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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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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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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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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 Em—molecules with the groups CN and CO,
~ ~ 3.3 . 35 Em—molecules with the groups CH2- and -CH3.
(2) Emission spectra:
11.3 Em - SiC,
30 Em - mixtures of MgS, CaS, FeS2,
~ 3.5 Em—formaldehyde (H2CO),
six emission bands with ~ ~ 3.28 . 11.2 Em . large organic molecules (No
20)
(3) ~ 2200& . graphite (?), carbines (-C—C-), amorphous and glassy
carbon.
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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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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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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.
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Clayton, R.N., N. Onuma, Y. Ikeda. 1983. Oxygen isotopic compositions of chondrules in
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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.
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Representative terms from entire chapter:
interstellar dust
