The earth based measurements and in situ sampling of Comet Halley have provided new data about the chemical composition of cometary grains. Recent progress in laboratory studies of interplanetary dust particles (IDPs) complement the comet data, allowing inferences about the mineralogy and physical structure of the comet dust to be drawn from the observed elemental composition and infrared spectra. The in situ dust composition measurements at Halley, the composition of IDPs and their relation to comet dust, and the origin of the 3.4 micron hydrocarbon feature is discussed. Related discussion is also presented on aromatic components in comets and the 3.4 micron feature. These topics are briefly summarized

Gehrz, R. D.

Hanner, M. S.

English

NASA Technical Reports Server

N89-13333
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CHAPTER 3
COMETARY DUST COMPOSITION
R. D. Gehrz
University of Minnesota
and
M. S. Harmer
Jet Propulsion Laboratory
with contributions from Session III speakers
1.0 INTRODUCTION
The earth-based measurements and in situ sampling of Comet Halley have provided
intriguing new data about the chemical composition of cometary grains. Recent progress
in laboratory studies of interplanetary dust particles (IDPs) complements the comet data,
allowing inferences about the mineralogy and physical structure of the comet dust to be
drawn from the observed elemental composition and infrared spectra.
Seven speakers presented talks in this session, discussing the in situ dust composition
measurements at Halley, the composition of IDPs and their relation to comet dust, and the
origin of the 3.4#m hydrocarbon feature. They were requested to prepare written versions
of their talks, which are included here in this chapter. Related poster papers on aromatic
components in comets (Allamandola et al.) and the 3.4#m feature (Danks et al., Encrenaz
et al.) are also included here for completeness.
The topics discussed in the session are briefly summarized below. Further discussion
and recommendations for future research are included at the end of the chapter. How well
the conclusions from independent research techniques fit together was one of the exciting
aspects of the Workshop.
2.0 IN SITU MEASUREMENTS
A time-of-flight mass spectrometer was carried on board the Vega (PUMA) and Giotto
PIA) spacecraft to record the composition of impacting dust particles (Kissel et al., 1986).
articles striking the target generated a cloud of ions which were accelerated down a drift
tube and counted by the detector, as a function of mass/charge ratio. Several thousand
mass spectra of dust particles in the mass range 10 -16 to 10 -12 g were recorded by the
three instruments.
From a sample of PUMA spectra, the relative abundances of the major rock -forming
elements are chondritic within a factor of 2 (Jessberger et al. 1986, 1987). Carbon,
however, is enhanced by a factor of--- 10 in this sample, compared to CI carbonaceous
chondrites. A class of particles containing primarily H,C,N,O was discovered ("CHON"
particles; Kissel et al. 1986), supporting the evidence from the infrared spectra that Halley
was rich in organic materials.
At the Workshop, Mason reported on correlation analyses of elemental abundances in
over 8,000 PIA spectra (Mason and Clark, this Chapter). Carbon is the most abundant
element, appearing in 74 percent of all the spectra, while highest correlation occurs for the
pair C,O. The presence of molecular ions, such as MgOH + and CN + is suggested by the
data.
Initially, it was thought that molecular ions would not be formed during particle
impact. The appearance of the spectra, however, particularly the peaks at large ainu,
5o
https://ntrs.nasa.gov/search.jsp?R=19890003962 2020-03-20T04:45:41+00:00Z
has caused a reconsideration of the theoretical model for the impact process. Kissel and
Krueger (1987) have concluded that molecular ions are present in the data and have con-
structed a model for the ion chemistry and the kinds of organic molecules present. They
argue that their model is indicative of the silicate cores with organic refractory mantles,
predicted for interstellar grains by Greenberg.
Further analysis of the PUMA and PIA data is in progress by several groups, and we
can expect interesting new results to emerge in the next few years. However, it must be
kept in mind that the instruments sampled only the smallest grains in the coma, whereas
most of the mass in the comet grains lies in particles larger than 10 -11 g (see Chapter 2).
3.0 INTERPLANETARY DUST PARTICLES
For more than 15 years, IDPs collected in the stratosphere have been available for
laboratory study. Recent advances in techniques for analyzing submicron sections of these
grains and for obtaining infrared spectra make possible a comparison with Halley data.
These techniques, including thin sections, EDX spectra, and ion-probe imaging, are re-
viewed in the paper by Walker, along with a discussion of results relevant to the comparison
with comet dust.
Using a different approach, Brownlee compared the degree of variability in the mineral
composition for the Halley dust, carbonaceous chondrites, and IDPs. The CI and CM
meteorite samples contain primarily hydrated silicates, with a narrow range in Mg/Si ratio,
in contrast to the broad dispersion in composition and prevalence of pure Mg silicates in
the Halley particles. Hydrated IDPs show a narrow range of Mg/Si similar to the meteorite
samples. The anhydrous chondritic aggregate IDPs, on the other hand, display a broad
compositional dispersion similar to that seen in the Halley spectra. These IDPs even
look like plausible cometary particles, perhaps with ice originally filling the voids. Walker
cautioned that one should not rule out other kinds of IDPs as potential cometary grains,
for example the Ca and A1 rich refractory particles and FSN particles as well as hydrated
silicates.
Relationships between CHON particles and IDPs were also discussed. The chondritic
aggregate IDPs contain dark, carbon-rich matrix material. Some IDPs may have a 3.4_m
absorption feature, although possible contamination is difficult to rule out (see discussion
by Walker). Raman spectra of IDPs, described by Allamandola and by Sandford, exhibit
bands characteristic of aromatic molecular units of size <25A, and some showed red lu-
minescence as well. The band positions are similar to the interstellar infrared emission
features. Some IDPs have strong D/H enrichment in localized areas correlated with high
carbon concentration. Allamandola et 02. associate the high D/H enrichment with poly-
cyclic aromatic hydrocarbons in the grains and suggest that these may be only slightly
modified interstellar grains. Walker presented an ion image of an IDP containing a frag-
ment rich in C,H,N, perhaps related to a subset of the "CHON" particles seen in Halley.
Thus, laboratory evidence regarding hydrocarbons in IDPs seems to be consistent with
what we have learned about the organic material in Comet Halley, although further in-
vestigations of both IDPs and comets are clearly important. Better understanding of the
organic material in the grains is one goal of the CRAF mission.
4.0 INFRARED SPECTRA OF IDPs AND IDENTIFICATION
OF SILICATES
Sandford reviewed the 5-20#m infrared spectra of IDPs. These fall into three groups,
identified respectively with terrestrial olivine, pyroxene and hydrated silicates. Compo-
sition analysis of the IDPs confirms the spectral identifications. A 6.8_m feature seen
in hydrated IDPs is associated with carbonate; a weak 6.8#m emission may have been
present in the spectrum of Halley (Chapter 1). The silicate feature observed in Halley can
be fitted with a combination of the three spectral types, primarily olivine and pyroxene.
This result is in agreement with Brownlee's conclusion that anhydrous silicates dominate
51
the dust composition. In contrast,the 10/_m spectrum of Comet Kohoutek at 0.3 AU
showed a broad smooth silicatefeature,resembling the hydrated silicatesand inconsistent
with more than a small per cent crystallineolivine.
Caution should be exercised,however, in attempting to make a detailedfitof labo-
ratory transmission spectra to cometary emission spectra. As discussed in Chapter 4, a
transmissionspectrum generallyincludesscatteringas well as absorption,which can dis-
tort the shape of the band. The temperature and sizeof the emitting grains also have
to be taken into account. One alsohas to consider the processing historyof IDPs since
theirejectionfrom a parent body and the extent to which they may have been alteredby
heating. Nevertheless,given the difficultyof predictingaccurately the band shape for a
mixture of inhomogeneous, irregularparticles(Chapter 2),the directlaboratory compari-
son isa valuablefirststep,and itissignificanthat crystallinesilicateseem to be present
in Halley.
5.0 THE 3.4-MICROIN FEATURE
The observed spectrum of the 3#m region was described in Chapter i. The main
emission peak occurs at 3.36_m, with weaker featuresat 3.29 and 3.52_m. Tokunaga and
Brooke stressedthat the 3.36_m featureseen in Halley and Wilson isnot matched by any
interstellaxsource. Although the Galactic Center has a featurecentered near 3.4#m, the
detailedshape differsand the feature isseen in absorption,not emission. Of the set of
unidentifiedinfraredemission featuresusually occurring together in interstellarsources,
only the 3.29_m band seems to be present in Halley. Thus, the task for theoreticiansis
to explain not only the originof the 3.36#m cometary emission,but alsowhy itdiffers
from that seen in the ISM and why no corresponding emission bands are present at longer
wavelengths.
Moreover, it appears that not all comets exhibit 3.4/_m emission (Tokunaga and
Brooke), and Danks pointed out that no feature was evident in a preperihelionCVF
spectrum of Halley in December 1985.
While the consensus isthat the emission probably arisesfrom C-H vibrationsin or-
ganic molecules, the specificmolecule(s)and emission mechanism are unknown, and it
isnot even clearwhether the carrierisin the gas or solidphase. Several possibleemis-
sion mechanisms were discussedat the Workshop and are described in the papers in this
chapter. These include resonance scatteringby gas molecules, UV-pumped fluorescence
in gas molecules or small grains,and thermal emission from small grains. Because the
derived carbon abundance depends strongly on the emission mechanism, clarificationof
the 3.36/_m featureisnecessaryfor understanding the carbon budget of the comet.
References
1. Kissel et al. 1986, Nature, 321, 280, 336.
2. Jessberger, E. K., Kissel, J., Fechtig, H. and Krueger, F. R. 1986,
The Comet Nucleus Sample Return, ESA SP-249, p. 27.
3. Jessberger, B. K., Christoforidis, A., Kissel, J. 1987, submitted to Nature (Dec 1987).
4. Kissel, J. and Krueger, F. R. 1987, Nature, 326, 755.
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