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Cosmochemistry
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Cosmochemistry (from Greek κόσμος kósmos, "universe" and χημεία khemeía)
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.
History
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
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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.
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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."
In February 2014, NASA announced a greatly upgraded database for tracking
polycyclic aromatic hydrocarbons (PAHs) in the universe. According to
scientists, more than 20% of the carbon in the universe may be associated with
PAHs, possible starting materials for the formation of life. PAHs seem to have
been formed shortly after the Big Bang, are widespread throughout the universe,
and are associated with new stars and exoplanets.
Meteorites
Meteorites are one of the most important tools that cosmochemists have for
studying the chemical nature of the Solar System. Many meteorites come from
material that is as old as the Solar System itself, and thus provides scientists
with a record from the early solar nebula. Carbonaceous chondrites are
especially primitive; that is they have retained many of their chemical properties
since their formation 4.56 billion years ago, and are therefore a major focus of
cosmochemical investigations.
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The most primitive meteorites also contain a small amount of material (< 0.1%)
which is now recognized to be presolar grains that are older than the Solar
System itself, and which are derived directly from the remnants of the
individual supernovae that supplied the dust from which the Solar System
formed. These grains are recognizable from their exotic chemistry which is
alien to the Solar System (such as matrixes of graphite, diamond, or silicon
carbide). They also often have isotope ratios which are not those of the rest of
the Solar System (in particular, the Sun), and which differ from each other,
indicating sources in a number of different explosive supernova events.
Meteorites also may contain interstellar dust grains, which have collected from
non-gaseous elements in the interstellar medium, as one type of composite
cosmic dust ("stardust").
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