Solar nebula evolution

Cross-calibration of the initial A1/ A1 records inferred for nebular components of chondrites with the absolute Pb-Pb ages of CAIs results in a self-consistent high-resolution chronology for the high temperature phases of solar nebula evolution. A plausible scenario and timeline can be constructed ... 

Class III objects are weak line T Tauri stars (T Tauri stars in which the characteristic emission lines are only weakly observed in their optical spectra) and have little or no evidence of a disk. At this stage of solar nebula evolution, which may last between 3 and 30 Ma, the sun has formed, and the material of the nebula is being dissipated by solar winds in the inner part. In the outer part of the nebula material is dissipated by photo-evaporation caused by UV radiation from the solar wind. A positive pressure gradient near the inner edge of the nebula facilitates planetesimal formation. 

The matter that made up the solar nebula from which the solar system was formed already was the product of stellar birth, aging and death, yet the Sun is 4.5 billion years old and will perhaps live to be 8 billion years but the Universe is thought to be 15 billion years old (15 Gyr) suggesting that perhaps we are only in the second cycle of star evolution. It is possible, however, that the massive clouds of H atoms, formed in the close proximity of the early Universe, rapidly formed super-heavy stars that had much shorter lifetimes and entered the supernova phase quickly. Too much speculation becomes worrying but the presence of different elements in stars and the subsequent understanding of stellar evolution is supported by the observations of atomic and molecular spectra within the light coming from the photosphere of stars. 

Such a measurement can tell us about the chemical evolution of oxygen, such as whether the isotopes differentiated via a thermal cycle in which lighter leO fractionates from the heavier lsO, much as Vostok ice-core oxygen ratios reveal the Earth s prehistoric climate. From this fixed point of the Sun s oxygen ratios, we can then trace the history of water in other planetary bodies since their birth in the solar nebulae through the subsequent cometary bombardment [13]. In NASA s search for water on the Moon, important for the establishment of a future Moon base, such isotopic ratios will determine whether the water is a vast mother lode or just a recent cometary impact residue. 

Isotope variations found in extraterrestrial materials have been classified according to different processes such as chemical mass fractionation, nuclear reactions, nucleosynthesis, and/or to different sources such as interplanetary dust, solar materials, and comet material. Various geochemical fingerprints point to the reservoir from which the planetary sample was derived and the environment in which the sample has formed. They can be attributed to a variety of processes, ranging from heterogeneities in the early solar nebula to the evolution of a planetary body. For more details the reader is referred to reviews of Thiemens (1988), Clayton (1993, 2004), and McKeegan and Leshin (2001). 

Extraterrestrial materials consist of samples from the Moon, Mars, and a variety of smaller bodies such as asteroids and comets. These planetary samples have been used to deduce the evolution of our solar system. A major difference between extraterrestrial and terrestrial materials is the existence of primordial isotopic heterogeneities in the early solar system. These heterogeneities are not observed on the Earth or on the Moon, because they have become obliterated during high-temperature processes over geologic time. In primitive meteorites, however, components that acquired their isotopic compositions through interaction with constituents of the solar nebula have remained unchanged since that time. 

Cosmochemislry places important constraints on models for the origin of the solar nebula and the formation and evolution of planets. We explore nebula constraints by defining the thermal conditions under which meteorite components formed and examine the isotopic evidence for interaction of the nebula with the ISM and a nearby supernova. We consider how planetary bulk compositions are estimated and how they are used to understand the formation of the terrestrial and giant planets from nebular materials. We review the differentiation of planets, focusing especially on the Earth. We also consider how orbital and collisional evolution has redistributed materials formed in different thermal and compositional regimes within the solar system. 

Discuss the evolution of ideas about temperatures in the solar nebula, and how that relates to the formation of CAIs. 

Ozima, M., Wieler, R., Marty, B., Podosek, F. A. (1998) Comparative studies of solar, Q-gas and terrestrial noble gases, and implications on the evolution of the solar nebula. Geochim. Cosmochim. Acta, 62, 301-14. 

While the MMSN provides insight into a plausible structure of the solar nebula, the concept can be a bit misleading. The MMSN is assumed to be a purely passive disk, and issues such as disk evolution and solid transport, both of which are discussed at length in this chapter, were not considered in developing these models... 

With the success of the Stardust mission, tests of our models for solar nebula, and thus protoplanetary disk evolution, are no longer limited to asteroidal bodies (meteorites), but now can be applied to cometary bodies as well. Stardust returned dust grains that were ejected from the surface of comet Wild 2, a Jupiter-family cometthatis thought to have formed at distances of >20 AU from the Sun (Brownlee et al. 2006). Thus, we now have samples of materials from the outer solar nebula that can be studied in detail. 

Besides water, the above evolution could occur for silicates (Cuzzi et al. 2003), organics, or more volatile ices in the outer solar nebula. The effects of these changes in abundances on the composition of primitive materials remains to be studied. 

Chemical and isotopic evolution of the solar nebula and protoplanetary disks... 

Abstract In this chapter we review recent advances in our understanding of the chemical and isotopic evolution of protoplanetary disks and the solar nebula. Current observational and meteoritic constraints on physical conditions and chemical composition of gas and dust in these systems are presented. A variety of chemical and photochemical processes that occur in planet-forming zones and beyond, both in the gas phase and on grain surfaces, are overviewed. The discussion is based upon radio-interferometric, meteoritic, space-borne, and laboratory-based observations, measurements and theories. Linkage between cosmochemical and astrochemical data are presented, and interesting research puzzles are discussed. 

The astrophysical models of protoplanetary disks based on optical observations and laboratory experiments and meteoritic measurements provide the basis for theories of nebular evolution. The best and most precise relevant measurements are from meteoritic analysis. Meteorites from the Asteroid Belt of our Solar System are the best record of the evolution of the solar nebula from a gas-dust mixture to an organized planetary system. The addition of cometary and solar-wind sample analysis complement these data. Combination of fundamental laboratory-based experiments and modeling efforts has led to a highly resolved understanding of the chemical conditions and processes in the primordial solar nebula (see Chapter 6). In this chapter an overview of recent advances in our understanding of the chemical and isotopic evolution of the early Solar System and protoplanetary disks is presented. 

For the purposes of this chapter, we will focus on the Class II stage because it is the best-studied phase of the evolution of the solar nebula. 

Several types of the early Solar System materials are available for laboratory analysis (see Chapter 1 and Table 1.1 and Fig. 1.1). Each material has unique characteristics and provides specific constraints on the chemistry of the solar nebula. Major components of this sample are meteorites, fragments of asteroids, that serve as an excellent archive of the early Solar System conditions. Primitive chondritic meteorites contain glassy spherical inclusions termed chondrules, some of the oldest solids in the Solar System. Most chondrites were modified by aqueous alteration or metamorphic processes in parent bodies but there are some chondrites that are minimally altered (un-equilibrated chondrites, UCs). They have yielded a wealth of information on the chemistry, physics, and evolution of the young Solar System. 

There is evidence from chondrites that the solar nebula was well mixed between 0.1 and 10 AU during its first several million years of the evolution, as shown by the homogeneity in concentrations of many isotopes of refractory elements (Boss 2004 Chapter 9). This is likely caused by the evaporation and recondensation of solids in the very hot inner nebula, followed by outward transport due to turbulent diffusion and angular momentum removal. Materials out of which terrestrial planets and asteroids are built have been heated to temperatures above 1300 K and are thus depleted in volatile elements. The inner solar nebula, with some exceptions, does not retain memories of the pristine interstellar medium (ISM) chemical composition (Palme 2001 Trieloff Palme 2006). 

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