FORMATION OF THE SOLAR NEBULA
Solar nebula from ionization of interstellar gas - star formation
STELLAR EVOLUTION↗
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Non-equilibrium chemistry in the solar nebula and early solar system: Implications for the chemistry of comets
Theoretical models of solar nebula and early solar system chemistry which take into account the interplay between chemical, physical, and dynamical processes have great utility for deciphering the origin and evolution of the abundant chemically reactive volatiles (H, O, C, N, S) observed in comets. In particular, such models are essential for attempting to distinguish between presolar and solar nebula products and for quantifying the nature and duration of nebular and early solar system processing to which the volatile constituents of comets have been subjected. The diverse processes and energy sources responsible for chemical processing in the solar nebula and early solar system are discussed. The processes considered include homogeneous and heterogeneous thermochemical and photochemical reactions, and disequilibration resulting from fluid transport, condensation, and cooling whenever they occur on timescales shorter than those for chemical reactions.
Fegley, Bruce, Jr.↗
Physical conditions in the primitive solar nebula.
Primitive solar nebula physical model considered under angular momentum conservation in collapsing fragment
Cameron, A. G. W.↗
Solar nebula chemistry - Implications for volatiles in the solar system
Current theoretical models of solar nebula chemistry which take into account the interplay between chemistry and dynamics are presented for the abundant reactive volatile elements including hydrogen, carbon, nitrogen, oxygen, and sulfur. Results of these models indicate that, in the solar nebula, the dominant carbon and nitrogen gases were CO and NO, whereas, in giant planet subnebulae, the dominant carbon and nitrogen gases were CH4 and NH3; in the solar nebula, the Fe metal grains catalyzed the formation of organic compounds from CO and H2 via the Fischer-Tropsch-type reaction. It was also found that, in solar nebula, bulk FeS formation was kinetically favorable, while FeO incorporation into silicates and bulk Fe3O4 formation were kinetically inhibited. Furthermore, clathrate formation was kinetically inhibited in the solar nebula, while it was kinetically favorable in giant planet subnebulae.
Fegley, Bruce, Jr.↗
A Simple Mechanism for Fractionating Oxygen Isotopes in the Solar Nebula
Lightning in the Solar Nebula is caused by the tribo-electric charging of dust grains carried by massive turbulent flows and driven by the accretion energy in the disk: it has long been one agent assumed responsible for the formation of chondrules. The degree to which charge separation can occur is dependent upon a number of factors, including the concentration of radioactive sources and the total level of ionization in the nebula, and these factors determine the maximum energy likely to be released by a single bolt. While chondrule formation requires a massive discharge, even a small lightning bolt can vaporize grains in the ionized discharge channel. Experimental studies have shown that silica, iron silicate and iron oxide grains formed from a high voltage discharge in hydrogen rich gas containing some oxygen produces solids that are enriched in O-17 and O-18 relative to the composition of the starting gas. Vaporization of silicates produces SiO, metal and free oxygen atoms in each discharge and these species will immediately begin to recondense from the hot plasma. Freshly condensed grains are incrementally enriched in heavy oxygen while the gas is enriched in O-16. Repeated evaporation and condensation of silicates in continuously occurring lightning discharges will monotonically increase the fractionation of oxygen isotopes between the O-17 and O-18 rich dust and the O-16 rich gas. The first mass independently fractionated refractory oxide particles were produced in the lab following the condensation of a flowing gas mixture containing variable amounts of hydrogen, silane, pentacarbonyl iron and oxygen that passed through a high voltage discharge powered by a Tesla coil. While the exact chemical pathway is still uncertain, the most probable reaction mechanisms involve oxidation of the growing refractory clusters by O3, OH or O atoms. This model has some interesting consequences for chemical processes in the early solar nebula. Chemical fractionation of recondensed dust evaporated via lightning discharges should be strongly time dependent. At earlier times, the accretion rate is maximal, thus driving strong turbulence, energetic grain-grain collisions, tribo-electric charging and charge separation, leading to frequent, powerful lightning discharges. As the accretion rate diminishes, turbulence decreases and lightning discharges will become both less powerful and less frequent, thus decreasing the rate of dust-gas fractionation. The most rapid increase in the formation of O-16 poor dust will occur early in nebular history. Generation of fractionated dust should be distributed throughout the inner disk. Once condensed, grain dispersal would average out any significant isotopic anomalies within the inner disk.
Nuth, Joseph A., III↗
A comparison of solar wind and estimated solar system xenon abundances - A test for solid/gas fractionation in the solar nebula
The solar Xe elemental abundance is determined here using solar wind measurements from lunar ilmenites which are normalized to Si by spacecraft data. The results are compared with estimated abundances assuming no fractionation. When corrected for solar wind/photospheric fractionation, the Xe-130 abundance given by surface layer oxidation of ilmenite from solid 71501 exposed within the last 200 m.y. is 0.24 +/- 0.09 normalized to Si = 10 exp 6. This is indistinguishable from estimates made assuming no solid/gas fractionation. Results from breccia 79035 ilmenite exposed at least 1 Gyr ago indicate that the solar wind Xe flux may have been significantly higher relative to other noble gases, perhaps due to more efficient Xe ionization. If this is true, fluxes of C and S, which have first ionization potentials similar to Xe, should also be higher in the ancient solar wind from the same time period.
Wiens, Roger C.↗
Organic Analysis of Catalytic Fischer-Tropsch Synthesis Products and Ordinary Chondrite Meteorites by Stepwise Pyrolysis-GCMS: Organics in the Early Solar Nebula
Abiotic generation of complex organic compounds, in the early solar nebula that formed our solar system, is hypothesized by some to occur via Fischer-Tropsch (FT) synthesis. In its simplest form, FT synthesis involves the low temperature (<300degC) catalytic reaction of hydrogen and carbon monoxide gases to form more complex hydrocarbon compounds, primarily n-alkanes, via reactive nano-particulate iron, nickel, or cobalt, for example. Industrially, this type of synthesis has been utilized in the gas-to-liquid process to convert syngas, produced from coal, natural gas, or biomass, into paraffin waxes that can be cracked to produce liquid diesel fuels. In general, the effect of increasing reaction temperature (>300degC) produces FT products that include lesser amounts of n-alkanes and greater alkene, alcohol, and polycyclic aromatic hydrocarbon (PAH) compounds. We have begun to experimentally investigate FT synthesis in the context of abiotic generation of organic compounds in the early solar nebula. It is generally thought that the early solar nebula included abundant hydrogen and carbon monoxide gases and nano-particulate matter such as iron and metal silicates that could have catalyzed the FT reaction. The effect of FT reaction temperature, catalyst type, and experiment duration on the resulting products is being investigated. These solid organic products are analyzed by thermal-stepwise pyrolysis-GCMS and yield the types and distribution of hydrocarbon compounds released as a function of temperature. We show how the FT products vary by reaction temperature, catalyst type, and experimental duration and compare these products to organic compounds found to be indigenous to ordinary chondrite meteorites. We hypothesize that the origin of organics in some chondritic meteorites, that represent an aggregation of materials from the early solar system, may at least in part be from FT synthesis that occurred in the early solar nebula.
Locke, Darren R.↗
Models of the formation of the solar nebula
Protostellar cloud collapse and solar nebula formation models indicate that the size of the nebula produced will be larger in terms of both gas centrifugal balance R(CF) and collapse time diffusion length R(V). From this, it can be deduced that low mass nebulas are produced if (R(V)/R(CF))-squared is much greater than unity, while nebulas result for values lower than approximately unity. The total angular momentum value distinguishes most current models of the solar nebula. Analytic expressions for the surface density, nebular mass flux and photospheric temperature distributions during the formation stage are presented for simple modes illustrating and general properties of growing protostellar disks.
Cassen, P.↗
A window on the composition of the early solar nebula: 2014MU69, Pluto, and Phoebe
The initial chemical composition of any solar nebula will depend upon the degree to which 1) organic and ice components form on dust grains, 2) organic and molecular species form in the gas phase, 3) organics and ices are exchanged between the gas and solid state, and 4) the precursor and newly formed (more complex) materials survive and are modified in the developing planetary system. Infrared and radio observations of star-forming regions reveal that complex chemistry occurs on icy grains, sometimes before stars even form. Additional processing, through the protosolar disk and within the solar nebula further modifies most, but probably not all, of the initial materials. In fact, the modern Solar System still carries a fraction of its interstellar inheritance [Alexander et al., 2017]. Here we focus on three examples of small bodies in our Solar System, each containing chemical and dynamical clues to its origin and evolution: the small, cold-classical Kuiper Belt object (KBO) 2014MU69 , Pluto and Saturn’s moon, Phoebe. The New Horizons flyby of 2014 MU69 has given the first view of an unaltered body composed of material originally in the solar nebula at ~45 AU. The spectrum reveals methanol ice (not commonly found), a possible detection of water ice (Stern et al. 2019). Pluto’s internal and surface inventory of volatiles and complex organics, together with active geological processes including cryo-volcanism, indicate a surprising level of activity on a body in the outermost region of the Solar System, and the fluid that emerges from subsurface reservoirs may contain material inherited from the solar nebula (Cruikshank et al. 2019). Meanwhile, Saturn's captured moon, Phoebe, carries high D/H in H2O [Clark et al. 2018)], and complex organics (Cruikshank et al. 2008), both consistent with its formation in, and inheritance from, the outer solar nebula. Together, these objects provide windows on the origin and evolution of our Solar System and constraints to be considered in future chemical and physical models of PPDs. The Spitzer Space telescope discovered the ring of dust around Saturn that emanated from the bombardment of Phoebe in the recent past, dislodging primitive Solar System material from deep within that satellite. In addition, Spitzer has provided detailed studies of the chemistry of star forming regions where methanol and other ices are readily observed. As a result of these and other observations, protoplanetary disk models are beginning to combine both dynamical and chemical complexities into their codes, resulting in new insights for exoplanetary systems.
Yvonne J. Pendleton↗
Nucleation and condensation in the primitive solar nebula
It is pointed out that the primitive solar nebula may be modeled using the frictionally induced transport theory of Lynden-Bell and Pringle (1974) if the principal frictional mechanism within the nebula is turbulent viscosity. The present investigation is concerned with the construction of a model of a section of the primitive solar nebula as a basis for the study of nucleation and condensation processes within this section. The construction involves a relatively simple application of the Lynden-Bell and Pringle theory subject to steady mass flow conditions. The calculations which are conducted in connection with the investigation indicate that by the time the gas in the primitive solar nebula has become sufficiently supercooled to nucleate condensation centers, several different compounds, including the magnesium silicates forsterite and enstatite (MgSiO3), will probably be able to condense on the growing condensation center.
Cameron, A. G. W.↗
Lyman Alpha Photochemistry in the Solar Nebula
The purpose of the project "Lyman Alpha Photochemistry in the Solar Nebula" was to model photochemistry in the primitive solar nebula and the early solar systems. As part of the modeling, it was necessary to model the composition of the gas and dust accreted by the solar nebula. This final report contains a list of publications where the results of this project have been published.
Fegley, Bruce, Jr.↗
Mass loss from the proto-sun - Formation and evolution of the solar nebula
It is argued that the solar nebula and hence the planetary system were formed from the mass ejected by the protosun, whose strong magnetic field controlled the azimuthal and radial flow of the ejected material. The formation and evolution of the solar nebula is discussed in the light of observations of T Tauri stars, oxygen-isotopic anomalies in meteorites, and the mass and angular momentum distribution in the present solar system.
Trivedi, B. M. P.↗
Interstellar molecules - Formation in solar nebulae
Herbig's (1970) hypothesis that solar nebulae might be the principal source of interstellar grains and molecules is investigated. The investigation includes the determination of physical and chemical conditions in the early solar system. The production of organic compounds in the solar nebula is studied, and the compounds in meteorites are compared with those obtained in Miller-Urey and Fischer-Tropsch-type (FTT) reactions, taking into consideration aliphatic hydrocarbons, aromatic hydrocarbons, purines, pyrimidines, amino acids, porphyrins, and aspects of carbon-isotope fractionation. It is found that FTT reactions account reasonably well for all well-established features of organic matter in meteorites investigated. The distribution of compounds produced by FTT reactions is compared with the distribution of interstellar molecules. Biological implications of the results are considered.
Anders, E.↗
Modeling Nucleation and Grain Growth in the Solar Nebula: Initial Progress Report
The primitive solar nebula was a violent and chaotic environment where high energy collisions, lightning, shocks and magnetic re-connection events rapidly vaporized some fraction of nebular dust, melted larger particles while leaving the largest grains virtually undisturbed. At the same time, some tiny grains containing very easily disturbed noble gas signatures (e.g., small, pre-solar graphite or SiC particles) never experienced this violence, yet can be found directly adjacent to much larger meteoritic components (chondrules or CAIs) that did. Additional components in the matrix of the most primitive carbonaceous chondrites and in some chondritic porous interplanetary dust particles include tiny nebular condensates, aggregates of condensates and partially annealed aggregates. Grains formed in violent transient events in the solar nebula did not come to equilibrium with their surroundings. To understand the formation and textures of these materials as well as their nebular abundances we must rely on Nucleation Theory and kinetic models of grain growth, coagulation and annealing. Such models have been very uncertain in the past: we will discuss the steps we are taking to increase their reliability.
Nuth, Joseph A.↗
Comparisons of solar nebula models
The relationship between meteorites and solar nebulae is examined. Turbulent models of the solar nebulae are compared and discussed.
Weidenschilling, S. J.↗
Chemical evolution of the solar nebula: A new model
The common notion of a hot solar nebula from which meteoritic minerals condensed is not supported by theories of star formation. A model is developed which can give the same sequence of condensation without recourse to hot solar nebula. In this model, the solar nebula was formed from the matter ejected by the Sun during its T Tauri phase and the chemical condensation took place in this outflowing matter. Isotopic anomalies and the unique minerals found in meteorites may be explained by this model.
Trivedi, B. M. P.↗
Chemical heterogeneity and imperfect mixing in the solar nebula
The 'standard' accretion-disk model of the solar nebula is examined together with its basic equations, which are solved for the steady-state case. It is shown that, for this model of the solar nebula, the efficiency of mixing the products of thermochemical processing from small to large disk radii depends not on the magnitude of the eddy diffusivity but only on the ratio k between the eddy diffusivity and the eddy viscosity. In most cases, this ratio k is probably of order of unity or less, implying that most of the solar nebula was not contaminated by the consequences of thermochemical equilibria that were established at 'small' disk radii. It is noted that, while this condition is almost certainly satisfied if the physical process responsible for viscosity is thermal convection or waves, it has uncertain validity during the early phases of disk evolution where accretion-induced shear instabilities may dominate.
Stevenson, David J.↗
Evolution of Grains in a Turbulent Solar Nebula
Planetesimal formation by gravitational instability of a dust layer requires a nonturbulent solar nebula. The assumption that the solar nebula is a turbulent, convective accretion disk is explored. Self consistent models of a convective disk depend on high opacity of the disk material, which must be provided by grains. Grain coagulation is implied by the requirement of forming planetesimals, as the turbulent velocities (approx. 1/3 sound speed) in the accretion disk would disrupt any dust layer. Collisional coagulation could form large planetesimals, simultaneously leaving a sufficient fraction of matter in small grains to maintain the nebula's opacity. Or, coagulation of grains into small (approx. cm) aggregates could lower the opacity enough for turbulence to decay. The evolution of a population of grains in a turbulent solar nebula is calculated numerically.
Weidenschilling, S. J.↗