Ices in Space

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J. Phys. Chem. 1983, 87, 4243-4260Ices in Space

4243

J. Mayo G reenberg, C. E. P. M. van de Bult, and L. J. AllamandolaLaboratory Astrophyslcs. Huygens Laboratory, University of Leiden, Leiden, The Netherlands (Recelved: November 17, 1982;In Final Form: April 4 , 1983)

The chemical and physical properties of ice grains in interstellar space have been studied in the laboratoryand theoretically modeled to compare with astronomical spectra between 2700 and 3700 cm-. The observedpolarization of starlight in this region clearly indicates that elongated particles are involved. Absorptioncharacteristics for various shaped grains whose radii vary from -0.1 to 1.0pm, containing either pure amorphousH 2 0or amorphous mixtures of H2O with NH3,have been calculated with the aim of narrowing the range ofacceptable grain parameters. By comparing the band shapes for spherical, spheroidal, and cylindrical grainswith astronomical spectra we show that elongated particles whose radii are -0.15 pm produce an acceptablematch and that both spherical and elongated particles whose radii are 20.5pm are definitely not consistentwith observations. Details of the band shape are shown to depend on particle size, shape, and composition.Similar profiies canbe produced by using different combinations of particle shape and composition. For example,the NH 3signature at 2.97 pm, which is prominent in a spherical grain, is greatly suppressed when in an elongatedgrain. This is exactly equivalent to reducing the concentration of NH3 in a spherical grain. A morphologicalgrain model is used to explain the large variations in the observed strength of the 3.07-pm ice band from oneregion of space to another.

1. IntroductionPerhaps the earliest notion that there was a considerableamount of water ice in space arose from the applicationby van de Hulstp2 of the idea suggested by Lindblad, thatthe small particles between the stars (interstellar dust)could have formed by accretion of a tom in space. Startingwith the most abundant condensible species-oxygen,carbon, and nitrogen-and letting them combine with thehighly abundant hydrogen after sticking on the alreadypresent small solid particles, van de Hulst derived a modelfor the dust grains consisting of the satura ted moleculesH2 0, CHI, and NH, with traces of other constituents.Since the interstellar grains are at temperatures of the

order of 10K, all the molecules are frozen and the mixturecame to be called dirty ice.About the same time as the dirty ice model was pro-posed, Whipple4 suggested that the cometary nucleusconsisted mostly of H20 ice. Although the origins ofcomets are somewhat uncertain, one of the current viewsis that comets have formed directly by coagulation of theinterstellar If this is so, then the cometary ices arethe same as the interstellar ices which we shall see mustbe a complex melange which includes many more com-plicated as well as simple molecules along with the HzO.The prediction of the dirty ice dust model was thatabout 170 f all the material between the stars consistedpredominantly of H20. Since this represents a really largeamount of H 2 0when one considers the vast dimensionsof space it was reasonable to expect to detect the Hz O inspace from its strong OH stretching absorption a t 3 pm.The initial results of such astronomical observations weresurprisingly negative.6 They set upper limits on theamount of HzO present in the grains which were, even bygenerous estimates, less than 10% of the expectation value.

(1)van de Hulst, H. C. N e d . Tijdschr. Nutuurkd . 1943, 10 , 25.(2) an de Hulst, H . C. Rech. Abstr . Obs. , Utrecht 1949, 11, part 2.(3) Lindblad, B., a t u r e (London) 1935, 135, 133.(4 ) Whipple, F. L. A s t r ophys . J. 1950, 1 1 1 , 375.(5) For recent dev elopments and references, see Greenberg, J. M.Comets; Wilkening, L. L., Ed.: University of Arizona Press: 1982;pp131-63.(6) anielson, E.R.; Woolf, J. N.; Gaustad, J. E. Astrophys . J. 1965,141, 116.

A subsequent observation was equally unsuccessful. Theobservations were not easy because they required not onlya great pathlength through dust but a strong infraredbackground source as well. These two requirements are,in the normal interstellar medium, quite difficult to satisfysimultaneously. The breakthrough in the discovery ofinterstellar ice came following the detection of a stronginfrared source in the Orion nebula. This, along with thehigh density of interstellar dus t in that region, made itpossible to provide the first 3-pm absorption evidence forH20 ce in space.* As a bonus, a strong featureless ab-sorption at 9.7 pm showed up which gave evidence for anew material in space. This has since become att ributedto the Si0 stretch in an as yet unidentified amorphoussilicate type material. This latter has come to be consid-ered as the most likely nucleus (which was lacking in theaccretion model of van de Hulst) on which the condensableatoms and molecules from the gas may stick; Le., it is theseed or core for the interstellar grains.In spite of the first success in the observation of H 2 0ice, continuing attempts met with highly variable results.There did not appear to be any predictable relationshipbetween the ice band absorption strength and the amountof extinction which is a measure of the totalamount of dustalong the line of sight. Most of the observations producednegative results and where the ice band was detected therewere distinct discrepancies in shape and peak positionwhen compared with what was then known about the in-frared properties of ice.g Although the situation wascertainly becoming confusing it was forcing the recognitionthat not only was the ice in space much more complicatedthan had been pictured, i t also required new laboratoryinvestigations to provide the relevant da ta for the inter-stellar conditions.In 1969Greenberg suggested that the negative 3-pm iceresult for such highly reddened stars as CIT 11and V ICygni No. 1 2 may possibly be explained away by effects

(7 ) Knacke, R. F.; Cudaback,D. D.; Gaustad, J. E. Astrophys. J . 1969,(8) Gillett, F. C.; Forrest, W. J. Astrophys . J . 1973, 179, 483.(9 ) Bertie, J.E.;Labb6, H. E.;Whalley, E. J . C h e n . P hys . 1958, 50,

158, 151.4501.

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4244of ultraviolet radiation on the dirty ice mixtures.O Thereduction in strength of the OH stretch at 3 fim was at-tributed to the breakup of the water molecules by photo-lysis.The use of a laboratory to study the photochemistry ofinterstellar ices actually arose not only because of thenegative ice observations, but it was also motivated by theapparently disconnected astronomical discovery of the thensurprisingly complicated molecule formaldehyde (H,CO)in the interstellar gas. The formation of complicatedmolecules and the reduction of the strength of the ice bandwere conjectured to have a commonorigin in the processingof grains resulting from photolysis of the ices by theubiquitous ultraviolet radiation in space. Since, not onlyOH , but other radicals are formed by photolysis as well,it is but one step further to seethe concomitant formationof complicated molecules in the grains so tha t HzCO wasprobably a simple example of the full complexity of whatactually existed in the small solid particles. There wasalready abundant literature on such problems as theproduction of complicated molecules by the photolysisof low-temperature solids.12 There was also the classicgas-phase experiment of Miller and Urey showing thatspark discharges which dissociated the molecules simu-lating a primitive earth atmosphere led to the formationof complex organic m01ecules.l~ But there had been noexperiments on complex, reactive, low-temperature mix-tures which could properly simulate the interstellar grains.This was attempted a t the Sta te University of New Yorkat Albany where temperatures approaching the required10 K level (actually T 2 28 K) were used in the earlyexperiment^.'^ But it was not until the foundation ofLaboratory Astrophysics at the University of Leiden in1975 that it became possible to combine all the principalrequirements for a labratory analogue of the interstellargrain pr0b1em.l~ It has indeed turned out tha t withoutthisapproach one cannot hope to answer the puzzles aboutH 2 0 ice and at the same time the connection betweengrains and the radio observations of complicated gaseousmolecules in space.In the subsequent sections we shall attempt to sum-marize the current sta tus of interstellar ice from the pointof view of observations, physical and chemical laboratorystudies, optical modeling of interstellar grains, and theunderlying astrophysics of grain evolution. In order tomaintain a reasonable degree of completeness in this re-view we shallhave to overlap somewhat with another paperin this issue.16a2. Basic Interstellar Medium

The space between the stars which is known as the in-terstellar medium provides the raw material from whichstars and planetary systems are born and is also the burialground for the remnants of stellar systems which haveexpired. This extremely tenuous distribution of matter,comprised of gas and dust, accounts for about 10% of thematerial in the Milky Way. The dust, small particles

The Journal of Physical Chemistry, Vol. 87, No. 21, 1983 Greenberg et ai.approximately 0.1 pm in radius, is extremely cold, beinggenerally at a temperature of about 10K. The molecules,which have been frozen onto this dust, comprise the icein space which is discussed in this article. The other majorconstituents of the interstellar medium in which this iceis found will now be described. A more complete de-scription of the interstellar medium is given in ref 17 and18.(a )Atoms. Most of the interstellar medium is hydrogenwhich was created in the earliest stages of our universe.The formation of the heavier elements has been an ongoingprocess ever since the first stars were born. What we seetoday, on the grand scale, is a distribution of the elementswhich have been produced by stars and ejected back intospace. Following helium, which is chemically inert, thegroup of elements comprising oxygen, carbon, and nitrogenconstitute about one part in a thousand, by number, rel-ative to hydrogen. The elements of the next most abun-dant group-magnesium, silicon, iron, and sulfur-arefurther down by a factor of ten. The mean hydrogendensity throughout the galaxy is about 1 ~ m - ~ ,( b )Molecules and Dust . Before the late 1960s, generalconsensus held that the interstellar radiation field was soharsh that no gaseous polyatomic molecule would survivelong enough to permit detection. The spell was brokenin 1968 with the detection of gaseous NH, toward thegalactic center.lg This discovery was startling for manyreasons; important among them is the fact tha t NH3 iseasily photodissociated. Millimeter wave astronomy wasnow properly launched and it was not long afterward thatformaldehyde was discovered, followed quickly by CO. Todate, a wide range of molecular species have been detectedin the gas, nearly 60 in total, of which carbon monoxideis the most abundant, though still consuming 510% of theavailable carbon. This is only the tip of the iceberg, since,in addition to the gas, many of these molecules, as well asothers which have been formed in situ in the grains, areto be found as an ice frozen onto the dust in abundancesgreater than seen in the gas. Thus, even though it is 10l2times less abundant by number than hydrogen, eachparticle can contain as many as lo9of the smaller mole-cules such as CO and NH,, so that there are more mole-cules in the solid than have been found in the gas. Al-though we can say tha t the dust and gas have a well-de-fined general distribution within the galaxy, a brief glanceat the night sky reveals a highly inhomogeneous andpatchy structure. This is not due to an uneven distributionof stars, but rather the presence of concentrations of dust,acting like a smoke screen and blocking the light of thebackground stars. This shows that the interstellar mediumactually consists of a chaotic distribution of gas/dustclouds with a variety of densities, always in motion, andpassing from one phase to another in their evolution, withthe most dramatic phase appearing when new stars areformed. The widest variety of molecules is associated withclouds in intimate association with star formation. Inaddition to blocking the light of stars, he grains sometimescause it to be partially linearly polarized. This means thatthe particles must be nonspherical and, to some extent,aligned.The extinction is quantitatively defined by astronomersin magnitudes as Am(X) E A(X) = -2.5 log [ I (A) / I , , (X)] ,where X is the wavelength and I the intensity of radiation.

(10)Greenberg, J. M. Molecules in the Galactic Environment;Gordon, M. A.; Snyde r, L. E., Ed.; Wiley: New York, 1973; 94.(11) Snyder, L. E.; Buhl, D.; Zuckerman, B.; Palmer, P. Phys . Reo.Lett. 1969, 22 , 679.(12) ee, or example, Vibrational Spectrcacopy of Trap ped Species;Hallam, H. E., Ed.; Wiley: Lon don, 1973; nd in particular referencesto the su bstantial work of Milligan and Jacox, mentioned therein.(13)Miller, S.L.Science 1953, 117, 528.(14)Greenberg, J. M.; Yencha, A. J.; Corbett, J. W.; Frisch, H. L.M e n . Soc. R. Sci. LiCge 1972,6th ser, Tome 111, 425.(15)Greenberg, J. M. Ned. Tijdschr. Nutuurkd. 1976, 2, 117.(16) a) Tielens, A. G. G. M.; Hagen., W.; Greenberg, J. M., this issue.(b) Aannestad, P. A.; Greenberg, J. M. Astrophys. J . , n press.

(17) Spitzer, L., Jr. Physical Processes in the Interstellar Medium;(18)Greenberg, J. M. Cosmic Dust; McDonnell, J. A. M., Ed.;Wiley:(19) heung, A. C.; Rank, D. M.; Townes, C. H.; Thornton, D. C.;

Wiley: N ew York, 1978.New York, 1978;Chapter 4, 187.Welch, W. J.Phys. Reu. Lett. 1968, 21 , 170.


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Ices in SpaceSince the passage of light through the interstellar mediumis attenuated by exp{-r},where 7 is the turbidity or opticaldepth, the extinction, in terms of the turbidity, is A(X) =1.0867(X).As was first pointed out some 50 years ago, theinterstellar extinction causes a reddening of the light fromdistant stars which is not to be confused with a Dopplereffect.z0 This reddening is commonly called the colorexcess and is defined as the difference in the extinctionsin the blue ( B = 435 nm) and the visual (V 547 nm);i.e., color excess E ( B - V) = A ( B )- A(V) .

By measuring the mean amount of starlight extinctionper unit distance (which is proportional to the total areaof particles per unit area) and from the mean size of thedust grains, which is inferred from the color dependenceof this extinction and polarization, a mean spatial densityof dust of one particle per 10l2 cm-3 is derived. This isapproximately the volume of a cube whose sides are afootball field in length. In darker, denser regions, thedensity may be higher by a factor of lo4 o lo6. The hy-drogen number density, nH, s about 10 cm-3 in the so-called diffuse cloud regions and can be as high as 105-106cm-3 in the denser molecular cloud regions. To providea point of reference, a density of lo6 is equivalent toa pressure of 3 X mbar! The denser regions are alsoknown as molecular clouds because it is in these that themolecules are detected.( c ) The Interstel lar R adiation Field. The mean ultra-violet radiation coming from the general stellar populationconsists of about 3 X photons cm-3 with energy greaterthan 6 eV ( A = 200 nm), where 6 eV is chosen as a roughthreshold value for photodissociation of molecules (someobvious exceptions being CO andNz). Thus, in the diffusemedium, the density of photons in the range 912-2000 Ais greater than the density of atoms and molecules. In thecores of dense clouds the radiation is attenuated by passagethrough the dust so that it is reduced enough to reversethis relationship.3. Ice Observations

In this section we shall give some examples of both thepositive and negative evidence for HzO in interstellargrains.( a ) P os i t iv e HzO Obseruations. The literature nowcontains many examples of the 3.07-pm absorption and,in a few cases, the degree of polarization through this band.So far most of the spectra in the 3-pm region have beenmeasured with resolution no better than Av N 50 cm-l, orequivalently, AX N 0.05 pm. Within this limitation thedata show a rather high degree of constancy in the valueof the peak absorption wavelength. Nevertheless sub-structure in the band which varies from object to objectis clearly evident. Several examples of ice bands are shownin Figure 1 along with the silicate absorptions. We notethat although curves 1-4 show a hint of structure near thepeaks their deepest absorptions are at X N 3.07 pmwhereas the peaks for objects 5-7 occur at wavelengthsshort of 3.07 pm. The differences are significantly greaterthan the resolution element. These observations weretaken from Willner et ai. and are t he infrared spectra ofprotostars.21a There exist other similar observations butthese constitute a fair sample for our purposes. It iswell-known that pure H 2 0also absorbs at 6 and - 2 pm.Although the 6-pm feature is evident in the astronomical

The Journal of Physical Chemistry, Vol. 87 , No. 21, 1983 4245spectra, it can be distorted or complicated by blending withabsorptions due to other molecules, e.g., HzCO and N H 3 .A good profile of the 6-pm region taken with moderateresolution is not yet available because of atmospheric ab-sorption and whatever measurements exist have been madefrom an airborne observatory with a relatively small tel-escope. It is interesting to note that where the 3-pm bandis shifted short of 3.07 pm, as in curves 5-7, the 6-pm bandis rather obscured. The 12-pm absorption, on the otherhand, does not seem to exist a t all in the interstellar icessurrounding protostellar objects.The common substructures in the ice band are a t X N2.9 and N 3.4 pm. Another common feature of the in-terstellar ice band is the absorption wing which extendsrather far on the long wavelength side. There are a fewexceptions to this; one example is shown by Soifer et al.in their Figure 1F2 Not only are the wing and substructureabsent, but the 6-pm band is sharper and the 12-pm bandis present, all characteristics of relatively pure, amorphousHzO ice.23 This is notably no t a protostellar source andconsequently there may be good reasons why the ice bandis different.23 We shall not discuss further small featuresin other regions of the astronomical spectra.We note, for future reference, that the strength of theice absorption relative to the 9.7-pm silicate absorption isobviously highly variable and, in fact, there may be no iceabsorption even with a strong silicate absorption (see, e.g.,curve 8 in Figure 1).A very interesting, and suggestive, observation has beenmade by Joyce and Simon24where it appears that, forcompact infrared sources imbedded in dense molecularclouds, the deepest 3.07-wm absorptions are associated withthe most highly polarized objects. This may be interpretedto imply tha t H 2 0 ce becomes a dominant feature onlyin an extremely dense cloud environment.A limited amount of polarimetry has been done withinthe ice band. The polarization of the BN source as ob-tained by two independent observations is shown in Figure2.25 The HzO absorption is shown for comparison.26( b )Negatiue HzO bseruations. First of all, it must bestated that no Hz O absorptions have ever been detectedin the diffuse cloud medium. Starting with the first ob-servations it had been assumed that the detectability ofHzO should be greater the greater the extinction by theobscuring dust. Although this is a necessary condition itis far from sufficient. Since some of the brightest infraredsources are at a great distance from us in the center of theMilky Way, they provide an excellent background forobserving an ice band if present in the dust along the lineof sight. As is obvious in Figure 3, although there is somesort of absorption feature at approximately 3 pm, it is quitedifferent from any of the H 2 0 features shown in Figure1; t is perhaps three times broader and also its maximumabsorbance, where determined, is at a wavelength sub-stantially short of 3 pm. We note also that there is con-siderably greater variety in this feature than is exhibitedby the ice features, and that the ratio of its strength to thetotal extinction is far less than that of the ice bandstrengths shown in Figure 1. Indeed this appears to be anabsorption by some material quite different from H,O.

(20) Trumpler, R. J. Lick Obs. Bull. 1930,24,154.(21) (a) Willner,S. .; Gillett, F. C.; Herter, T. L. ; Jones, B.; Krasemer,J.; Merrill, K. M.; Pipher, J. L.; Puetter, R. C.; Rudy, R. J.; Russell, R .W.; Soifer, B. T. Astrophys .J . 1982,253, 174. (b) Willner,S. .; Pipher,J. L. Proceedings of Workshop on the Galactic Center; California In-stitute of Technology, 1982.

(22) Soifer,B. .; W illner, S.P.; Capps, R. W.; Rudy, R. J. Astrophys.J. 981, 250, 31.(23) Hagen, W .; Tielens, A. G. G. M .; Greenberg, J. M . Astron. As -trophys. S u p p l . Ser. 1983, 51, 389.(24) Joyce, R. .; Simon, T. Astrophys . J. 1978,260, 04.(25) (a) Kobayashi, Y.; awara, K.; Sato, S.;Okuda, H. Publ. Astron.SOC. p n . 1980, 32, 295. (b)Capps, R.W.; Gillett, F. C.; Knacke, R. F.Astrophys. J.1978,226, 63.(26) Gillett, F.C.; Jones, T . W.; Merrill, K. M .; Stein, W. A. Astron.Astrophys. 1975, 45, 17 .


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4246Some rather higher resolution observations of one of thegalactic center sources shows quite clearly that the materialcontains little if any H20and, indeed, as inferred from itsdetailed structure in the 3.4-pm region, must be due tomuch more complex molecules.27The attempts to detect solid H,O in molecularclouds-other than those associated with protostellarobjects-have met with almost as litt le success as thediffuse cloud observations. One such example is shownin Figure 4.28 Although the total extinction in the Tauruscloud is not great, one might expect a noticeable ice bandbecause the grains in this cloud are larger than average andmay therefore have sufficient H 2 0per grain to compensatefor the lack of large extinction. The observational resultobtained is certainly far below what the authors expected.However, we show (see section 6b and ref 45c) that basedon an evolutionary picture of grains the level of detectionof a 3-pm band is quite consistent with a substantialpresence of H,O in the grain mantles. (See Note Addedin Proof.)4. The Astronomical Problem-The LaboratorySolution

Grains interact with atoms, molecules, and electro-magnetic radiation in space. Let us consider first thephotophysical and photochemical interactions with elec-tromagnetic radiation in the vacuum ultraviolet. If weassume, for the moment, that a grain of -0.1-Fm sizeconsists of a dirty ice mixture of HzO, CH,, NH3 and CO,what would happen to it in space? The ultraviolet photonswith energies sufficientto photolyze these molecules wouldpenetrate the particle (only -2-3% of the photons areabsorbed in the outer few surface layers) and break themolecular bonds.29 This is schematically illustrated in thetop frame of Figure 5. Considering the reaction H,O +hv - H + H, we see that, if a photon with energy greaterthan the photodissociation energy is absorbed, the hy-drogen atom could come flying off and even escape fromthe grain leaving the radical OH, which could either reactwith a neighboring atom or molecule, or become immo-bilized before reacting because of the low grain tempera-ture. Similarly for the other molecules. In general, theenergies required to photodissociate molecules are of theorder of 4 eV or greater. The frozen radicals are chemicallyvery reactive, and should two of them be adjacent to eachother they would generally combine since radical-recom-bination reactions proceed with zero activation energy, andin so doing release energy to the grain. The possibility ofrecombination is shown in the second frame of Figure 5where, for example, combining the hydroxyl radical withthe methyl radical leads to the new, and more complex,molecule CH30H. The continuation of this type of processleads to a grain with new molecules and frozen radicals aspictured in the last of the sequence shown in Figure 5. Theconditions in interstellar space are such that this phe-nomenon should be important almost everywhere becausethe time scales for this process are generally very shortcompared with the interstellar time scales. An extremecase is given by considering placing the grains in the diffusecloud (DC) medium where the ultraviolet flux given inTable I for Ehv> 6 eV is = lo 8 cm-2s-l. In a cloud,this flux is reduced by an attenuation factor e-w. A lowerlimit of 6 eV is used as a convenient and perhaps con-

The Journal of Physical Chem istry, Vol. 87 , No. 21, 1983 Greenberg et al.

(27) Allen, D . A.; Wickramasinghe, D. T. Nature (London)1981,94,(28) W hittet, D. C. B.; Bode, M . F.; Evans, A.; Butchart, I. M on. Not .(29) Greenberg, J. M. Infrared Astronomy; Set ti, G .; Fazio, G. G. ,

539.R. Astron. SOC. 981,196, 819.Ed. ; Reidel Dordrecht: Holland, 1978; p 51 .

TABLE I : C o m p a r is o n b e t w e e n L a b o r at o r y andInterste l l ar Condi t ionslaboratory interste l l ar

grain mantl ein i t ia l composi t ion C O , H,O, N H , , C H , al l condensib les p e c i e sth i c k n e s s , p m 0.5-10 p m EO . 1t e m p , K 2 10 2 10gas: pressure of 5 x l o -@ 3 n [ H ] xc o n d e n s i b l e s p e c i e s ,m b a rul travio l et f lux (A < 10l5 10 d i f fu s e200 n m ) , c m - s - m e d iu m )t ime scalesd i f fu s e c l o u d s l h l o 3 yrm o l e c u l a r c l o u d s 1 h -104-106 rservative critical value for a photodissociation energy.Reducing this to 4.5 eV results in a doubling of the ul-traviolet We define the photoprocessing time (rpp)as the time i t takes for the number of photons absorbedby the grain to be equal to the number of bonds in thegrain. This photoprocessing time is given by

- d 3 ~ a 2 @ D Ce - r ~d3@Dce-Tu~where a is the grain radius and d is a molecular diameter.For a = 0.1 pm and d = 3 A, the value of T ~ , ,n the diffusecloud medium where 7uv = 0 is only about 200 years whichis indeed very short. The production of complex moleculesand radicals in grains leads to physical and chemicalprocesses which play a role, not only in determining thechemical constituents of the grains, but also those of thegas. In order to understand and quantify the relevantphenomena one must study the effect of ultraviolet pho-tons on materials under conditions which prevail in in-terstellar space. Early attempts to do this at temperatures(2 N 28 K) approachingthat of the grains and with photonenergies -7.5 eV proved that this method would work.l0J4Similar experiments performed at rather high tempera-tures (2 77 K) and somewhat lower photon energies (EI eV) also showed interesting results in terms of theproduction of large, complex molecules.30The Astrophysical Laboratory at the University ofLeiden which was established in 1975is the first to succeedin simulating the essential conditions in interstellar spaceas they affect the evolution of interstellar grains. Aschematic of the main elements of the experimental setupis shown in Figure 6. The key components are the cryostatand the ultraviolet sources. The low temperature isachieved by means of a closed cycle helium cryostat withwhich one reaches temperatures as low as 10K on a coldfinger which can variously be an aluminum block ortransparent window mounted in a metal ring. Variousgases may be controllably allowed to enter the vacuumchamber of the cryostat starting pressure is 5 X mbar)via a small tube. These gases condense as a solid on thecold finger which acts then like the core of the interstellargrains. On one port of the chamber is mounted a vacuumultraviolet radiation source which, until now, has almostexclusively been a microwave excited hydrogen flow lampwhich has a sharp emission peak a t 121.6 nm (Lyman a)and a broad component centered at about 160.0 nm, aspectrum which reproduces quite well that in the diffuseinterstellar medium.31 The normal flux of vacuum ul-

4a-4/ )na3-

(30) Khare, B.; Sagan, C. Molecules in the Galactic Environment;(31) Grewing Diffuse Matter in Galaxies; NATO Advanced StudyGordon, M. A.; Snyder, L. E., Ed.; Wiley: New York, 1973;p 399.Institute; Cargese: France, in press.


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Ice s in Space

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4240 The Journal of Physical Chemistry, Vol. 87, No. 21, 1983

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I I30003500 v (cm-1) hlFlgure 2. Observed extinction and polarizatlon in the Becklin-Neu-gebauer infraredsource n the Orbn nebula. Polarization from ref 25a(open circles) and ref 25b (fllled circles). Extinction from ref 26.chemiluminescence, mass spectra, and visible absorption.Further details of the equipment may be foundA comparison between laboratory and interstellar con-ditions is made in Table I. The most important, butnecessary, difference is in time scales for photolysis. Onehour of radiation in the laboratory is equivalent to 1000years in the diffuse medium and longer in denser clouds.The basic mode of operation consists of the depositionof mixtures of simple volatile molecules-CH4, CO, HzO,COz, NH3, Nz, 02-with and without simultaneous irra-diation as they freeze on the cold finger. Sometimes ir-radiation is continued after deposition is stopped. Wesimulate in this way the accretion and photoprocessing ofgrains in molecular clouds. The principal laboratory se-quences and operations are the measurement of the fol-lowing:(1) infrared absorption spectra of pure substances andmixtures at 10K to study how molecular interactions affect

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(32) Hagen, W.; Allamandola, L.J.; Greenberg, J. M. Astrophys .Space Sci. 1979, 65, 215.

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The Journal of Physical Chemistry, Vol. 87, No. 21, 1983 4249

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2Flgure 5. Schematic evolution sequence for a grain mantle at 10 Ksubjected to ultraviolet photolysis. The processes illustrated arephotodissociation, radical-radlcal recombination, and the productlonof new molecules and radlcals.

\I Vacuum ,Detector

AbsorptionSpectrum HO .CO.NH,

U 0Loboro ory InterstellarFigure 8. A schematic of the aboratory analogue method for studyinginterstellar grain evolution Is shown in the upper haif. Molecules aredeposited onto a 10 K cold finger and Irradiated by uitraviolet photons.The optical properties of the 3.1-pm ice band are deduced from theinfrared abswptlon spectrum. The lower half compares the conflgu-ration of the ice prepared in the laboratory with that in space.(3) infrared spectra of irradiated material followingwarmup to follow the disappearance of frozen radicals andformation of new molecules(4 ) visible and ultraviolet absorption spectra of irradi-ated and warmed up samples(5 ) chemiluminescence (visible) and vapor pressure si-multaneously during warmup of irradiated and, for com-

Loo0 3500 v Irmi'l 2500Figure 7. Absorption of amorphous ice, H,O(as), at 10 K and crys-talline ice, H20 IC). The dots show the shape of the 3.1-~m bsorptionband in the Becklin-Neugebauer (BN) object.parison, unirradiated samples(6 ) explosions produced in the warmup period

(7) infrared and mass spectrometric analyses of complexnonvolatile residues remaining after warmup to roomtemperature(8) visible absorption spectra of nonvolatile residuesWe shall indicate briefly in the following some sampleresults from the laboratory.(a) The HzO ce Band. Interpretation of the observa-tions of the 3-pm ice band (0-H.stretch) requires aknowledge of the absorption properties of H 2 0 n variousmixtures and a t various temperatures relevant to inter-stellar dust. The first complete measurements of the op-tical properties of pure solid H2 0 9provided an importantguide to the early observations. However, because theywere made for pure crystalline ice rather than for ice asit occurs naturally in interstellar space they led to some

apparent inconsistencies in shape and position of the iceband which even led to suggestions that the ice band maynot be due to H,O at all.33In the Leiden Astrophysics Laboratory it has beenpossible to study ices under conditions which match thoseof interstellar space. We have started first with pure HzOice even though generally H20 must occur in mixturesalong with other molecules in interstellar grains. This workhas served as a bench mark or standard with which tocompare various mixtures prepared under similar condi-tions. These studies have appeared in detail in a numberof p ~ b l i c a t i o n s . ~ ~ ~ ~ ~ ~ ~ ~e shall summarize here for com-pleteness a few of the critical results with emphasis on thespectral features around 3 pm. Further details may befound in Tielens et al. (this issue).One of the most important aspects of interstellar ice isthat it forms and exists mostly at extremely low temper-atures, T N 10K. The HzO ice deposited very slowly atthis temperature is extremely amorphous (there are variousdegrees of amorphicity) and accounts for the fact that theabsorption due to the OH stretch is about twice as broadas hat of crystalline ice (Figure 7). Annealing the sampleup to 80K (still amorphous) results in both a shift in peak(33) Mukai, T.;Mukai, S.;Noguchi, K. A s t r ophys . Space Sc i . 1978,(34) Hagen, W.; Allamandola, L. J.; Greenberg, J. M. A s t r on . Astro-(35) Hagen, W.; Tielens , A. G. G. M.; Greenberg, J. M. C hem. P hys .

53, 77 .p h y s . 1980, 86, L3 .1981, 56, 367.


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4250 The Journalof Physical Chemistry, Vol. 87, No. 21, 1983 Greenberg et al.nT = 3p(1 - )13[4 + (1- ) ]

For f = 0.15 only about 10% of all the solute moleculesform trimers so that the expected value of the relativeabsorptivity of Hz O (at -3.1 pm) is f e f f = mi/ml = (f - 0.15)/0.85 shown in Figure 8. With rarepossible exceptions (as might be produced by high abun-dances of NH 3 n the mixture) the relative strength of theexpected ice absorption in mixtures should be well belowthat of equal amounts of pure HzO and this effect becomesmuch stronger as f gets smaller.( b ) In-S i tu Product ion of N ew Mo l ecu l es a n dRadicals-Infrared Spectra. In Figure 9 are shown someresults of the laboratory analogue studies of dust evolution.The absorption spectra in this figure show first an unir-radiated sample and then, for comparison, the spectra afterirradiation where the appearance of new molecules andfrozen radicals is evident. We see, for example, thatmolecules such as formaldehyde and formamide are readilycreated and it may be inferred that much more compli-cated molecules are also being produced at the low tem-peratures. Their presence is clearly indicated afterwarmup as shown in the upper sequence where, as themore volatile molecules are evaporated, the absorptionspectrum takes on the very different character shown inthe two upper right spectra. Both samples show that theHCO radical is easily created and probably plays an im-portant role in subsequent stages. The sample illustratedin the lower half shows that HzCO is produced in the coldsolid in quantities which may even become comparable (inthis sample) with the H 2 0 and N H 3 as indicated by thecomparable absorption intensities in the 1600-cm- region.(c ) Explos ions o f Irradia ted Sample s . It was earlydemonstrated from both chemiluminescence and pressureenhancement in irradiated materials during warmup thatenergy was released not only as visible light, but also inthe form of heat produced by radical-radical or radical-molecule reactions. Only about of the energy is re-leased as visible light. The fact th at these reactions arediffusion controlled is also clear from the observation thatthe luminescence stops immediately upon recooling anddoes not resume until a subsequent warmup again reachesthe temperature at which the luminescence has stopped.Explosive events can be systematically produced in thelaboratory by ensuring that the reaction energy is notconducted away from the sample too rapidly.38 An ex-ample of the pressure and luminescence behavior in sucha sample is given in Figure 10 showing simultaneouspressure spikes along with light flashes. When such eventsoccur, essentially all the sample is blown off of the coldfinger. It has been noted tha t the explosions appear tooccur for a variety of different samples at temperaturesof T = 27 K. From infrared measurements of the shapeof the NH, absorption in the remaining material we haveestablished that the temperature overshoots to at least- 0 K during the explosion thus clearly demonstratingindependently the tremendous energy release which occurs.An important criterion for the explosion to take place isthat the number of ultraviolet photons absorbed in thesample be at least 1/10 of the number of molecules in thesample.( d ) Complex Organic Residue. As was illustrated inFigure 9, during the photoprocessing of the grain mantleAstron. Astrophys. Lett . 1982, 109, 21 .(38) dHendecourt, L.; Allamandola, L. J.;Baas, F.; Greenberg, J. M.

0 0.5f 1.0Figure 8. Approximate dependenceof the peak value of on dilutionfor ice mixtures containing a fraction, f, of H,O.T A B L E 11: O p t i c a l C o n s ta n t s for V a r i o u s H,OMixturesat Peak Absorpt ion (- 3 fim)

~~ ~hal f -w i d thA v ,

f H , Oa f o r m c m - Im e a s I T h1 crysta l l ine 150 0.8151 a m o r p h o u s 1 0 K 300 0.5 0.50.75 a m o r p h o u s 1 0 K 320 0.29 0.350.42 a m o r p h o u s 1 0 K 350 0.12 0.16a f ~ , os th e f r a c t i o n of H,O i n th e m i x tu r e .

absorption and an increase in intensity (at the peak) aswell as a decrease in width. Thus the optical propertiesmeasured at liquid nitrogen temperature^^^ differ notice-ably from those at 10 K. It is important to note tha t the6-pm band isnarrower a t 10 than at 80 K (opposite to thebehavior of the 3-pm band) and that the librational ab-sorption at -800 cm-l of unannealed HzO is shifted tolower frequencies, broadened, and reduced in intensityrelative to that of annealed forms of solid H 2 0 . Theseeffects are enhanced in mixtures.The values of mand m (the real and imaginary partsof the index of refraction) at 3280 cm- for pure amorphousice (deposited and maintained at 10 K) are 1.31 and 0.477,respectively. These are to be compared with the values1.365 and 0.663 obtained by Ldger et alasat 3240 cm-. InTable I1 are shown some measured and theoretical ex-pectation values of the strength of the 3.07 pm H 2 0 ab-sorption in pure (f = 1)and mixed ( f < 1)samples wheref is the fraction of HzO in the mixtures. We see that themeasured value of the ratio m i / m rl which is the ratio ofthe absorption per unit mass of HzO in the mixture relativeto that for an equivalent amount of pure HzO is, as ex-pected, less than unity. Thi s is because, as the ice is di-luted, larger fractions of the molecules of H20 annotcombine to form the oligomers which give the 3.07-pmabsorption.37a An extreme case of this is illustrated the-oretically by a very dilute system. Be l~ in ge l. 3~ ~as shown,for example, tha t in a simple cubic lattice (and similarlyfor other structures) the statistical concentrations of mo-nomers, dimers, and trimers as a function o f f are

n M = f(1 - f yn D = 3 f ( l - f)

(36) Jkger, A.; Klein, J.;De Chevergne,S.; Guinet, C.; Defourneau, D.;(37) (a ) van Thiel, M.; Becker, E. D.; Pimentel, G. C. J.Chem. Phys .Belin, M. Astron. Astrophys. 1979, 79, 56.1957, 27, 86 . (b) Behringer, R. E. J . Chenz. Phys. 1958, 29, 37.


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Ices in Space The Journal of Physical Chemistry, Vol. 87 , No. 21, 1983 4251

I I 1 Ib l I 1 1 12 HOUR i n . s i l u PHOTOLYSIS

A = 1400-3000

F R E Q U E N C Y c m ) 11800 1800 1400 1200 1000

F R E Q U E N C Y c m ) 1

ICWUJIIOM .e4 v [WI] -Figure B. Infrare d absorption spectra of tw o different ice mixtures showing the effect of photolysis. Left side of upper sequence and the twolower s pectra show first the features in unirradiated samples and then the s pectra of the irradiated samples showing the in-situ production ofnew m olecules and radicals. The upper right-hand pair of spectra clearly indicate the presence of complex molecules whose spectra becomeevident as the more volatile species are evaporated away during warmup.analogue material more and more complex molecules arecreated. When the volatile components in the sample areevaporated away by warming there always remains anonvolatile residue material. If one starts with a mixtureof CO:H20:NH,:CH4 (10:1:2:6) the ultimate residue ap-pears yellow. We have obtained infrared absorptionspectra of nonvolatile residues with various initial com-positions. However, we have not followed up on thephotoprocessing by examining the results of continuedultraviolet irradiation of the residues themselves. Nev-ertheless what we have already done provides some im-portant qualitative answers to the grain composition. Oneof our samples had a molecular weight of 514 and all ofour samples do not evaporate at temperatures less than-400 K with at least one pyrolyzing, without evaporating,at 600 K. The infrared absorption spectrum of a residue

is shown in Figure 11 and comparison is made with thespectrum of the original unirradiated mixture. The relativeabsorption strengths have been normalized by equatingthe integrated absorptions of the unirradiated sample andthe residue between 2000 and 1000 cm-. This region ischosen to avoid the H 20absorption enhancement whichappears in the 3000-cm-l region. We identify the verybroad absorption from 3500 to 2000 cm- as due to car-boxylic acid groups and a number of absorptions betweenabout 1200 and 1500 cm- as due to amino groups. Thefeatures around 3.4 pm indicate the presence of -CH3 and-CH2- groups.A high-resolution mass spectrometer analysis of thelowest pressure component in the residue whose infraredspectrum is given in Figure 11showed, after warmup andC 0 2 elease of about 60 C, a mass corresponding to C4H6N2


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4252 The Journal of Physical Chemistty, Vol. 87, No.27, 1983 Greenberg et al.the clouds. From the fact that the clouds can not bestatic-either because they are observed to be in motion,or because we infer or see such dramatic energetic eventsas star formation occurring within them-it is obvious thatthe physical conditions of density and temperature rep-resent different stages in their evolution (see middle col-umn of Figure 12). The densest part of clouds seems tocorrespond to times just before, during, or just after starformation. Diffuse clouds become dense by a number ofmechanisms, perhaps as a result of collisions or somesource of external pressure.40 Within the dense cloudscritical densities may be reached which lead to instabilitiesand further contraction and finally to star f ~ r m a t i o n . ~ ~After the stars form-if they happen to be large hot starsor if they develop high material ejection speeds by pro-cesses other than radiation (accretion disks for exam-ple)-the remaining local material from which they havebeen formed is ejected into the surrounding space.42 Muchof this material, being heated and finding itself in a verytenuous low-pressure environment, expands to reappearas diffuse clouds.Should the silicate particles appear first in a diffusecloud region no gaseous material will accrete (o r remain)on them because of the harsh environment. Sticking ofatoms or molecules from the gas on the grains only beginswithin molecular clouds where it proceeds simultaneouslywith the process of ultraviolet photolysis. We are now atthe s tar t shown at the top of the left sequence in Figure12 . After some period of accretion and photoprocessingtwo grains will collide with each other at suprathermalspeeds resulting from turbulent gas m0tions,4~which raisesthe temperature enough to trigger grain explosions asdescribed in section 4. Following the explosions-whichmay be complete or partial-the grains now have mantlesof various composition and thickness all the way down tothe original silicate core. In general, a residue of complexorganic material will be gradually built up on the silicatecores, each successive generation within the cloud leadingto an additional layer. Since the indication from thelaboratoryis that the order of 2 % to 20% of the condensedmaterial is converted to the nonvolatile residue each lo7years we may assume that a substantial mantle thicknesssay a significant fraction of the 0.07 pm required in themean for diffuse cloud grains, will have been accumulatedin the course of time the grain is in the dense cloud, whichis of the order of lo8years.We shall assume that the total cycle time for a grain topass through the diffuse cloud phase and the subsequentmolecular cloud phase is -2 X lo8years with about halfof this time spent in each. The maximum age of a grainis limited because ultimately all interstellar material isrecycled through the birth of stars in which all particlesare fully evaporated. The turnover time, based on starformation rate estimates, is of the order of 5 X lo9years,44so that a typical grain should go into and out of a molecular

(40) (a) Field, G. .; Saslaw, W. C. Astrophys. J . 1965, 142, 568. (b )Kwan, J. Ibid. 1979, 229, 567. (c) Oort, J. H. Bul l . As t ron. Inst. N e d .1954 ,12,177 . (d) Scoville, N. Z. ; Hersh, K. Astrophys . J . 1979,29,578.(e) Taff, L.; Savedoff, M . M onthly Notices of the Royal AstronomicalSociety 1972, 160, 89. (0 Taff, L.; Savedoff, M. Ibid. 1972, 164, 357.(41) (a) Woodward,P. . Annu. Rev. Astron. Astrophys. 1978,16,555.(b) Bash, F.N. strophys. J . 1979, 233, 524.(42) (a)Blitz, L.; Shu, F. H. Astrophys . J. 980, 238, 148. (b) Lada,C. J.; Harvey, P. M. bid. 1981 ,245 ,58. (c) Heydari-Malayeri, M.; Testor,G.; Baudry, A.; Lufon, G.; de laNoe, J. Astron. Astrophys. 1982,113,118.(43) (a) Greenberg, J. M . In Stars and Star Systems; Westerlund, B .E. , Ed.; Reidel: Dordrecht, 1979, p 173. (b) Volk, H. J.; Jones, F. C.;Morfd, G. E.; and Roser,S. Astron. Astrophys. 1980,85,316. (c) Lichten,S.M. Astrophys. J . 1982, 255, L119.(44) Oort, J. H. In Recent Radio Studies of Bright Galaxies,Shakeshaft, J. R., Ed.; eidel: Dordrecht, 1974; p 375.

C 0 : H 2 0 : N H 3 : C H4 (10:1:1:4)3x110 MINUTE DEPOSIT A N D 2 HOUR P H O T O L Y S I S )

P R E 5 5 U R E I

2 6 2 8 30T E M P E R A T U R E ( K )Flgure 10. Correlation of chemiluminescence flashes with pressurebursts during warmup of the mixture CO:H,O:NH,:CH, (10 1:1:4) whichwas prepared by three cycles of a 10-min deposition followed by 2 hof photolysis at 10 K .and traces of urea. The intensity ratios suggest that am-inopyroline rings make up a substantial part of this ma-terial. Undoubtedly this material will undergo furthermodification when subjected to continued ultravioletbombardment.High molecular weight molecules in liquid or solid formhave significantly higher real values of the index of re-fraction than the volatile ices. We have not yet measuredthis for these residues but a survey of the data for othercomplex molecules shows that we can expect a value of mof at least 1.40 and perhaps greater than 1.5.39 We haveadopted a preliminary estimate of m 1.45 in our cal-culations.5. Grain EvolutionThe birth, growth, and death of interstellar grains is acontinuing process so that what we see in various regionsof the Milky Way is always a passing phase. Whether inthe vast, almost empty region between the stars or in thedense active regions of new star formation, the grains arealways undergoing change. So, how is it that there arecertain uniform observational properties of the dust whichone can associate with diffuse clouds and, on the otherhand, what are the processes which lead to the differenceswe observe in molecular clouds? How can we account forthe presence or absence of H 2 0 ice; how can we accountfor the variations in the infrared absorption spectra whichindicate not only H20 but also other molecular species?These and many other questions can only be answered bya full theory of the evolution of grains in space. One ofthe principal new factors which is making it possible tobegin to answer these questions is the application of thelaboratory results as explicitly shown in the followingscheme.We star t with the bi rth of a grain, assumed here to bein the form of an elongated silicate particle of -0.05-pmradius. Such small particles are formed in the atmosphereof cool evolved stars and blown out by radiation pressureinto the surrounding space. Ultimately they are swept upinto the general gaseous matter and, being coupled to thegas atoms and ions, begin to partake in the evolution of

(39) Handbook of Chemistry and P hysics; The Chemical RubberCo.: Cleveland, 1966-7; 47th ed.


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Ices in Space The Journal of Physical Chemistry, Vol. 87, No. 21, 7983 4253I I I I I Ico

1 1 I 1 I 1 1 I3000 2000 1000F R E Q U E N C Y cm"

F W e 11. Comparison of the infrared spectrum of a mixture at 10 K, before photolysis, with that of the nonvolatile resMue remaining after photolysisa i d subsequent' warmup to room temperature under vacuum.cloud-diffuse cloud cycle approximately 20 times duringits lifetime. Because of this it becomes possibletomaintaina steady state of particle types in diffuse clouds. Fur-thermore it is statistically more realistic to have started(as shown in the left sequence of Figure 12) with a core-mantle particle than with a bare core particle. In the nextsection we shall look more closely at the individual stepswe have proposed above for the chemical and physicalprocessing of grains and what the grains probably look likeat various key stages. Whatever processes are expectedto take place in each region are limited to those which canoccur in the maximum time interval of - o8years unlessotherwise stated.6. Appearance of Grains at Various StagesWhat we shallemphasize here is the observational statusof HzO in the interstellar grains based on their changingchemical and physical structure from one region of spaceto another. The following models are derived from thegrain evolution scheme described above. They provide thefoundation for quantitative calculations some of whichhave already been done,16 and some of which are presentedin section 7.( a )D us t in Diffuse Clouds. Very few volatile moleculesmay be expected to remain on a grain during and subse-quent to ejection from the star formation region. If anydo remain they should have been sputtered or evaporatedaway relatively rapidly in the diffuse medium so that allthat can remain is the organic residue. We thus picturethe grains in diffuse clouds as consisting of silicate coreswith mantles exclusively made up of an organic refractorymaterial. Under normal interstellar conditions this ma-terial willcontinue to be subjected to ultraviolet photolysisso that its composition may undergo still further changesbut it is tough enough to remain as a substantial mantlefor times longer than the normal lo8years spent in thediffuse matter.45 The spectrum of a first generationresidue is already very different from that of any H,O icemixture. For one, its broadest structure, centered at about3 pm, is at least 800 cm-' wide as compared with the -300cm-' characteristic of the interstellar ice absorption.Secondly it exhibits a higher degree of complexity in the

(45) (a) Draine, B.T.;Salpeter, E. E. Astrophys. J. 1979,231,77. (b )Ibid. 1979, 231, 438. (c) Greenberg, J. M. "Submillimeter WaveAstronomy"; Beckman, J. E.; Phillips, J. P., Ed.; Cambridge UniversityPress: Cambridge, England, 1982, 261.

absorptions in the 3.4-pm region. I t is of interest to notethat the spectra of residues which are produced from ir-radiation of dirty ices in which the initial amount C in CHIis comparable with the C in CO, bear a closer resemblanceto some of the galactic center featuresn than do the spectraof residues resulting from irradiation of ices in which Cis mostly in the form of CO. This leads us to expect thatwhat we see in the interstellar grains which have passedthrough n cycles (n< 20 ) has been so greatly photolyzedduring the 2n X los years spent in the diffuse cloud me-dium that the carboxyl groups which show up as the verybroad 3-pm feature in the first generation residue shownin Figure 9 are a t least partially destroyed leaving a ma-terial which structurally approaches complex hydro-carbons.In any case, the evolutionary picture of diffuse cloudgrains having generally emerged violently from protostellarregions leads to the expectation that there is no H20 obe found in their mantles even though the grains probablycontain a substantial fraction of oxygen bound up in theresidue. A schematic representation of a typical diffusecloud grain is shown in Figure 13.( b ) D us t in Molecular Clouds. From the kinds ofchanges observed in the wavelength dependence of ex-tinction and polarization, one readily deduces that grainsin molecular clouds are larger than those in diffuseclouds.18 Therefore the already observed diffuse cloudgrains must be the cores on which additional accretiontakes place within the molecular clouds. The question iswhat is the likely chemical composition of the extra ac-creted material? It turns out that the diffuse cloud grainsas modeled in Figure 13 contain a smaller fraction of thecosmically available oxygen than of the carbon and ni-t r ~ g e n . ~ ? ~ ~n fact so little of the oxygen is tied up indiffuse cloud grains that, whereas the cosmic abundanceratio of oxygen to carbon is OCA:CCA = 6.8:3.7 e :1, thegas which is in the state of contracting toward the densemolecular cloud probably has 0 : C = 5:1! It is thereforeto be expected that the mantle material in molecularclouds must be superabundant in oxygen and consequentlyone might expect a significant portion of this oxygen tohave formed as HzO either within the gas or on the grainsurface. Why, then, has it been difficult to detect solid

(46) De Boer, K. S.Astrophys. J.1980,224, 848.


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4254 The Journal of Physical Chemistry, Vol. 87 , No. 21, 1983 Greenberg et al.

\

\ccf+hoto-R D c e s s e d m+ Stored R adcal s

lgoiocdl isionlChain reoctmn

by Ehpomtionor ExplosionI

\ I\ I

J

I /UltmViOW photon

I rrat iat ion

Accretion of A t o m s tMobcules. I rradiat ion

Grain - GrainCOl l lS lon

Molecule EjectionF a r m t i o n ofNonvolat ik YellowResidue

Grains with NmbdatileH ont l es

Flgure 12. Schematic diagram of grain and cloud evolution. The sequence on the lefl corresponds to the m olecular cloud phase and that onthe right shows how the grains evolve through the molecular cloud and star formation phase and then back to the diffuse cloud phase.H20 n molecular clouds? The answer to this question willdepend on several factors: ( 1 ) the use of a proper modelfor grains in a molecular cloud; (2 ) the relatively small totalextinction available in a molecular cloud not surroundinga protostellar object; ( 3 ) the effect of molecular dilutionon the absorption strength of H20 n a mixture.The schematic representation of a molecular cloud grainis shown in the middle of Figure 13 . We shall apply thismodel to the specific example already mentioned in section3, namely, the star HD 29647 in the Taurus dark cloud.The color excess (amount of reddening) of this star isE (B -V ) = 1.00. For normal sized grains in diffuse cloudsthis would imply a total extinction in the visual of 3. 1magnitudes. However, what is observed is an extinctionof 3.5 magnitudes.28This implies that the grains are largerthan the diffuse cloud grains by about 10% so tha t on top

of the base organic refractory mantle of -0.12-pm radiusis an additional layer which is possibly almost pure H2 0with thickness about Aa = 0.01 pm; i.e., the total grain hasa radius of -0.13 pm. The strength of the ice absorptionis defiied as A(3.07) . However, in ref 28 the measured icestrength is defined in terms of [A (3 . 0 7 )- A ( V ) ] / E ( B - V )E E ( 3 . 0 7 - V ) / E ( B - V ) . Theoretically one can relate thevalue of A ( 3 . 0 7 ) / A ( V )o the quantity E (3 . 0 7 -V ) / E (B -V )by

Since the absorption by smal l grains is proportionalto theirvolume and their extinction in the visual is proportionalto their area one may show that, for the molecular cloud


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Ices in Space The Journal of Physical Chemistry, Vol. 87 , No. 21, 1983 4255

MANTLEu-0.2Lwm --.3vm - 4 5 0.Lp-n cGRAINS IN DIFFUSE CLOUDS GRAINS IN MOLECULAR CLOUDS GRAINS AROUND PROTOSTELLAR OBJECTSFigure 13. Schematic representation of the types of grains expected in different regions of the interstellar medium.grain model of Figure 13, the ratio of ice absorption to theextinction is5A(3.07) 16.rr a z 3 - ai3 e2 EE - -A(V) (3.07) a; ( e l + 2)2 + t2 2

t 2(e1 + 2)2 + t2 2 (6.2)167- Aa(3.07)

Aa = a 2- al


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4256 The Journal of Physical Chemistty, Vol. 87 , No. 27, 1983 Greenberg et al.appears to be in the range of ob se r~ ab il it y. ~~owever,as discussed in ref 48, the detection limit depends not onlyon the column density but also on how the molecules areexcited in the cloud itself, so that here again we may befaced with a difficult observational problem. Is it just asdifficult to detect gaseous H 2 0as it is to observe solidH20in nonprotostellar clouds? Actually it is inferred tha twhere H2 0 s observed in the gas phase i t is considerablyless abundant than in the solid grain^.^(e ) D us t in Protostellar Regions. As stated in section3, it is only around protostellar objects that one generallyfinds substantial H 2 0 ice absorption features. Two bigfactors which favor this observation are the following: (1)there is a strong continuum infrared background sourceand (2 ) most of the line of sight extinction originatesreasonably close to the object and is always high. Anotherfavorable factor is that the grains appear to be substan-tially larger than the average, thus implying the presenceof thick grain mantles. For example, in the BN object theyare at least 25% larger in radius than the diffuse cloudgrains.How the grain mantles have attained their structuredepends on how grains evolve from the molecular cloudthrough the prestellar through to the early protostellarphase. During the not too dense molecular cloud phasethe grains tend tomaintain a steady-state size distributionbased on the equilibrium between grain growth and grainexplosions. Ultimately as the cloud becomes very densein the approach to final collapse, the grain explosions areno longer possible because the ultraviolet radiation isdrastically reduced. In this collapse stage the grains mayaccrete all the remaining condensable constituents fromthe gas. In the ensuing stage of star formation those grainswhich are very close to the star will have their volatilemantle constituents evaporated while those grains whichare further away from the young star will be shielded fromthe stellar radiation and can be cool enough to maintainthe H 20 n their outer mantles and, for some time perhaps,even more volatile species. This leads to the grain modelpictured on the right in Figure 13.If we use such a model, along with grain size parametersand extinctions deduced for the BN source in Orion-which appears in the infrared much like the ones shownin Figure ?--an approximate calculation leads to the ex-pectation d u e of A(3.07) of about 3.5 magnitudes if thetotal outer mantle is pure H20.45cThe observed value isabout 1.5 rmgnitudes so that we deduce a mean effectivefractional concentration of H 2 0 of 0.43 which, accordingto Figure 8, gives an actual H 2 0 concentration of about5290,a value which is quite reasonable considering the highabundance of oxygen relative to all other species in mo-lecular clouds.An interesting corollary can be derived from this result.Recalling that the observed protostellar dust is what hasremained in the environs of the new star after its birth,we may assume that in its earlier form during the verydense cloud stage it was the material out of which objectslike comets could have coagulated from the cool materialin the protostellar neighborhood. Thus if this picture iscorrect the ices in interstellar dust would be the directprogenitors of the ices in comets and we have returned fullcircle to conclude that all the ices in space have a commonorigin.There is another possible source of H,O ice which wehave not considered here, namely, the ice which may

condense as pure amorphous H2 0 n circumstellar shellsaround late type stars. This form has been briefly con-sidered in ref 16. We note tha t such ice is ephemeral inthe sense that it is quite local and is not likely to maintainits integrity long enough in interstellar conditions to con-tribute significantly to the material composition of acometary nucleus.7. Grain Modeling for the Ice Band

The ingredients which are required to model the iceband in a grain are (1)grain dimensions-core and mantleradii, (2 ) optical constants of core and organic refractorymaterials, (3) optical constants of various ice mixtures inthe 3-pm region, and (4) a computational scheme forcalculating the scattering of electromagnetic waves bylayered nonspherical particles. Because the grains arenonspherical as shown by their polarization of starlight inthe 3-pm as well as in the visible regions, it is necessary,for a complete picture, to calculate the extinction crosssections as a function of wavelength for aligned non-spherical particles. The question of how, and to whatextent, the grains are aligned will not be considered here.This is briefly outlined in ref 16a and discussed in moredetail in Aannestad and Greenberg.lGbOur approach willbe to consider rather the extinction effects of grain mantlestructure on perfectly aligned elongated particles.In general a particle removes light from a beam eitherby deflecting it (scattering) or by attenuating it within theparticle (absorption). We denote the cross sections forscattering and absorption by C,,, and Cabs, respectively.The total (extinction) cross section is the sum of the two:C = C + Cabs.For spherical and spheroidal absorbingparticles, small compared to the wavelength (27~a/X5 0.3where a is some semidiameter), Cabs >> C,,, so that C,,,N Cab For larger particles C,, may become comparablewith Cabsnd, since the wavelength dependences of cabsand C are different across an absorption band, we expectto find that the shape of the extinction will depend onparticle size aswell as on particle optical properties. Since,for simplicity, many calculations have been performed forspherical particles (for extinction only, of course) it willbe of some interest to compare the results for spheres withnonspheres. Fo r nonspheres we also introduce a simpli-fication by limiting ourselves to core-mantle, infinitecylinders although, with considerably more calculationalcomplexity, one may obtain equivalent results for spher-oidal grains.49 It is customary to express a cross sectionin terms of an efficiency, Q, which is defined as the crosssection C divided by the geometrical cross section.Fo r spheres we shall calculate and present the quantitiesQZXtand Q,,, (superscript s denotes sphere) while, for in-finite c linders, we shall calculate the basic cross sectionsQfxt, d , :,, and QEawhere E and H indicate that theparticles are aligned with their axis parallel respectivelyto the electric and magnetic fields of incident plane po-larized radiation. We shallpresent the results for cylindersin the form of extinction, scattering, and polarization byperfectly aligned particles for an unpolarized incidentsource. These are, respectively

(48) Waters, J. W.; Gustincie, J. J.; Kaken, R. K.; Kuiper, T. W . B.;Roscoe, H. K.; Swanson, P. N. ; Rodrigues Kuiper, E. N.; Kerr., A. R. ;Thaddeus, P. Astrophys. J. 980, 236, 57 . (49) Onaka, T. Ann. Tokyo Astron. Obs., Second Ser . 1980,18, No . 1.


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Ices in Space The Journal of Physical Chemistry, Vol. 87, No. 27, 1983 4257Q% - Q%

2c =We have calculated the shapes of the 3-pm absorptionbands for a variety of particle types. As representative ofthe molecular cloud and protostellar cloud grains we haveconsidered both spheres and cylinders with ice mixturemantles on cores which are the compound diffuse cloudcore residue grains (see Figure 13). The calculationalcomplexity has been reduced for the ice mantle grains by

replacing the silicate core-organic refractory (OR) mantle(compound core) by an equivalent homogeneous grainwhose size is the same as the organic refractory but whoseindex is a mean of tha t of the silicate and the OR material.The latter has some absorption in the 3-pm region whichwe have not considered here but which will be includedin a later paper. Thus we let the mean index of refractionof the compound core be given in terms of the phase shiftalong the particle diameter by (0.12)m = (0.05)msil+(0.07)moRwhere 0.05 pm is the core radius and 0.07 pmis the OR mantle thickness. Using msil= 1.6 and mOR=1.45 (an estimate based on the range of - .4to - .5 givenfor complex organic molecules in ref 39), we get m N 1.5.As representative of the complex index of refraction of theice mantle in dense clouds we have used that of a mixtureH20:NH3= 3:l deposited and maintained at 10 K (un-annealed) and tha t of the same mixture warmed to 50Kand recooled to 10 K hereafter called the annealed icemixture. As representative of H20as it might appear ifaccreted directly from the gas we have also used pure H20unannealed and annealed to 80 K. We have systematicallyfollowed the variation in the strength and shape of theabsorption band for spheres and cylinders with both pureand complex ice mantles ranging from 0.13- to 1-pm radiusand, for cylinders, the shape of the polarization as well.We have also shown under what conditions the scatteringmakes a significant contribution. It is beyond the scopeof this paper to present allof our results in graphical form.However, we have selected a number of examples to il-lustrate how the principal ice features vary with index ofrefraction of the pure ice or ice mixture, shape of theparticle, and size of the particle. It is obvious that the mostimportant single parameter defining the shape of an ab-sorption band is the optical property of the material. Thisis certainly true when we limit ourselves to the range ofnormal interstellar grain sizes which are 50.2 pm, suchthat, for the range 2.5 pm < X < 4 pm the value of 2?ra/Xis


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4250 The Journal of Physical Chemistry, Vol. 87, No. 21, 1983...

Q :Greenberg et al.

Q !0. I -

n n-

0. I ]

0 . 375 0 . 350 0 . 325 0 . 3 00 0 . 275 0 .h- ' (pm-')

Flgure 18. Extinction and scattering efficiencies of compound (s k a tei-organic refractory) core-ice mantle spheres with various forms ofice mantles. Silicate radius = 0.05 pm , OR radius = 0.12 pm, icemantle radius = 0.14 pm.0 .2 ex t po l sca....... Iy1 - ~ Inh ----r/q

0 . 375 0 . 350 0 . 325 0 . 30 0 0 . 275 0 . 250h-' ( p m - ' )

Flgure 17. Extinction, polarization (where applicab le), and scatteringefficiencie s of compound core-pu ice mantle sphere and alignedcircular cylinder. Compound core radius = 0.12 pm, mantle radius= 0.14 pm.Comparing the curves for mn in Figure 14 with theshapes of the extinction curves in Figure 15 we see thatthe latter approximately follow the shapes of the respectiveabsorptive parts of the index of refraction of the ice

mantles (although, see section 7b). This is seen in Figure16 to hold also for the particles with compound cores madeup of core plus organic refractory mantles, although thereare some minor differences between the shapes in Figures15 and 16. Thus for particles whose sizes are like thoseof typical interstellar grains, the wavelength dependence,or shape, of the observed extinction follows the shape ofthe material absorption. Note that the ratio of the ab-sorption efficiencies between Figures 15 and 16 is almostexactly equal (within6% )o their respective ice volumeratios.The ratios of the peak absorption efficiencies in Figure16 are proportional to the ratios of their respective valuesof m".( b )Effe ct of Shape. Comparing the extinctions by thesphere and the cylinder in Figure 17 we see that the peakposition shifts toward smaller X-' with increasing elonga-tion. For the cylinder itself the notable features are asfollows: (1 ) the maximum in the polarization is a t a sig-nificantly smaller X - l than is the maximum in the ex-tinction, (2 ) there appears to be a scattering contributionwhich is not zero although it is essentially zero (


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Ices in Space The Journal of Physical Chemistry, Vol. 87 , No . 21, 1983 4259J

Q !2-

0 . 375 0 . 350 0 . 325 0 . 30 0 0 . 275 0 . 250A" (pm-')

Flgwe 20. Extinctlon and scattering efficiencies of compound core-puice man tle spheres of various sizes. Compound core radius = 0.12pm , m antle radius = 0.14, 0.5, 1.0 pm .to H 2 0 . It is notable tha t the wavelength dependence ofextinction by elongated particles follows the wavelengthdependence of "'much more closely than that by spheresas has been shown earlier.52 Rayleigh approximationcalculations which are shown in Figure 19 for homogeneousspheres and spheroids can be used to qualitatively verifythe results in Figure 18. We note that both the elongationeffect on the ratio of the H 2 0 and NH 3peaks and on theshift in the H 2 0 peak to smaller A-' are confirmed. Alsothe peak polarization is at smaller A-' than t he peak ex-tinction. Note that the scattering contribution relative tothe extinction is indeed small (Cl%) s already shown inthe exact spheroid results.( c )Effect of Size. As the principal particle dimensionbegins to be large enough (2aalA - 1, rather than 2aalAC 1) he extinction profile deviates more and more fromthe material absorption profile. This is a well-known effectand has been amply demonstrated in the past by numerousnumerical calculations on interstellar grains related toabsorption bands in the visible (A - 0.5 pm). See, forexample, ref 2 and 53. Figure 20 shows thi s effect forspheres of radii 0.14, 0.5, and 1.0pm, the latter two beingsignificantly larger than interstellar grains. There maybe-and indeed must be-regions where such size particlesabound as can be inferred from the existence of cometswhich have probably aggregated from these particles.However, we do not believe tha t this has yet been dem-onstrated for the grains which exhibit the ice band and,in fact, the results shown in Figures 20-23 would appearto preclude such a possibility. Since the polarization inthe 3-pm band (as shown in Figure 2) demands non-spherical particles we should consider, say, cylinders withice mixture mantles rather than spheres with pure icemantles. Particularly striking in Figures 22 and 23 is thatthe long wavelength wing which is already present in the0.5- and 1-pm grains for pure unannealed ice (Figure 22)is grossly exaggerated when the large grain mantles consistof ice mixtures (Figure 23). In fact there is no longer thecharacteristically observed band width of about 300 cm-'but rather it is -500 cm-I for the 0.5-pm cylinder and evenlarger for the 1-pm cylinder. Finally we see again, com-paring the 0.5- and 1-pm cylinders with the 0.14-pm cyl-inder and the small prolate spheroid tha t, with increasingsize, the -2.9-pm structure is suppressed almost beyond

(52) Greenberg,J. M. J.Colloid Interface Sc i . 1972, 39, 513.(53) Greenberg, J. M.; Hong, S. . s trophys . Space Sci . 1978,39,31.

1 oo --0.275 0. ;. 375 0 . 350 0 . 325 0.300

A-' (pm-')0

-re 21. Extinction and scattering efficiencies of compound core-maice mantle spheres of various sizes. Compound core radius = 0.12pm, mantle radius = 0.14, 0.5, 1.0 pm.e x t pol sca

0 . 375 0 . 350 0 . 325 0 . 300 0 . 275 0 . 250

A-' (pm- ' )Figure 22. Extinction and scattering efficiencies and polarization byaligned compound core-pu ice mantle circular cylinders of varioussizes. Compound core radius = 0.12 pm, mantle radius = 0.5, 1.0pm .recognition. This structure is always suppressed in thepolarization even for small sizes. In Figures 24 and 25 weshow the kinds of variations in extinction and polarizationlimited to a range of mantle thicknesses which is about aswide as can be produced b y accretion in the interstellarmedium. Details in the shape vary in a small but signif-icant way as is, for example, seen by comparing the relativeheights of the NH , and H,O peaks.8. SummaryCombining laboratory measurements of complex iceswith theoretical models of interstellar grains based on apicture of how they evolve in space enables us to explainthe observed variability in the interstellar 3-pm water iceband. Thus an evolutionary tracking of interstellar grainswhich takes them from diffuse clouds through molecularclouds and back as part of a repeating cycle is shown tobe consistent with observations. All grains are presumedto have small silicate cores. The mantles of grains intenuous low density clouds are shown to consist of anorganic refractory material whose laboratory spectrumexhibits no H 2 0 ce band. The mantles in molecular cloudsconsist of an additional layer, or layers, on top of theorganic refractory material. Models of such layered dust


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4260 The Journal of Physical ch8mistty, Vol. 87, No. 21, 1983 Greenberg et ai.

e x t po l sca0.5 ~ -1 0

......... --.----- I0.375 0.350 0.325 0.300 0.275 0.250

A-' (pm- ' )Flgure 23. Extinction and scattering efficiencies and polarization bycompound core-mu ice m antle aligned circular cylinders of varioussizes. Compound core radius = 0.12 pm, mantle radius = 0.5, 1. 0Pm .

0.1 40.16 - - -0.18 .......

0.3 0.20 -

0.0 -. . . . . .0.375 0.350 0.325 0.300 0.275 0.250

h-' ( pm- ' )Figure 24. Extinction efficiencies and polarization by compoundcore-mu ice mantle aligned circular cylinders of various sizes. Com-pound core radius = 0.12 pm , mantle radii = 0.14, 0.16, 0.18, 0.20pm .grains are combined with laboratory measurements of thevariability of the 3-pm H20absorption in pure water iceand in mixtures to show how the observed ice band pro-vides a diagnostic probe of the evolution of grains. Asystematic study has been made of the effects of particleshape and size as well as index of refraction on the strengthand shape of all the principal observed infrared structuralfeatures in the 3-pm band. We have demonstrated thefollowing:

(1)How the nondetection or limited detectability of H 20ice in nonprotostellar molecular clouds is consistent witha core-mantle-mantle grain model whose outer mantle isalmost pure H 2 0 .(2 ) That the maximum ice absorption strength of amixture containing a fraction, f , of H20must generally beless than that given by simply multiplying the pure H 20absorption by f , and that this effect is stronger with in-creasing dilution.(3) That, within a few percent, the peak H 20absorptionby a complex grain is simply proportional to the volume

ext sca0.16 -- -0.1 8 .......

0.3 0.20- --Q !

0.2-

0.1-

0.00.375 0.350 0.325 0.300 0.275 0.

h-' k m - 0 50Flgure 25. Extinction and scattering efficiencies by the same particlesas in Figure 24.of the mantle and to the peak value of m", he absorptivepart of the index of refraction of the mixture.(4) That a theoretical derivation of the optical propertiesof an ice mixture which neglects intermolecular interac-tions by simply proportioning fractions of the index ofrefraction t o the respective fractions of the molecules inthe mixture (index of refraction averaging) produces avariety of spurious results including not only incorrectestimates of the H20absorption strength but also signif-icant errors in band structure.(5) That the long wavelength wing in the ice absorptionwhich is characteristic of protostellar dust can only bereproduced by elongated interstellar particles, within therange of normal sizes, whose outer mantles consist of H 20mixed with a strong base such as, for example, NH,.(6 ) That the long wavelength wing in the ice absorptioncharacteristic of protostellar dust cannot be produced bysuperlarge (Z0.5 pm) particles of either pure ice or icemixtures without introducing substantial distortions in theband structure and polarization.(7) The observed detailed structures in the band pro-duced by an H20-NH3 mixture (as an example) dependon the shape of the grains producing them as well as theoptical properties of the mantle in the following ways: (a)For spheres, the H 2 0 (3.07 pm) absorption peak is de-pressed with respect to the NH, (2.97 pm) absorption peakcompared with the relative peak values of "'(A). (b) Fo rsubstant idy elongated particles the relative heights of theH,O and NH, peaks approach more closely the relativeheights of the values of m"(A).(8) Combining point 7 with the fact that strong inter-stellar polarization at 3 pm implies elongated (and well-aligned) grains, comparison of observations of the relativeabsorption strengths of the 2.97- and 3.07-pm peaks withcalculations using spherical grains leads to underestimatesof NH, relative to H20.Note Added in Proof: ObservationsMof some molecularcloud stars with high extinctions show ice bands withinthe expected range of absorption strengths.45c

Registry No. W a t e r , 7732-18-5; a m m o n i a , 7664-41-7.(54) Whittet, D. C. B.; Bode, M. F.;Longmore,A. J.;Baines, D. W. T.;

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