Galactic chemical evolution and the oxygen isotopic composition of the solar system

Abstract– We review current observational and theoretical constraints on the galactic chemical evolution (GCE) of oxygen isotopes to explore whether GCE plays a role in explaining the lower ¹⁷O/¹⁸O ratio of the Sun, relative to the present‐day interstellar medium, or the existence of distinct ¹⁶O‐rich and ¹⁶O‐poor reservoirs in the solar system. Although the production of both ¹⁷O and ¹⁸O are related to the metallicity of progenitor stars, ¹⁷O is most likely produced in stars that evolve on longer timescales than those that produce ¹⁸O. Therefore, the ¹⁷O/¹⁸O ratio need not have remained constant over time, contrary to preconceptions and the simplest models of GCE. An apparent linear, slope‐one correlation between δ¹⁷O and δ¹⁸O in the ISM need not necessarily reflect an O isotopic gradient, and any slope‐one galactocentric gradient need not correspond to evolution in time. Instead, increasing ¹⁷O/¹⁸O is consistent both with observational data from molecular clouds and with modeling of the compositions of presolar grains. Models in which the rate of star formation has decelerated over the past few Gyr or in which an enhanced period of star formation occurred shortly before solar birth (“starburst”) can explain the solar‐ISM O‐isotopic difference without requiring a local input of supernova ejecta into the protosolar cloud. “Cosmic chemical memory” models in which interstellar dust is on average older than interstellar gas predict that primordial solar system solids should be ¹⁶O‐rich, relative to the Sun, in conflict with observations. However, scenarios can be constructed in which the ¹⁶O‐rich contribution of very massive stars could lead to ¹⁶O‐poor solids and a ¹⁶O‐rich bulk Sun, if the solar system formed shortly after a starburst, independent of the popular scenario of photochemical self‐shielding of CO.     

Particles and fields in the outer solar system

Space probe missions to the outer planets can provide in situ measurements of the electromagnetic fields, the charged and neutral particles, and the cosmic rays in the unexplored regions of the outer solar system and the local interstellar medium. The physics of these distant regions is now known only through theoretical and model extrapolation of observations made near 1 AU and through remote sensing. These methods have inherently large uncertainties and shortcomings. We review here the relevant solar system physics, the astrophysics, and the plasma-physics questions associated with the media in the remote solar system, and we indicate what in situ measurements are needed to resolve remote sensing ambiguities, to verify or reject theoretical predictions, and to provide data not obtainable by existing methods.     

Near-parabolic cometary flux in the outer solar system

The flux of near-parabolic comets in the outer-planetary region is estimated on the presumption that the major planets and the galactic tide control the dynamics of comets. It is found that the flux of the Oort cloud comets (semi-major axis > 20000 AU) is similar to the case of a strong comet shower derived on the presumption that the galactic tidal force were not operative. On the other hand, the flux of comets with semi-major axes <- 20000 AU is found to be an increasing function of q (perihelion distance) until q reaches 20 AU, while for a 45000 AU it is a rapidly increasing function for q 12 AU. In other words, for comets of the inner extension of the Oort cloud the planetary perturbation acts as a strong barrier for them to penetrate into the inner planetary region.     

The solar system evolution

The concepts on the spatially-periodic condensation in the solar system have been considered in the light of the general theory of the evolution of the solar system. It has been shown that as protodisks arise and compress, the role of hydromagnetic effects weakens. After the stage of spatially-periodic condensation and accretion, the concentration of gas in protodisks decreases and the role of hydromagnetic effects increases again. Specific features of the formation of planets near the Sun and satellites near the planets can be explained if these peculiarities of the evolution are taken into account. The corresponding role of the above processes has been evaluated numerically.The accretion of gas molecules both by jet streams arising after spatially-periodic condensation and by planet embryos has also been considered. Characteristic times of these processes have been estimated.The results obtained show that the general concept on the solar system evolution (Alfvén and Arrhenius, 1976) is in good agreement with the mechanism of spatially-periodic condensation, which takes place during the formation of primary rings of the solar and satellite systems (Gladyshev, 1977).     

The chemical evolution of the solar neighbourhood: the effect of binaries

In this paper we compute the time evolution of the elements (⁴He, ¹²C, ¹⁴N, ¹⁶O, ²⁰Ne, ²⁴Mg, ²⁸Si, ³²S, ⁴⁰Ca and ⁵⁶Fe) and of the supernova rates in the solar neighbourhood by means of a galactic chemical evolutionary code that includes in detail the evolution of both single and binary stars. Special attention is payed to the formation of black holes.Our main conclusions:

• •in order to predict the galactic time evolution of the different types of supernovae, it is essential to compute in detail the evolution of the binary population,
• •the observed time evolution of carbon is better reproduced by a galactic model where the effect is included of a significant fraction of intermediate mass binaries,
• •massive binary mass exchange provides a possible solution for the production of primary nitrogen during the very early phases of galactic evolution,
• •chemical evolutionary models with binaries or without binaries but with a detailed treatment of the SN Ia progenitors predict very similar age–metallicity relations and very similar G-dwarf distributions whereas the evolution of the yields as function of time of the elements ⁴He, ¹⁶O, ²⁰Ne, ²⁴Mg, ²⁸Si, ³²S and ⁴⁰Ca differ by no more than a factor of two or three,
• •the observed time evolution of oxygen is best reproduced when most of the oxygen produced during core helium burning in ALL massive stars serves to enrich the interstellar medium. This can be used as indirect evidence that (massive) black hole formation in single stars and binary components is always preceded by a supernova explosion.

     

Spectra of hydrate frosts: Their application to the outer solar system

Reflectance spectra from 1 to 6 microns were taken of CH₄ and CO₂ gas hydrates and were found to be very similar to H₂O frost spectra over the entire wavelength region. H₂O clathrates have a gas to H₂O ratio of about $\text{1}\text{6}$, hence a surface may contain 17% (by number) gas and appear spectroscopically similar to an H₂O frost covered surface. This is important in the pressure-temperature regime of the outer solar system where hydrates, which often have vapor pressures 10^?5 (or less) that of the pure gas component, are marginally stable as solids (e.g., the vapor pressure in Torr at 60 K for CH₄·6H₂O = 10^?8 while for CH₄ = 10^?1). We may conclude that reflectance spectroscopy (especially Earth-based) is useful for positive identification of some components of the surface, but does not set stringent limits for spectroscopically active hydrate forming substances in the presence of water frost.     

The stability and evolution of the solar system

The known history of the solar system is discussed, also the types of dynamical problems exhibited by members of the solar system and the solutions suggested for a number of such problems. The recent work of Walker, Emslie and Roy, on Empirical Stability Criteria in Many Body Problems is also mentioned.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy.     

Quantum physics exploring gravity in the outer solar system: the SAGAS project

We summarise the scientific and technological aspects of the Search for Anomalous Gravitation using Atomic Sensors (SAGAS) project, submitted to ESA in June 2007 in response to the Cosmic Vision 2015–2025 call for proposals. The proposed mission aims at flying highly sensitive atomic sensors (optical clock, cold atom accelerometer, optical link) on a Solar System escape trajectory in the 2020 to 2030 time-frame. SAGAS has numerous science objectives in fundamental physics and Solar System science, for example numerous tests of general relativity and the exploration of the Kuiper belt. The combination of highly sensitive atomic sensors and of the laser link well adapted for large distances will allow measurements with unprecedented accuracy and on scales never reached before. We present the proposed mission in some detail, with particular emphasis on the science goals and associated measurements and technologies.     

Origin and evolution of the solar system,II

(7)Formation of celestial bodies. The basic concepts of the accretional process are discussed, and the inadequacy of the contractional model is pointed out. A comparison is made between the general pre-planetary state on the one hand and the present state in the asteroidal region on the other. A model for accretion of resonance-captured grains leading to the formation of resonance-captured planets and satellites is suggested.(8)Spin and accretion. The relation between the accretional process and the spin of planets is analyzed.(9)Accretion of planets and satellites. It is shown that jet streams are a necessary intermediate stage in the formation of celestial bodies. The time sequence of planet formation is analyzed, and it is shown that the newly accreted bodies have a characteristic internal heat structure; the cases of the Earth and the Moon are considered in detail. A region of high initial temperature is found at 0.4 of the present Earth radius, whereas the culminating temperature of the Moon is near its present surface. An accretional heat wave is found to proceed outwards, and may produce the observed differentiation features.     

Water ice on outer solar system surfaces: Basic properties and radiation effects

This paper reviews the properties of vapor-deposited water ice in connection with icy surfaces in the outer solar system. The emphasis is on knowledge gained during the last decade, and on the properties of the amorphous phase, especially those affected by the presence of microporosity. The paper discusses the role played by the properties of different phases of ice and the effect of irradiation on the icy surfaces of satellites in the outer solar system: sputtering, phase transformation, the production and trapping of molecular radiation products, and stress induced cracking. The understanding of how growth and irradiation processes affect the optical properties of ice will lead to extract better information from optical remote sensing than is possible today. It is argued that cracks in ice induced by stresses are the main reason causing low-temperature ices to be strongly scattering.     

On the sputter alteration of regoliths of outer solar system bodies

This paper discussed several processes that are expected to occur when the porous regoliths on bodies without atmospheres in the outer Solar System are subjected to energetic ion bombardment. The conclusions reached in many of the papers involving sputtering published in the planetary literature are qualitatively or quantitatively incorrect because effects of soil porosity have been neglected. It is shown theoretically and experimentally that porosity reduces the effective sputtering yield of a soil by more than an order of magnitude. Between 90 and 97% of the sputtered atoms are trapped within the regolith, where they are fractionated by differential desorption. Experiments indicate that more volatile species have higher desorption probabilities. This process is the most important way in which alteration of chemical and optical properties occurs when a regolith is sputtered. When a basic silicate soil is irridiated these effects lead to sputter-deposited films enriched in metallic iron, while O, Na, and K are preferentially lost. The Na and K are present in the atmosphere above the sputtered silicate in quantities much greater than their abundances in the regolith. Icy regoliths of SO₂ should be enriched in elemental S and/or S₂O. This prediction is supported by the probable identification of S₂O and polysulfur oxide bands in the IR spectra of H-sputtered SO₂ reported by M. Moore (1984, Icarus 59, 114–128). When porous mixtures of water, ammonia, and methane frosts are sputtered, the loss of H and surface reactions of C, N, and O in the deposits should produce complex hydrocarbons and carbohydrates, some of which may be quite dark. Such reactions may have played a role in the formation of the matrix material of carbonaceous chondrites prior to agglomeration.     

The Kozai resonance in the outer solar system and the dynamics of long-period comets

We consider the secular evolution of the orbits of bodies in the Outer Solar System under the perturbations of the jovian planets assumed on coplanar and circular orbits. Through the approach used for asteroidal belt by Yoshihide Kozai in 1962, we obtain that the Kozai resonance do not affect the behavior of bodies belonging to the Kuiper belt but concerns the long-timescale evolution of long-period comets. In particular this resonance appears as a process contributing to produce Sun-grazer comets.     

Production of organic molecules in the outer solar system by proton irradiation: Laboratory simulations

In the past few years considerable attention has been given to the determination of likely compounds that could account for the various colors observed in the outer solar system: and to possible formation mechanisms for these compounds. Many experiments have been done using electrical discharges (Chadha, M. S., et al., 1971, Icarus15, 39) and ultraviolet light (Khare, B. N., and Sagan, C., 1973, Icarus20, 311) on mixtures of CH₄, NH₃, and H₂S, which are most likely the dominant minor constituents of the atmospheres of Jupiter, Saturn, Titan, and possibly the other satellites early in their histories. Colored polymers, usually brownish-red, have been produced in these experiments. With the passage of Pioneer 10 around Jupiter, there is another source of energy worthy of consideration, energetic protons (and electrons). Preliminary experiments to investigate the formation of colored polymers and other interesting molecules by the irradiation of gas mixtures by protons are discussed. Two to four Mev protons were used, with corresponding beam fluxes (as measured at 6R_J from the planet) equivalent to approximately 80 Earth years at Jupiter per hour of exposure. As in the other types of experiments, colored polymers have been produced. An important feature of this work is the presence or absence of absorption at 5 μm in the different materials produced; Titan is quite dark at this wavelength and Io is fairly bright. Such features may provide criteria for accepting or rejecting various materials produced in these experiments as reasonable coloring agents for the outer solar system.     

Investigation of the long-periodic orbits of Trojan-type asteroids in the outer solar system

The present paper demonstrates the results of the numerical integration of equations of motion of a infinitesimal mass pleased in the neighborhood of the triangular point of the Sun-planet system. There are presented the results for the outer solar system, i.e. for Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. The long-periodic solutions were searched for the distance from the Lagrangian point changing from ±0.01 to ±0.10 in canonical units. The Trojans of those planets have the circle, tadpole, horseshoe and irregular shape of their orbits. Same of those test particles showed a close approach to planet. Other of those collided with planet and then was removed from the solar system. The tadpole, circle and same trajectories surveyed integration for 100,000 years. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structure and evolution of solar radio bursts at 26.4 MHz

Two dimensional source brightness distributions at 26.4 MHz for solar bursts of spectral type II, III, IV, and V are derived from observations with a multiple-baseline, time-sharing interferometer system. It was designed explicitly to study the large angle (40 halo) component of low frequency solar bursts first reported by Weiss and Sheridan (1962). Thirty-two bursts occurring in the interval of June–August, 1975, were fit with a circular gaussian core and an elliptical gaussian halo component. Half-power halo diameters (E-W×N-S) averaged 30×28 for type III bursts and 42×27, 28×37, 30×25 for type V, II and IV bursts respectively. Typical core sizes fell in the range of 10±4 giving 31 halo to core size ratio. All burst types were found to have some large angle structure: the specific intensity was 10% compared to the core but the total power in each component was comparable. Two processes for producing the core-halo structure of type III bursts are compared: scattering and refraction of a point source and refraction from many sources over an extended region. It is concluded that the core can be explained by either model but the halo is more consistent with emission from an extended source region of 40° in longitude.