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Astrochemistry, the overlap of the disciplines of astronomy and chemistry, is the study of the abundance and reactions of chemical elements and molecules in the universe, and their interaction with radiation. The word astrochemistry can refer to both the Solar System, and the interstellar medium. The study of the abundance elements and isotope ratios in Solar System objects (such as meteorites), is also called cosmochemistry, and the study of interstellar atoms and molecules and their interaction with radiation is sometimes also called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of molecular gas clouds, is of special interest because it is from these clouds that solar systems form.

One important aspect of astrochemistry is spectroscopy, 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 scientists are able to infer the elemental abundances, chemical composition, and temperatures of stars and interstellar clouds. This is possible because ions, atoms, and molecules have characteristic spectra: that is, the absorption and emission of 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 first polyatomic organic molecule detected in the interstellar medium.

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Spectroscopy

Perhaps the most powerful techniques for detection of individual molecules is radio astronomy which has resulted in the detection of over a hundred interstellar species, including radicals and ions, and organic (i.e., carbon) compounds, such as alcohols, acids, aldehydes, and ketones. One of the most abundant interstellar molecules, and among the easiest to detect with radio waves (due to its strong electric dipole moment) is CO (carbon monoxide). In fact, CO is such a common interstellar molecule that it is used to map out molecular regions[1] The radio observation of perhaps greatest human interest is the claim of interstellar glycine,[2] the simplest amino acid, but with considerable accompanying controversy.[3] One of the reasons why this detection was controversial is that though radio (and some other methods like rotational spectroscopy) are good for the identification of simple species with large dipole 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 don't have a dipole. For example, by far the most common molecule in the universe is (H2, hydrogen gas), but does not have a dipole moment, so it is invisible to radio telescopes. Moreover, such methods simply cannot detect species that are not in the gas-phase. Since dense molecular clouds are very cold (10-50 K = -263 to -223 C = -440 to -370 F) most molecules (other than hydrogen) are frozen out as solids. Instead hydrogen and these other molecules are detected using other wavelengths of light. Hydrogen is easily detected in the UV and visible from its absorption and emission of light hydrogen line. Moreover, most organic compounds absorb and emit light in the infrared (IR) so, for example, the recent detection of methane in the atmosphere of Mars[4] was achieved using an IR ground based telescope, NASA's 3-meter Infrared Telescope Facility atop Mauna Kea, Hawaii. NASA also has an airborne IR telescope called SOFIA and a IR space telescope called Spitzer

Infrared astronomy has also revealed that the interstellar medium contains a suite of complex gas-phase carbon compounds called polyaromatic 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 the galaxy. 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, carry deuterium and isotopes 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 rich red giant stars).

Infrared astronomy has also been used to assess the composition of the solid materials in the interstellar medium, including silicates, kerogen-like carbon-rich solids, and ices. 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[5] 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 hydrogen 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 Oxygen, Carbon, and Nitrogen 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 and cosmic rays, which results in complex radiation driven chemistry.[6] Lab experiments on the photochemistry of simple interstellar ices have produced amino acids.[7] 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 the stardust mission but the minerals also indicated a surprising contribution from high temperature chemistry of the solar nebula.

Research

Research is progressing on the way interstellar and circumstellar molecules form and interact, and this research could have a profound impact on our understanding of the suite of molecules that were present in the molecular cloud when our solar system formed, and 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, since symmetry-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 the H3+ ion. Astrochemistry overlaps with astrophysics and nuclear physics in characterizing the nuclear reactions which occur in stars, the consequences for stellar evolution, as well as stellar 'generations'. Indeed, the nuclear reactions in stars produce every naturally-occurring chemical element. As the stellar 'generations' advance, the mass of the newly-formed elements increases. A first-generation star uses elemental hydrogen (H) as a fuel source and produces helium (He). Hydrogen is the most abundant element, and it is the basic building block for all other elements as its nucleus has only one proton. Gravitational pull toward the center of a star creates massive amounts of heat and pressure, which cause nuclear fusion. Through this process of merging nuclear mass, heavier elements are formed. Lithium, carbon, nitrogen and oxygen are examples of elements that form in stellar fusion. After many stellar generations, very heavy elements are formed (e.g. iron and lead).

See also

References

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