Chemical evolution directed

Before addressing the main topic of chemical evolution, I would like to discuss briefly the rather curious story of the Belgian school of thermodynamics, often called the Brussels school. It took shape at the end of the 1920s and during the 1930s. At a time when the great schools of thermodynamics, such as the Californian school founded by Lewis and the British school with Guggenheim, directed their efforts almost exclusively to the study of equilibrium systems, the point of view presented by the Brussels school appeared as quite unorthodox and somewhat controversial. Indeed, the Brussels school tried to approach equilibrium as a special case of nonequilibrium and concentrated its efforts on the presentation of thermodynamics in a form that would be applicable also to nonequilibrium situations. This story is rather curious from the point of view of the history of science, so let me go into a little more detail. 

Study of the chemical evolution of chirality started in 1809 with the discovery of Haiiy [4], who postulated from crystal cleavage observations that a crystal and each of its constituent space-filling molecules are images of each other in overall shape. Later, in 1848, Pasteur reported the different destruction rates of the dextro- and levorotatory forms of ammonium tartrate by the mold Penicillium glaucum [5]. These observations could not be explained properly at that time, but in 1874 Le Bel [6] and van t Hoff [7] independently proposed that the four valences of the carbon atom are directed toward the vertices of an atom-centered tetrahedron. This finding allowed the development of the theory of the three-dimensional structure of molecules by which the phenomenon of chirality and Pasteur s discovery were explained scientifically. 

Other models directly couple chemical reaction with mass transport and fluid flow. The UNSATCHEM model (Suarez and Simunek, 1996) describes the chemical evolution of solutes in soils and includes kinetic expressions for a limited number of silicate phases. The model mathematically combines one- and two-dimensional chemical transport with saturated and unsaturated pore-water flow based on optimization of water retention, pressure head, and saturated conductivity. Heat transport is also considered in the model. The IDREAT and GIMRT codes (Steefel and Lasaga, 1994) and Geochemist s Workbench (Bethke, 2001) also contain coupled chemical reaction and fluid transport with input parameters including diffusion, advection, and dispersivity. These models also consider the coupled effects of chemical reaction and changes in porosity and permeability due to mass transport. 

Many critical uncertainties must still be addressed to assess quantitatively the sources and sinks of atmospheric nanoparticles, their physical and chemical evolution during atmospheric aging, their direct and indirect impacts on atmospheric chemistry and physics, and their connections to adverse effects on human health. Several important and complex areas that need much additional study include ... 

As discussed earlier, the stellar and Galactic chemical evolution uncertainties afflicting 3He are so large as to render the use of 3He to probe or test BBN problematic therefore, I will ignore 3He in the subsequent discussion. There are a variety of equally valid approaches to using D, 4He, and 7Li to test and constrain the standard models of cosmology and particle physics (SBBN). In the approach adopted here deuterium will be used to constrain the baryon density (r] or, equivalently, if>h 2 ) Within SBBN, this leads to predictions of Yp and [Li]p. Indeed, once the primordial deuterium abundance is chosen, r/ may be eliminated and both Yp and [Li] p predicted directly, thereby testing the consistency of SBBN. 

Further implications of the protosolar He/" He and D/H ratios are discussed by Gloeckler and Geiss (2000). The local interstellar cloud (LIC) represents a galactic sample having experienced chemical evolution for 4.6 Gyr longer than the protosun. D/H in the LIC is lower than the protosolar value whereas He/" e in the LIC is higher than the protosolar value (see also the Elementary particles in interplanetary space section). The direction of both these changes is expected, because stars only destroy deuterium but destroy and produce He. The observed increase of the He abundance is mainly due to production of this isotope in small stars. 

National Research Council. The Search for Life s Origins Progress and Future Directions in Planetary Biology and Chemical Evolution. Washington,... 

During the whole chemical evolution, water is the most important substance. As a reactant in the Miller experiment and the only solvent in Level 2, water directly participated in the early prebiotic chemical evolution. In addition, as a selector from Level 3 to Level 4, most of the macromolecules in Level 3 were screened out by the water environment. Water is the primary enviroimient of terrestrial life. 

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