Workshop on the Analysis of Interplanetary Dust Particles

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Summary of the Workshop on the Analysis of Interplanetary Dust Particles

Held in May of 1993 at the Lunar and Planetary Institute

The abstracts from this meeting have been published by the Lunar and Planetary Institute (LPI), as Workshop on the Analysis of Interplanetary Dust Particles, LPI Technical Report 94-02. This publication (available from the LPI) is the source of the following workshop summary. A proceedings volume for this workshop has also been published by the American Institute of Physics.

Great progress has been made in the analysis of interplanetary dust particles (IDPs) over the past few years, as reckoned by short presentations made at meetings of the Meteoritical Society, Lunar and Planetary Science Conferences, and electron microscopy conclaves. However, dust workers had been desirous of a more focused showcase for recent IDP results, and opportunity for consolidation of past work and the forging of new research collaborations. The recent availability of larger IDPs from the Large Area Collectors, and consequent particle analysis consortia, had made the necessity of a dedicated workshop even more acute. To satisfy this need, this first workshop dedicated to the analysis of IDPs was organized by Don Brownlee (University of Washington), John Bradley (MVA Associates), George Flynn (SUNY Plattsburgh), Alfred Nier (University of Minnesota), Frans Rietmeijer (University of New Mexico), and Michael Zolensky (NASA Johnson Space Center). From the start the principal goal of the workshop was to provide a forum for free and relatively uninterrupted discussion. To provide for the maximum degree of participant interplay and productive discussion, the workshop was designed around a few review talks. Each of these talks was intended to review past results in a specific branch of IDP research, and suggest future potentially fruitful directions. Following each of these presentations, workshop participants were free to discuss any aspects of the specific subject, and introduce and discuss their own results and ideas. Contributed presentations were made in the form of posters, although these results were folded into the discussions at appropriate times.

For each discussion, one workshop participant served as a summarizer. These summaries, and recordings of the talks and discussions, are now being used to facilitate production of the workshop proceedings volume, which has been published by the American Institute of Physics. These summary presentations were grouped in the following order:

  1. Introduction
  2. Observation and modeling of dust in the solar system
  3. Mineralogy and petrography of IDPs
  4. Processing of IDPs in the solar system and terrestrial atmosphere
  5. Comparisons of IDPs to meteorites and micrometeorites
  6. Composition of IDPs (including isotopes)
  7. Classification schemes of IDPs
  8. Collection of IDPs

We present here a brief summary of the major points raised at the workshop.


Don Brownlee made the points that IDPs are probably the only samples now available of outer-belt asteroids and comets, including Kuiper belt objects. IDPs could have turned out to be merely additional fragments of H6 chondrites, but (fortunately for us) detailed work over the past two decades has shown them to be compositionally and mineralogically distinct from meteorites. These important distinctions include greater porosity, aggregate structure, higher volatile content, and unique mineralogy. Great progress has been made in the development of new analytical techniques, permitting measurements to be made of bulk compositions (including trace elements by four different and complementary techniques, noble gasses, and some organic compounds), infrared spectra (both transmittance and reflectance), isotopes and physical properties. Important problems remain, including the establishment of a useful classification scheme, resolving differences in terminology (e.g. tar balls vs. granular units, etc.), elucidation of the relationships between different types of IDPs (chondritic vs. refractory vs. basaltic; hydrous vs. anhydrous). Finally, future collection technologies were discussed, including replacement of silicone oil in stratospheric collection, and the role of dust collection in space.

Observation and modeling of dust in the solar system

Stan Dermott presented the results of analysis of IRAS and COBE data, which indicate that main-belt asteroids are the source of approximately 40% of the dust providing zodiacal light. The remainder must be provided by near-Earth asteroids, comets and interstellar sources. In particular it would seem to be useful to make detailed dynamical calculations of the evolution of grains from near-Earth asteroids. Apparently, dust grains derived from some asteroid families are distinguishable from one another, in inclination space. Differences in orbital elements for asteroidal grains from different families also cause different encounter velocities at Earth, and hence differential atmospheric entry-heating levels which will vary temporally. Al Jackson and Herb Zook also presented results of calculations of which indicate that cometary and asteroidal particles can be distinguished, if their velocities and trajectories are measured in space. This is a critical requirement for proposed dust collection efforts in low-Earth orbit. Herb Zook presented data from the Ulysses spacecraft cosmic dust experiment indicating the presence of streams of interstellar dust focused by interaction with Jupiter.

Martha Hanner reviewed the available spectroscopic information on comets, of which there is all too little. The main point of this presentation was that the comets observed to date appear to differ significantly from one another in mineralogy. While it is possible that these differences are due to differential aging of comets, it is also likely that there is considerable inherent intercomet heterogeneity. Recent measurements of the reflectance spectra of chondritic IDPs by John Bradley reveal some features found in some comets, including features possibly due to non-crystalline phases. Definite ties between specific comets and IDPs have yet to be demonstrated by spectroscopic work, although this is clearly a promising line of research. Still unknown is the relationship between cometary dust and interstellar grains, and whether hydrated materials are found on comets. This latter point is currently the source of contentious discussion.

Mineralogy and petrography of IDPs

John Bradley summarized what has been learned concerning the mineralogy of chondritic IDPs. A fascinating aspect of IDP petrography is the frequent occurrence of fine-grained aggregates, which can have bulk chondritic composition at the femtogram scale. These objects are still the subject of nomenclaturial disagreement; Bradley has called them tar balls, Rietmeijer has used the term granular units. Bradley has now identified three flavors of aggregates, which he calls unequilibrated-, equilibrated- and reduced aggregates (UAs, EAs and RAs), depending upon the mineral assemblage. (Frans Rietmeijer presented al alternate classification scheme in much the same vein, see Classification, below) Are these aggregates the products of nebular accretion, with subsequent processing in the cases of the EAs? Lindsay Keller has suggested that some of these aggregates could be agglutinates from parent body regoliths. These objects clearly will receive much more attention in the future, although it is unfortunate that they are so small, being right at the analytical limit for present electron beam instruments. Bradley also reviewed the evidence for hydration on the IDP parent bodies, concluding that the frequent occurrence of hydrated and anhydrous phases within the same grain is likely due to the incipient nature of the alteration event. Mike Zolensky described work on the compositions of olivines and pyroxenes in chondritic IDPs. The common occurrence of diopside in hydrous IDPs probably indicates parent body metamorphism. Considerable attention is also centering on the non-crystalline component of IDPs.

Processing of IDPs in the solar system and terrestrial atmosphere

Al Nier has been measuring noble gas contents of individual IDPs by step heating. He has realized that examination of the temperature - gas release profile for a particle will reveal the peak temperature of atmospheric entry heating. This Workshop Summary continued from page 3latter value can be used to infer cometary vs. asteroidal origin for the particle, as the cometary grains enter the atmosphere at considerably higher velocities (on the average). This technique appears to be the only current course for distinguishing cometary from asteroidal particles, since velocitiy and trajectory information for particles is otherwise lost upon atmospheric entry.

George Flynn reviewed the evidence indicating the degree of atmospheric entry-heating experienced by IDPs. Despite the obfuscatory effect this has on nebular and parent body processes, recognition of the degree of heating can provide unique source information (see above). Documented entry-heating effects include formation of magnetite rims, depletion of volatile elements like zinc, dehydration of phyllosilicates, and the changes in the release temperatures of noble gasses mentioned above. At one time it was hoped that inspection of solar flare track densities and their density variation across a grain would provide useful information on space-exposure duration and entry-heating level, however track densities appear to be too low to permit useful estimates to be made of these values. The explanation of these heating effects is still somewhat controversial. Some researchers believe that magnetite rims could form by sublimation onto the particles from the atmospheric E-layer gases. However, most workshop participants felt that this sublimation process would be unlikely, due to the low iron concentration of the E-layer, and the short atmospheric residence time of the particles. Some researchers are unsure of the proposed nebular origin of the bromine and zinc enrichments reported for many chondritic IDPs, making heating/depletion estimations potentially uncertain. All participants concluded that we need to better understand the concentration and sources of zinc, bromine and iron in the upper atmosphere.

Discussion also centered on the potential for particulate contamination during collection in silicone oil on the inertial inpaction collectors currently used by NASA. Although workers have not found indications of contamination from this material (except for Si, which can apparently be removed), lingering uncertainties on this subject indicate that care must be taken during the interpretation of compositional analyses.

Comparisons of IDPs to meteorites and micrometeorites

This topic is well covered by Wolfgang Klock's presentation. Basically, most IDPs appear to have significant differences from meteorites, although certain IDPs appear to be identical to some carbonaceous chondrite matrix materials. These differences do not preclude derivation from the same parent bodies as meteorites, however, but require that different materials from them are being sampled. In general, low-strength, poorly-consolidated materials like IDPs would not be expected to survive as meteorite-sized bodies, only as dust. Klock also compared IDPs to the larger (>100 µm) micrometeorites collected in the oceans (deep sea spheres), and polar ice caps. These latter materials are generally highly contaminated, and severely heated by atmospheric entry, which often precludes useful comparison. However, the most pristine of these larger materials bear considerable resemblance to CI, CR and CM chondrites. There are sufficient differences between these micrometeorites and IDPs to warrant continued study of the former, particularly since they are considerably easier to handle and are available in abundance (thanks largely to the efforts of Michel Maurette).

Composition of IDPs

Steve Sutton's presentation was a good introduction to this subject. Numerous complementary techniques are now routinely utilized for the measurement of the bulk compositions of IDPs. Problems linger concerning potential contamination during atmospheric residence and collection, however progress is being made in understanding the actual dimensions of these hazards. Lindsay Keller and Kathie Thomas find that chondritic IDPs contain up to approximately 50 wt% carbon, far higher than for bulk chondrites. The highest carbon concentrations appear to always come in IDPs containing pyroxenes.

Robert Walker summarized ion probe analyses of chondritic IDPs, with a major goal being the location of preserved interstellar material. Isotopic measurements have been made of H, C, Mg, N and Si on fewer than 100 IDPs. No large isotopic anomalies have been found in C, Mg or Si. Approximately half of the analyzed particles exhibited deuterium enrichments (up to 2000ä). Some IDPs have shown 15N enrichments (up to 411ä), usually correlated with a deuterium enrichment. Hydrous and pyroxene-dominated IDPs appear to be isotopically similar, however no anomalies have been located in olivine-dominated IDPs. Basically, no IDPs have yet been located which are entirely interstellar in origin (as we understand them). While this is a disappointing result, the fact is that only a very small total mass of IDPs has yet to be examined; if interstellar materials are present among IDPs in the same concentration as in the Murchison CM chondrite then (statistically) we should not expect to have seen even one entirely interstellar grain yet. However, isotopic anomalies are found in IDPs, and these remain to be explained satisfactorily.


Major problems remain with IDP classification. The most widely used scheme discriminates between four types, based upon the dominant crystalline silicate phase: olivine, pyroxene, saponite and serpentine. Not included in this framework are the refractory IDPs. Frans Rietmeijer reviewed these schemes, proposing a new classification based on the composition and petrography of the micrometer-sized aggregates present in most chondritic IDPs (see above). Rietmeijer proposes that these objects be called granular units and polyphase units, in contrast to Bradley's suggestion. Actually, both classifications have common features. No common classification scheme for IDPs was found acceptable to everyone, although participants agreed that a radically new classification scheme along the lines of those proposed was desirable. However, lively discussion of these classification schemes established that for any new one to be accepted it will have to (1) be based on measurements which can be made by numerous investigators, (2) not rely on inferred processes, and (3) not include unsavory terms (you had to be there). It is clear that further development work will be necessary here.

Collection of IDPs

At present, IDPs are actively being collected in the stratosphere, from polar ices, and within impact features on spacecraft. Presentations by Maurette, Zolensky, and Dardano covered basic aspects of these efforts. It is fair to say that all techniques being used are complementary, and that no one is clearly superior in all aspects. The stratospheric collections provide the least contaminated and heated particles, but recovered particles are generally less than 70 microns in diameter. Particles from polar ices can be larger than the stratospheric particles, but are more contaminated and heated, and may actually represent a different population of objects. Particles collected in space can be collected with velocity and trajectory information, unlike those collected at Earth, but are generally highly shocked and/or melted at best, and at worst are vaporized. All of these collection techniques are being improved, as resources permit. In the stratosphere collection with a medium other than silicone oil has being attempted (so far without success). Cleaner equipment is now being used in the Antarctic to collect particles. Better capture media are being developed for less disruptive collections in space. Future flight opportunities for these collectors include LDEF II, EURECA II, MIR and the U.S. Space Station.