The 10 micron spectra of comets Halley (1982i), Wilson (1986l), Kohoutek (1973f) and Bradfield (1987s) are presented and compared. The silicate emission profiles of Halley and Bradfield are seen to be remarkably similar in that both contain a sharp break in the spectrum at 11.3 microns. Comet Bradfield does not show the same double peak structure seen in olivine and reported in Comet Halley be Campins and Ryan (1988) and Bregman, et al. (1987). The authors interpret the 11.3 micron signature as being due to olivine-type dust grains with at least some degree of crystallinity. Olivine alone is not enough to reproduce the shape of the 10 micron structure. However, in view of the authors' past success in fitting interstellar dust features with the emissivity profile obtained from amorphous grains produced by laser-vaporizing olivine, this is a very appealing identification. They note that there are significant variations in olivine spectra due to compositional differences, grain size distribution and related grain temperature variations to make the olivine identification tentative. They further tentatively identify the 9.8 micron feature in Halley as being due to either amorphorous olivine or a phyllosilicate (layer lattice). Neither the spectra of Halley, Kohoutek, nor Bradfield exhibited the 12.2 micron feature seen in Comet Wilson, which may prove diagnostic of the composition or thermal history differences between these comets. IR spectra of various mineral samples are discussed in terms of their match to cometary spectra

Campins, Humberto

Lynch, David K.

Russell, Ray W.

English

NASA Technical Reports Server

N91 " 15 00. .
10 pm SPECTRAL STRUCTURE IN COMETS
David K. Lynch*, Ray W. Russell*, and Humberto Campins**
*The Aerospace Corporation, Space Sciences Laboratory
**Planetary Science Institute
Abstract
The 10 pm spectra of comets Halley (1982i), Wilson (19861), Kohoutek (1973f) and
Bradfield (1987s) are presented and compared. The silicate emission profiles of Halley
and Bradfield are seen to be remarkably similar in that both contain a sharp break in the
spectrum at 11.3 )am. Comet Bradfield does not show the same double peak structure
seen in olivine and reported in Comet Halley by Campins and Ryan (1988) and Bregman, et
al. (1987). We interpret the 11.3 pm signature as being due to olivine-type dust grains
with at least some degree of crystallinity. Olivine alone is not enough to reproduce the
shape of the 10 pm structure. However, in view of our past success in fitting
interstellar dust features with the emissivity profile obtained from amorphous grains
produced by laser-vaporizing olivine, this is a very appealing identification. We note that
there are significant variations in olivine spectra due to compositional differences, grain
size distribution and related grain temperature variations to make the olivine
identification tentative. We further tentatively identify the 9.8 pm feature in Halley as
being due to either amorphorous olivine or a phyllosilicate ('layer lattice"). Neither the
spectra of Halley, Kohoutek, nor Bradfield exhibited the 12.2 pm feature seen in Comet
Wilson, which may prove diagnostic of the composition or thermal history differences
between these comets. IR spectra of various mineral samples are discussed in terms of
their match to cometary spectra.
[This work was supported in the Space Sciences Laboratory by the Aerospace Sponsored
Research Program and NASA contract NAS2-12370, and at the Planetary Science
Institute by NASA and the NSF.]
I. Introduction
The composition of interstellar dust is so poorly known that, until recently, any
statement about its make-up needed to be heavily qualified. Indeed, identifications in the
10 pm thermal window have to date amounted to little more than vague associations of
the "silicate" emission feature to grossly similar features in terrestrial (and some lunar)
silicates. It was shown theoretically by Gilman (1969) that it is plausible that silicates
could condense from a hot cloud as it cools. This work has recently been extended by, for
example, Grossman and Larimer (1974). Several workers (e.g. Day and Donn, 1978)
obtained laboratory spectra which exhibited absorptivity (i.e., emissivity) peaks which
were similar to the astrophysical feature at 9.8 pro, but which either had a central
wavelength that did not match that seen in astronomical spectra, or did not have enough
of a long wavelength tail. Stephens and Russell (1976), using laser vaporized sample of
olivine were able to match the 10 pm feature in the Trapezium, and later the twenty
pm feature as well (Cohen, et al., 1980). The first in situ experimental evidence that
condensed silicates might indeed be present in comets was obtained by the mass
spectrometer on the Halley flyby which samples comet dust (Kissel, et al., 1986).
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https://ntrs.nasa.gov/search.jsp?R=19910005691 2020-03-19T19:32:49+00:00Z
Suchcompellingresultsaddscientificweighto butnotproofof thelargerassertionthat
interstellarmaterialhasalargeproportionof siliceous material (or any of the other
proposed celestial grain materials, such as polycyclic aromatic hydrocarbons,
hydrogenated amorphous carbons, etc.). An additional concern is that silicate-to-silicate
variations in the type of fine spectral structure (features whose widths tD, are of the
order of k/100) necessary to uniquely identify the composition is complex and subtle. In
view of the now well established laboratory demonstration that fine spectral features are
often lost when the crystals are heated and become amorphous, the need to proceed
carefully cannot be over emphasized.
Comets provide a rare astrophysical opportunity to identify the chemical composition
of solar system dust because their thermal emission originates in simple, nearly
isothermal, optically thin dust clouds. In this paper we analyze a simple subset of thermal
emission from comets near 10 pm. The goal is to identify those features which are: 1)
real, 2) present in more than one spectra, in an effort to help identify the mineralogical
species responsible for thermal dust emission in comets. As an intermediate step in this
analysis one derives temperatures for the cometary grains from the (assumed) grey
continuum and compares the results with the blackbody radiative equilibrium temperature
for the appropriate heliocentric distance to derive limits on particles sizes.
II. Five Spectra of Four Comets
Figure 1 shows five spectra taken of four comets: two of Halley, and one each of
Bradfield (1987s), Wilson (19861) and Kohoutek (1973f). Although the true shape of the
spectra are accurately represented, they have been multiplied by appropriate intensity
factors for convenient placement on the graph. Also note the overlay of a 300 K grey
body for shape comparison. Table 1 gives the source of data and the continuum color
temperature.
Table 1
T(K)
A. Bradfield 1987s (Lynch and Russell, 1988) 475
B. Kohoutek 1973f (Merrill, 1974) 600
C. Halley 1982i (Campins and Ryan, 1988) 385
D. Halley 19821 (Bregman, et al., 1987) 320
E. Wilson 1986s (Lynch, et al., 1988) 300
While it is obvious that the slope of any portion of a spectrum are seriously
influenced by the temperature of the blackbody spectral shape it follows, the locations
where the slope changes abruptly will be independent of the underlying blackbody
temperature. With this in mind, we note two aspects common to most of the spectra of
figure 1, and possible additional structure in the spectra of Comet Halley (see also Fig. 3).
1. A break in spectra A,C,D and possibly B at about 11.3 lain.
2. A break in spectra A,B,C and possibly D at about 9 l_m.
3. Double-peak structure (9.8 and 11.3 pro) in C and D.
The meaning of these features depends on where the continuum is drawn. They
could be either: a) two broad emission features at 9 or 9.8 and 11.3 lain, or b) a single
broad dip at 10 lain. Campins and Ryan (1988) have proposed that olivine interplanetary
dust particles (IDPs) can explain the shape of the Halley spectra longward of 10 lain.
418
III. Spectra of Olivine
Olivine is common rock-forming mafic (rich in Mg and Fe) mineral occurring widely
on Earth. Its composition varies within two well-defined limits, being an isomorphous
solid solution between forsterite (MgzSiO 4) and fayalite (Fe2SiO4).
From the low pressure gas phase olivine condenses at around 1300 K (Grossman and
Larimer, 19"/4) although lower temperature condensates with closely related chemical
composition occur down to around 800 K. Figure 2 shows several IR spectra of olivine
digitized from publications of A. Hunt and Salisbury (1974), B. Zaikowski (1975), C. Day
(1975), and D. Koike, et al. (1981). Several properties of the spectra are evident:
1. Two prominent peaks occur in the 10 pm region, at around 10.1 and 11.4 l_m.
2. The peaks occur at somewhat different wavelengths in different samples.
3. The shapes of the peaks are different in different samples.
These spectra, though showing some differences, suggest that the spectra of olivine could
be used to identify mineralogical content of cometary dust, providing that olivine's
spectral variations cannot be duplicated by spectra of other minerals.
IV. Is Comet Halley's Dust Made of Olivine?
Figure 3 shows Campins and Ryan's fit of the IRTF Halley spectrum to Sandford and
Walker's (1985) olivine interplanetary dust particle. It is evident that:
a. Halley's spectrum's peaks ................ = 9.8 and = ll.3 pm
b. Olivine IDP spectrum's peaks ........... = 10.3 and = 11.3 pm
c. Olivine (Fig. 2) spectrum's peaks ...... = 10.1 and = 11.3 pm
From this analysis we see that the structure of Halley's spectrum and olivine's spectrum,
though similar, are significantly different, shortward of 10 pm. Although the 10.1 to
10.3 pm olivine features are observed to change wavelength in various samples, no
crystalline olivine sample was found that had the short wavelength peak shifted to
= 9.8 pm. The discrepancy is enough to suggest that olivine is either not the sole
content of the dust, or if it is, it's lattice structure (and thus spectral structure) has been
altered in a significant manner. An equally plausible scenario involves a mineral
component as yet unidentified. The main appeal of olivine as a major constituent in
cometary dust is that a large fraction of IDPs contain olivine-like material and comets
are believed to be the main source of IDPs (Sandford and Walker 1985). Furthermore, an
amorphous olivine emission spectrum has been shown to match the 10 and 20 pm
emission profile (Stephens and Russell, 1979).
V. What Causes the 9.8 pm Feature?
Our approach to identifying the 9.8 pm feature in Comet Halley's spectrum is
two-fold: 1) locating minerals with known 9.8 pm emission features and compositions
similar to olivine, and 2) locating minerals with the 9.8 pm feature that occur in IDPs.
Taking our cue from the tentative olivine identification of the 11.3 pm feature, we
have searched for minerals whose spectra show the similar structure and which would be
expected to condense from a low density plasma around 1300 K as olivine does. The
following minerals are possible candidates for the mineral responsible for the 9.8 pm
feature.
419
1. Andalusite....... AISiO
5
2. Pectolite ......... NaCa2Si3OsOH
3. Cordierite ........ (Mg,Fe3+)2A14SisO18
4. Garnierite ........ (Ni,Mg)3Si2Os(OH)_
5. Hisingerite ....... Fe2Si20 (OH)4o2H20
The spectra of these minerals are shown in figure 4.
Andalusite is considered a metamorphic mineral and, being trimorphic with
neosilicates kyanite and sillimanite, would not normally be expected to form directly from
a low density plasma. However, it is not known what changes would take place in
corundum (A1203) to alter it to andalusite on solar system time scales. Kyanite
shows a feature too broad to reproduce the 9.8 pm cometary feature and sillimanite has
a completely different 10 pm spectrum. Pectolite is an inosilicate which has both 9.8
and 11.3 pm features. Cordierite is a cyclosilicate and is commonly found with
polymorphs of A1205 (see #1 andalusite above). Except for being metamorphic,
cordierite might be considered a suitable candidate. Garnierite, though possessing the
proper spectral shape, is probably too rare to be seriously considered because of the low
nickel abundance in the solar system. Little is known about hisingerite.
The cometary feature at around 9.8 --. 0.2 tam (figure 3) may well be due to the
Si-O stretching mode in phyllosilicates (Farmer, 1974). Such a feature has been seen in
IDPs (Sandford and Walker, 1985), some classes of which also show the 11.3 l_m feature
attributed to olivine (Campins and Ryan, 1988).
Sandford and Walker found that the best IDP fit to Merrill's Kohoutek spectrum was a
roughly equal mixture of layer-lattice silicates and pyroxenes (phyllosilicates and
inosilicates). We find the best fit to Campins and Ryan's spectrum of Comet Halley would
be roughly equal mixtures of phyllosilicates and neosilicates (olivines).
Finally, we note that amorphous olivine (Stephens and Russell, 1979) has a single
feature at _ 9.8 pm (see Fig. 5). In view of the identification of the 11.3 lain
feature with crystalline olivine, it is reasonable to suggest that both peak in Comet
Halley's spectrum can be explained by a mixture of amorphous and crystalline olivine.
VI. Conclusions
Five spectra of four comets are shown to contain enough common spectral features
to begin making tentative mineralogical identifications of the dust. Crystalline olivine is
a likely candidate for reproducing the 11.3 lam feature in some comets. Other minerals
acting together (possibly amorphous olivine and phyllosilicates similar to those identified
in IDP must be present to explain the 9.8 l_m feature in Comet Halley's spectrum.
420
References
Bregman, J.D., et al., 1987, Astron. and Astrophys., 187, 616.
Campins, H. and Ryan, E.V., 1988, (preprint).
Cohen, N.L., McCarthy, J.F., Russell, R.W. and Stephens, J.R., 1980, B.A.A.S., 11, 610.
Day, K.L., 1975, Astrophys. J., 199, 660.
Day, K.L., and Donn, B., 1978, Astrophys. J. (Letters), 222, L45.
Farmer, V.C., 1974, The Infrared Spectra of Minerals, Mineralogical Society of London.
Ferraro, J.R., 1982, The Sadtler Infrared Spectra Handbook of Minerals and Clays,
Sadtler Research Laboratories, Philadelphia.
Gilman, R.C., 1969, Astrophys. J. (Letters), 155, L185.
Grossman, L. and Latimer, J.W., 1974, Rev. Geophys. and Space Phys., 12, 71-101.
Hunt, G.R. and Salisbury, J.W., 1974, AFCRL-TR-74-0625.
Kissel, J., et al., 1986, Nature, 321,280-282.
Koike, C., Hasegawa, H., Asada, N. and Hattori, T., 1981, Astrophy$. Space Sci., 79, 77.
Lynch, D.K., et al., 1988, (preprint).
Merrill, K.M., 1974, Icarus, 23, 566-567.
Russell, R.W., Lynch, D.K., and Chatelain, M., 1988, Bull. A.A.S., 20, 724.
Sandford, S.A. and Walker, R.M., 1985, Astrophys. J., 291,838.
Zaikowski, A., Knacke, R., and Porco, C.C., 1975, Astrophys. Space Sci., 35, 97.
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The 10 micron spectral structure in comets

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