Astrochemistry is the study of the abundance and reactions of molecules in the Universe, their interaction with radiation. The discipline is an overlap of chemistry; the word "astrochemistry" may be applied to both the interstellar medium. The study of the abundance of elements and isotope ratios in Solar System objects, such as meteorites, is called cosmochemistry, while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics; the formation and chemical composition and fate of molecular gas clouds is of special interest, because it is from these clouds that solar systems form. As an offshoot of the disciplines of astronomy and chemistry, the history of astrochemistry is founded upon the shared history of the two fields; the development of advanced observational and experimental spectroscopy has allowed for the detection of an ever-increasing array of molecules within solar systems and the surrounding interstellar medium. In turn, the increasing number of chemicals discovered by advancements in spectroscopy and other technologies have increased the size and scale of the chemical space available for astrochemical study.
Observations of solar spectra as performed by Athanasius Kircher, Jan Marek Marci, Robert Boyle, Francesco Maria Grimaldi all predated Newton's 1666 work which established the spectral nature of light and resulted in the first spectroscope. Spectroscopy was first used as an astronomical technique in 1802 with the experiments of William Hyde Wollaston, who built a spectrometer to observe the spectral lines present within solar radiation; these spectral lines were quantified through the work of Joseph Von Fraunhofer. Spectroscopy was first used to distinguish between different materials after the release of Charles Wheatstone's 1835 report that the sparks given off by different metals have distinct emission spectra; this observation was built upon by Léon Foucault, who demonstrated in 1849 that identical absorption and emission lines result from the same material at different temperatures. An equivalent statement was independently postulated by Anders Jonas Ångström in his 1853 work Optiska Undersökningar, where it was theorized that luminous gases emit rays of light at the same frequencies as light which they may absorb.
This spectroscopic data began to take upon theoretical importance with Johann Balmer's observation that the spectral lines exhibited by samples of hydrogen followed a simple empirical relationship which came to be known as the Balmer Series. This series, a special case of the more general Rydberg Formula developed by Johannes Rydberg in 1888, was created to describe the spectral lines observed for Hydrogen. Rydberg's work expanded upon this formula by allowing for the calculation of spectral lines for multiple different chemical elements; the theoretical importance granted to these spectroscopic results was expanded upon the development of quantum mechanics, as the theory allowed for these results to be compared to atomic and molecular emission spectra, calculated a priori. While radio astronomy was developed in the 1930s, it was not until 1937 that any substantial evidence arose for the conclusive identification of an interstellar molecule - up until this point, the only chemical species known to exist in interstellar space were atomic.
These findings were confirmed in 1940, when McKellar et al. identified and attributed spectroscopic lines in an as-of-then unidentified radio observation to CH and CN molecules in interstellar space. In the thirty years afterwards, a small selection of other molecules were discovered in interstellar space: the most important being OH, discovered in 1963 and significant as a source of interstellar oxygen, H2CO, discovered in 1969 and significant for being the first observed organic, polyatomic molecule in interstellar spaceThe discovery of interstellar formaldehyde - and other molecules with potential biological significance such as water or carbon monoxide - is seen by some as strong supporting evidence for abiogenetic theories of life: theories which hold that the basic molecular components of life came from extraterrestrial sources; this has prompted a still ongoing search for interstellar molecules which are either of direct biological importance – such as interstellar glycine, discovered in 2009 – or which exhibit biologically relevant properties like Chirality – an example of, discovered in 2016 - alongside more basic astrochemical research.
One important experimental tool in astrochemistry is spectroscopy through the use of telescopes to measure the absorption and emission of light from molecules and atoms in various environments. By comparing astronomical observations with laboratory measurements, astrochemists can infer the elemental abundances, chemical composition, temperatures of stars and interstellar clouds; this is possible because ions and molecules have characteristic spectra: that is, the absorption and emission of certain wavelengths of light not visible to the human eye. However, these measurements have limitations, with various types of radiation able to detect only certain types of species, depending on the chemical properties of the molecules. Interstellar formaldehyde was the first organic molecule detected in the interstellar medium; the most powerful technique for detection of individual chemical species is radio astronomy, which has resulted in the detection of over a hundred interstellar species, including radicals and ions, organic compounds, such as alcohols, acids and ketones.
One of the most abundant int
Abiogenesis, or informally the origin of life, is the natural process by which life has arisen from non-living matter, such as simple organic compounds. While the details of this process are still unknown, the prevailing scientific hypothesis is that the transition from non-living to living entities was not a single event, but a gradual process of increasing complexity that involved molecular self-replication, self-assembly and the emergence of cell membranes. Although the occurrence of abiogenesis is uncontroversial among scientists, there is no single accepted model for the origin of life, this article presents several principles and hypotheses for how abiogenesis could have occurred. Researchers study abiogenesis through a combination of molecular biology, astrobiology, biophysics and biochemistry, aim to determine how pre-life chemical reactions gave rise to life; the study of abiogenesis can be geophysical, chemical, or biological, with more recent approaches attempting a synthesis of all three, as life arose under conditions that are strikingly different from those on Earth today.
Life functions through the specialized chemistry of carbon and water and builds upon four key families of chemicals: lipids, amino acids, nucleic acids. Any successful theory of abiogenesis must explain the origins and interactions of these classes of molecules. Many approaches to abiogenesis investigate how self-replicating molecules, or their components, came into existence. Researchers think that current life on Earth descends from an RNA world, although RNA-based life may not have been the first life to have existed; the classic 1952 Miller–Urey experiment and similar research demonstrated that most amino acids, the chemical constituents of the proteins used in all living organisms, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth. Scientists have proposed various external sources of energy that may have triggered these reactions, including lightning and radiation. Other approaches focus on understanding how catalysis in chemical systems on the early Earth might have provided the precursor molecules necessary for self-replication.
Complex organic molecules occur in the Solar System and in interstellar space, these molecules may have provided starting material for the development of life on Earth. The biochemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the age of the universe was only 10 to 17 million years. The panspermia hypothesis suggests that microscopic life was distributed to the early Earth by space dust, meteoroids and other small Solar System bodies and that life may exist throughout the universe; the panspermia hypothesis proposes that life originated outside the Earth, but does not definitively explain its origin. Earth remains the only place in the universe known to harbour life, fossil evidence from the Earth informs most studies of abiogenesis; the age of the Earth is about 4.54 billion years. In May 2017 scientists found possible evidence of early life on land in 3.48-billion-year-old geyserite and other related mineral deposits uncovered in the Pilbara Craton of Western Australia.
However, a number of discoveries suggest that life may have appeared on Earth earlier. As of 2017, microfossils, or fossilised microorganisms, within hydrothermal-vent precipitates dated from 3.77 to 4.28 billion years old found in Quebec, Canadian rocks may harbour the oldest record of life on Earth, suggesting life started soon after ocean formation 4.4 billion years ago. According to biologist Stephen Blair Hedges, "If life arose quickly on Earth … it could be common in the universe." The Hadean Earth is thought to have had a secondary atmosphere, formed through degassing of the rocks that accumulated from planetesimal impactors. At first, it was thought that the Earth's atmosphere consisted of hydrogen compounds—methane and water vapour—and that life began under such reducing conditions, which are conducive to the formation of organic molecules. According to models, suggested by study of ancient minerals, the atmosphere in the late Hadean period consisted of water vapour and carbon dioxide, with smaller amounts of carbon monoxide and sulfur compounds.
During its formation, the Earth lost a significant part of its initial mass, with a nucleus of the heavier rocky elements of the protoplanetary disk remaining. As a consequence, Earth lacked the gravity to hold any molecular hydrogen in its atmosphere, lost it during the Hadean period, along with the bulk of the original inert gases; the solution of carbon dioxide in water is thought to have made the seas acidic, giving it a pH of about 5.5. The atmosphere at the time has been characterized as a "gigantic, productive outdoor chemical laboratory." It may have been similar to the mixture of gases released today by volcanoes, which still support some abiotic chemistry. Oceans may have appeared first in the Hadean Eon, as soon as two hundred million years after the Earth was formed, in a hot 100 °C reducing environment, the pH of about 5.8 rose towards neutral. This has been supported by the dating of 4.404 Ga-old zircon crystals from metamorphosed quartzite of Mount Narryer in the W
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 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 but not 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 would not yield an accurate overall picture of the chemical composition of the cosmos.
Therefore, Goldschmidt concluded that extraterrestrial material must 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 considered to be one of the fathers of cosmochemistry, engaged in research that 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 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.
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, as old as the Solar System itself, thus provide scientists with a record from the early solar nebula. Carbonaceous chondrites are primitive; the most primitive meteorites contain a small amount of material, now recognized to be presolar grains that are older than the Solar System itself, 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, alien to the Solar System, they often have isotope ratios which are not those of the rest of the Solar System, which differ from each other, indicating sources in a number of different explosive supernova events. Meteorites may contain interstellar dust grains, which have collected from non-gaseous elements in the interstellar medium, as one type of composite cosmic dust Recent findings by NASA, based on studies of meteorites found on Earth, suggests DNA and RNA components, building blocks for life as we know it, may be formed extraterrestrially in outer space.
On 30 July 2015, scientists reported that upon the first touchdown of the Philae lander on comet 67/P's surface, measurements by the COSAC and Ptolemy instruments revealed sixteen organic compounds, four of which were seen for the first time on a comet, including acetamide, methyl isocyanate and propionaldehyde. In 2004, scientists reported detecting the spectral signatures of anthracene and pyrene in the ultraviolet light emitted by the Red Rectangle nebula; this discovery was considered a confirmation of a hypothesis that as nebulae of the same type as the Red Rectangle approach the ends of their lives, convection currents cause carbon and hydrogen in the nebulae's core to get caught in stellar winds, radiate outward. As they cool, the atoms bond to each other in various ways and form particles of a million or more atoms; the scientists inferred that since they discovered polycyclic aromatic hydrocarbons —which may have been vital in the formation of early life on Earth—in a nebula, by necessity they must originate in nebulae.
In August 2009, NASA scientists identified one of the fundamental chemical building-blocks of life in a comet for the first time. In 2010, fullerenes were detected in nebulae. Fullerenes have been implicated in the origin of life. In October 2011, scientists reported that cosmic dust contains complex organic matter that could be created and by stars. On August 29, 2012, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glyc
Stellar nucleosynthesis is the theory explaining the creation of chemical elements by nuclear fusion reactions between atoms within stars. Stellar nucleosynthesis has occurred continuously since the original creation of hydrogen and lithium during the Big Bang, it is a predictive theory that today yields excellent agreement between calculations based upon it and the observed abundances of the elements. It explains why the observed abundances of elements in the universe grow over time and why some elements and their isotopes are much more abundant than others; the theory was proposed by Fred Hoyle in 1946, who refined it in 1954. Further advances were made to nucleosynthesis by neutron capture of the elements heavier than iron, by Margaret Burbidge, Geoffrey Burbidge, William Alfred Fowler and Hoyle in their famous 1957 B2FH paper, which became one of the most cited papers in astrophysics history. Stars evolve because of changes in their composition over their lifespans, first by burning hydrogen helium, progressively burning higher elements.
However, this does not by itself alter the abundances of elements in the universe as the elements are contained within the star. In its life, a low-mass star will eject its atmosphere via stellar wind, forming a planetary nebula, while a higher–mass star will eject mass via a sudden catastrophic event called a supernova; the term supernova nucleosynthesis is used to describe the creation of elements during the evolution and explosion of a pre-supernova massive star. Those massive stars are the most prolific source of new isotopes from carbon to nickel; the advanced sequence of burning fuels is driven by gravitational collapse and its associated heating, resulting in the subsequent burning of carbon and silicon. However, most of the nucleosynthesis in the mass range A = 28–56 is caused by the upper layers of the star collapsing onto the core, creating a compressional shock wave rebounding outward; the shock front raises temperatures by 50%, thereby causing furious burning for about a second. This final burning in massive stars, called explosive nucleosynthesis or supernova nucleosynthesis, is the final epoch of stellar nucleosynthesis.
A stimulus to the development of the theory of nucleosynthesis was the discovery of variations in the abundances of elements found in the universe. The need for a physical description was inspired by the relative abundances of isotopes of the chemical elements in the solar system; those abundances, when plotted on a graph as a function of atomic number of the element, have a jagged sawtooth shape that varies by factors of tens of millions. This suggested a natural process, not random. A second stimulus to understanding the processes of stellar nucleosynthesis occurred during the 20th century, when it was realized that the energy released from nuclear fusion reactions accounted for the longevity of the Sun as a source of heat and light. In 1920, Arthur Eddington, on the basis of the precise measurements of atomic masses by F. W. Aston and a preliminary suggestion by Jean Perrin, proposed that stars obtained their energy from nuclear fusion of hydrogen to form helium and raised the possibility that the heavier elements are produced in stars.
This was a preliminary step toward the idea of nucleosynthesis. In 1928, George Gamow derived what is now called the Gamow factor, a quantum-mechanical formula that gave the probability of bringing two nuclei sufficiently close for the strong nuclear force to overcome the Coulomb barrier; the Gamow factor was used in the decade that followed by Atkinson and Houtermans and by Gamow himself and Edward Teller to derive the rate at which nuclear reactions would proceed at the high temperatures believed to exist in stellar interiors. In 1939, in a paper entitled "Energy Production in Stars", Hans Bethe analyzed the different possibilities for reactions by which hydrogen is fused into helium, he defined two processes. The first one, the proton–proton chain reaction, is the dominant energy source in stars with masses up to about the mass of the Sun; the second process, the carbon–nitrogen–oxygen cycle, considered by Carl Friedrich von Weizsäcker in 1938, is more important in more massive main-sequence stars.
These works concerned the energy generation capable of keeping stars hot. A clear physical description of the proton–proton chain and of the CNO cycle appears in a 1968 textbook. Bethe's two papers did not address the creation of heavier nuclei, however; that theory was begun by Fred Hoyle in 1946 with his argument that a collection of hot nuclei would assemble thermodynamically into iron Hoyle followed that in 1954 with a large paper describing how advanced fusion stages within massive stars would synthesize the elements from carbon to iron in mass. This is the first work of stellar nucleosynthesis, it and Hoyle's 1954 paper provided the roadmap to how the most abundant elements on Earth had been synthesized within stars from their initial hydrogen and helium, making clear how those abundant elements increased their galactic abundances as the galaxy aged. Hoyle's theory was expanded to other processes, beginning with the publication of a review paper in 1957 by Burbidge, Burbidge and Hoyle.
This review paper collected and refined earlier research into a cited picture that gave promise of accounting for the observed relative abundances of the elements.