Astrochemistry is the study of the abundance and reactions ofmolecules in theuniverse, and their interaction withradiation.[1] The discipline is an overlap ofastronomy andchemistry. The word "astrochemistry" may be applied to both theSolar System and theinterstellar medium. The study of the abundance of elements andisotope ratios in Solar System objects, such asmeteorites, is also calledcosmochemistry, while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate ofmolecular 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 experimentalspectroscopy has allowed for the detection of anever-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 thechemical space available for astrochemical study.
Observations of solar spectra as performed byAthanasius Kircher (1646),Jan Marek Marci (1648),Robert Boyle (1664), andFrancesco Maria Grimaldi (1665) all predated Newton's 1666 work which established thespectral nature of light and resulted in the firstspectroscope.[2] Spectroscopy was first used as an astronomical technique in 1802 with the experiments ofWilliam Hyde Wollaston, who built a spectrometer to observe the spectral lines present within solar radiation.[3] These spectral lines were later quantified through the work ofJoseph von Fraunhofer.
Spectroscopy was first used to distinguish between different materials after the release ofCharles Wheatstone's 1835 report that thesparks given off by different metals have distinct emission spectra.[4] This observation was later built upon byLéon Foucault, who demonstrated in 1849 that identicalabsorption andemission lines result from the same material at different temperatures. An equivalent statement was independently postulated byAnders Jonas Ångström in his 1853 workOptiska 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 theBalmer Series. This series, a special case of the more generalRydberg Formula developed byJohannes Rydberg in 1888, was created to describe the spectral lines observed forhydrogen. Rydberg's work expanded upon this formula by allowing for the calculation of spectral lines for multiple different chemical elements.[5] The theoretical importance granted to these spectroscopic results was greatly expanded upon the development ofquantum mechanics, as the theory allowed for these results to be compared to atomic and molecular emission spectra which had been calculateda priori.
Whileradio astronomy was developed in the 1930s, it was not until 1937 that any substantial evidence arose for the conclusive identification of an interstellarmolecule[6] – up until this point, the only chemical species known to exist in interstellar space were atomic. These findings were confirmed in 1940, when McKellaret al. identified and attributed spectroscopic lines in an as-of-then unidentified radio observation to CH and CN molecules in interstellar space.[7] 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,[8] and H2CO (formaldehyde), discovered in 1969 and significant for being the first observed organic, polyatomic molecule in interstellar space[9]
The discovery of interstellar formaldehyde – and later, other molecules with potential biological significance, such as water orcarbon monoxide – is seen by some as strong supporting evidence forabiogenetic theories of life: specifically, 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 interstellarglycine, discovered in a comet within the Solar System in 2009[10] – or which exhibit biologically relevant properties likechirality – an example of which (propylene oxide) was discovered in 2016[11] – alongside more basic astrochemical research.
One particularly important experimental tool in astrochemistry isspectroscopy through the use oftelescopes to measure the absorption and emission oflight from molecules and atoms in various environments. By comparing astronomical observations with laboratory measurements, astrochemists can infer the elemental abundances, chemical composition, and temperatures of stars andinterstellar clouds. This is possible becauseions,atoms, and molecules have characteristic spectra: that is, the absorption and emission of certain wavelengths (colors) of light, often not visible to the human eye. However, these measurements have limitations, with various types of radiation (radio,infrared, visible,ultraviolet etc.) able to detect only certain types of species, depending on the chemical properties of the molecules.Interstellar formaldehyde was the firstorganic molecule detected in the interstellar medium.
Perhaps the most powerful technique for detection of individualchemical species isradio astronomy, which has resulted in the detection of over a hundredinterstellar species, includingradicals and ions, andorganic (i.e.carbon-based) compounds, such asalcohols,acids,aldehydes, andketones. One of the most abundant interstellar molecules, and among the easiest to detect with radio waves (due to its strong electricdipole moment), is CO (carbon monoxide). In fact, CO is such a common interstellar molecule that it is used to map out molecular regions.[12] The radio observation of perhaps greatest human interest is the claim of interstellarglycine,[13] the simplestamino acid, but with considerable accompanying controversy.[14] One of the reasons why this detection was controversial is that although radio (and some other methods likerotational spectroscopy) are good for the identification of simple species with largedipole moments, they are less sensitive to more complex molecules, even something relatively small like amino acids.
Moreover, such methods are completely blind to molecules that have nodipole. For example, by far the most common molecule in the universe is H2 (hydrogen gas, or chemically better saiddihydrogen), but it does not have a dipole moment, so it is invisible to radio telescopes. Moreover, such methods cannot detect species that are not in the gas-phase. Since dense molecular clouds are very cold (10 to 50 K [−263.1 to −223.2 °C; −441.7 to −369.7 °F]), most molecules in them (other than dihydrogen) are frozen, i.e. solid. Instead, dihydrogen and these other molecules are detected using other wavelengths of light. Dihydrogen is easily detected in the ultraviolet (UV) and visible ranges from its absorption and emission of light (thehydrogen line). Moreover, most organic compounds absorb and emit light in the infrared (IR) so, for example, the detection ofmethane in the atmosphere of Mars[15] was achieved using an IR ground-based telescope, NASA's 3-meterInfrared Telescope Facility atop Mauna Kea, Hawaii. NASA's researchers use airborne IR telescopeSOFIA and space telescopeSpitzer for their observations, researches and scientific operations.[16][17] Somewhat related to the recent detection ofmethane in theatmosphere of Mars. Christopher Oze, of theUniversity of Canterbury inNew Zealand and his colleagues reported, in June 2012, that measuring the ratio of dihydrogen and methane levels on Mars may help determine the likelihood oflife on Mars.[18][19] According to the scientists, "...low H2/CH4 ratios (less than approximately 40) indicate that life is likely present and active."[18] Other scientists have recently reported methods of detecting dihydrogen and methane inextraterrestrial atmospheres.[20][21]
Infrared astronomy has also revealed that the interstellar medium contains a suite of complex gas-phase carbon compounds calledpolyaromatic hydrocarbons, often abbreviated PAHs or PACs. These molecules, composed primarily of fused rings of carbon (either neutral or in an ionized state), are said to be the most common class of carbon compound in theGalaxy. They are also the most common class of carbon molecule in meteorites and in cometary and asteroidal dust (cosmic dust). These compounds, as well as the amino acids,nucleobases, and many other compounds in meteorites, carrydeuterium andisotopes of carbon, nitrogen, and oxygen that are very rare on Earth, attesting to their extraterrestrial origin. The PAHs are thought to form in hot circumstellar environments (around dying, carbon-richred giant stars).
Infrared astronomy has also been used to assess the composition of solid materials in the interstellar medium, includingsilicates,kerogen-like carbon-rich solids, andices. This is because unlike visible light, which is scattered or absorbed by solid particles, the IR radiation can pass through the microscopic interstellar particles, but in the process there are absorptions at certain wavelengths that are characteristic of the composition of the grains.[22] As above with radio astronomy, there are certain limitations, e.g. N2 is difficult to detect by either IR or radio astronomy.
Such IR observations have determined that in dense clouds (where there are enough particles to attenuate the destructive UV radiation) thin ice layers coat the microscopic particles, permitting some low-temperature chemistry to occur. Since dihydrogen is by far the most abundant molecule in the universe, the initial chemistry of these ices is determined by the chemistry of the hydrogen. If the hydrogen is atomic, then the H atoms react with available O, C and N atoms, producing "reduced" species like H2O, CH4, and NH3. However, if the hydrogen is molecular and thus not reactive, this permits the heavier atoms to react or remain bonded together, producing CO, CO2, CN, etc. These mixed-molecular ices are exposed to ultraviolet radiation andcosmic rays, which results in complex radiation-driven chemistry.[22] Lab experiments on the photochemistry of simple interstellar ices have produced amino acids.[23] The similarity between interstellar and cometary ices (as well as comparisons of gas phase compounds) have been invoked as indicators of a connection between interstellar and cometary chemistry. This is somewhat supported by the results of the analysis of the organics from the comet samples returned by theStardust mission but the minerals also indicated a surprising contribution from high-temperature chemistry in the solar nebula.
Research is progressing on the way in which interstellar and circumstellar molecules form and interact, e.g. by including non-trivialquantum mechanical phenomena for synthesis pathways on interstellar particles.[25] This research could have a profound impact on our understanding of the suite of molecules that were present in the molecular cloud when theSolar System formed, which contributed to the rich carbon chemistry of comets and asteroids and hence the meteorites and interstellar dust particles which fall to the Earth by the ton every day.
The sparseness of interstellar and interplanetary space results in some unusual chemistry, sincesymmetry-forbidden reactions cannot occur except on the longest of timescales. For this reason, molecules and molecular ions which are unstable on Earth can be highly abundant in space, for example theH3+ ion.
Astrochemistry overlaps withastrophysics andnuclear physics in characterizing the nuclear reactions which occur in stars, as well as the structure of stellar interiors. If a star develops a largely convective envelope,dredge-up events can occur, bringing the products of nuclear burning to the surface. If the star is experiencing significant mass loss, the expelled material may contain molecules whose rotational and vibrational spectral transitions can be observed with radio and infrared telescopes. An interesting example of this is the set of carbon stars with silicate and water-ice outer envelopes. Molecular spectroscopy allows us to see these stars transitioning from an original composition in which oxygen was more abundant than carbon, to acarbon star phase where the carbon produced by helium burning is brought to the surface by deep convection, and dramatically changes the molecular content of the stellar wind.[26][27]
In October 2011, scientists reported thatcosmic dust containsorganic matter ("amorphous organic solids with a mixedaromatic-aliphatic structure") that could be created naturally, and rapidly, by stars.[28][29][30]
On August 29, 2012, and in a world first, astronomers atCopenhagen University reported the detection of a specific sugar molecule,glycolaldehyde, in a distant star system. The molecule was found around theprotostellar binaryIRAS 16293-2422, which is located400 light years from Earth.[31][32] Glycolaldehyde is needed to formribonucleic acid, orRNA, which is similar in function toDNA. 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.[33]
In September, 2012,NASA scientists reported thatpolycyclic aromatic hydrocarbons (PAHs), subjected tointerstellar medium (ISM) conditions, are transformed, throughhydrogenation,oxygenation andhydroxylation, to more complexorganics – "a step along the path towardamino acids andnucleotides, the raw materials ofproteins andDNA, respectively".[34][35] 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 ininterstellar icegrains, particularly the outer regions of cold, dense clouds or the upper molecular layers ofprotoplanetary disks."[34][35]
In February 2014,NASA announced the creation of an improved spectral database[36] for trackingpolycyclic aromatic hydrocarbons (PAHs) in theuniverse. According to scientists, more than 20% of thecarbon in the universe may be associated with PAHs, possiblestarting materials for theformation oflife. PAHs seem to have been formed shortly after theBig Bang, are widespread throughout the universe, and are associated withnew stars andexoplanets.[37]
On August 11, 2014, astronomers released studies, using theAtacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution ofHCN,HNC,H2CO, anddust inside thecomae ofcometsC/2012 F6 (Lemmon) andC/2012 S1 (ISON).[38][39]
For the study of the recourses of chemical elements and molecules in the universe is developed the mathematical model of the molecules composition distribution in the interstellar environment on thermodynamic potentials by professor M.Yu. Dolomatov using methods of the probability theory, the mathematical and physical statistics and the equilibrium thermodynamics.[40][41][42] Based on this model are estimated the resources of life-related molecules, amino acids and the nitrogenous bases in the interstellar medium. The possibility of the oil hydrocarbons molecules formation is shown. The given calculations confirm Sokolov's and Hoyl's hypotheses about the possibility of the oil hydrocarbons formation in Space. Results are confirmed by data of astrophysical supervision and space researches.
In July 2015, scientists reported that upon the first touchdown of thePhilae lander oncomet67/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, includingacetamide,acetone,methyl isocyanate andpropionaldehyde.[43][44][45]
In December 2023, astronomers reported the first time discovery, in theplumes ofEnceladus, moon of the planetSaturn, ofhydrogen cyanide, a possible chemical essential forlife[46] as we know it, as well as otherorganic molecules, some of which are yet to be better identified and understood. According to the researchers, "these [newly discovered] compounds could potentially support extantmicrobial communities or drive complexorganic synthesis leading to theorigin of life."[47][48]
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