Cosmic Dust N98-18554

D. E. Brownlee and S. A. Sandford

galaxy and the solar system. Its average spatial density is less than one particle per i ustcubicis akilometerubiquitous but componentthe total numberof our of

particles is large. The particles are so prevalent that both interstellar and interplanetary dust can readily be seen with the naked eye under dark sky conditions. Zodiacal light, a glow usually seen in the east before dawn twilight and in the west following sunset, is sunlight reflected off dust particles orbiting the Sun. The dark band marking the central plane of the Milky Way is caused by absorption of background starlight by dust concentrated in the plane of our galaxy. In fact, dust is so prominent that light typically travels only a thousand years in the plane of our galaxy before it is absorbed by dust. The distance traversed in this time is only 1% of the diameter of the galaxy.

Interstellar dust is the predominant form of the condens- able elements in the galaxy that are not in stars. The grains form in gas outflows from stars and they are processed, perhaps even destroyed and reformed, in the interstellar medium and molecular clouds. Interplanetary dust is debris recently liberated from comets and asteroids within the solar system. There is a strong link between interplanetary and interstellar dust. Prior to COLOR F'HO-iOSi_;A_H

145 ] formation of the solar nebula, tant roles in the origin and experience melting from most of the atoms heavier development of life. Inter- atmospheric drag but, because than helium that found their planetary dust particles (IDPs) of their smaller size, they tend way into our solar system that are collected in the to have isothermal interiors were contained in interstellar stratosphere and analyzed in and they melt completely, grains. Some of these grains the laboratory are preserved resulting in the loss of were incorporated into fragments from asteroids and volatiles such as sulfur, comets and asteroids. Comets comets, bodies which formed sodium, and organic matter. forming in the outer fringes in the outer regions of the Such particles form melt of the nebula presumably solar nebula, are relatively spheres (cosmic spherules) contain relatively higher rich in volatiles, and which that have been collected in abundances of presolar grains. are a source of abiotic organic abundance from the ocean Many of the grains preserved molecules that have fallen to floor and polar ice deposits. in comets may then be older Earth throughout its history. than the Sun and planets. Particles smaller than 0.1 mm, Comets are thus vehicles Interplanetary dust particles however, enjoy a special potentially capable of carry- accreted by the Earth must advantage over conventional ing the products of interstellar first enter the atmosphere. meteorites and larger dust chemical processes directly to They enter the atmosphere at particles. These grains can planets. They are believed to velocities ranging from usually enter the atmosphere contain substantial organic 11.2 km/s, the Earth's escape without melting. This is components produced in velocity, to a maximum of possible because the particles space by nonequilibrium 72 km/s, the velocity of a only travel at high velocity at reactions such as catalysis head-on impact with a par- altitudes above 90 km where and radiation processing of ticle in a retrograde parabolic the air is very tenuous. For a condensed volatiles. orbit. The kinetic energies given velocity, the frictional corresponding to these power density generated on The collection and analysis of velocities are capable of the face of a particle depends extraterrestrial dust particles is melting or vaporizing the only on the local air density. important to exobiology incoming particles if the At 90 kin, the frictional because it provides informa- energies are converted to energy is generated slowly tion about the sources of internal heat. In the case of enough that it can be ther- biogenically significant conventional meteorites, mally reradiated without the elements and compounds the surfaces are heated to particle being heated to its that accumulated in distant vaporization temperatures, melting point. Smaller par- regions of the solar nebula although the short time scale ticles decelerate at higher and that were later accreted of heating and the combined altitudes and for a given onto the planets. Both inter- effects of thermal inertia and initial velocity there is a stellar and interplanetary dust conductivity confine melting limiting size below which no seeded the early Earth with and strong heating to the melting will occur. Particles elements and compounds outer millimeter or so of the that do not melt are called that may have played impor- body. The interiors of the micrometeorites and are meteorites remain essentially unheated. Dust particles in the 0.1 mm to several mm size range typically also

146 genuine samples of interplan- observed to fragment imply tors worldwide. Most of the etary dust. Most particles typical crushing strengths of particles are easily identified below 50 _m in size do not 105 dynes/cm 2 and in the as extraterrestrial on the basis melt upon atmospheric entry extreme case of the Draconid of their elemental composi- and the survival of solar flare meteors (the youngest meteor tion which is similar to that cosmic ray tracks and miner- stream) the pressure is only of primitive meteorites but als with low thermal stabili- 103 dynes/cm 2. Micrometeor- quite different from most ties in the particles indicate ites this fragile can enter the terrestrial particles. Typical that typical 10 _m particles atmosphere without fragmen- particles are composed of Mg, are not heated above 600°C. tation whereas larger objects Si, Fe, S, Ca, Na, A1, Ni, Mn, cannot. Cr, and Ti in ratios that An additional factor favoring match their relative abun- the survival of interplanetary Thus, typical cometary rocks dance in the Sun. Oxygen and dust during atmospheric entry cannot enter the atmosphere carbon are also major is the fact that they are not without being crushed. Once constituents but, as in mete- subjected to large ram pres- crushed, most fragments orites, their relative abun- sures during atmospheric should either melt or vaporize dances are less than solar. An deceleration. The ram pres- because fragmentation occurs extraterrestrial origin can be sure experienced by dust at lower altitudes where proven for any given particle particles is orders of magni- frictional heating of small by detection of He implanted tude smaller than must be hypervelocity particles is by the solar wind or tracks in survived by larger objects severe. If comets do not mineral grains produced by such as conventional meteor- contain any strong centimeter solar flare cosmic rays. As ites. This allows fragile materi- and larger components, then previously mentioned, larger als to enter as dust but pre- dust would be the only particles are also recovered vents them from surviving as meteoroid type that could from deep sea sediments and millimeter or larger sized carry cometary molecules to from polar ice (Greenland and objects. At a given velocity the Earth's surface. Antarctica). Particles larger the ram pressure is propor- than 0.1 mm are usually tional to the ambient air Micrometeorites ranging in melted, but a few particles up density. The maximum size from 5 to 50 _m are to 1 mm survive without any ram pressure experienced routinely collected in the melting. These rare, giant by a micrometeorite that stratosphere using high- micrometeorites are presum- decelerates near 90 km is altitude aircraft. The particles ably particles that entered at 100,000 times smaller than are characterized and curated relatively low velocity and that experienced by a 10-kg for distribution to investiga- very low incident angles. In rock that has retained its low-angle impacts the hyper- cosmic velocity down to an velocity interaction with the altitude of 40 kin. Even the atmosphere occurs at higher most fragile conventional altitudes and results in pro- meteorite is a factor of longed but less severe heating. 100 times stronger than the materials observed in cometary meteor showers. The altitudes and velocities where cometary meteors are

147 General Properties of Microns 2.5 3 4 5 7 10 15 20 Interplanetary I I I I I I Dust

r21-M4-3A he most common (Olivine) IDPs collected from the stratosphere are r21 -M3-SA black, fine-grained (Layer-lattice silicate) materials that have major and minor element compositions similar to those of carbonaceous chondrites, i.e., primitive meteorites. Particles with this composition are referred to as being "chondritic" even though they may have no u23-M4-4A (Pyroxene) actual relation to chondritic meteorites. Some micromete- I I I orites do not have chondritic 4000 3200 2400 1600 800 compositions, but many of Wavenumbers these are large single-mineral grains that have surficial debris indicating that they were previously embedded in Figure 8-1. The three major infrared spectral IDP classes. From top to larger samples of fine-grained bottom, these spectra art" representative of lDPs dominated by the minerals chondritic material. The olivine, layer-lattice silicates, and pyroxenes, respectively. The spectra can be "chondritic" particles are easily separated by the profiles of their characteristic I0 pm (I000 cm -1) small (typically less than Si-O stretching features. The spectra also show longer wavelength features 20 _m in diameter) and due to Si-O-Si bending vibrations in the silicates, hi addition to the Si-O classifying them into mean- bands, spectra from the particles dominated by layer-lattice silicates also ingful groups is much more show bands centered near 3.0 and 6.012m (3330 and 1670 cm -1) due to difficult than with con- O-H stretching and H-O-H bending vibrations in adsorbed and absorbed ventional kilogram-sized water, and bands near 6.8 and 11.4 pm ('1470 and 875 cm -]) due to C-O meteorites. The primary stretching and CO3 scissoring vibrations in carbonates. Note the minor properties that can be readily features at 3.37, 7.94, and 12.53 pm (2970, 1260, and 798 cm -1) due to silicon oil in the middle spectnlm. measured are morphology, elemental composition, and mineralogical composition as determined by both electron microscopy and infrared lattice) silicates and those common. These differences spectroscopy. The particles dominated by anhydrous are clearly seen in infrared can be grouped into two silicates, of which pyroxene spectra of individual particles major classes: those domi- and olivine are the most (fig. 8-1). nated by hydrated (layer-

148 he hydrated particles silicate sheets and fibers. minimal pore spaces (fig. 8-2). are similar to two There are two subgroups of Like the CI and CM chon- types of primitive hydrous IDPs, one dominated drites, many of these particles meteorites known as by serpentine-like minerals have properties consistent the CI and CM carbonaceous with 7 A basal spacings with processing by aqueous chondrites. These meteorites typical of CI and CM meteor- alteration on a parentbody. are dominated by layer-lattice ites, and one dominated by a These include the existence silicates and they are the only layer-lattice silicate with of carbonates, clusters chondrites that have carbon 10-12 _ spacings not seen in (framboids) of rounded abundances appreciably above most meteorites. Like the magnetite grains, and redistri- 1% by weight. The bulk of conVentional meteorites, bution of Ca and Mg, ele- hydrated IDPs is composed of most of the hydrated IDPs are ments that often are compo- tangled masses of layer-lattice compact objects having only nents of water-soluble

Figure 8-2. A scanning electron microscope photograph of a particle dominated by hydrated (layer-lattice) silicates. These particles are generally compact and contain little void space. The white scale bar on the photograph is 1012m long.

BI_ACK AND WfliTE PHOTOGI.4'Ai,h_

149 minerals. The CI and CM The second general class The Carbonaceous chondrites are the most of IDPs is dominated by primitive meteorites known anhydrous minerals such as Component of on the basis of match with olivines, pyroxenes, and iron IDPs the elemental composition of sulfides. These particles are the Sun, but they are heavily similar to CI and CM chon- Carbon is a major element altered rocks that have been drites in bulk elemental in IDPs, but it is difficult to processed in a relatively warm composition, but in terms of study because of fundamental and wet parentbody. Their mineralogy and structure they analytical limitations and parentbodies were almost are unique and have no close ever present contamination certainly asteroids as it is analogs among meteorites. problems. Studies of the difficult to imagine aqueous These IDPs should be consid- carbon in IDPs are in an early alteration occurring on ered a new type of carbon- stage of development and the comets, small bodies which aceous chondrite that appar- nature of its form and distri- sublime when warmed above ently does not survive bution in this material is not cryogenic temperature. By atmospheric entry in the form yet well established. The analogy, it is likely that many of conventional, large meteor- carbon abundance in the and possibly all of the ites. They are carbon-rich and particles is not well deter- hydrated IDPs are also of composed of roughly equi- mined but it appears to be in asteroidal origin. dimensional grains ranging in size from micrometers to less the 5-15% range by weight for most particles. In terms of than I00/_. They also contain atom fraction this makes amorphous materials such as carbon the second most glass and disordered carbon. abundant element after Some of the anhydrous oxygen in IDPs. This carbon particles are highly porous content is intermediate (as in the Frontispiece) and between that of the most resemble gravel aggregates or carbon-rich meteorites known clusters of grapes. The high and that measured in dust porosity suggests a similarity from comet Halley by Soviet to the porous, fragile materi- and European spacecraft. als observed in cometary meteor streams. High porosity and weak structure are evidence against compaction processes that probably occur in asteroids due to meteoroid impact and, in some cases, self-gravitation. The submicrometer and micrometer pore spaces in IDPs are likely to be voids produced by sublimination of volatile materials such as ice or perhaps organic compounds with high vapor pressures.

15o Electron microscope studies Part of the problem with Additional information about have shown that most of the studying the carbonaceous the form of the carbonaceous carbon in IDPs is amorphous. material in IDPs by electron material in IDPs can be It occurs in micrometer and microscopy is that this tech- gleaned from their infrared submicrometer grains, as a nique is most sensitive to transmission and Raman component of submicrometer crystalline phases, while most spectra. As was shown earlier lumps composed of 100 of the carbon in IDPs is (fig. 8-1), the infrared trans- mineral grains and carbon, amorphous. The most com- mission spectra of individual and as 100 A films on mineral mon carbon-bearing crystal- IDPs are dominated by their grains. Oxygen and nitrogen line phases found in IDPs abundant silicate minerals. do not appear to be major include Mg- and Ca-rich However, some spectra also constituents of the carbon- carbonates, iron carbides, show the presence of a weak aceous matter and from an lonsdaleite, and trace absorption feature near electron microscopy view- amounts of graphite. The iron 3.4 _tm (2940 cm-1). This point the material looks like carbide is _-iron carbide and it spectral position is character- elemental carbon (although is found in association with istic of C-H stretching vibra- the techniques used could not elemental carbon, Fe-Ni tions and suggests the pres- detect hydrogen). The H, C, metal, and magnetite. The ence of hydrocarbons. N, and O containing "CHON" carbides are believed to have Unfortunately, the strato- grains identified as abundant been formed by catalytic spheric IDPs are collected on micrometer-sized particles in reaction of CO gas on grain impact collectors coated with the Halley coma have not surfaces. This most probably a thin layer of silicone oil, been identified in IDP occurred in the solar nebula. which itself produces an samples. It is possible that the Graphite is easy to identify in absorption feature near cometary CHON grains the electron microscope 3.4 13m. Thus, it is difficult to consist of relatively volatile because of its distinctive 3.4 ascertain what fraction of the materials and they do not basal spacings. Yet graphite is observed 3.4 p.m band in the survive the prolonged rarely seen in IDPs. If the IDP spectra is due to indig- exposure to space and subse- anhydrous particles are enous hydrocarbons and how quent entry into the Earth's cometary, it is clear that much is due to residual atmosphere. graphitic carbon is not a silicone oil that was incom- major constituent of comets. pletely removed from the If preserved grains are abun- particle during curation. The dant components of comets, layer-lattice silicate in fig- then it would also follow that ure 8-1 provides a good graphite is not abundant in example of a particle in which interstellar grains either. the majority of the observed 3.4 p.m feature is probably due to silicone oil. The presence of features at 7.94 and 12.53 12m (1260 and 798 cm -1) are also characteristic of silicone oil. There are particles, however, in which a 3.4 I_m feature is

I51 present and unaccompanied Unfortunately, our inability individual IDPs. Comparisons by the corresponding silicone to rule out contamination by of the widths and relative oil features at 7.94 and silicon oil or other lab materi- strengths of the features in 12.$3 gin. In these cases, we als makes uncertain any both sets of spectra show that may be seeing evidence of conclusions derived from the the carbonaceous material in indigenous carbonaceous observed 3.4 _m IDP features. IDPs contains aromatic materials, although it has Further work will be needed domains which are generally not been possible up to the before the infrared spectra can smaller than 25 _. present time to eliminate the place any strong constraints possibility of other forms of on the composition of the In addition to the vibrational contamination. carbonaceous compounds features, many of the Raman in IDPs. spectra of IDPs also show a If the 3.4 gm features broad photoluminescence observed in IDP spectra are feature arising from electronic due to indigenous materials, has been somewhat transitions (see, for example, aman spectroscopy their positions and profiles more successful in the bottom spectrum in indicate that aliphatic com- providing information figure 8-3). The position and pounds are present. The about the state of the carbon strength of the luminescence overall IDP feature, when in IDPs. As can be seen in varies from particle to par- seen, is similar to that figure 8-3, Raman spectros- ticle, but typically the short observed in spectra taken copy is particularly sensitive wavelength limit of the toward the galactic center and to the CC vibrations within feature is around 5400 _ and from residues produced in the aromatic molecular units the feature peaks between 5900 and 6400 A. This red laboratory experiments in in carbonaceous materials. which simple mixed molecu- The top five spectra, taken luminescence is of particular lar ices are irradiated with from laboratory standards, interest since it has been ultraviolet photons or ions. demonstrate how the Raman known for some time that All three cases represent spectra of carbonaceous small bodies in the outer solar material which has had materials change as the system show a general ten- volatile components removed. material dency to grow darker and In the case of the IDPs, the becomes less ordered and the redder with heliocentric more volatile components size of the aromatic domains distance. Photoluminescence have been removed by expo- decreased. Graphite, a mate- associated with carbonaceous sure to the Sun in inter- rial in which the aromatic materials similar to those in planetary space and by domains are very large, the IDPs may contribute to heating during atmospheric produces a single intense first- the observed reddening. entry. In the case of the order band at 1581 Acm -1 and galactic center, the volatiles a weaker second-order feature It is also interesting to note have been removed by long near 2700 Acm -1. As the that, while the particles are duration exposure to the aromatic domains decrease dominated by silicate miner- diffuse interstellar medium. in size, both of these bands als (as evidenced by the grow broader and new fea- infrared spectra and the tures appear near 1350 and electron microscope studies), 2950 Acm -_. The bottom two spectra were taken from

152 the Raman spectra seldom Firstorder Secondorder La = structural contain bands due to these unitdomainsize minerals. This suggests that the carbonaceous material in Natural graphite IDPs effectively "screens" the silicates from visible photons. This explains how the IDPs La=l pm can have relatively dark k, A.._._ appearances and yet still be dominated by silicate miner- .__ _.__. La = 130A als. This observation has implications for the many observations obtained La = 25A during comet Halley's recent I apparition.

r-,

¢. r- \ \ m IDPs are likely to be E derivedany of thefromcollectedcomets. re Figure 8-4 shows comparison between a 5-13 I.tm (2000-770 cm -1) spectrum of comet Halley and - r21-M4-3B a composite spectrum derived I I I I from IDP data. The IDP composite spectrum consists of a mixture of roughly 65% olivine-rich, 35% pyroxene- rich, and 10% layer-lattice silicate-rich IDPs. Note that r21-M2-4 not only is the 10 _m (1000 cm -1) silicate feature reasonably well fit, but

-- b the Halley spectrum also

I ] I l I shows a feature near 6.8 lam 0 1000 2000 3000 4000 (1470 cm -1) which is consis- Ramanshift(Acm-1) tent with the presence of

Figure 8-3. Raman spectra of several laboratory standards and IDPs. The top five spectra were taken from laboratory materials having decreasing degrees of crystalline order. The bottom two spectra were taken from individual IDPs. Comparison between the two sets of spectra show that the aromatic domains in IDPs are mostly smaller than 25/[. Note the broad photoluminescence apparent in the lowest IDP spectnlm.

153 lengths than similar features 2.0 observed in IDPs and toward I I I I the galactic center. As the Halley Dec 1985 1.8 collected dust and the dust

o C] 0 toward the galactic center has 1.6 undergone some thermal processing, this suggests that 1.4 comets contain some carbonaceous materials that have 0[30o 1.2 relatively low volatilities. This o oo°o [] material may remain with the 1.0 dust long enough to be observed in the telescopic .8 I I i l data but not survive long 4 6 8 10 12 14 enough to be observed in the Wavelength (IJm) collected interplanetary dust.

In summary, while the

Figure 8-4. A comparison between the spectrum of comet Halley (points) presently available informa- and a composite spectnan of several IDPs (solid line). The IDP composite tion about the chemical state contains contributions for all three IDP infrared classes in the relative of the carbonaceous material proportions of 55% olivines, 35% pyroxenes, and 10% layer-lattice silicates. in IDPs is quite limited, there is presently nothing inconsistent with the view that the majority of the carbonaceous material in IDPs is similar to carbonates in the abundances may be screened from visible they are normally found in photons by less abundant the kerogen-like materials in carbonaceous meteorites, i.e., the IDPs dominated by layer- carbonaceous materials. lattice silicates. Similar fits to it consists of a network of the IDP data have also been There is some indication, small aromatic domains found for comets Wilson and however, that the carbon- randomly interlinked by Giocobini-Zinner. The match aceous component in comets more-aliphatic bridges. This material contains minor between the IDP spectra and includes a component that is amounts of O and N and the cometary spectrum and not present in the collected the detailed structure within dust. Infrared spectra of resides in the particles both as the cometary 10 _m feature comets Halley and Wilson separate clumps and as matrix demonstrate that these have been shown to contain material. comets contain large amounts emission features near 3.4 pm of silicates and that these (2940 cm -1) that are diagnos- silicates are crystalline. Thus, tic of C-H stretching vibra- analogous to the collected tions in hydrocarbons. These IDPs, the low observed albedo features fall at shorter wave- of Halley does not necessarily imply the comet is dominated by carbonaceous materials. Instead, the abundant silicates

154 The Presence of component. The answer, carrier of the deuterium provided by isotopic studies, excess in IDPs is not uniform, an Interstellar is yes! but instead seems to be Component confined to "hot spots" whose dimensions are smaller IDPs are found to than a micrometer (fig. 8-5). The actual formation sites of containany of thelargecollecteddeute- This suggests that the carrier the carbonaceous material in rium enrichments of the deuterium consists of IDPs are unknown. Certainly, which are inconsistent with small "grains" which are some of the material must an origin in the solar system. distributed randomly within have formed in the early solar The spatial distribution of the nebula. However, given that much of the dust may come from comets and that comets are likely to be relatively Figure 8-5. "Pictures" of an IDP taken at different masses using an ion primitive bodies, the intrigu- microprobe. Note that while the elements oxygen, carbon, and hydrogen are distributed more or less uniformly throughout the particle, the deuterium is ing possibility exists that a concentrated in a localized region. The size of the observed deuterium-rich fraction of the material has "hot spot" (approximately 1 Ixm in diameter) is on the order of the spatial an interstellar origin. This resolution of the ion probe. As a consequence, only a lower limit to the possibility is of special interest deuterium enrichment in this spot can be obtained. The D/tt ratio in the hot to biogenic studies since the spot is found to be greater than a factor of I0 times the terrestrial value, interstellar medium offers a making this the most deuterium-rich naturally occurring sample ever rich variety of environments examined in the laboratory. The scale bar is 10 prn long. (Picture courtesy in which carbon chemistry of R. Walker, Washington University at St. Louis). can occur. In addition to normal gas phase chemistry, Hydrogen Deuterium carbon-containing compounds can form in the interstellar medium via ion- molecule reactions, reactions between gas phase molecules • catalyzed on grain surfaces, and reactions in ice mantles induced by ultraviolet photons and cosmic rays. This wide variety of chemical Carbon Oxygen processes potentially provides for the production of a rich assemblage of biogenically interesting compounds. Given the diversity of chemical processes operating in the interstellar medium, it is worth asking whether there is any evidence that the carbonaceous material in IDPs lO _m contains an interstellar I I

155 the overall particles. Unfortu- molecule reactions taking presumably derived from nately, because of the small place in dense, cold interstel- more or less normal interstel- size of the carrier, it is not yet lar clouds may be the source lar material, we might expect clear whether the deuterium- of the observed deuterium most interstellar material to rich hot spots represent true, enrichments. An additional have "normal," i.e., solar unaltered interstellar grains means by which the deute- system-like isotopic abun- that have been incorporated rium enrichment could have dances. Thus, the interstellar into the larger IDP, or occurred in the interstellar material evidenced by the whether they represent medium involves selective deuterium enrichments material partially altered in photodissociation reactions. represents a lower limit to the the solar nebula which still Interactions of ultraviolet abundance of interstellar has a "molecular memory" of photons with aromatic material present. such a grain. The deuterium molecules in the interstellar enrichments seem to correlate medium should result in the loosely with carbon concen- photodissociation of some of Summary tration, suggesting a carbon- the molecules' peripheral aceous carrier phase is respon- hydrogen atoms. Since the Collected samples of inter- sible. However, the carbon zero point energy of the C-D planetary dust are of exobio- isotopic system is clearly bond in polycyclic aromatic logical interest since these decoupled from the deute- hydrocarbons is lower than particles contain a variety of rium enrichments. The D/H that of the C-H bond, deute- carbon-bearing compounds ratio is not found to correlate rium is less likely to be disso- and offer an efficient means with the 13C/12C ratio in the ciated. The dissociation site is of seeding planets in the early same particles, nor does the then free to capture a new solar system with these t3c/12C ratio show the same atom. Over an extended materials. Because of their heterogeneity exhibited by period of successive photodis- small size, IDPs can be decel- the D-H system. This implies sociation and recapture, a erated from cosmic velocities that, while the deuterium is deuterium enrichment of the at high altitudes in planetary carried by a carbonaceous molecular aromatic popu- atmospheres. As a result, phase, the carrier and the lation is expected. In any much of the dust is captured enrichment need not have event, whether the observed without experiencing thermal formed in the same place, at enrichments are produced via excursions sufficient to melt the same time, or by the same ion-molecule reactions or or vaporize them. Any car- process(es). selective photodissociation, bonaceous materials indig- their presence in IDPs clearly enous to the particles are then The chemical origin of the points to material having an deposited on the surface of deuterium excess is uncertain. interstellar origin. the planet. Deuterium enrichment via It should be noted, however, chemical reactions requires While several minor carbon- very low temperatures that while the presence of an bearing phases have been (T < 100 K), but at these exotic isotopic component identified in IDPs, the major- temperatures the reaction rate can prove that interstellar ity of the carbonaceous material is present, the lack of is extremely slow. The reac- material in IDPs seems to an exotic isotopic signature tion threshold is greatly consist of a disordered mate- reduced, however, if one of does not necessarily imply rial rich in C and H and the reactants is an ion. For formation in the solar system. containing minor amounts of this reason, it is felt that ion- Since the solar system is

156 O and N. Spectroscopic "memory" of original inter- Additional evidence suggests that this stellar grains. In any event, it material contains aromatic is clear that the collected IDPs Reading molecular units smaller than contain molecular material 25 A in size and possibly not only from the early solar Bradley, J. P.: Analysis of aliphatic hydrocarbons as system, but also from the Chondritic Interplanetary well. In so far as a comparison interstellar medium. Since the Dust Thin- Sections. is possible, the majority of the interstellar medium contains Geochim. Cosmochim. Acta, carbonaceous material in IDPs a wide variety of environ- vol. $2, 1988, p. 889. appears to be similar to the ments in which many differ- kerogen-like material found ent chemical processes can Bradley, J. P.; Sandford, S. A.; in primitive carbonaceous occur, it would not be surpris- and Walker, R. M.: Interplan- meteorites, a material consist- ing if IDPs contain a varied etary Dust Particles. In Mete- ing of small aromatic moieties assemblage of compounds of orites and the Early Solar that are randomly interlinked exobiological interest. System, J. Kerridge and M. by aliphatic structures. Matthews, eds., University of Finally, we note that there are Arizona Press, Tucson, 1988, The collected dust can provide a number of proposed space- p. 861. reasonable matches to astro- craft missions that could nomical spectra of comets. provide important new Brownlee, D. E.: Cosmic Dust: The known extraterrestrial insights into the nature of Collection and Research. Ann. nature of these particles and interplanetary dust and Rev. Earth Planet. Sci., vol. 13, their spectral similarity to comets. The most spectacular 1985, p. 147. primitive astronomical objects of these is the Rosetta Comet all argue that the collected Sample Return Mission Fraundorf, P.; Brownlee, D. E.; IDPs contain "primitive" planned by the European and Walker, R. M.: Laboratory materials that may have been Space Agency. This Mission Studies of Interplanetary present during the formation will be flown in the next Dust. In Comets, L L. of the solar system. century and is designed to Wilkening, ed., University of land on a short period comet, Arizona, Tucson, 1982, p. 383. Isotopic studies of the IDPs collect a core sample, and demonstrate that many return the sample to Earth Mackinnon, I. D. R.; and of them contain small while retaining cryogenic Rietmeijer, F. J. M.: Mineral- (<1 micron) components that temperatures. A nearer term ogy of Chondritic Interplan- are greatly enriched in deute- mission is the Comet Rendez- etary Dust Particles. Rev. rium. The magnitudes of the vous Asteroid Flyby (CRAF) Geophys., vol. 25, 1987, observed enrichments are not Mission which is tentatively p. 1527. explainable in terms of solar planned for a 1996 launch system processes and indicate that will result in matching Sandford, S. A.: The Collec- the presence of interstellar orbits with a short period tion and Analysis of Extrater- material. It is not clear at this comet. The spacecraft will be restrial Dust Particles. Fund. time whether the carriers of in close proximity to the Cosmic Phys., vol. 12, 1987, the isotopic anomalies repre- comet for over a year and will p. 1. sent true, unaltered interstel- measure the composition of lar dust grains, or whether the gases and dust emitted by they represent an altered the comet over much of its component with a molecular orbital cycle.

157 COLOR F'HOiOGi_APH

158