, and have 

 elemental compositions identical to CI and CM carbonaceous chondrites. Among 

 the chondritic composition particles there are two clear subdivisions: those that 

 contain hydrated minerals and those that are anhydrous. The hydrated particles 

 are often rather compact and the bulk of their mass is contained in clay miner- 

 als. In some cases the abundant hydrated phases are serpentine minerals that 

 closely resemble those in CI/CM chondrites. However, in other particles the 

 hydrated minerals are distinct from those common in carbonaceous meteorites 

 and they also contain minor anhydrous phases, such as low Ni-pentlandite, that 

 have not been identified in meteorites. Some of the hydrated cosmic dust par- 

 ticles may be samples of the same parent bodies that produce the CI and CM 

 meteorites, but others in this class are mineralogically distinct and probably did 

 not come from the CI/CM parent bodies. The anhydrous particles are unique and 

 are unlike any established meteorite type. They are the only known case of a 

 carbon-rich chondritic composition material that is composed entirely of anhy- 

 drous phases. These particles are often exceedingly porous and are similar in 

 strength to the fragile materials that are observed as cometary meteors. The pore 

 spaces in the particles may have originally been filled with ice. The anhydrous 

 particles are aggregates of grains ranging in size from ~50 A to micrometers. 

 Some grains are single minerals (enstatite, olivine, iron sulfide, or carbides), but 

 others are themselves microaggregates of very tiny crystals embedded in carbon- 

 aceous material. Carbon occurs as binding material for the grains, as discrete 

 amorphous clumps, and as coatings. Isotopic analysis of hydrogen associated 

 with carbon in the particles has shown D/H enhancements as high as a factor of 

 10 relative to solar. The bulk D/H enhancement is higher than that found in 

 most meteorites. High D/H fractionation in interstellar environments is usually 

 attributed to ion-molecule reactions. 



4.2 Science Questions 



The primary goals of future dust-collection experiments in Earth orbit are to 

 collect materials that cannot be recovered as conventional meteorites and to 

 directly associate this material with specific comets, with specific parent bodies 

 (particular asteroids), or with interstellar origins. The samples should then be 

 returned to Earth for detailed elemental, isotopic, molecular, mineralogic, and 

 structural study, with exobiological interest focusing on the nature and abun- 

 dances of the biogenic elements (carbon, hydrogen, nitrogen, oxygen, sulfur, 

 phosphorus) and their compounds. The particles obtained would always be 

 small, and depending on the collection technique, they would be at least some- 

 what altered during acquisition. Even if the particles could not be collected in 

 pristine condition from Earth orbit, they could at least be collected in a form 

 that would give accurate elemental and isotopic compositions for grains over a 

 large size range. 



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