\
PERGAMON
Planetary and Space Science 36 "0888# 676Ð684
All comets are born equal] infrared emission by dust as a key to
comet nucleus composition0
J[ Mayo Greenberga\\ Aigen Lib
a
b
Laboratory Astrophysics\ University of Leiden\ Postbus 8493\ 1299 RA Leiden\ The Netherlands
Beijin` Astronomical Observatory\ Chinese Academy of Science\ Beijin` 099901\ People|s Republic of China
Received 14 March 0887^ received in revised form 03 August 0887^ accepted 5 September 0887
Abstract
The infrared emission of various comets can be matched within the framework that all comets are made of aggregated interstellar
dust[ This is demonstrated by comparing results on Halley "a periodic comet#\ Borrelly "a Jupiter family short period comet#\ HaleÐ
Bopp "a long period comet#\ and extra!solar comets in the b Pictoris disk[ Attempts have been made to generalize the chemical
composition of comet nuclei based on the observation of cometary dust and volatiles and the interstellar dust model[ Finally\ we
deduce some of the expected dust and surface properties of comet Wirtanen from the interstellar dust model as applied to other
comets[ Þ 0888 Elsevier Science Ltd[ All rights reserved[
0[ Introduction
Classically\ the composition of comet nuclei was
derived primarily from the coma volatile molecules domi!
nated by water "or OH#[ The dust was considered mostly
in terms of its scattering properties from which empirical
approximations were used to deduce a dust!to!gas ratio[
The discovery of the silicate emission feature "Maas et
al[\ 0869# con_rmed the existence of refractory material
in comets along with volatiles "ices#[ The idea of organic
refractories as a major comet nucleus constituent was _rst
quantitatively introduced in the interstellar dust model of
comets "Greenberg\ 0871#[ But it was the mass spec!
troscopic evidence of the Giotto:Vega space probes which
provided the _rst proof that the refractory material in
comet dust consisted of both the organic elements "O\ C\
N# as well as the rocky elements "Mg\ Si\ Fe# "Kissel et
al[\ 0875a\b^ Jessberger and Kissel\ 0880#[ While the visual
and ultraviolet emission of coma molecules excited\ pho!
tolyzed and ionized by the solar radiation is used to
deduce the volatile composition of the nucleus\ it is the
infrared radiation by the dust which is the remote obser!
vational data used to deduce the refractory components[
A major advance in our understanding of comet dust
Corresponding author[ E!mail] greenberÝstrw[leidenuniv[nl
0
Presented in the Workshop on the Rosetta Targets*35P:Wirtanen]
Observations\ Modeling and Future Work\ 09Ð00 December 0886\
Napoli\ Italy[
has resulted from the infrared emission observation both
of the 8[6 mm spectral feature as well as the continuum[
While there are di}erences from comet to comet in the
details of the emission\ a uniform approach in terms of
aggregates of submicron size interstellar dust "Greenberg
and Hage\ 0889# which is widely believed to be silicate
core!organic refractory mantle particles "Li and Green!
berg\ 0886# provides a coherent theoretical structure[ It
should be noted here that the in situ mass spectra con!
_rmed the core!mantel structure "Krueger and Kissel\
0876^ Lawler and Brownlee\ 0881# so that the pre!
sumption of interstellar dust grains as the basic units of
comet dust aggregates is most reasonable[
In this paper we show how the infrared emission for
several distinctly di}erent types of comets bear a general
resemblance to each other[ While the dust distributions
contain sizes as high as milligrams or higher\ in no case
are there compact particles with mass higher than
½09−03 g] the mean mass of an interstellar core!mantle
particle[ In fact it is di.cult\ if not impossible\ to explain
how large compact particles*silicates or otherwise*can
have emerged from the comet nucleus in view of the fact
that the interstellar volatile mantles are so well repre!
sented in the coma[ One must infer that the preservation
of the volatiles automatically guarantees preservation of
what the volatiles originally covered[ Furthermore\ the
temperature of the core!mantel particles during aggre!
gation never exceeded the evaporation temperature of
H1O as attested to by the low temperature of formation
9921!9522:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[
PII] S 9 9 2 1 ! 9 5 2 2 " 8 7 # 9 9 0 9 1 ! 9
677
J[M[ Greenber`\ A[ Li : Planetary and Space Science 36 "0888# 676Ð684
of cometary H1O inferred from the ortho!para ratio "see
e[g[\ Mumma et al[\ 0882^ Crovisier et al[\ 0886#[
1[ Comet Halley] a periodic comet
The uniqueness of comet Halley with regard to the
dust was that for the _rst time three properties were
simultaneously observed] chemical composition\ size
"mass# distribution\ infrared emission[ It was shown
"Greenberg and Hage\ 0889# that\ in order to satisfy
simultaneously such independent properties of Halley
coma dust as] "0# 8[6 mm emission "amount and shape#\
"1# dust mass distribution\ and "2# mass spectroscopic
composition\ one must represent the dust as very ~u}y
aggregates of submicron interstellar dust silicate core!
organic refractory mantle particles[
The individual particle size and the organic refractory
absorptivity in the visual provide the high temperatures
required for the emission by the silicates which\ alone
"bare#\ are too cold to emit e.ciently[ An additional
criterion required for the organic mantles is to account
for the distributed CO "Eberhardt et al[\ 0876# by evap!
orating its C1O bearing molecules "Greenberg and Li\
0887a\b#[ The required degree of porosity "~u.ness of
the aggregates# is de_ned by the fact that for a given mass
the more porous the aggregate the more it acts like a sum
of small particles rather than a compact particle of that
mass\ so that a larger fraction of the observed mass dis!
tribution provides silicate emission as if by submicron
particles[ For example\ the temperature of a 09−00 g com!
pact grain made of silicate core!organic refractory mantle
materials "with a mass ratio of the mantle to the core 0:1#
at 0 AU is ¼305 K\ while at a porosity of P 9[864 it is
¼695 K[ The temperature for a pure silicate grain is
¼239 K "compact#\ ¼340 K "P 9[864#\ respectively[
The major thrust of this is that comet dust consists
of intimately related silicate and carbonaceous materials
"core!mantel structure# rather than separate silicate and
carbon components[ One of the observational supports
of the model is that the in situ mass spectra of Halley
dust with high dynamic range show that\ except for the
very small "attogram# grains "Utterback and Kissel\
0889#\ neither pure organic "so!called {CHON|# nor pure
silicate particles exist^ instead\ they are intimately mixed
on a very _ne scale in such a manner that they form the
subunits with a core!mantle structure in the aggregates
"Lawler and Brownlee\ 0881# as additionally re~ected by
the fact that the CHON ions have on the average a higher
initial energy than the silicate ions in measuring the mass
spectra "Krueger and Kissel\ 0876#[
In summary\ the result of the intertwining of the three
basic observations is] "0# comet dust consists of aggre!
gates of ½9[0 mm silicate core!organic refractory mantel
particles^ "1# the average porosity of the comet dust is
9[82 ³ P ³ 9[864[ The inferred comet dust density is
9[97 ¾ rCD ¾ 9[05 g cm−2^ i[e[\ rCD ¼ 9[0 g cm−2 is a
reasonable canonical value[ Note that rCD "rsol!
id×"0−P#\ where rsolid\ is the mass density of compact
particles# is only determined by the porosity of the aggre!
gate\ not size "mass# dependent "see Greenberg and Hage\
0889#[ By considering the dust as comet nucleus material
out of which all the volatiles\ the very small "interstellar
dust# particles\ and about 0:1 of the original "relatively
volatile# organic refractories were removed\ the recon!
stituted comet nucleus density was inferred to be
9[15 ¾ rC ¾ 9[40 g cm−2[ Later works "Greenberg and
Li\ 0887a^ Greenberg\ 0887# modify these results slightly
but the bottom line is that comet Halley dust has a density
rCD ¼ 9[0 g cm−2 and its nucleus has a density rC ¼ 9[2
g cm−2[ This is consistent with the low density suggestion
proposed by Rickman "0875# based on the analysis of
non!gravitational forces[
2[ Comet P:Borrelly "0883l#] a Jupiter family short!
period comet
The ~u}y aggregate comet dust model has also been
shown to be applicable to short!period comets "see Li
and Greenberg\ 0887a#[ As an example\ we have cal!
culated the dust thermal emission spectrum of comet
P:Borrelly "0883#\ a Jupiter family short!period comet
"with an orbital period P ¹ 6 years#\ from 2Ð03 mm as
well as the 09 mm silicate feature in terms of the comet
modeled as a porous aggregate of interstellar dust "Li
and Greenberg\ 0887a#[ The ~u}y aggregate model of
silicate core!amorphous carbon mantle grains with a
porosity P 9[74 can match the observational data
obtained by Hanner et al[ "0885# quite well "see Fig[ 0#[
It seems that\ compared to the Halley dust\ the dust
grains of P:Borrelly appear to be relatively more pro!
cessed "more carbonized#\ less ~u}y\ and richer in smaller
particles[
Since P:Borrelly has passed through the inner solar
system many more times than P:Halley and therefore
been subjected much more to the solar irradiation\ the
dust grains within the surface layer of the nucleus could
have been signi_cantly modi_ed[ In particular\ the
organic refractory materials could have undergone fur!
ther carbonization^ namely\ the organic materials\ would
partially lose their H\ O\ N atoms and thus become car!
bon!rich "Jenniskens et al[\ 0882#[ This is supported by
the results of the EURECA space experiments which
have indicated the carbonization of the {_rst generation|
organic refractory materials by solar irradiation "Green!
berg et al[\ 0884#[ Observations do show that some Jup!
iter!family short!period comets are depleted in C1 and C2
but approximately constant in CN "A|Hearn et al[\ 0884#[
This is consistent with the idea of carbonization since CN
is mostly produced from grains while some C1 and C2
come directly from the volatile nuclear ices which are
J[M[ Greenber`\ A[ Li : Planetary and Space Science 36 "0888# 676Ð684
678
Fig[ 0[ The theoretical spectra calculated for the porous aggregate comet dust model of silicate core!amorphous carbon mantle grains with a porosity
P 9[74[ The dotted line is a black!body "T 164 K# emission "Hanner et al[\ 0885#[ The ordinate unit is 09−03 W m−1 mm−0[
relatively depleted in SP comets "A|Hearn et al[\ 0884#[
The solar irradiation can also lead to a lower porosity
than that of Halley dust due to the packing e}ect "Mukai
and Fechtig\ 0872^ also see Smoluchowski et al[\ 0873#[
The dust size distribution could be weighted toward
smaller grains^ i[e[\ smaller grains are enhanced as a
consequence of evaporation and subsequent frag!
mentation in the coma[ There are both observational and
theoretical indications of dust fragmentation in the coma
of comet P:Halley "see Greenberg and Li\ 0887b and
references therein#[ As the volatile ice sublimates from
the nucleus\ it leaves behind the refractory particles and
loosens the aggregates[ If the fragmentation indeed
results from the sublimation of volatile materials which
act as {glue|\ one may expect relatively more drastic and
more complete fragmentation in the coma of SP comets
since volatiles are relatively depleted in SP comets "Weiss!
man and Campins\ 0882#[ We should note that signi_cant
uncertainties in determining the Halley dust size dis!
tribution still remain\ nevertheless\ a statistical analysis
of _fteen comets indeed seems to suggest that the dust
size distribution is somewhat steeper for short!period
comets than for long!period comets "Fulle\ 0887#[
Since up to now only two SP comets were known to
have silicate emission "P:Borrelly and P:Fay^ see Hanner
et al[\ 0885#\ at this point we are not able to generalize
the dust properties of short!period comets[ Systematic
observations of the thermal emission spectra and the
silicate features for a large set of samples are needed[
3[ Comet Hale!Bopp "C:0884 O0#] a very large long!
period comet
Comet HaleÐBopp "C:0884 O0# is an exceptionally
bright long!period comet "P ¼ 1999 years#[ It was so
689
J[M[ Greenber`\ A[ Li : Planetary and Space Science 36 "0888# 676Ð684
active and so bright that it became visible even at a
heliocentric distance of ½6 AU[ Its strong activity and
strong thermal emission features provide a rare oppor!
tunity to study the origin of comets and to constrain the
comet dust morphology\ composition and size[ For this
purpose\ in particular\ to better understand the nature of
HaleÐBopp dust\ we should perform detailed modelling
on the heliocentric evolution of dust properties based on
the evolution of the silicate feature and of the thermal
continuum emission as well as the dust properties during
sporadic events such as outbursts and jets[
As a starting point\ we have only modeled the 2Ð19 mm
emission spectrum of HaleÐBopp of February 19\ 0886\
obtained by Williams et al[ "0886#\ when it was at a
heliocentric distance rh 0[04 AU[ Its spectrum "of Feb!
ruary 19\ 0886# was characterized by the strongest 09
mm silicate feature ever observed for a comet[ Its strong
silicate emission feature has been generally interpreted as
indicating the presence of an unusually high abundance
of small "³0 mm\ or equivalently ³0[4×09−00 g for
solid silicate and ³9[5×09−01 g for solid carbon# grains
"Hanner\ 0886#[ Equally special was the highest ratio
of the color temperature to the blackbody temperature
"de_ned as superheat by Gehrz and Ney "0881## derived
by Williams et al[ "0886#[ This has led to the suggestion
that Hale!Bopp contains the smallest grains yet observed
for any comet[ Williams et al[ "0886# estimated the mean
HaleÐBopp dust size to be ¼9[3 mm "corresponding to
5[1×09−02 g for amorphous carbon\ 8[3×09−02 g for
silicate\ respectively# in terms of solid separated sili!
cate:carbon grains[ This result did not take into account
that the superheated thermal continuum and the strength
of the silicate feature are not solely determined by grain
sizes[ The dust morphology\ the optical properties of the
dust components and the way in which the di}erent dust
components are mixed "Hanner et al[\ 0885# are equally
important[
We have calculated the dust thermal emission spectrum
based on the model of comet dust consisting of porous
aggregates of interstellar dust[ As shown in Fig[ 1\ both
the continuum emission and the 09 mm silicate feature
are well matched "see Li and Greenberg\ 0887b\ for
details#[ The mean grain masses "derived by averaging
over the size distributions# are ½09−09Ð09−8 g\ sig!
ni_cantly di}erent from the suggestion that HaleÐBopp
is rich in submicron grains "Hanner\ 0886^ Williams et
al[\ 0886^ see above#[ The presence of large numbers of
very large particles in HaleÐBopp was con_rmed by the
submillimeter continuum emission observation "Jewitt
and Matthews\ 0887#[ It was argued that these large par!
ticles may dominate the total dust mass of the coma
"Jewitt and Matthews\ 0887#[ Assuming a spherically
symmetric dust coma with uniform radial out~ow\ adopt!
ing the water production rate on February 12[8\ 0886
"Russo et al[\ 0886# 3[2×0929 mols:s\ and adopting an
average dust out~ow velocity of 9[01 km s−0 "calculated
from vd ¼ 9[94 "rh:5[71#−9[4 km:s where rh the heliocentric
distance in AU "Sekanina\ 0885#\ the dust!to!water pro!
duction rate ratio was estimated to be as high as 30 or
even higher "see Li and Greenberg\ 0887b#[ If a higher
dust out~ow velocity\ 9[59 km s−0\ which may be more
realistic\ is adopted\ the dust!to!water ratio would be
about 199; However\ one should keep in mind that\ the
IR emission alone can not give a reliable dust production
rate since very large particles are too cold to contribute
to the limited wavelength range of the infrared radiation
considered here "as long as the size distribution for those
cold particles is not too ~at# and therefore the total mass
of the large particles is not well constrained\ as already
noted by Crifo "0876#[
The presence of a crystalline silicate component in
comet HaleÐBopp was explicitly demonstrated by both
space "Crovisier et al[\ 0886# and ground based "Hayward
and Hanner\ 0886# observations[ The fact that the crys!
talline silicate emission features are quite strong even
when HaleÐBopp is at such large heliocentric distances
as rh 1[8 AU "Crovisier et al[\ 0886#\ rh 3[1 AU "Hay!
ward and Hanner\ 0886#\ is of particular interest because
it may invoke serious questions on the role played by
solar insolation[ At this point\ we are not going to address
where and how cometary silicates are crystallized[
However\ it seems likely to us that the crystallization did
not occur before comet nucleus formation since\ on the
one hand\ no evidence exists for the presence of crystalline
silicates in the interstellar medium^ and on the other hand\
circumstellar dust is unlikely to be directly incorporated
into comets without _rst passing through the interstellar
medium[ We have also carried out calculations in which
crystalline silicates are included in the model[ In our
calculation\ we adopted the e}ective medium theory and
the Mie theory "Bohren and Hu}man\ 0872# to obtain the
grain temperatures and therefore the emerging emission
spectrum[ We did not apply the approach
s fi×ki"l#×B"l\ Ti#
i
"Ti is the temperature for the i!th dust component^ ki"l#\
B"l\ Ti#\ fi are the mass absorption coe.cient\ the Planck
function\ the mass fraction of the i!th dust component\
respectively# by adjusting Ti and fi as commonly used in
the literature "see e[g[\ Colangeli et al[\ 0884#[ One should
be cautious when adjusting the dust temperature Ti that\
in so doing\ the adopted temperatures remain within a
physically acceptable range\ i[e[\ not lower than the cor!
responding black!body temperature and not unphysically
too much higher[ Our calculations imply that the partially
crystallized silicate model with the same dust parameters
derived for the amorphous silicate model gives rise to
prominent crystalline silicate features while the _t to the
overall thermal emission spectrum is still maintained "see
Li and Greenberg\ 0887b#[
J[M[ Greenber`\ A[ Li : Planetary and Space Science 36 "0888# 676Ð684
680
Fig[ 1[ Fits of the thermal emission spectrum of comet HaleÐBopp at rh 0[04AU "Williams et al[\ 0886# with a variety of dust size distributions] the
single!component power law distribution model "solid#\ the two!component power law model "dotted#\ and the Hanner size distribution model
"dashed^ see Hanner\ 0874#[ For details we refer to Li and Greenberg "0887b#[
4[ The b Pictoris disk] comets in an external {solar
system|
One of the big surprises from the Infrared Astro!
nomical Satellite "IRAS# was the so!called {Vega
Phenomenon|^ i[e[\ that some main sequence stars exhibit
large infrared "IR# excesses over the black body emission
of their photospheres "Aumann et al[\ 0873#[ It is gen!
erally believed that these IR excesses are attributed to the
dust thermal emission of their circumstellar dust grains[
Now it has become well established that the {Vega
Phenomenon| is not exceptional but rather quite common
among the main sequence stars "see Backman and Pare!
sce\ 0882#[ Beta Pictoris is an A4 star with the largest
infrared excess among the {Vega!type stars| and an edge!
on circumstellar dusk disk "Smith and Terrile\ 0873^ Lag!
age and Pantin\ 0883#[
Various observational and theoretical evidence indi!
cate that comets may exist in the disk "see Li and Green!
berg\ 0887c and references therein#[ First of all\ dynamical
studies indicate that the grain destruction time scales due
to grain!grain collisions\ PoyntingÐRobertson drag and
radiation pressure are shorter than the lifetime of b Pic!
toris "Backman and Paresce\ 0882^ Artymowicz\ 0883#[
This indicates that there must be some dust source which
continually replenishes the lost particles[ Such a source
was _rst suggested as due to comet!like bodies by Weiss!
man "0873#[ Moreover\ its silicate emission feature\ show!
ing a crystalline silicate feature at 00[1 mm superimposed
on the broad amorphous silicate feature at 09 mm
"Knacke et al[\ 0882#\ resembles that of some comets
including comet Halley and the most recent comet HaleÐ
Bopp "Campins and Ryan\ 0878^ Hanner et al[\ 0883^
Hayward and Hanner\ 0886^ Crovisier et al[\ 0886#[ This
indicates the similarity of cometary dust with the b Pic!
toris dust[ Furthermore\ the systematic spectroscopic
observations of gaseous elements in the visible or in the
ultraviolet carried out by a French group showed that
the spectral lines CaII\ MgII\ FeII\ AlIII\ AlII\ CIV\ CI
and CO show strong time variations and are almost
always redshifted relative to the stellar spectra "Vidal!
Madjar et al[\ 0883 and references therein#[ These vari!
ations can be explained to result from the evaporation of
the dust grains shed from comet!like bodies falling on the
star as a consequence of planet perturbation "Beust et al[\
0889#[
We have developed a model for the b Pictoris dust disk
which shows that the ~u}y aggregate comet dust model
is also applicable to extrasolar comets "see Li and Green!
berg\ 0887c#[ The basic idea is that the dust in the disk
plane is continually replenished by comets orbiting the
star where the dust may be quickly swept out by radiation
pressure or spiral onto the star as a result of PoyntingÐ
Robertson drag[ The initial dust shed by the comets is
taken to be the ~u}y aggregates of interstellar silicate
core!organic refractory mantle dust grains "with an
additional ice mantel in the outer region of the disk#[ The
heating of the dust is primarily provided by the organic
refractory mantel absorption of the stellar radiation[ The
temperature of some of the particles close to the star is
su.cient to crystallize the initially amorphous silicates[
The dust grains are then distributed throughout the disk
by radiation pressure[ The steady state dust distribution
of the disk then consists of a mixture of crystalline silicate
681
J[M[ Greenber`\ A[ Li : Planetary and Space Science 36 "0888# 676Ð684
aggregates and aggregates of amorphous silicate core!
organic refractory mantel particles "without:with ice
mantles# with variable ratios of organic refractory to
silicate mass[ The whole disk which extends inward to
½0 AU and outward to ½1199 AU is divided into three
components which are primarily responsible respectively\
for the silicate emission\ the mid!infrared emission and
the far infrared:millimeter emission[ As a starting point\
the grain size distribution is assumed to be like that
observed for comet Halley dust while in the inner regions
the distribution of small particles is relatively enhanced
which may be attributed to the evaporation and:or frag!
mentation of large ~u}y particles[ The dust grains which
best reproduce the observations are highly porous\ with a
porosity around 9[84 or as high as 9[864[ The temperature
distribution of a radial distribution of such particles pro!
vides an excellent match to the 09 mm amorphous and
the 00[1 mm crystalline silicate spectral emission as well
as the excess continuum ~ux from the disk over a wide
range of wavelengths "see Figs 2\ 3\ and 4#[ These models
result in a total mass of dust in the whole disk ½1×0916
g of which only 09−4Ð09−3 is required to be heated enough
to give the silicate excess emission[
Figure 4 implies that\ to obtain a prominent crystalline
silicate feature at 00[1 mm\ a silicate mass ratio of the
crystalline silicate model to the amorphous silicate model
f Mcrystalline:Mamorphous 9[39 is needed "actually\ as
shown in _g[ 5A of Li and Greenberg\ "0887c#\ even
f 19) is not quite su.cient#[ This indicates that ½39)
of the silicates in the disk have been converted into the
Fig[ 2[ The spectral energy distribution predicted from the amorphous silicate model with a porosity of P 9[864 together with various observational
data as summarized in Li and Greenberg\ "0887c#[ For illustrative purposes\ the near!infrared "NIR# and the 09 mm silicate emission spectra are
presented in "b# with the mid!infrared "MIR# spectra in "c#\ in addition to the overall spectral energy distribution from the NIR to the millimeter
plotted in "a#[
J[M[ Greenber`\ A[ Li : Planetary and Space Science 36 "0888# 676Ð684
682
Fig[ 3[ Same as Fig[ 2 except for the crystalline silicate model[
crystalline phase[ It is obvious that very large particles
are cold and thus are not crystallized as e.ciently as
small particles[ It is likely that the crystalline silicate
aggregates distributed in the disk constitute only those in
the low mass part of the size distribution[ Crystalline
silicates are needed only to produce the 00[1 mm feature
and possibly the mid!infrared "MIR# bands with the far
infrared "FIR#:millimeter emission dominated by the
amorphous silicate aggregates[ This will not a}ect the
model _t of the MIR and FIR spectra since both the
crystalline silicate model and the amorphous silicate
model show a very similar behavior in the MIR and FIR
"see Figs 2 and 3#[ In other words\ the above derived mass
fraction need only be valid in the inner components of
the disk\ so that only about 0) of the total silicates are
required to be converted into the crystalline phase[
5[ Generalized comet nucleus
We present here the suggested canonical composition
of comet nuclei based on observation of the dust and
volatiles of a variety of comets[ According to Greenberg
"0887#\ the chemical composition of a comet nucleus can
be very strictly constrained by combining the latest results
on] the core!mantel interstellar dust model\ the solar sys!
tem abundances of the elements\ the space observed com!
position of the dust of comet Halley\ and the latest data
on the volatile molecules of comet comae[ The dis!
tribution of the components in the comet nucleus fall
naturally into two basic categories*refractories and vol!
atiles[ The refractory components are tightly constrained
to consist of about 15) of the mass of a comet as silicates
"a generic term for combinations of the elements Si\ Mg\
683
J[M[ Greenber`\ A[ Li : Planetary and Space Science 36 "0888# 676Ð684
Fig[ 4[ The spectral energy distribution derived from a mixture of the amorphous silicate model "dotted line^ same as Fig[ 2# and the crystalline
silicate model "dot!dashed line^ same as Fig[ 3# with an assumption of 39) crystalline silicates[ "a# The overall spectrum from the NIR to the
millimeter^ "b# the 09 mm silicate feature^ "c# the MIR emission bands[
Fe#\ 12) complex organic refractory material "domi!
nated by carbon#\ and about 8) in the form of extremely
small "attogram# carbonaceous:large molecule "PAH#
particles[ The remaining atoms are in an H1O dominated
mixture containing of the order of 1Ð2) each of CO\
CO1\ CH2OH plus other simple molecules[ The H1O
abundance itself if very strictly limited to ½29) of the
total mass of a comet*not much more nor much less[
The refractory to volatile "dust to gas# ratio is about 0 ] 0\
while the dust to H1O ratio is ¼1 ] 0[ The maximum mean
density of a fully packed nucleus would be ¼0[54 g cm−2[
The morphological structure of the component materials\
following the interstellar dust into the _nal stage of the
presolar cloud contraction\ is as tenth micron silicate
cores with organic refractory inner mantles and outer
mantels of {ices| with each grain containing many thou!
sands of the attogram carbonaceous:large molecule par!
ticles embedded in the icy and outer organic fraction[
6[ Concluding remarks*application to comet
Wirtanen
It has been shown that a unifying theory of comet
dust properties is based on their consisting of porous
aggregates of silicate core!organic refractory mantel
interstellar dust particles[ Some of the consequences of
this are that] "0# ~u}y dust\ which can break up into
submicron particles\ is expected to be coming o} and be
part of the environment of comet Wirtanen^ "1# mass
spectra of both comet nucleus and comet dust material
will on the average exhibit\ simultaneously\ charac!
J[M[ Greenber`\ A[ Li : Planetary and Space Science 36 "0888# 676Ð684
teristics of both organics and silicates^ "2# the mor!
phological structure of the comet nucleus will exhibit
optical features down to the submicron level^ "3# the
nucleus surface is probably a ~u}y but chemically bonded
structure whose material and chemical properties are
derived from sintering of comet dust fragments and of
residual comet surface material[
Acknowledgements
We are grateful for the support by NASA grant NGR
22!907!037 and by a grant from the Netherlands Organ!
ization for Space Research "SRON#[ We thank Dr R[D[
Gehrz for kindly providing us with the IR thermal emis!
sion data of comet HaleÐBopp^ Dr S[B[ Fajardo!Acosta
and Dr R[F[ Knacke for the b Pictoris data^ Dr M[S[
Hanner for the comet P:Borrelly data^ and Dr C[ Koike
for the optical constants of crystalline silicates[ We are
also indebted to Dr M[ Fulle and Dr M[ Mueller for their
useful suggestions[ One of us "AL# wishes to thank Prof[
G[V[ Coyne\ S[J[\ for his kind support in participating in
the Vatican Observatory summer school[ AL also would
like to thank Leiden University for an AIO fellowship
and the World Laboratory for a Scholarship and the
National Science Foundation of China for _nancial sup!
port[
References
A|Hearn\ M[F[\ Millis\ R[L[\ Schleicher\ D[G[\ et al[\ 0884[ Icarus 007\
112[
Artymowicz\ P[\ 0883[ In] Ferlet\ R[\ Vidal!Madjar\ A[ "Eds#[ Cir!
cumstellar Dust Disks and Planet Formation\ Editions Frontieres]
Gif sur Yvette\ p[ 36[
Aumann\ H[H[\ et al[\ 0873[ Astrophys[ J[ 167\ L12[
Backman\ D[E[\ Paresce\ F[\ 0882[ In] Levy\ E[H[\ Lunine\ J[E[\
Mathews M[S[ "Eds#[ Protostars and Planets III\ University of Ari!
zona Press\ Tucson[ p[ 197
Beust\ H[\ Lagrange!henri\ A[M[\ Vidal!Madjar\ A[\ Ferlet\ R[\ 0889[
Astron[ Astrophys[ 125\ 191[
Bohren\ C[F[\ Hu}man\ D[R[\ 0872[ Absorption and Scattering of
Light by Small Particles[ Wiley\ New York[
Campins\ H[\ Ryan\ E[\ 0878[ Astrophys[ J[ 230\ 0948[
Colangeli\ L[\ Mennella\ V[\ Di Marino\ C[\ Rotundi\ A[\ Bussoletti\
E[\ 0884[ Astron[ Astrophys[ 182\ 816[
Crifo\ J[F[\ 0876[ In] Ceplecha\ Z[\ Pecina\ P[ "Eds#[ Interplanetary
Matter\ Czech[ Academy of Sciences Report G6\ 48[
Crovisier\ J[\ Leech\ K[\ Bockelee!Morvan\ D[\ et al[\ 0886[ Science 164\
0893[
684
Eberhardt\ P[\ Krankowsky\ D[\ Schulte\ W[\ et al[\ 0876[ Astron[ Astro!
phys[ 076\ 370[
Fulle\ M[\ Planet[ Space Sci[\ submitted\ 0887[
Gehrz\ R[D[\ Ney\ E[P[\ 0881[ Icarus 099\ 051[
Greenberg\ J[M[\ 0871[ In] Wilkening\ L[L[ "Ed[#[ Comets\ University
of Arizona Press\ Tucson[ p 020[
Greenberg\ J[M[\ Hage\ J[I[\ 0889[ Astrophys[ J[ 250\ 159[
Greenberg\ J[M[\ Li\ A[\ Mendoza!Gomez\ C[X[\ Schutte\ W[A[\ Gerak!
ines\ P[A[\ de Groot\ M[\ 0884[ Astrophys[ J[ 344\ L066[
Greenberg\ J[M[\ Li\ A[\ 0887a[ In] Schmitt\ B[\ de Bergh\ C[\ Festou\
M[ "Eds#[ Solar System Ices\ Kluwer\ Chapter 02\ p[ 226[
Greenberg\ J[M[\ Li\ A\ 0887b[ Astron[ Astrophys[ 221\ 263[
Greenberg\ J[M[\ 0887[ Astron[ Astrophys[ 229\ 264[
Hanner\ M[S[\ 0874[ Adv[ Space Res[ 3 "8#\ 078[
Hanner\ M[S[\ Lynch\ D[K[\ Russell\ R[W[\ 0883[ Astrophys[ J[ 314\
163[
Hanner\ M[S[\ Lynch\ D[K[\ Russell\ R[W[\ Hackwell\ J[A[\ Kellogg\
R[\ 0885[ Icarus 013\ 233[
Hanner\ M[S[\ 0886[ Bull[ Am[ Astron[ Soc[ 18\ 0931[
Hayward\ T[L[\ Hanner\ M[S[\ 0886[ Science 164\ 0896[
Jenniskens\ P[\ Baratta\ G[A[\ Kouchi\ A[\ de Groot\ M[S[\ Greenberg\
J[M[\ Strazzulla\ G[\ Astron[ Astrophys[ 0882[ 162\ 472[
Jessberger\ E[K[\ Kissel\ J[\ 0880[ In] Newburn\ R[L[\ Neugebauer\ M[\
Rahe\ J[ "Eds#[ Comets in the Post!Halley Era\ Kluwer\ Dordrecht\
p[ 0964[
Jewitt\ D[C[\ Matthews\ H[E[\ 0887[ Science\ submitted[
Kissel\ J[\ Sagdeev\ R[Z[\ Bertaux\ J[L[\ et al[\ 0875a\ Nature 210\ 179[
Kissel\ J[\ Brownlee\ D[E[\ Buchler\ K[\ et al[\ 0875b[ Nature 210\ 225[
Knacke\ R[F[\ Fajardo!Acosta\ S[B[\ Telesco\ C[M[\ et al[\ 0882[ Astro!
phys[ J[ 307\ 339[
Krueger\ F[R[\ Kissel\ J[\ 0876[ Naturwissenschaften 63\ 201[
Lagage\ P[O[\ Pantin\ E[\ 0883[ Nature 258\ 517[
Lawler\ M[E[\ Brownlee\ D[E[\ 0881[ Nature 248\ 709[
Li\ A[\ Greenberg\ J[M[\ 0886[ Astron[ Astrophys[ 212\ 455[
Li\ A[\ Greenberg\ J[M[\ 0887a[ Astron[ Astrophys[ 227\ 253[
Li\ A[\ Greenberg\ J[M[\ 0887b[ Astrophys[ J[ 387\ L72[
Li\ A[\ Greenberg\ J[M[\ 0887c[ Astron [ Astrophys[ 220\ 182[
Maas\ R[W[\ Ney\ E[P[\ Woolf\ N[F[\ 0869[ Astrophys[ J[ 059\ L090[
Mukai\ T[\ Fechtig\ H[\ 0872[ Planet[ Space Sic[ 20\ 544[
Mumma\ M[\ Weissman\ P[\ Stern\ S[A[\ 0882[ In] Levy\ E[H[\ Lunine\
J[I[\ Mathews\ M[S[ "Eds#[ Protostars and Planets III\ University of
Arizona Press\ Tucson\ p[ 0066[
Rickman\ H[\ 0875\ In] Melitta\ O[ "Ed[#[ Comet Nucleus Sample
Return\ ESA SP!138\ p[ 084[
Russo\ N[D[\ 0886[ IAU Circ[ 5593[
Sekanina\ Z[\ 0885[ Astron[ Astrophys[ 203\ 846[
Smith\ B[A[\ Terrile\ 0873[ Science 115\ 0310[
Smoluchowski\ R[\ Marie\ M[\ McWilliam\ A[\ 0873[ Earth\ Moon\ and
Planets 29\ 170[
Utterback\ N[G[\ Kissel\ J[\ 0889[ Astron[ J 099\ 0204[
Vidal!Madjar\ A[\ Lagrange!Henri\ A[!M[\ Feldman\ P[D[\ et al[\ 0883[
Astron[ Astrophys[ 189\ 134[
Weissman\ P[R[\ 0873[ Science 113\ 876[
Weissman\ P[R[\ Campins\ H[\ 0882\ In] Lewis\ J[\ Mathews\ M[S[\
Guerrieri\ M[L[ "Eds#\ Resources of Near!Earth Space\ University
of Arizona Press\ Tucson\ p[ 458[
Williams\ D[M[\ mason\ C[G[\ Gehrz\ R[D[\ et al[\ 0886[ Astrophys[ J[
378\ L80[