Some stories told from cometary dust and IDP compositions
Elmar K. Jessberger
Institute for Planetology, Westfälische Wilhelms-Universität, Münster

This talk attempts to place extraterrestrial dust particles into their proper planetologicaland astrophysical proportion by illustrating examples of dust reseach.
First a short overview into the history of the “world” from the Big Bang until now is given
with examples of galaxies and their dust rich star forming regions. Then it is shown how
the building block of the “world”, nuclei, are formed in solar type and heavier stars byhigh temperature nuclear fusion processes. As a special example the calculated siliconisotopic composition in the core of a 25 solar mass supernova is presented, which ischaracterized by a mono-isotopic overabundance of 28Si of at least a factor of two. This
leads to the introduction pre-solar dust particles that are identified by correlated nonsolar isotopic composition of many elements. As an example the results of a TOF-SIMS
analysis of a pre-solar 1.7 µm-sized SiC-type X grain extracted from the Murchison meteorite are presented. The isotopic composition of Mg – only 26Mg from extinct 26Al
(T1/2=720.000 y) which was incorporated into the grain with 26Al/27Al=0.5 – shows that thegrain was formed shortly after nuclear synthetic 26Al-production, which, as is clear fromthe factor two 28Si-enrichment, occurred in a 25-solar-mass-supernova. Since the 28Si
overabundance is synthesized near its core and the 26Al overabundance in the outer half(mass-wise) of the supernova, our result implies that during and after the SN explosion
its material (ions) were not well mixed but that the inner material moved as “clumps” tothe outside to condense later together with more outside-material. This interpretation issupported by the N isotopic composition (formation region same as Al) and also by astronomical evidence that Fe “clumps” stick together long after the Supernova explosion(Science 287; 203). Therefore, the study of dust particles that are supernova condensates extractable from certain meteorites allow to understand the condensation processes in supernova remnants.
This chapter is followed by the introduction of the solar system and the sizes and distances of its planets. To elucidate the history of the solar system an interlude on the science of meteorites follows because they provide input of outmost significance to ourknowledge. This portion covers definitions, modern ways to collect meteorites and theirsignificance, pre-terrestrial orbits ranging mostly into the asteroid belt, radiometric ages,
classifications and their implications for parent body histories, the importance of Calcium-Aluminum-rich inclusions as the oldest solar system rocks, the onion-shell model ofparent body thermal history, the oxygen isotope systematics of the solar system bodies
that probably now (after the Genesis mission) indicate that CAIs were formed close tothe sun and that allow to distinguish classes of meteorites by specific O isotopic compositions, and the significance of CI chondrites as representing the average chemical composition of the solar system. This portion of the talk ends with wrap-ups on lunar andMartian meteorites and their significance followed by a short account of impact craters
for dating Moon (mentioning the possible Terminal Lunar Cataclysm) and Mars.
Returning to the dust theme, Interplanetary Dust Particles (IDPs) are introduced andmethods and results of their chemical analyses. The difficulties of analyzing these typically only 15 µm-sized particles are demonstrated as well the inferences of the lack ofcomplete data sets. It is mentioned that we have shown experimentally that at least one
element, Br, is prone to be largely affected by contamination in the stratosphere. A few
results from the isotope analyses of dust particles collected while the Earth moved
through the dust stream produced by comet Grigg-Skjellerup are presented focusing on
O and Si.
The next topic is the in-situ chemical analysis of Halley’s dust particles in 1986 in theGiotto and Vega-1/-2 missions by impact-ionization time-of-flight mass spectrometry. It isshown that Halley’s dust is even more primitive that CI chondrites (= richer in volatiles),that the abundance of the CHON elements (indicating the abundance of refractory organic substances) correlates with lower density particles (compared to particles rich inrock forming elements) and that attempts to extract information on the mineralogy aredifficult and resulted only on the notion that the particles are dominated by Fe-poor, Mgrich silicates. The only element for which isotopic information could be obtained is carbon. Unequivocally, 16% of Halley’s analyzed dust particles are dramatically enriched in12C (up to 50-fold!) indicating their circum-stellar origin.
20 years after the missions to comet Halley the next big step in cometary dust analysis
was brought about by the STARDUST mission to comet Wild-2 that returned dust particles to Earth using aerogel as catcher medium. A few results of the many and diverse
interdisciplinary studies are presented: The presence of CAIS (absent among Halley’sdust), the very few pre-solar grains (identified by O isotopic composition), the notion thatthe bulk elemental compositions of the many different particles and their fragments arehighly variable and need further study, and the presence of organic matter similar to thatin IDPs and carbonaceous chondrites. A highlight of the analyses certainly was the finding of tiny (20 nm) pebbles composed of ultra-refractory metals (Fe, Ni, Ru, Pt, Os, Ir)
and inclusions of TiN, an ultra- high-temperature (>1600 K) mineral. They were found inparticles that by their O isotopic compositions are linked to the most refractory minerals
in CAIs. If this O stems from near the forming sun (as indicated by new Genesis results),
the refractory materials need to have been transported from the inner solar system toregions beyond Neptune, possibly by the X-wind process (Shu et al.), and mixed therewith local (cold) organics and ices.
The talk ends with a gross comparison of the results obtained on Halley’s comet, on
comet Grigg-Skjellerup and comet Wild-2 emphasizing that comets appear to be very
different and are true “individuals”. In conclusion, more comet missions are mandatory to
get close to a better understanding of these enigmatic bodies that – in any case – best
witness the early history of our solar system and thus even may yield insights into thehistories of other solar systems.