Astrochemistry

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Astrochemistry is the study of molecules in the universe, their diversity, their reactions and their interaction with radiation .

Solar system ( not to scale )

The discipline is an intersection of chemistry and astronomy . Astrochemistry includes both the solar system and the interstellar medium . The investigation of the elements and isotope ratios in objects of the solar system, such as meteorites , is called cosmochemistry, while the investigation of interstellar atoms and molecules and their interaction with radiation is usually included in molecular astrophysics . The formation, atomic and chemical composition and development of molecular clouds is of particular interest, as solar systems can arise from these clouds.

history

As a branch of the two disciplines of astronomy and chemistry, the history of astrochemistry emerged as a sub-area of ​​the history of both subjects. The development of experimental spectroscopy allows the recording of a growing number of molecules in the solar system and in the surrounding interstellar medium. Simultaneously with the increasing number of chemical molecules characterized - due to advances in spectroscopy and other techniques - the size and scope of astrochemical studies of the chemical universe grew.

History of Spectroscopy

Observations of solar spectra, such as those made by Athanasius Kircher (1646), Jan Marek Marci (1648), Robert Boyle (1664), and Francesco Maria Grimaldi (1665), all preceded Newton's 1666 work , who discovered the natural spectrum of sunlight and built the first optical spectrometer. Spectroscopy was initially used as an astronomical technique with experiments by William Hyde Wollaston , who built the first spectrometer with which he could present the spectral lines of the sun's rays. These spectral lines were later characterized by Josef von Fraunhofer .

First, spectroscopy was used to distinguish different materials, according to Charles Wheatstones (1835) observations on atomic emission spectroscopy that different metals have different emission spectra . This observation was later expanded upon by Léon Foucault , who in 1849 showed that the same absorption and emission lines result from the same material at different temperatures. The same was postulated independently by Anders Jonas Ångström in 1853 in his work "Optiska Undersökningar", where he stated that luminous gases emit light rays of the same frequency as the light they absorb.

These spectroscopic data were important after Johann Balmer's observation that spectral lines emitted by hydrogen samples follow a simple empirical rule called the Balmer series . This series is a special case of the more general Rydberg formula , developed by Johannes Rydberg in 1888, which is used to describe the spectral lines of hydrogen. Rydberg's work on the basis of this formula made it possible to calculate spectral lines for many different chemical experiments. The importance of this theory forms the basis that the results of spectroscopy could become the basis for quantum mechanics , as it enabled a comparison between measured atomic and molecular emission spectra and values ​​calculated in advance (a priori).

History of astrochemistry

While radio astronomy , which is based on the recording of radio waves, was developed in the 1930s, it wasn't until 1937 that evidence of the existence of interstellar molecules became available. Until then, it was believed that only atomic chemical species existed in interstellar space. This discovery of molecules was confirmed in 1940 when A. McKellar was able to assign spectroscopic lines to unidentified radio observations of CH and CN molecules in interstellar space. In the following 30 years a small selection of other molecules were discovered in space: the most important: OH, discovered in 1963 and recognized as an important source of interstellar oxygen, and H 2 CO ( formaldehyde ), discovered in 1969 and classified as important, since it is the first observed organic polyatomic molecule in interstellar space.

The discovery of interstellar formaldehyde - and later other molecules of potential biological importance, such as water or carbon monoxide - is seen as evidence of abiogenetic theories of life: especially theories that claim that the basic molecular components of life come from extraterrestrial sources. This results in the constant search for biologically important interstellar molecules, such as B. interstellar glycine was found in 2009; - or the search for molecules that have biologically relevant properties such as chirality, such as propylene oxide , which was found in 2016, - accompanied by basic research in the field of astrochemistry.

Spectroscopy

CSIRO ScienceImage 11144 Parkes Radio Telescope

A particularly important experimental tool in astrochemistry is spectroscopy and the use of telescopes to measure absorption and emission of light from molecules and atoms in different environments. By comparing astronomical observations with laboratory experiments, astrochemists can determine the number, chemical composition, and temperatures of fixed stars and interstellar clouds. This possibility arises from the fact that ions, atoms and molecules have characteristic spectra: that means absorption and emission of certain wavelengths of light, also outside the range visible to the human eye. The measurement methods are limited to different wavelength ranges, such as radio waves, infrared radiation, ultraviolet radiation ... which only certain species can detect, depending on the chemical properties of the molecules. Formaldehyde was the first organic molecule to be detected in the interstellar medium .

Perhaps the most powerful technique for the detection of individual chemical species is radio astronomy , which results in the detection of over a hundred interstellar species such as radicals and ions , organic compounds (i.e., hydrocarbons such as alcohols, organic acids, aldehydes, and ketones). The most important interstellar molecule, which is easiest to detect with radio waves due to its dipole, is CO (carbon monoxide). Indeed, carbon monoxide is such a distinctive interstellar molecule that it is used to detect regions of the molecule. Probably the most important species observed by radio waves for humans is interstellar glycine, the simplest amino acid.

In addition, these methods are blind to molecules without a dipole moment. For example, the most abundant element in the universe, hydrogen (H 2 ), is invisible to radio telescopes because it does not have a dipole. In addition, the method cannot detect any molecules outside the gas phase. When dense molecular clouds are cold, most of the molecules are frozen; H. firmly. Therefore one has to observe these molecules with light of a different wavelength. Hydrogen is visible in the UV range of the spectrum through its emission and absorption line ( hydrogen line ). In addition, most organic molecules absorb and emit light from the infrared (IR) region of the spectrum, for example the methane in the Martian atmosphere was discovered with an IR ground-based telescope, NASA's 3-meter infrared telescope on Mauna Kea, Hawaii . NASA researchers use the Air-IR Telescope SOFIA and the Spitzer Space Telescope for their observations, research and scientific work. These recently led to the discovery of methane in the Martian atmosphere.

Mars Orbiter Mission Over Mars (15237158879)

Christopher Oze from the University of Canterbury in New Zealand and his colleagues reported in June 2012 that measuring the proportions of hydrogen and methane in the Martian atmosphere helps to find out whether life is possible on Mars . According to the scientists, low hydrogen / methane ratios (<40%) mean that life is possible. In addition, other scientists recently reported methods to detect hydrogen and methane in the extraterrestrial atmosphere . Infrared astronomy has shown that the interstellar medium contains a number of complex gas-phase hydrocarbon compounds, such as polyaromatic hydrocarbons (PACs). These molecules, mostly made up of condensed aromatics, are believed to be the most common carbon compounds in the galaxy . They are also the most common compounds in meteorites , comets , asteroids, and cosmic dust . These compounds, as well as amino acids and nucleobases from meteorites, contain deuterium and isotopes of carbon, nitrogen and oxygen, which are rarely found on Earth, suggesting their extraterrestrial origin. It is assumed that PACs form in the hot environment of fixed stars ( red giants ).

Infrared astronomy has also been used to study the composition of solids in the interstellar medium, such as silicates , kerogen- like carbon-rich solids, and ice. This is because, in contrast to visible light, which is scattered or absorbed by solids, the IR radiation penetrates the microscopic interstellar particles, but the absorption corresponds to a wavelength that is specific / characteristic of the composition of the rock. As with the radio wave astronomy mentioned above, there are limits, e.g. B. Nitrogen is difficult to detect with either method.

Such IR observations have shown that in dense clouds, where there are enough particles to shield the molecules from UV radiation, a thin layer of ice covers the particles so that low-temperature chemistry can take place. Since hydrogen is the most common molecule in the universe, the chemistry of this ice is determined by the chemistry of hydrogen. When the hydrogen is atomic, it can react with oxygen, carbon, and nitrogen atoms and produce species like H 2 O, CH 4 , and NH 3 . However, if it is molecularly present as H 2 and is therefore not as reactive, then other atoms can react with one another, so that, for example, CO, CO 2 , CN are formed. These ice lumps made up of mixtures of molecules are exposed to UV radiation and cosmic radiation, which results in a complex radiation-dependent chemistry. Amino acids could be produced in photochemical laboratory experiments with simple interstellar ice. The similarity between interstellar and cometic ice has shown that there is a connection between interstellar and cometary chemistry. This thesis is supported by the results of the analyzes of comet samples that came from the Stardust mission, but the minerals also showed a surprising amount of “high temperature chemistry” in the solar nebula.

research

Transition from atomic to molecular gas at the boundary layer of the Orion Nebula.

Research is making advances in the field of interstellar and circumstellar molecules forming and their reactions, e.g. B. Astrochemistry, which records the synthesis pathways of interstellar particles. This research has had a profound impact on our understanding of the formation of molecules in molecular clouds found in our solar system, including the chemistry of comets, asteroids and meteorites and the space dust that falls on Earth every day.

The size of the interstellar and interplanetary space leads to some unusual chemical results, since the reactions take place on a long time scale. That is why molecules and ions that are unstable on earth are often found in space, such as protonated hydrogen, the H 3 + ion. Astrochemistry overlaps with astrophysics and nuclear physics by characterizing the nuclear reactions that take place in stars as a result of evolution. In fact, the stars' nuclear reactions create every naturally occurring element on Earth. As the universe expands and the generation of stars advances, the mass of the newly formed elements grows. A first generation star has its origin in the element hydrogen and produces helium. Hydrogen is the most common element and is the basic substance for all other elements as it only has one proton. The gravitational force causes a mass attraction in the center of the fixed star and generates heat and pressure, which causes a nuclear fusion in which other, heavier elements are formed. Carbon, oxygen, and silicon are examples of elements created from the fusion of stars. It was only after several generations that elements such as iron and lead came into being.

In October 2011, scientists reported on the cosmic dust, which contains organic compounds (amorphous organic solids with mixed aromatic-aliphatic structures), produced by fixed stars.

On August 29, 2012, astrochemists from the University of Copenhagen reported for the first time the discovery of the sugar molecule glycolaldehyde in a distant star system. The molecule was found near the star "IRAS 16293-2422", which is about 400 light years from Earth. Glycolic acid is used to build RNA (ribonucleic acid). This discovery suggests that complex organic molecules form in star systems before the planets were formed.

In September 2012, NASA scientists reported that polyaromatic hydrocarbons subjected to the conditions of the interstellar medium can be converted into more complex organic molecules through hydrogenation, oxygenation and hydroxylation - a step towards amino acids and nucleic acids , the starting materials for proteins and DNA . In addition, as a result of these transformations, the polyaromatic hydrocarbons change their typical spectroscopic properties, which explains the lack of their detection in interstellar ice and cosmic dust, especially in the cold regions. "

In February 2014 NASA announced the creation of a database for spectra. According to the scientists, 20% of the carbon in space is contained in polyaromatic hydrocarbons, which could form the basis of extraterrestrial life. Polyaromatic hydrocarbons were formed shortly after the Big Bang, are widespread in space and are linked to the formation of new stars.

On August 11, 2014, astronomers published studies recorded with the Atacama Large Millimeter / Submillimeter Array (ALMA) that provided a detailed distribution of HCN , HNC , H 2 CO and comet dust from comet C / 2012 F6 (Lemmon) and des Comet C / 2012 S1 (ISON) showed.

For the theoretical studies of chemical elements and molecules in space, M.Yu. Dolomatov developed a mathematical model for the distribution of molecular composition in the interstellar environment, taking into account mathematical and physical statistics and equilibrium thermodynamics. Based on this model, he estimates the resources of vital molecules, amino acids and nitrogen bases in the interstellar medium. The possibility of the formation of hydrocarbons is given. The calculations support the hypotheses of Sokolov's and Hoyl's. The results have been confirmed by astrophysical research and observations in space.

In July 2015, scientists reported that the Philae spacecraft (probe) landed on the surface of Comet 67 / P and, during measurements with COSAC and Ptolemy, sixteen organic compounds (four of which were first discovered on a comet - including acetamide, acetone , Methyl isocyanate and propionaldehyde).

Web links

Wikiversity: Astrochemistry (English)  course materials

Individual evidence

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