\

PERGAMON Planetary and Space Science 36 "0888# 676Ð684

All are born equal] infrared emission by dust as a key to nucleus composition0 J[ Mayo Greenberga\ \ Aigen Lib

a Laboratory Astrophysics\ University of Leiden\ Postbus 8493\ 1299 RA Leiden\ The Netherlands b 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 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 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 has resulted from the infrared emission observation both of the 8[6 mm spectral feature as well as the continuum[ Classically\ the composition of comet nuclei was While there are di}erences from comet to comet in the derived primarily from the volatile molecules domi! details of the emission\ a uniform approach in terms of nated by water "or OH#[ The dust was considered mostly aggregates of submicron size interstellar dust "Greenberg in terms of its scattering properties from which empirical and Hage\ 0889# which is widely believed to be silicate approximations were used to deduce a dust!to!gas ratio[ core!organic refractory mantle particles "Li and Green! The discovery of the silicate emission feature "Maas et berg\ 0886# provides a coherent theoretical structure[ It al[\ 0869# con_rmed the existence of refractory material should be noted here that the in situ mass spectra con! in comets along with volatiles "ices#[ The idea of organic _rmed the core!mantel structure "Krueger and Kissel\ refractories as a major constituent was _rst 0876^ Lawler and Brownlee\ 0881# so that the pre! quantitatively introduced in the interstellar dust model of sumption of interstellar dust grains as the basic units of comets "Greenberg\ 0871#[ But it was the mass spec! comet dust aggregates is most reasonable[ troscopic evidence of the Giotto:Vega space probes which In this paper we show how the infrared emission for provided the _rst proof that the refractory material in several distinctly di}erent types of comets bear a general comet dust consisted of both the organic elements "O\ C\ resemblance to each other[ While the dust distributions N# as well as the rocky elements "Mg\ Si\ Fe# "Kissel et contain sizes as high as milligrams or higher\ in no case al[\ 0875a\b^ Jessberger and Kissel\ 0880#[ While the visual are there compact particles with mass higher than and ultraviolet emission of coma molecules excited\ pho! ½09−03 g] the mean mass of an interstellar core!mantle tolyzed and ionized by the solar radiation is used to particle[ In fact it is di.cult\ if not impossible\ to explain deduce the volatile composition of the nucleus\ it is the how large compact particles*silicates or otherwise*can infrared radiation by the dust which is the remote obser! have emerged from the comet nucleus in view of the fact vational data used to deduce the refractory components[ that the interstellar volatile mantles are so well repre! A major advance in our understanding of comet dust sented in the coma[ One must infer that the preservation of the volatiles automatically guarantees preservation of what the volatiles originally covered[ Furthermore\ the Corresponding author[ E!mail] greenberÝstrw[leidenuniv[nl 0 Presented in the Workshop on the Targets*35P:Wirtanen] temperature of the core!mantel particles during aggre! Observations\ Modeling and Future Work\ 09Ð00 December 0886\ gation never exceeded the evaporation temperature of Napoli\ Italy[ 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

−2 −2 of cometary H1O inferred from the ortho!para ratio "see 9[97 ¾ rCD ¾ 9[05 g cm ^ i[e[\ rCD ¼ 9[0gcm is a e[g[\ Mumma et al[\ 0882^ Crovisier et al[\ 0886#[ 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! 1[ Comet Halley] a periodic comet gate\ not size "mass# dependent "see Greenberg and Hage\ 0889#[ By considering the dust as comet nucleus material The uniqueness of comet Halley with regard to the out of which all the volatiles\ the very small "interstellar dust was that for the _rst time three properties were dust# particles\ and about 0:1 of the original "relatively simultaneously observed] chemical composition\ size volatile# organic refractories were removed\ the recon! "mass# distribution\ infrared emission[ It was shown stituted comet nucleus density was inferred to be −2 "Greenberg and Hage\ 0889# that\ in order to satisfy 9[15 ¾ rC ¾ 9[40 g cm [ Later works "Greenberg and simultaneously such independent properties of Halley Li\ 0887a^ Greenberg\ 0887# modify these results slightly coma dust as] "0# 8[6 mm emission "amount and shape#\ but the bottom line is that comet Halley dust has a density −2 "1# dust mass distribution\ and "2# mass spectroscopic rCD ¼ 9[0gcm and its nucleus has a density rC ¼ 9[2 composition\ one must represent the dust as very ~u}y gcm−2[ This is consistent with the low density suggestion aggregates of submicron interstellar dust silicate core! proposed by Rickman "0875# based on the analysis of organic refractory mantle particles[ non!gravitational forces[ 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 2[ Comet P:Borrelly "0883l#] a Jupiter family short! "bare#\ are too cold to emit e.ciently[ An additional period comet criterion required for the organic mantles is to account for the distributed CO "Eberhardt et al[\ 0876# by evap! The ~u}y aggregate comet dust model has also been orating its C1O bearing molecules "Greenberg and Li\ shown to be applicable to short!period comets "see Li 0887a\b#[ The required degree of porosity "~u.ness of and Greenberg\ 0887a#[ As an example\ we have cal! the aggregates# is de_ned by the fact that for a given mass culated the dust thermal emission spectrum of comet the more porous the aggregate the more it acts like a sum P:Borrelly "0883#\ a Jupiter family short!period comet of small particles rather than a compact particle of that "with an orbital period P ¹ 6 years#\ from 2Ð03 mmas mass\ so that a larger fraction of the observed mass dis! well as the 09 mm silicate feature in terms of the comet tribution provides silicate emission as if by submicron modeled as a porous aggregate of interstellar dust "Li particles[ For example\ the temperature of a 09−00 g com! and Greenberg\ 0887a#[ The ~u}y aggregate model of pact grain made of silicate core!organic refractory mantle silicate core!amorphous carbon mantle grains with a materials "with a mass ratio of the mantle to the core 0:1# porosity P  9[74 can match the observational data at 0 AU is ¼305 K\ while at a porosity of P  9[864 it is obtained by Hanner et al[ "0885# quite well "see Fig[ 0#[ ¼695 K[ The temperature for a pure silicate grain is It seems that\ compared to the Halley dust\ the dust ¼239 K "compact#\ ¼340K"P 9[864#\ respectively[ grains of P:Borrelly appear to be relatively more pro! The major thrust of this is that comet dust consists cessed "more carbonized#\ less ~u}y\ and richer in smaller of intimately related silicate and carbonaceous materials particles[ "core!mantel structure# rather than separate silicate and Since P:Borrelly has passed through the inner solar carbon components[ One of the observational supports system many more times than P:Halley and therefore of the model is that the in situ mass spectra of Halley been subjected much more to the solar irradiation\ the dust with high dynamic range show that\ except for the dust grains within the surface layer of the nucleus could very small "attogram# grains "Utterback and Kissel\ have been signi_cantly modi_ed[ In particular\ the 0889#\ neither pure organic "so!called {CHON|# nor pure organic refractory materials could have undergone fur! silicate particles exist^ instead\ they are intimately mixed ther carbonization^ namely\ the organic materials\ would on a very _ne scale in such a manner that they form the partially lose their H\ O\ N atoms and thus become car! subunits with a core!mantle structure in the aggregates bon!rich "Jenniskens et al[\ 0882#[ This is supported by "Lawler and Brownlee\ 0881# as additionally re~ected by the results of the EURECA space experiments which the fact that the CHON ions have on the average a higher have indicated the carbonization of the {_rst generation| initial energy than the silicate ions in measuring the mass organic refractory materials by solar irradiation "Green! spectra "Krueger and Kissel\ 0876#[ berg et al[\ 0884#[ Observations do show that some Jup!

In summary\ the result of the intertwining of the three iter!family short!period comets are depleted in C1 and C2 basic observations is] "0# comet dust consists of aggre! but approximately constant in CN "A|Hearn et al[\ 0884#[ gates of ½9[0 mm silicate core!organic refractory mantel This is consistent with the idea of carbonization since CN particles^ "1# the average porosity of the comet dust is is mostly produced from grains while some C1 and C2 9[82 ³ P ³ 9[864[ The inferred comet dust density is 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 Wm−1 mm−0[

relatively depleted in SP comets "A|Hearn et al[\ 0884#[ uncertainties in determining the Halley dust size dis! The solar irradiation can also lead to a lower porosity tribution still remain\ nevertheless\ a statistical analysis than that of Halley dust due to the packing e}ect "Mukai of _fteen comets indeed seems to suggest that the dust and Fechtig\ 0872^ also see Smoluchowski et al[\ 0873#[ size distribution is somewhat steeper for short!period The dust size distribution could be weighted toward comets than for long!period comets "Fulle\ 0887#[ smaller grains^ i[e[\ smaller grains are enhanced as a Since up to now only two SP comets were known to consequence of evaporation and subsequent frag! have silicate emission "P:Borrelly and P:Fay^ see Hanner mentation in the coma[ There are both observational and et al[\ 0885#\ at this point we are not able to generalize theoretical indications of dust fragmentation in the coma the dust properties of short!period comets[ Systematic of comet P:Halley "see Greenberg and Li\ 0887b and observations of the thermal emission spectra and the references therein#[ As the volatile ice sublimates from silicate features for a large set of samples are needed[ the nucleus\ it leaves behind the refractory particles and loosens the aggregates[ If the fragmentation indeed results from the sublimation of volatile materials which 3[ Comet Hale!Bopp "C:0884 O0#] a very large long! act as {glue|\ one may expect relatively more drastic and period comet more complete fragmentation in the coma of SP comets since volatiles are relatively depleted in SP comets "Weiss! Comet HaleÐBopp "C:0884 O0# is an exceptionally man and Campins\ 0882#[ We should note that signi_cant bright long!period comet "P ¼ 1999 years#[ It was so 689 J[M[ Greenber`\ A[ Li : Planetary and Space Science 36 "0888# 676Ð684

−9[4 active and so bright that it became visible even at a from vd ¼ 9[94 "rh:5[71# km:s where rh the heliocentric heliocentric distance of ½6 AU[ Its strong activity and distance in AU "Sekanina\ 0885#\ the dust!to!water pro! strong thermal emission features provide a rare oppor! duction rate ratio was estimated to be as high as 30 or tunity to study the origin of comets and to constrain the even higher "see Li and Greenberg\ 0887b#[ If a higher comet dust morphology\ composition and size[ For this dust out~ow velocity\ 9[59 km s−0\ which may be more purpose\ in particular\ to better understand the nature of realistic\ is adopted\ the dust!to!water ratio would be HaleÐBopp dust\ we should perform detailed modelling about 199; However\ one should keep in mind that\ the on the heliocentric evolution of dust properties based on IR emission alone can not give a reliable dust production the evolution of the silicate feature and of the thermal rate since very large particles are too cold to contribute continuum emission as well as the dust properties during to the limited wavelength range of the infrared radiation sporadic events such as outbursts and jets[ considered here "as long as the size distribution for those As a starting point\ we have only modeled the 2Ð19 mm cold particles is not too ~at# and therefore the total mass emission spectrum of HaleÐBopp of February 19\ 0886\ of the large particles is not well constrained\ as already obtained by Williams et al[ "0886#\ when it was at a noted by Crifo "0876#[ heliocentric distance rh  0[04 AU[ Its spectrum "of Feb! The presence of a crystalline silicate component in ruary 19\ 0886# was characterized by the strongest 09 comet HaleÐBopp was explicitly demonstrated by both mm silicate feature ever observed for a comet[ Its strong space "Crovisier et al[\ 0886# and ground based "Hayward silicate emission feature has been generally interpreted as and Hanner\ 0886# observations[ The fact that the crys! indicating the presence of an unusually high abundance talline silicate emission features are quite strong even of small "³0 mm\ or equivalently ³0[4×09−00 g for when HaleÐBopp is at such large heliocentric distances −01 solid silicate and ³9[5×09 g for solid carbon# grains as rh  1[8 AU "Crovisier et al[\ 0886#\ rh  3[1 AU "Hay! "Hanner\ 0886#[ Equally special was the highest ratio ward and Hanner\ 0886#\ is of particular interest because of the color temperature to the blackbody temperature it may invoke serious questions on the role played by "de_ned as superheat by Gehrz and Ney "0881## derived solar insolation[ At this point\ we are not going to address by Williams et al[ "0886#[ This has led to the suggestion where and how cometary silicates are crystallized[ that Hale!Bopp contains the smallest grains yet observed However\ it seems likely to us that the crystallization did for any comet[ Williams et al[ "0886# estimated the mean not occur before comet nucleus formation since\ on the HaleÐBopp dust size to be ¼9[3 mm "corresponding to one hand\ no evidence exists for the presence of crystalline 5[1×09−02 g for amorphous carbon\ 8[3×09−02 g for silicates in the interstellar medium^ and on the other hand\ silicate\ respectively# in terms of solid separated sili! circumstellar dust is unlikely to be directly incorporated cate:carbon grains[ This result did not take into account into comets without _rst passing through the interstellar that the superheated thermal continuum and the strength medium[ We have also carried out calculations in which of the silicate feature are not solely determined by grain crystalline silicates are included in the model[ In our sizes[ The dust morphology\ the optical properties of the calculation\ we adopted the e}ective medium theory and dust components and the way in which the di}erent dust the Mie theory "Bohren and Hu}man\ 0872# to obtain the components are mixed "Hanner et al[\ 0885# are equally grain temperatures and therefore the emerging emission important[ spectrum[ We did not apply the approach We have calculated the dust thermal emission spectrum based on the model of comet dust consisting of porous s fi×ki"l#×B"l\ Ti# aggregates of interstellar dust[ As shown in Fig[ 1\ both i the continuum emission and the 09 mm silicate feature are well matched "see Li and Greenberg\ 0887b\ for "Ti is the temperature for the i!th dust component^ ki"l#\ details#[ The mean grain masses "derived by averaging B"l\ Ti#\ fi are the mass absorption coe.cient\ the Planck over the size distributions# are ½09−09Ð09−8 g\ sig! function\ the mass fraction of the i!th dust component\ ni_cantly di}erent from the suggestion that HaleÐBopp respectively# by adjusting Ti and fi as commonly used in is rich in submicron grains "Hanner\ 0886^ Williams et the literature "see e[g[\ Colangeli et al[\ 0884#[ One should al[\ 0886^ see above#[ The presence of large numbers of be cautious when adjusting the dust temperature Ti that\ very large particles in HaleÐBopp was con_rmed by the in so doing\ the adopted temperatures remain within a submillimeter continuum emission observation "Jewitt physically acceptable range\ i[e[\ not lower than the cor! and Matthews\ 0887#[ It was argued that these large par! responding black!body temperature and not unphysically ticles may dominate the total dust mass of the coma too much higher[ Our calculations imply that the partially "Jewitt and Matthews\ 0887#[ Assuming a spherically crystallized silicate model with the same dust parameters symmetric dust coma with uniform radial out~ow\ adopt! derived for the amorphous silicate model gives rise to ing the water production rate on February 12[8\ 0886 prominent crystalline silicate features while the _t to the "Russo et al[\ 0886# 3[2×0929 mols:s\ and adopting an overall thermal emission spectrum is still maintained "see average dust out~ow velocity of 9[01 km s−0 "calculated 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 Bopp "Campins and Ryan\ 0878^ Hanner et al[\ 0883^ system| Hayward and Hanner\ 0886^ Crovisier et al[\ 0886#[ This indicates the similarity of cometary dust with the b Pic! One of the big surprises from the Infrared Astro! toris dust[ Furthermore\ the systematic spectroscopic nomical Satellite "IRAS# was the so!called {Vega observations of gaseous elements in the visible or in the Phenomenon|^ i[e[\ that some main sequence stars exhibit ultraviolet carried out by a French group showed that large infrared "IR# excesses over the black body emission the spectral lines CaII\ MgII\ FeII\ AlIII\ AlII\ CIV\ CI of their photospheres "Aumann et al[\ 0873#[ It is gen! and CO show strong time variations and are almost erally believed that these IR excesses are attributed to the always redshifted relative to the stellar spectra "Vidal! dust thermal emission of their circumstellar dust grains[ Madjar et al[\ 0883 and references therein#[ These vari! Now it has become well established that the {Vega ations can be explained to result from the evaporation of Phenomenon| is not exceptional but rather quite common the dust grains shed from comet!like bodies falling on the among the main sequence stars "see Backman and Pare! star as a consequence of planet perturbation "Beust et al[\ sce\ 0882#[ Beta Pictoris is an A4 star with the largest 0889#[ infrared excess among the {Vega!type stars| and an edge! We have developed a model for the b Pictoris dust disk on circumstellar dusk disk "Smith and Terrile\ 0873^ Lag! which shows that the ~u}y aggregate comet dust model age and Pantin\ 0883#[ is also applicable to extrasolar comets "see Li and Green! Various observational and theoretical evidence indi! berg\ 0887c#[ The basic idea is that the dust in the disk cate that comets may exist in the disk "see Li and Green! plane is continually replenished by comets orbiting the berg\ 0887c and references therein#[ First of all\ dynamical star where the dust may be quickly swept out by radiation studies indicate that the grain destruction time scales due pressure or spiral onto the star as a result of PoyntingÐ to grain!grain collisions\ PoyntingÐRobertson drag and Robertson drag[ The initial dust shed by the comets is are shorter than the lifetime of b Pic! taken to be the ~u}y aggregates of interstellar silicate toris "Backman and Paresce\ 0882^ Artymowicz\ 0883#[ core!organic refractory mantle dust grains "with an This indicates that there must be some dust source which additional ice mantel in the outer region of the disk#[ The continually replenishes the lost particles[ Such a source heating of the dust is primarily provided by the organic was _rst suggested as due to comet!like bodies by Weiss! refractory mantel absorption of the stellar radiation[ The man "0873#[ Moreover\ its silicate emission feature\ show! temperature of some of the particles close to the star is ing a crystalline silicate feature at 00[1 mm superimposed su.cient to crystallize the initially amorphous silicates[ on the broad amorphous silicate feature at 09 mm The dust grains are then distributed throughout the disk "Knacke et al[\ 0882#\ resembles that of some comets by radiation pressure[ The steady state dust distribution including comet Halley and the most recent comet HaleÐ 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! distribution of a radial distribution of such particles pro! organic refractory mantel particles "without:with ice vides an excellent match to the 09 mm amorphous and mantles# with variable ratios of organic refractory to the 00[1 mm crystalline silicate spectral emission as well silicate mass[ The whole disk which extends inward to as the excess continuum ~ux from the disk over a wide ½0 AU and outward to ½1199 AU is divided into three range of wavelengths "see Figs 2\ 3\ and 4#[ These models components which are primarily responsible respectively\ result in a total mass of dust in the whole disk ½1×0916 for the silicate emission\ the mid!infrared emission and g of which only 09−4Ð09−3 is required to be heated enough the far infrared:millimeter emission[ As a starting point\ to give the silicate excess emission[ the grain size distribution is assumed to be like that Figure 4 implies that\ to obtain a prominent crystalline observed for comet Halley dust while in the inner regions silicate feature at 00[1 mm\ a silicate mass ratio of the the distribution of small particles is relatively enhanced crystalline silicate model to the amorphous silicate model which may be attributed to the evaporation and:or frag! f  Mcrystalline:Mamorphous  9[39 is needed "actually\ as mentation of large ~u}y particles[ The dust grains which shown in _g[ 5A of Li and Greenberg\ "0887c#\ even best reproduce the observations are highly porous\ with a f  19) is not quite su.cient#[ This indicates that ½39) porosity around 9[84 or as high as 9[864[ The temperature 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 5[ Generalized comet nucleus are cold and thus are not crystallized as e.ciently as small particles[ It is likely that the crystalline silicate We present here the suggested canonical composition aggregates distributed in the disk constitute only those in of comet nuclei based on observation of the dust and the low mass part of the size distribution[ Crystalline volatiles of a variety of comets[ According to Greenberg silicates are needed only to produce the 00[1 mm feature "0887#\ the chemical composition of a comet nucleus can and possibly the mid!infrared "MIR# bands with the far be very strictly constrained by combining the latest results infrared "FIR#:millimeter emission dominated by the on] the core!mantel interstellar dust model\ the solar sys! amorphous silicate aggregates[ This will not a}ect the tem abundances of the elements\ the space observed com! model _t of the MIR and FIR spectra since both the position of the dust of comet Halley\ and the latest data crystalline silicate model and the amorphous silicate on the volatile molecules of comet comae[ The dis! model show a very similar behavior in the MIR and FIR tribution of the components in the comet nucleus fall "see Figs 2 and 3#[ In other words\ the above derived mass naturally into two basic categories*refractories and vol! fraction need only be valid in the inner components of atiles[ The refractory components are tightly constrained the disk\ so that only about 0) of the total silicates are to consist of about 15) of the mass of a comet as silicates required to be converted into the crystalline phase[ "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! sands of the attogram carbonaceous:large molecule par! nated by carbon#\ and about 8) in the form of extremely ticles embedded in the icy and outer organic fraction[ 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\ 6[ Concluding remarks*application to comet

CO1\CH2OH plus other simple molecules[ The H1O Wirtanen abundance itself if very strictly limited to ½29) of the total mass of a comet*not much more nor much less[ It has been shown that a unifying theory of comet The refractory to volatile "dust to gas# ratio is about 0 ] 0\ dust properties is based on their consisting of porous while the dust to H1O ratio is ¼1 ] 0[ The maximum mean aggregates of silicate core!organic refractory mantel density of a fully packed nucleus would be ¼0[54 g cm−2[ interstellar dust particles[ Some of the consequences of The morphological structure of the component materials\ this are that] "0# ~u}y dust\ which can break up into following the interstellar dust into the _nal stage of the submicron particles\ is expected to be coming o} and be presolar cloud contraction\ is as tenth micron silicate part of the environment of comet Wirtanen^ "1# mass cores with organic refractory inner mantles and outer spectra of both comet nucleus and comet dust material mantels of {ices| with each grain containing many thou! will on the average exhibit\ simultaneously\ charac! J[M[ Greenber`\ A[ Li : Planetary and Space Science 36 "0888# 676Ð684 684 teristics of both organics and silicates^ "2# the mor! Eberhardt\ P[\ Krankowsky\ D[\ Schulte\ W[\ et al[\ 0876[ Astron[ Astro! phological structure of the comet nucleus will exhibit phys[ 076\ 370[ Fulle\ M[\ Planet[ Space Sci[\ submitted\ 0887[ optical features down to the submicron level^ "3# the Gehrz\ R[D[\ Ney\ E[P[\ 0881[ Icarus 099\ 051[ nucleus surface is probably a ~u}y but chemically bonded Greenberg\ J[M[\ 0871[ In] Wilkening\ L[L[ "Ed[#[ Comets\ University structure whose material and chemical properties are of Arizona Press\ Tucson[ p 020[ derived from sintering of comet dust fragments and of Greenberg\ J[M[\ Hage\ J[I[\ 0889[ Astrophys[ J[ 250\ 159[ residual comet surface material[ 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[ Acknowledgements Greenberg\ J[M[\ Li\ A\ 0887b[ Astron[ Astrophys[ 221\ 263[ Greenberg\ J[M[\ 0887[ Astron[ Astrophys[ 229\ 264[ We are grateful for the support by NASA grant NGR Hanner\ M[S[\ 0874[ Adv[ Space Res[ 3 "8#\ 078[ 22!907!037 and by a grant from the Netherlands Organ! Hanner\ M[S[\ Lynch\ D[K[\ Russell\ R[W[\ 0883[ Astrophys[ J[ 314\ 163[ ization for Space Research "SRON#[ We thank Dr R[D[ Hanner\ M[S[\ Lynch\ D[K[\ Russell\ R[W[\ Hackwell\ J[A[\ Kellogg\ Gehrz for kindly providing us with the IR thermal emis! R[\ 0885[ Icarus 013\ 233[ sion data of comet HaleÐBopp^ Dr S[B[ Fajardo!Acosta Hanner\ M[S[\ 0886[ Bull[ Am[ Astron[ Soc[ 18\ 0931[ and Dr R[F[ Knacke for the b Pictoris data^ Dr M[S[ Hayward\ T[L[\ Hanner\ M[S[\ 0886[ Science 164\ 0896[ Jenniskens\ P[\ Baratta\ G[A[\ Kouchi\ A[\ de Groot\ M[S[\ Greenberg\ Hanner for the comet P:Borrelly data^ and Dr C[ Koike J[M[\ Strazzulla\ G[\ Astron[ Astrophys[ 0882[ 162\ 472[ for the optical constants of crystalline silicates[ We are Jessberger\ E[K[\ Kissel\ J[\ 0880[ In] Newburn\ R[L[\ Neugebauer\ M[\ also indebted to Dr M[ Fulle and Dr M[ Mueller for their Rahe\ J[ "Eds#[ Comets in the Post!Halley Era\ Kluwer\ Dordrecht\ useful suggestions[ One of us "AL# wishes to thank Prof[ p[ 0964[ G[V[ Coyne\ S[J[\ for his kind support in participating in Jewitt\ D[C[\ Matthews\ H[E[\ 0887[ Science\ submitted[ Kissel\ J[\ Sagdeev\ R[Z[\ Bertaux\ J[L[\ et al[\ 0875a\ Nature 210\ 179[ the Vatican Observatory summer school[ AL also would Kissel\ J[\ Brownlee\ D[E[\ BuÃchler\ K[\ et al[\ 0875b[ Nature 210\ 225[ like to thank Leiden University for an AIO fellowship Knacke\ R[F[\ Fajardo!Acosta\ S[B[\ Telesco\ C[M[\ et al[\ 0882[ Astro! and the World Laboratory for a Scholarship and the phys[ J[ 307\ 339[ National Science Foundation of China for _nancial sup! Krueger\ F[R[\ Kissel\ J[\ 0876[ Naturwissenschaften 63\ 201[ Lagage\ P[O[\ Pantin\ E[\ 0883[ Nature 258\ 517[ port[ 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[ References Li\ A[\ Greenberg\ J[M[\ 0887c[ Astron [ Astrophys[ 220\ 182[ Maas\ R[W[\ Ney\ E[P[\ Woolf\ N[F[\ 0869[ Astrophys[ J[ 059\ L090[ A|Hearn\ M[F[\ Millis\ R[L[\ Schleicher\ D[G[\ et al[\ 0884[ Icarus 007\ Mukai\ T[\ Fechtig\ H[\ 0872[ Planet[ Space Sic[ 20\ 544[ 112[ Mumma\ M[\ Weissman\ P[\ Stern\ S[A[\ 0882[ In] Levy\ E[H[\ Lunine\ Artymowicz\ P[\ 0883[ In] Ferlet\ R[\ Vidal!Madjar\ A[ "Eds#[ Cir! 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