User:AstroChemist/sandbox

Astrochemistry is the science devoted to the study of chemical processes at work in astrophysical environments, including the interstellar medium, stellar and planetary regions. Astrochemistry is a kind of overlap between its two parent disciplines, Astronomy and Chemistry. Astronomy is concerned with detecting molecules in the interstellar medium and stellar environments by research of the sky, where chemistry is interested in processes that transform molecular species. Molecular spectroscopy is responsible for making these discoveries. Aspects of current research include trying to understand the processes likely to lead to the formation of these molecules, as a function of the properties of their environment. Particular interest is focused on the formation, composition, evolution and fate of molecular gas clouds, because it is from these clouds that solar systems form.

Spectroscopy
One particularly important experimental tool in 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, astrochemists can 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 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 first polyatomic organic molecule detected in the interstellar medium.

Perhaps 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, and organic (i.e. carbon-based) 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. The radio observation of perhaps greatest human interest is the claim of interstellar glycine, the simplest amino acid, but with considerable accompanying controversy. One of the reasons why this detection was controversial is that although 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 have no dipole. For example, by far the most common molecule in the universe is H2 (hydrogen gas), 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 (10-50 K = -263 to -223 C = -440 to -370 F), most molecules in them (other than hydrogen) are frozen, i.e. solid. Instead, hydrogen and these other molecules are detected using other wavelengths of light. Hydrogen is easily detected in the ultraviolet (UV) and visible ranges from its absorption and emission of light (the hydrogen line). Moreover, most organic compounds absorb and emit light in the infrared (IR) so, for example, the detection of methane in the atmosphere of Mars 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 an IR space telescope called Spitzer. Somewhat related to the recent detection of methane in the atmosphere of Mars, scientists reported, in June 2012, that measuring the ratio of hydrogen and methane levels on Mars may help determine the likelihood of life on Mars. According to the scientists, "...low H2/CH4 ratios (less than approximately 40) indicate that life is likely present and active." Other scientists have recently reported methods of detecting hydrogen and methane in extraterrestrial atmospheres.

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 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. 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 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 and cosmic rays, which results in complex radiation-driven chemistry. Lab experiments on the photochemistry of simple interstellar ices have produced amino acids. 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 in the solar nebula.

Stellar Chemistry
Stars form in clusters that fall into two general types: globular clusters and open clusters. The globular clusters contain many thousands of stars but looking at their spectra suggests that they are deficient in heavy atoms such as iron.

A typical mass of a giant molecular cloud that we can see today is of order 106 MSun and some 100 ly in diameter with a density of one billion hydrogen molecules per cubic metre,

The nuclear fusion processes within the core of the star are the only source of the heavier nuclei and hence all elements with greater mass than H, He and Li.

Sun: this is also the energy source driving photosynthesis and the conversion of CO2 and H2O into sugars.

It was from an analysis of the spectrum of the Sun that helium was first identified.(helios)

Star formation
Star formation occurs in clouds of interstellar matter when, due to gravitational attraction, the density of a molecular cloud increases past the critical mass required for star formation. This point is referred to as Jeans mass. Millions of years can pass prior to formation of clouds from material in the interstellar medium. Once critical mass is achieved gravity attracts the matter together, converting the gravitational potential energy into kinetic energy or temperature. The collapse near the centre is rapid and the temperature begins to increase to form a core of material with a temperature of a few hundred Kelvin. As the pressure increases, the gravitational attraction is balanced by the heat of expansion and the contraction stops, forming a star.

Stellar Evolution
Big Bang nucleosynthesis produced only H and He atoms with a little Li, from which nuclei the first generation of stars must have formed.

Energy and mass Nuclear fusion processes derive energy from the formation of low-mass nuclei, which have a different binding energy. Fusion of two nuclear particles produces a new nucleus that is lighter in mass than the masses of the two fusing particles. This mass defect is then interchangeable in energy via Einstein’s equation E = mc2.

nuclear fusion process is thus an exothermic process for the formation of nuclei with a mass defect

For the heavier elements fission processes are exothermic, where splitting a heavy nucleus into its two daughter nuclei results in a mass defect and the release of energy. The fission/fusion line is shown in Figure 4.6 and marks the divide for the synthesis of nudei in stars: nuclei with masses heavier than 26Fe56 (the most abundant isotope on Earth) are not formed in stars so what is the formation process of the heavier nuclei that we find on Earth, such as 92U235,238?

The basic energy source in young stars is the fusion of protons into the 4He nucleus

A Sun-like star spends 80 per cent of its life on the main sequence, called its main-sequence lifetime. The main-sequence phase ends when almost all of the H in the core has been converted to He. There is a greater energy flow to the surface during this phase and the luminosity of the star increases. The star is now poised to become a red giant. Hydrogen fusion reactions stop in the core but continue in the shell around the core. With no heating in the core it begins to contract under the force of gravity to produce more energy, an increase in the luminosity and an increase in the radius of the star. The Sun will then become a red giant.

Origin of the Elements
Big Bang nucleosynthesis was responsible for producing the first elements, primarily hydrogen and helium, with smaller amounts of lithium. However, all atoms that make up life on Earth are the remainder of atomic nuclei forged by nuclear fusion in a star, nucleosynthesis. Elements heavier than Fe were formed by neutron capture events during supernova; examples include Ag, Au,Br, Cu, I, Pd, Zn, lanthanides and actinides.

Elements and molecules which make up the solar system originated from a solar nebula that had already undergone a complete stellar cycle involving birth, aging and death of a star. Creation of the Sun 4.5 billion years ago (expecting to last another 8 billion years) and creation of universe at 13.8 billion years agy suggests the solar system is only in the second cycle of star evolution.

Research
Research is progressing on the way in which 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, 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, 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. Carbon, oxygen and silicon are examples of elements that form in stellar fusion. After many stellar generations, very heavy elements are formed (e.g. iron and lead).

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.

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."

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 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.

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."

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.

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")

Recent findings by NASA, based on studies of meteorites found on Earth, suggests DNA and RNA components (adenine, guanine and related organic molecules), building blocks for life as we know it, may be formed extraterrestrially in outer space.