Cosmochemistry-An Outlook
COSMOCHEMISTRY- AN OUTLOOK
Cosmochemistry or chemical cosmology is the study of the chemical composition of matter in the universe and the processes that led to those compositions. This is done primarily through the study of the chemical composition of meteorites and other physical samples. Given that the asteroid parent bodies of meteorites were some of the first solid material to condense from the early solar nebula, cosmochemists are generally, but not exclusively, concerned with the objects contained within the solar system.
In 1938, Swiss mineralogist Victor Goldschmidt and his colleagues compiled a list of what they called “cosmic abundances” based on their analysis of several terrestrial and meteorite samples. Goldschmidt justified the inclusion of meteorite composition data into his table by claiming that terrestrial rocks were subjected to a significant amount of chemical change due to the inherent processes of the Earth and the atmosphere. This meant that studying terrestrial rocks exclusively would not yield an accurate overall picture of the chemical composition of the cosmos. Therefore, Goldschmidt concluded that extraterrestrial material must also be included to produce more accurate and robust data. This research is considered to be the foundation of modern cosmochemistry.
During the 1950s and 1960s, cosmochemistry became more accepted as a science. Harold Urey, widely considered to be one of the fathers of cosmochemistry, engaged in research that eventually led to an understanding of the origin of the elements and the chemical abundance of stars. In 1956, Urey and his colleague, German scientist Hans Suess, published the first table of cosmic abundances to include isotopes based on meteorite analysis.
The continued refinement of analytical instrumentation throughout the 1960s, especially that of mass spectrometry, allowed cosmochemists to perform detailed analyses of the isotopic abundances of elements within meteorites. in 1960, John Reynolds determined, through the analysis of short-lived nuclides within meteorites, that the elements of the solar system were formed before the solar system itself which began to establish a timeline of the processes of the early solar system.
In October 2011, scientists reported that cosmic dust contains complex organic matter (“amorphous organic solids with a mixed aromatic-aliphatic structure”) that could be created naturally, and rapidly, by stars.
On August 29, 2012, and in a world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light years from Earth. Glycolaldehyde is needed to form ribonucleic acid, or RNA, which is similar in function to DNA. This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.
In September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics – “a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively”. Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons “for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks.”
During the second half of the twentieth century, the domain of geochemistry has greatly expanded and the field is today often seen as a branch of an extended chemistry of the Earth, called cosmochemistry. This paper is a historical introduction to cosmochemistry in which the wider cosmic aspects are surveyed up to about 1915, when nuclear physics changed the scene. These wider aspects or themes include, firstly, the attempts to determine the relative abundances of the elements, secondly, the extension of geochemistry to include physical geochemistry, thirdly, the study of meteorites and, fourthly, the spectroscopic study of the stars within the astrochemical tradition. Because of the lack of reliable data, a great deal of the protocosmochemistry described in the present paper was speculative. Nonetheless, by 1915 the contours of the cosmochemistry of the future were just visible and the developments here singled out can thus be seen as belonging to the prehistory of modern cosmochemistry.
The basic chemical structure of the Solar System is a result of the chemistry of just a handful of major elements. Combined, just five elements (in order of abundance, Fe, O, Mg, Si, and Ni) make up 95% of the mass of the Earth and the other terrestrial planets (McDonough and Sun 1995). Thus a planet composed of an Fe–Ni metallic core surrounded by a Mg-silicate mantle is a perfectly acceptable first-order model for planet Earth. The addition of nine other elements (Ca, Al, S, Cr, Na, Mn, P, Ti, and Co) brings the total to over 99.9% of the mass of our planet. The story of how our planet, and ourselves, came to be, however, is often unraveled by studying the details of the remaining 0.1%.
The bulk elemental abundances in the Solar System reflect the astrophysical evolution of the Universe (Fig. 1). Almost 99.9% of the Solar System is composed of hydrogen (H) and helium (He). These elements, along with trace amounts of Li, Be, and B, are remnants from the Big Bang (Copi et al. 1995). Hydrogen and helium were the feedstock for the first stars, which were massive and ran through their evolution in a few million years. It was not until the formation of these stars that carbon and the heavier elements came into existence.
Hydrogen burning is the principal energy source in main sequence stars. This process fuses four H nuclei together to make 4He. In solar-mass stars, H burning proceeds via proton–proton chain reactions. In stars of somewhat higher mass, core temperatures are greater and the burning proceeds by the CNO cycle, in which the net reaction is catalysed by isotopes of C, N, and O. Side reactions in H burning generate rare but important isotopes, including 14N, 17O, and the short-lived radioisotope 26Al.
After a star has consumed the H in its core, it contracts and heats, resulting in the onset of He burning. This process produces 12C in the “triple-α process,” in which three 4He nuclei collide simultaneously. In addition, 16O is produced during burning when 12C nuclei capture alpha particles (which are simply 4He nuclei). Neutrons are also liberated during He burning, resulting in “s-process” neutron-capture nucleosynthesis (Gallino et al. 1998). Helium burning is the last stage in the life of stars below eight solar masses. At the end of this stage, strong stellar winds blow off the stellar envelope, delivering gas and dust back to the interstellar medium (ISM), creating a planetary nebula (which, by the way, has nothing to do with planets). Once the envelope of the star is completely lost, a white dwarf star is left behind. Accretion of material given off by a companion star can result in explosive H burning on the surface of the white dwarf, forming a nova. If enough mass is accreted, C and O may burn explosively and completely disrupt the star, forming a type-Ia supernova.
In stars greater than eight solar masses, C burning occurs, producing an excited nucleus of 24Mg by fusion of two 12C nuclei. This nucleus decays into 20Ne and an α particle, 23Na and a proton, or 24Mg and a gamma ray. The 20Ne is the next nuclear fuel in the star’s life. In this stage, 20Ne nuclei disintegrate into 16O nuclei and α particles. Other 20Ne nuclei may capture these α particles to produce 24Mg. Ne burning is thus the major producer of 24Mg.
As Ne burning ends, the star contracts and heats until O burning is initiated. This process produces an excited nucleus of 32S. This nucleus decays into 28Si and an α particle, or 31P and a proton, or relaxes down to 32S through gamma ray emission. Thus, 28Si and 32S are the principal products of O burning. As the star contracts further, Si burning occurs. In this process, some of the nuclei disintegrate into lighter nuclei, neutrons, protons, and α particles. The remaining nuclei capture these light particles to produce a range of isotopes, including 40Ca, 56Fe (the primary product), and the short-lived radioisotope 60Fe. These massive stars end their lives in gigantic explosions as type-II supernovae, expelling these newly synthesized elements into the ISM. Ejection of neutron-rich matter from these supernovae results in r-process neutron-capture nucleosynthesis, in which elements heavier than Fe are created (Arnould et al. 2007; Schatz 2010).
When the Solar System formed, it incorporated material from many of these stellar sources, including supernovae, late-stage stars, and novae. The evidence for this diversity is recorded in presolar grains, which preserve detailed chemical signatures of the end stages of stellar evolution and processes in the ISM. In their article in this issue, Ann Nguyen and Scott Messenger (2011) provide an overview of the study of presolar materials, which record the detailed history of the end of stellar evolution and the beginning of Solar System formation.
The starting material for the Solar System was mixed extremely well, such that the system as a whole has a nearly uniform elemental and isotopic composition. In addition to H and He, the primary products of stellar nucleosynthesis are C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, and Fe. These elements account for 99.5% of the matter in the ISM that is not H and He. With the exception of the noble gases, they form the chemical basis of the bodies of our Solar System and all planetary systems. C/O ratios determine the ultimate chemistry of planetary systems, with C-rich systems dominated by graphite and carbides (Bond et al. 2010). O-rich systems, like our Solar System, are dominated by silicates and oxides.
Cosmochemistry is a dynamic field with a bright future. It promises to be at the forefront of Solar System exploration, which is following a natural progression from flyby missions, to orbiters, to landers that perform in situ analyses, to missions that return samples to Earth for detailed analyses. The Stardust mission, which returned samples of comet Wild 2, and the Genesis mission, which returned samples of solar wind, have yielded unprecedented scientific returns. With many national space agencies developing future sample-return missions, we can look forward to samples from asteroids, comets, the Moon, and Mars over the coming decades. Someday we may even see samples returned from more challenging targets, such as Venus, Mercury, and the Galilean satellites. Only when humanity has acquired a complete geologic inventory of the Solar System will the entire history of our planetary system be understood. In the meantime nature continues to deliver new and amazing samples, each one of which reveals a heretofore-unknown chapter in the history of our Solar System.
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