Comets may be our best probes of the physical and chemical conditions in the outer regions of the solar nebula during that crucial period when the planets formed. The volatile composition of cometary nuclei can be used to decide whether comets are the product of a condensation sequence similar to that invoked to explain the compositions of the planets and asteroids, or if comets are simply agglomerations of interstellar grains which have been insignificantly modified by the events that shaped the other bodies in the solar system. Although cometary nuclei are not generally accessible to observation, observations of cometary comae can illuminate at least some of the mysteries of the nuclei provided one has a detailed knowledge of the excitation conditions in the coma and also has access to basic atomic and molecular data on the many species present in comets. Examined here is the status of our knowledge of the volatile composition of cometary nuclei and how these data are obtained

Weaver, H. A.

English

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1990006555
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tTHE VOLATILE COMPOSITION OF COMETS
¢
H. A. WEAVER
Space Telescope Science Institute
3700 San Martin Drive
Baltimore, MD 21218
U.S.A.
4"
To be published in the 1989 issue of the Highlights o/Astronomy _
1
1990006555-002
ABSTRACT
Comets may be our best probes of the physical and chemical conditions in the outer
regions of the solar nebula during that crucial period when the planets formed. The volatile
composition of cometary nuclei, in particular, can be used to decide whether comets are the
product of a condensation sequence similar to that invoked to explain the compositions of
the planets and asteroids, or if comets are simply agglomerations of interstellar grains which
' have been insignificantly modified by the events that shaped the other bodies in the solar
system. Although cometary nuclei are not generally accessible to observation, observations
of comet_) comae can illuminate at least some of the mysteries of the nuclei provided one
has a detailed knowledge of the excitation conditions in the coma and also has access to basic
atomic and molecular data on the many species present in comets. This paper examines the
status of our knowledge of the volatile composition of cometary nuclei and discusses how
these data are obtained.
1. INTRODUCTION
The volatile composition of a comet is a sensitive indicator of its formation environment.
In particular, the relative abundances of molecules in cometary nuclei can be used to infer
the physical and chemical state of either the solar nebula or the interstellar cloud from which
the nebula condensed. However, measuring the volatile inventory in comets is a demanding
task that involves combining in situ spacecraft data with observations from ground-based,
airborne, rocket, and Earth-orbiting observatories.
2. MOLECULES IN COMET NUCLEI
_.I THE DOMINANT VOLATILE: H20
During the past two decades cometary observers built a _txong circumstantial case for i
H20 ice a_ the dominant volatile in nuclei. However, the apparition of comet Halley provided
the first opportunity to observe the H20 molecule directll./. Infrared (IR) observations of
Halley in December 1985 from the Kuiper Airborne Observatory (KAO) [1] revealed intense,
solar-pumped fluorescent emission in the v3 band of H20 near )_ ,,_2.65#m. This same band
was subsequently observed in Halley post-perihelion by the IR spectrometer on Vega (the IKS
experiment) [2] and from the KAO [3,4]. Measurements by the neutral mass spectrometer
(NMS) on Giotto demonstrated that H20 comprised _>80% of the total volatile inventory of
comet Halley [5]. ,_
Water is not particularly well-suited for constraining the formation conditions of comets
because its sublimation temperature is relatively high (Tr_ b _150 K), and H20 is expected
" to be a relatively abundant molecule in the gas phase throughout much of the solar nebula.
Thus, measuring the relative abundances of the more volatile trace species in cometary
nuclei i._required for establishing the connection between comets and interstellar grains and _'
between comets and possible solar nebula condensates.
f
r* I
1990006555-003
2.2 MAJOR CARBON-BEARING SPECIES: CO, CH4, C02, H2CO t
_. Carbon monoxide is ubiquitous in the interstellar medium and is also predicted to be the
dominant carbon-bearing gas in non-equilibrium modelsof the solar nebula [6]. As illustrated
"| in Table 1, CO may also be the second-most abundant volatile in cometary nuclei. Prior to
.: Halley, CO had been observed definitely in only two comets: the CO fourth positive group
at )t ,-4500/_ was detected during sounding rocket observations of comet West (1976 VI) [7]
•_ and during IUE observations of comet Bradfield (1979 X) [8]. The derived CO abundance in
comet West was huge (,,_30% of theH20 abundance). Similar observations were conduct,..d
on comet Halley [9], also showing a high CO/H20 abundance (,,-17% in this case). On ,he
other hand, the UV observations of comet Bradfield yielded CO/H20-,,0.02 [10]. Does the
relative abundance of CO vary from comet to comet? The Giotto NMS measurements of CO
-: in the coma of comet Halley provided some interesting insight into this question [11]. The
NMS observations showed that only ~7% (or less) of the CO in the coma of Halley could
': be derived by sublimation from the nucleus. The rest of the CO in the coma is apparently
i produced from a distributed source, possibly due to the evaporation of organic mantles on
cometary grains (the so-called CHON particles). Viewed from this new perspective, both '
the high CO abundances derived from UV observations of comets West and Halley, and _'
: the low CO abundance derived from the UV observation of comet Bradfield, are consistent.
Whenever a large field-of-view (FOV) is used and the comet has a large dust production
rate (the UV rocket observations of both West and Halley satisfied these conditions), a high
abundance of CO will be observed in the coma due to extra CO production from grain
evaporation. Whenever the FOV is small and/or the dust production rate is low, then
the coma CO observation should be sampling something close to the "true" abundance in
the nucleus (we are making a distinction here between the CO stored in the ice versus the
presence of C-O bonds in complex hydrocarbons that may be coa*]ng cometary grains).
Equilibrium condensation models of the solar nebula indicate that CH4 should be the
dominant carbon-bearing gas in the outer regions of the nebula. Indeed, for many years
people often referred to comets as being composed of "water, methane, and ammonia" al-
though the latter two molecules had never been observed in comets and the indirect evidence
for their presence indicated that they were only trace constituents. Recent non-equilibrium i
chemical models of the solar nebula demonstrate that CH4 should be extremely depleted
relative to CO in the nebula, except near the sub-nebulae associated with the Giant planets
[6].
Spectroscopic observations of the v3 band of methane near A _3.3#m have yielded the
" first direct evidence for the presence of CH4 in comets. The R(1) line of this band was :
• marginally detected in comet Halley [12] while an average over six R-branch Linesin this
'_ band gave a tentative detec'don during KAO observations of comet Wilson (1986/) [13]. The
interpretation of these results in terms of the CH4 abundance is somewhat problematical.
The derived abundance can vary significantly depending on the choice of the "effective"
- ¢,
excitation temperature of the CH4 molecules in the coma, especially if only one line is used.
Some of this uncertainty is reflected in the range of values given in Table 1. We simply note - , :
that the above-referenced KAO observations of H20 indicate that the appropriate excitation I
temperature of CH4 is probably -,,50 K implying that the CH4 abundance is closer to 1%
than 5%.
Strong evidence for the presence of CH4 in the coma of comet Halley was also obtained
by analysis of data taken by the ion mass spectrometer (IMS) on Giotto [21]. Using the
CH+ ion as a probe of the CH4 abundance in the coma, and by comparing the observed I_
1990006555-004
ion spectrum to a model spectrum containing all known important ion-molecule reactions, i
4 the IMS data indicate that CH4/H20,,_0.02. The uncertainty in th_s number is di_cult
_ to establish owing to the complexity of the modeling, but the nominal value reported is
consistent with the results from the spectroscopic investigations.
Both the NMS experiment on Giotto and the IR spectrometer on Vega detected CO2 in
'_1 the coma of comet Halley and both measurements yi, _d a CO:_ relative abundance of ,-,,3%
•[ [5,15]. Assuming that the CO2 observe:l in these experiments originated in the nucleus, the
' COs abundance was approximately one-half of the CO abundance in Halley. UV observations
, of several comets indicate that the CO2/H20 abundar':e does not vary significantly from
comet-to-comet (Feldman, private communication). If this is true, then CO2 and CO may
_ have comparable abundances in some comets (e.g., comet Bradfield [1979X]).
There was a tentative detection of H2CO in Halley via IR fluorescence in a vibrational
band near A ,-,3.5/zm by the IKS experiment [15]. The derived abundance from the IKS is
given in Table 1 as an upper limit. If the detection stands, it indicates that H2CO is an
important trace constituent in comets and may contribute significantly to the "extended"
source of CO in the coma.
_.8 M,,!.70R NITROGEN-BEARING SPECIES: NHs, N2, HCN
There has only been one direct observation of NH s in a comet: a rad;.o llne was marginally
detected in comet IRAS-Araki-Alcock (1983d) [16]. The derivation of an NH 3 abundance
from these data is extremely model-dependent; the radio observers reported NHs/H20,,,0.06
while I have derived a ratio of --,0.003 from the same data.
Since >90% of the photodissociation of NHs by solar UV radiation results in the produc-
tion of NH2, visible observations of the NH2 molecule should provide reasonably accurate
measurements of the NH3 abundance. Observation., of the NH 2 band at A ,_5700 A in comet
Halley gave NHa/H20=0.003 [17].
Both NH s and CH4 are required to explain the Giotto IMS data [14] (see the earlier
discussion on CH4). An NHs relative abundance of ,,,1-2% gives the best fit of the model
to the data. However, the accuracy of this result has not yet been determined due to the
complexity of the modeling, and the uniqueness of the interpretation has also been challenged
[IS].
The abundance of N2 in comets is very difficult to establish with confidence. There are
no known spectroscopic signatures of the neutral molecule and since its mass is coincident
with that of CO, even in $i_u observations with mass spectrometers suffer from a confusion
problem. Using the small difference in ionization potential between N 2 and CO, the Giotto
NMS data were used to obtain N2/H20<0.1 [11].
The N+ ion is obsereed in the visible spectra of comets near A ,-_3914 A. Since the
ionization properties of N 2 and CO are similar, and since the CO/H_O abundance can be t
determined by other means, the ratio of the intensities of the visible bands of N2 and CO +
can be used to infer the ratio of N2/H20. Using this technique for the Halley observations
yields N2/H20<_0.001 (Feldman and Wyckoff, private communication).
" Fluorescent emission from CN is usually the strongest feature in the visual spectra of
comets. Most of the CN in the coma is probably produced by the photodissociation of
• HCN that has sublimated from the nucleus, although some may have been produced as a
result of evaporation of organic-coated grains [19] or from photodissociation of more complex
molecules [20]. The HCN molecule is detected via a rotational transition at radio frequencies,
and the apparition of Halley witnessed the first systematic investigation of HCN in comets _
[21]. The derived abundance from these observations is HCN/H_O,-,0.001. _-
4 ,_ ,
1990006555-005
2._ MAJOR SULFUR-BEARING SPECIE, S: CS2, $2
Fluorescent emissions from CS and S are a prominent featu:es in UV spectra of comets
[22]. The most plausible parent for CS is the CS2 molecule [23], although CS2 has never
been observed directly. Assuming that all of the CS observed in the coma comes from ca2,
then CS2/H20,-&.001, i.e., CS2 is a trace constituent of the nucleus.
UV observations of the "Earth-grazing" comet IRAS-Araki-Alcock (1983d) dramatically
revealed the presence of S2, a molecule which had never been observed previously in any
astrophysical environment [24]. Since the S2 discovery coincided with an "outburst" in
cometary activity, and since the signal virtually disappeared during late_ observations, the
presence of S2 in IRAS-Araki-Alcock may not be typical. The abundance derived from the
UV observations during the outburst is S2/H20-.,0.001.
The presence of the OCS molecule has been invoked to explain the spatial distribution
of S atoms in the coma [25]. Although this hypothesis is speculative since OCS has never
been detected in comets, it is also clear that if OCS is present it cannot be more than a trace
constituent of the nucleus.
3. CONCLUSION
Table 1 summarizes the abundance data on cometary volatiles discussed in this paper.
All values are expressed as volume abundances relative to H20. Although the ability to
compile a table like this with reasonable confidence represents a significant improvement in
our knowledge of the nature of cometary matter, several cautionary remarks must be made.
First, *..heabundances of trace species have been measured in very few comets. Table 1 is
heavily weighted toward results from Halley since the best data generally come from this
comet. Systematic observations of trace species in both "new" and "old" comets are desper-
ately needed to establish whether the results from Halley, and the handful of other comets
mentioned here, are "typical" of comets as a class. Second, even when identifications of
species are secure, the conversion from raw data into nucleus abundances frequently intro-
duces significant uncertainties (factors of two or more are common). Finally, comets often
exhibit dramatic temporal variability which may suggest that the observed composition is
not constant in time (e.g., the case of $2). Despite these present shortcomings, we are well on
our way toward a more quantitative understanding of the link between comet compositions
and cometary formation environments.
4. ACKNOWLEDGEMENTS
I would like to express thanks to Bruce Fegley, Paul Feldman, Jonathar. Lunine, Hal
Larson, and Mike Mumma for many stimulating discussions on the topic of this paper.
Q
1990006555-006
!TABLE 1. VOLATILE ABUNDANCES ,1
IN COMET NUCLEI :'
Molecule Abundance
H20 1
CO 0.02- 0.07
' CH4 0.01- 0.05 .
CO2 ,--,0.03
H2CO <O.O5
NH3 0.003 - 0.02
N 2 <0.001
CS 2 <0.001 "
S2 <_0.001
HCN ,,.,0.001
i
5. REFERENCES
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The volatile composition of comets

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