Evolution and composition of the Solar System

Conditions in the Earth s atmosphere and interior are, of course, much better known than conditions on the other planets. At this time, the forefront of composition studies in the Earth s atmosphere concerns the concentrations of man-made and natural pollutants. In particular, photochemical reactions as well as dynamic effects in the stratosphere are the subject of intense investigations. The steady 

Part a of this section summarizes present theories of Solar System formation. Parts b and c provide brief discussions of evolutionary processes of the terrestrial and the outer planets, respectively. In these discussions we give a few examples where composition measurements have contributed to the understanding of planetary evolution. 

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No description of our sofar system s formation wouid be complete without a discussion of the profound changes wrought by its orbital and collisional evolution. Although these physical processes may not seem to be related to cosmochemistry, they have changed the spatial distribution of planets and small bodies of differing compositions within the solar system, and in some cases, even the bulk compositions of large bodies. Understanding these processes can help us appreciate how some of the cosmochemical conundrums and complexities of the solar system arose. 

Tiny solid cosmic particles - often referred to as dust - are the ultimate source of solids from which rocky planets, planetesimals, moons, and everything on them form. The study of the dust particles genesis and their evolution from interstellar space through protoplanetary disks into forming planetesimals provides us with a bottom-up picture on planet formation. These studies are essential to understand what determines the bulk composition of rocky planets and, ultimately, to decipher the formation history of the Solar System. Dust in many astrophysical settings is readily observable and recent ground- and space-based observations have transformed our understanding on the physics and chemistry of these tiny particles. 

Models of planetary evolution assume that at the time of planetary formation the solar system had a single universal and well-mixed composition from which aU parts of the solar system were derived (see Podosek, 1978). Information as to the elemental and isotopic characteristics of this primordial composition is presently available from the Sun, meteorites, and the atmospheres of the giant planets (Wider, 2002). In the case of the Sun, distinction is usually made between the present-day composition, which is available via spectral analysis of the solar atmosphere and capture of the solar wind, either directly in space or by using metallic foU targets, and the proto-Sun (the composition at the time of planetary accretion) whereby the lunar regolith and/or meteorites are utilized as archives of ancient solar wind. As discussed below, the distinction is only really important for helium due to production of He by deuterium burning. 

The compositions of the atmospheres of the planets and their evolution since the formation of the solar system are the result of a complex array of Interacting chemical and dynamical processes driven by the absorption of photons and energetic particles from the sun and of cosmic rays from the galaxy. Photochemistry and plasma chemistry exercise a major influence oh the composition of planetary clouds and on the nature of the liquid and solid surfaces. 

The collective study of the various atmospheres and surfaces in the Solar System constitutes the field of comparative planetology. Wide ranges in surface gravity, solar flux, internal heat, obliquity, rotation rate, mass, and composition provide a broad spectrum of boundary conditions for atmospheric systems. Analyses of data within this context lead to an understanding of physical processes applicable to all planets. Once the general physical principles are identified, the evolution of planetary systems can be explored. 

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. 

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