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Physical-Chemical Processes in a Protoplanetary Cloud AVGUSTA K. LAVRUKHINA V.I. Vernadskiy Institute of Geochemistry and Analytic Chemistry ABSTRACT According to current views, the protosun and protoplanetary disk were formed during the collapse of a fragment of the cold, dense molecular interstellar cloud and subsequent accretion of its matter to a disk. One of the most critical cosmochemical issues in this regard is the identification of relics of such matter in the least altered bodies of the solar system: chondrites, comets, and interplanetary dust. The presence of deuterium- enriched, carbon-containing components in certain chondrites (Pillinger 1984) and radicals and ions in comets (Shulman 1987) is evidence that this area holds great promise. If a relationship is established between solar nebula and interstellar matter, we can then identify certain details, such as the interstellar cloud from which the Sun and the planets were formed. We can also come to a deeper understanding of the nature of physico-chemical processes in the protoplanetary cloud which yielded the tremendous diversity of the chemical and mineralogical compositions of the planets and their satellites, meteorites, and comets. CHARACTERISTICS OF THE CHEMICAL COMPOSITION OF MOLECULAR INTERSTELI^R CLOUDS One would expect that chemical compositions of interstellar clouds are significantly varied and are a function of such physical parameters as temperature and density. Moreover, one would expect that they are also dependent on the age of a cloud, its history, the impoverishment of 61

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62 PI~ETARY SCIENCES

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AMERICAN AND SO~IET RESEARCH TABI-E 1 Identification of Interstellar Molecules (Irvine 1988) Sunple *ydrides, o~ades, sulfutes and other molecules H2 CO NH3 CS Naclx HC1 SiO SiH4X SiS AlClX H2O so2 CC H2S Kc~x OCS CH,,~ PN AlFX HNO ? Nitrides, derivative acetylenes, and other molecules HCN HC--C-CN H3C-C_C-CN H3C-CH2-CN H2C CH2X H3CCN H(C_C)2-CN H3C-C_CH H2C-CH-CN HC~CHX CCCO H(C=C)3-CN H3C-(C-C)2-H HNC CCCS H(C=C)4-CN H3C-(C=C)2-CN? HN~-O HC_CCHO H(C=C)5-CN HN~S ~3CNC Aldehydes, alcohols, ethers, ketones, amides, ar~d other molecules H2C--O H3COH HO-CH=0 H2CNH H2C=S H3C-CH2-OH H3C-O-CH=0 H3CNH2 H3C-CH=0 H3CSH H3C-O-CH3 H2NCN NH2-CH=0 (CH,l,CO? H2C~ Cycl~cal molecules Ions Radicals C3H2 SiC2X _-C`H CH+ HCO+ HCNH+ H3O+? HN2+ HOCO+ SO+ HOC+? HCS+ H2D+? OH C3H CN HCO C,S CH C4H C3N NO NS C2H CsH H,CCN SO (a) New molecules discovered after 1986 are underlined. (x) Present only in clouds of evolving stars. (?) Not yet ca~lrmed. 63

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64 PLANETARY SCIENCES unsaturated bonds, despite the fact that hydrogen distribution rates are three to four orders greater than for C, N. and O. This is evidence of the predominance of kinetic over thermodynamic factors in chemical reactions in the interstellar medium and of the large contribution of energy from cosmic rays and UV radiation to these processes. Chemically saturated compounds such as CH3CH2CN are only present in the "warmer" sources (i.e., in Orion). HNCO, CH3CN, HC3N, C2H3CN, C2HsCN levels are higher in warm clouds, possibly owing to higher NH3 parent molecule levels. One interesting feature pertaining to the distribution rates for certain interstellar molecules is their uniformity for dark molecular clouds with wide variation in P and T parameters. Furthermore, an inverse dependence of the amount of gas molecules on dust density is absent. This would have been an expected consequence of molecules freezing into the ice mantle of particles. This is confirmed by data on the constancy of the CO/dust ratio in three clouds. It is further supported by the absence of a drop in H2CO levels as dust density rises in dark molecular clouds. An indication that the efficiency rate of this in-freezing is not uniform for various molecules has also not been confirmed. A high degree of homogeneity of the Hi3Co/~3CO and C2H/~3CO ratios for many clouds has been found. Clearly, the processes involved in the breakdown of particle ice mantles are highly efficient. Their efficiency may be enhanced when dust grain density increases as the grains collide with each other. Data on the distribution of different interstellar molecules are in general agreement with calculations in which ion-molecular reactions in gas are the primary process. However, there is a question as to the reliability of calculations with a value of CO ~ 1 in a gas phase. It has been found that the abundance values for many C-rich molecules and ions are extremely low in steady-state conditions. Despite the fact that various explanations of these facts have been offered, an alternative hypothesis suggests that C/O > 1 in the gas phase. Other facts were already indicated above which can be explained by such a composition of the gas phase. Enhanced carbon levels may be attributed to the fact that CH4 (being a nonpolar molecular) is more easily volatilized from the surface of the grain mantle than NH3 and H2O. Therefore, we can hypothesize that in certain dark molecular clouds or in different portions of them, the gas phase has a C/O ratio which departs from the cosmic value. This is fundamentally critical to understanding the processes in the early solar system. It has been found that many unique minerals of enstatite chondrites (including enstatite, silicon-containing ka- masite, nainingerite, oldgamite, osbornite, and carbon) could only have been formed during condensation from gas with C/O > 1 (Petaev et al.

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AMERICAN AND SOVIET RESEARCH 65 1986~. SiC and other minerals, which were condensed in highly reduc- ing conditions, have also been found in CM-type carbonaceous chondrites (Lavrukhina 1983~. PROPERTIES AND PlIYSICO-CHEMICAL PROCESS IN THE GENESIS OF INTERSTELLAR DUST GRAINS According to current thinking (Voshchinnikov 1986), the total sum of molecules in a "dense," not-too-hot gaseous medium of complex molecular composition precipitates into a solid phase, thereby forming embryos of dust grains. These grains then begin to grow through accretion of other molecular compounds or atoms. The grains may in turn act as catalysts for reactions to form new types of molecules on their surface. A portion of these remains as particles, and the rest converts to the gaseous phase. Laminated interstellar grains are formed in this manner. Their cores are made up of refractory silicate compounds, metal iron, and carbon. The grain mantle is formed from a mixture of ices of water, ammonia, methane, and other low-temperature compounds with varying admixtures. Atomic carbon may also be adsorbed on the mantle surface at the low temperatures of dark molecular clouds. These dust grains are often aspherical. Their size is approximately 0.3 ~m. Generation of the finest particles ~ ~ 0.01 ~m) also takes place. They have no mantle due to the increased temperature of these grains as a single photon is absorbed or a single molecule is formed. The dust grains are usually coalesced as a result of photoelectron emission and collisions with electrons and ions. Mean grain temperature is approximately 10 K The following data are evidence of the chemical composition of dust grains. (1) IR- absorption band: 3.1 Em - ice H2O (NH3), 9.7 and 18 Em - amorphous silicates, 4.61 and 4.67 Emmolecules with the groups CN and CO, ~ ~ 3.3 . 35 Emmolecules with the groups CH2- and -CH3. (2) Emission spectra: 11.3 Em - SiC, 30 Em - mixtures of MgS, CaS, FeS2, ~ 3.5 Emformaldehyde (H2CO), six emission bands with ~ ~ 3.28 . 11.2 Em . large organic molecules (No 20) (3) ~ 2200& . graphite (?), carbines (-CC-), amorphous and glassy carbon.

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66 PLANETARY SCIENCES According to current views, at least part of the interstellar molecules is formed from reactions on dust grain surfaces. At low-grain surface temperatures and moderately high gas temperatures, atoms and molecules coming into contact with the surface may adhere to it. Van-der-Vaals effects determine a minimum binding energy value. However, significantly high values are also possible with chemical binding. Migration along the grain surface of affixed atoms generates favorable conditions for molecule formation. A portion of the released binding energy (Ec) of atoms in a molecule is taken up by the crystal grid of the grain surface. If the remaining portion of the molecule's energy is greater than Ec, the molecule "comes unglued" and is thrown into the gas phase. This process is accelerated when the dust grains are heated by cosmic rays. H2, CH4, NH3, and H2O molecules are formed in this manner. Since the binding energy of C, N. O. and other atoms is on the order of 800 K, they adhere to the grain surface where they enter into chemical reactions with hydrogen atoms. The aforementioned molecules are thus formed. Part of these then "comes unglued," such as the H2 molecule. A portion freezes to the grain surface. The temperatures at which molecules freeze are equal to (K): H2 - 2.5, N2 - 13, CO - 14, CH4 - 19, NH3 - 60, and H2O - 92. Hence, the formation of the mantle of interstellar dust grains and certain molecules in the gaseous phase of dense, gas-dust clouds occurs contemporaneously. liable 2 lists certain data on the characteristics of the basic physical and chemical processes involved in the formation and subsequent evolution of interstellar dust grains and the astrophysical objects in which these processes occur. With these data we can evaluate the nature of processes occurring in the protoplanetaty cloud during the collapse and subsequent evolution of the Sun. These basic processes triggered: 1) the breakdown and vaporization of dust grains under the impact of shock waves at collapse and the accretion of primordial cloud matter onto the protoplanetary disk; 2) the collision of particles; and 3) particle irradiation by solar wind ions. The role of these processes varied at different distances from the protosun. Yet the main outcome of the processes is that organic, gas-phase molecules and the cores of dust grains (surrounded by a film of high-temperature polymer organic matter) are present throughout the entire volume of the disk. They are obvious prima~y-starting material for the formation of a great varietr of organics which are observed in carbonaceous chondrites (Lavrukhina 1983~. Dust grains at great distances from the protosun (R ~ 2 AU) will be screened from the impact of high temperatures and solar radiation. They will therefore remain fairly cold in order to conserve water and other volatile molecules in the mantle composition. Comets, obviously, contain such primary interstellar dust grains.

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AMERICAN AND SOVIET RESEARCH 67 TABLE 2 Characteristics of the Basic Physico-Chemical Processes of Interstellar Dust Grain Genesis Process Parameters Proposed Chemical Astrophysical Compounds or Processes Objects 1. Condensation of T=1400- high temperature -1280K embryos Amorphous silicates, mixes 1) Atmospheres of cold stars of oxides MgO, SiO, CaO, (10'-10~tcm~3), FeO, Fe, Ni-particles, 2) Planetary nebulae SiC, carbines, graphite(?) 3) Envelopes of novae amorphous & glassy carbon and supemovae, 4) Envelopes of red giants 2. Formation of mantle on embryos T=70~25K FeS, H2O, NH3-H2O, CH4 x H2O Solid clatrates Ar, Kr, Xe. Carbines 1) Upper layers of cold stars and interstellar space, 2) Dispersed matter of old planetary nebulae, 3) GMC,~ 4) Old supernovae envelopes 3. Coalescence of t - 10yrs fine particles with formation of "sleeve" pooding type particles Ice with phenocrysts Turbulent gas from silicates, metals of protostellar graphite (?) clouds 4. Destruction of dust grains (primarily in mantles) Particle life- span: ice- (107-5-108) 2) Sublimation, years, silicate- 3) Physical and Chemical (4-108-2-10l) destruction, years 4) Photodeso~ption 1) Collision of particles with V > 20km~c~l, 1) Envelopes of red giants and novae, 2) Shock waves from super- novae flash, 3) Irradiation by high velocity ions of stellar wind and by high energy cosmic rays in GMC~> and planetary nebulae 5. O'cidation-reduc- tion reactions . ~ on gram sunace Formation of hydrides Low - temperature Fe oxidation by monatomic oxygen FeO, Fe:O3, Fe3O4 T=2.5-5 K FeH, FeH:, hydrides of transitional metals Diffusion interstellar medium Daric GMC~, zones 6. Formation of envelopes and dust from solid organic compounds Radiation poly- Tolines, hexamethylent- merization of etramine, cellulose, organic complex organic or compounds with prebiological compounds T24K on grains (PAC) with subsequent breakdown into fragments UV-radiation, cosmic rays, shock waves in GMC~, (x) Gigantic dark molecular interstellar clouds

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68 PLANETARY SC ENCES THE ISOTOPE COMPOSITION OF VOLATILES IN BODIES OF THE SOLAR SYSTEM Investigation of the isotope composition of H. O. C, N. and the inert gases in meteorites, planets, and comets is extremely relevant as we attempt to understand the processes involved in the genesis of the preplanetary cloud. The majority of these elements had a high abundance in the interstellar gas and the gas-dust, initial protosolar cloud. Great variation in the isotope composition for various cosmic objects is also characteristic of these elements. Such variation has made it possible to refute outmoded views of the formation of the protosolar cloud from averaged interstellar material (Lavrukhina 1982; Shukolyukov 1988~. From detailed studies of meteorites, we have been able to discover a number of isotonically anomalous components and identify their carrier phases (Anders 1987~. These studies have demonstrated that the pro- tomatter of the solar system was isotonically heterogeneous. For example, examination of the hydrogen isotope has shown that objects of the solar system can be subdivided into three groups in terms of the hydrogen isotopy (Eberhardt et al. 1987~. (1) Deuterium-poor interstellar hydrogen, proto- solar gas, and the atmospheres of Jupiter and Saturn; (2) deuterium-rich interstellar molecules (HNC, HCN, and HCO+) of dark molecular clouds of Orion A; and (3) the atmospheres of Earth, Titanus, and Uranus, the water of Halley's comet, interplanetary dust, and certain chondrite frac- tions of Orgueil C1 and Semarkona LL3 occupy an intermediary position. Clearly, the isotopic composition of hydrogen in these components is de- termined by the mixing of hydrogen from two sources: a deuterium-poor and a deuterium-rich source. A single gas reservoir is thus formed. A similar situation has been found for oxygen. Ho oxygen compo- nents have been discovered in meteorites: impoverished and enriched i60 of nucleogenetic origin ~avrukhina 1980~. The relative abundance of oxy- gen isotopes in chondrules tells us that chondrite chondrules of all chemical groups are convergent in relation to a single oxygen reservoir, characterized by the values ~ t80 = 3.6 ~ 0.2L% and ~ 170 = 1.7 ~ 0.2L% (Lavrukhina 1987; Clayton e! al. 1983~. They are similar to the corresponding values for Earth, the Moon, achondrites, pallasites, and mesosiderites. On the basis of these data and the dual-component, isotopic composition of nitrogen, carbon, and the inert gasses (Levskiy 198~, Anders 1987), workers have raised the idea that protosolar matter was formed from several sources. For example, two reservoirs of various nucleosynthesis are proposed that differ in terms of their isotopic composition and the degree of mass fractioniza- tion (Lavru~ina 1982; Levskiy 1980~. Shukolyukov (1988) proposes three sources: ordinary interstellar gas; material injected into the solar system by

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AMERICAN AND SOVIET RESEARCH 69 an explosion of an adjacent supernova; and interstellar dust made up of a mixture of different stages of stellar nucleosynthesis. The presence of at least two sources of matter in protoplaneta~y matter may be evidence of the need to reconsider the hypothesis of the contemporaneous formation of the Sun and the protoplanetary cloud from a single fragment of a gigantic molecular interstellar cloud. REFERENCES Anders, E.A. 1987. Local and exotic components of primitive meteorites and their origin. Itans. R Soc. Land. A323~1~:287-304. Bell, M.B., LW. Avery, H.E. Matthews et al. 1988. A study of C3HD in cold interstellar clouds. Astrophys. J. 326~23:924-930. Clayton, R.N., N. Onuma, Y. Ikeda. 1983. Oxygen isotopic compositions of chondrules in Allende and ordinary chondrites. Pages 3743. In: King, E.A. (add. Chondrules and Their Origins. Lunar Planet. Institute, Houston. Eberhardt, P., R.R. Hodges, D. Krankowsky et al. 1987. The D/H and 180/160 isotopic ratios in comet Halley. Lunar and Planet. Sci. XVIII:251-252. Inrine, W.M. 1988. Observational astronomy: recent results. Page 14. Preprint, Five College Ratio Astronomy Observatory. University of Massachusetts, Amherst. Irvine, W.M., P.F. Goldsmith, and A. Hjalmamon. 1987. Chemical abundances in molecular clouds. Pages 561609. In: Hollenbach, DJ, and H.A. Thronson (eds.~. Interstellar Processes. D. Reidel Publishing Company, Dordrecht. Lavrukhina, A.K. 1987. On the origin of chondrules. Pages 75-77. In: XX Nat. Meteorite Conference: Thes. Paper. GEOKHI AS USSR, Moscow. Lavrukhina AK. 1983. On the genesis of carbonaceous chondrite matter. Geokhimiya 11:1535-1558. Lavrukhina, A.K. 1982. On the nature of isotope anomalies in the early solar system. Meteoritika 41:78-9Z Levskiy, L K 1989. Isotopes of inert gases and an isotonically hetereogeneous solar system. Page 7. In: VIII Nat. Symposium on Stable Isotopes in Geochemistry: Thes. Paper. GEOKHI AS USSR, Moscow. Petaev, M.I., A.K. Lavrukhina, and I.L. Khodakovskiy. 1986. On the genesis of minerals of carbonaceous chondrites. Geokhimiya 9:1219-1232. Pillinger, C.T. 1984. Light element stable isotopes in meteorites from grams to picograms. Geochim. et Cosmochim. Acta 48(12~:2739-2766. Shukolyukov, Yu.A. 1988. The solar system's isotopicnonuniformity: principles and conse- quences. Geokhimiya 2 200-211. Shulman, LM. 1987. Comet Nuclei. Nauka, Moscow. Voshchinnikov, N.V. 1986. Interstellar dust. Pages 98-202. In: Conclusions of Science and Technology. Space research, vol 25. VINITI, Moscow.