Astrochemistry, the overlap of the disciplines of
astronomyAstronomy is the scientific study of celestial objects and phenomena that originate outside the Earth's atmosphere...
and
chemistryChemistry is the science concerned with the composition, behavior, structure, and properties of matter, as well as the changes it undergoes during chemical reactions...
, is the study of the abundance and reactions of chemical elements and molecules in space, and their interaction with radiation. The word
astrochemistry can refer to both the
Solar SystemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
, and the interstellar medium. The study of the abundance elements and isotope ratios in
Solar SystemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
objects (such as meteorites), is also called
cosmochemistryCosmochemistry is concerned with the origin and development of the elements and their isotopes, primarily within the Solar System. The term was coined by Harold Urey...
, 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 cloudA molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
s, is of special interest because it is from these clouds that solar systems form.
One important aspect of astrochemistry is
spectroscopySpectroscopy was originally the study of the interaction between radiation and matter as a function of wavelength . In fact, historically, spectroscopy referred to the use of visible light dispersed according to its wavelength, e.g. by a prism. Later the concept was expanded greatly to comprise...
, the use of
telescopeA telescope is an instrument designed for the observation of remote objects by the collection of electromagnetic radiation. The first known practically functioning telescopes were invented in the Netherlands at the beginning of the 17th century...
s 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
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
s of
starA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
s and
interstellar cloudInterstellar cloud is the generic name given to an accumulation of gas, plasma and dust in our and other galaxies. Put differently, an interstellar cloud is a denser-than-average region of the interstellar medium. Depending on the density, size and temperature of a given cloud, the hydrogen in it...
s. 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 formaldehydeInterstellar formaldehyde was first discovered in 1969 by L. Snyder et al.. using the National Radio Astronomy Observatory...
was the first polyatomic organic molecule detected in the interstellar medium.
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
dipoleIn physics, there are two kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some, usually small, distance. A permanent electric dipole is called an...
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
glycineGlycine is an organic compound with the formula NH2CH2COOH. With only a hydrogen atom as its side chain, glycine is the smallest of the 20 amino acids commonly found in proteins....
, the simplest
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
, but with considerable accompanying controversy. One of the reasons why this detection was controversial is that though radio (and some other methods like
rotational spectroscopyRotational spectroscopy or microwave spectroscopy studies the absorption and emission of electromagnetic radiation by molecules associated with a corresponding change in the rotational quantum number of the molecule...
) 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 (H
2,
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
gas), but does not have a
dipoleIn physics, there are two kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some, usually small, distance. A permanent electric dipole is called an...
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 lineThe hydrogen line, 21 centimeter line or HI line refers to the spectral line created by changing in the energy state of neutral hydrogen and occurs at a frequency of 1420.40575177 MHz, equivalent to a vacuum wavelength of around 21.10611405413 cm...
. 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 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
SOFIASofia is the capital and largest city of Bulgaria and the 12th largest city by population in the European Union, with 1.4 million people living in the Capital Municipality...
and a IR space telescope called
SpitzerThe Spitzer Space Telescope is an infrared space observatory launched in 2003...
Infrared astronomy has also revealed that the interstellar medium contains a suite of complex gas-phase carbon compounds called aromatic hydrocarbons, often abbreviated (
PAHPAH or Pah may refer to:In chemistry and biology:* Polycyclic aromatic hydrocarbon, one of a class of chemical compounds, organic pollutants* Phenylalanine hydroxylase, an enzyme involved in breaking down phenylalanine...
s 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
meteoriteA meteorite is a natural object originating in outer space that survives an impact with the Earth's surface. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
s and in cometary and asteroidal dust (
cosmic dustCosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 mm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust , interplanetary dust and circumplanetary dust .In our...
). These compounds, as well as the amino acids, nucleobases, and many other compounds in meteorites, carry deuterium and
isotopeIsotopes are different types of atoms of the same chemical element, each having a different number of neutrons. Correspondingly, isotopes differ in mass number but not in atomic number. The difference in the number of nucleons comes from a difference how many neutrons are in the atomic nucleus...
s 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 giantA red giant is a luminous giant star of low or intermediate mass that is in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
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 As above with radio astronomy, there are certain limitations, e.g., N
2 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 H
2O, CH
4, and NH
3. However, if the hydrogen is molecular and thus not reactive, this permits the heavier atoms to react or remain bonded together, producing CO, CO
2, 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 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+Protonated molecular hydrogen, trihydrogen cation, or H3+, is one of the most abundant ions in the universe. It is stable in the interstellar medium due to the low temperature and low density of interstellar space. The role that H3+ plays in the...
ion. Astrochemistry overlaps with
astrophysicsAstrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects such as galaxies, stars, planets, exoplanets, and the interstellar medium, as well as their...
and
nuclear physicsNuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei.The most commonly known applications of nuclear physics are nuclear power and nuclear weapons, but the research has provided wider applications, including those in medicine , materials...
in characterizing the nuclear reactions which occur in stars, the consequences for
stellar evolutionStellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years , considerably more than the age of the universe.Stellar evolution is not...
, as well as stellar 'generations'. Indeed, the nuclear reactions in stars produce every naturally-occurring
chemical elementA chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. The term is also used to refer to a pure chemical substance composed of atoms with the same number of protons.Common examples of elements...
. As the stellar 'generations' advance, the mass of the newly-formed elements increases. A first-generation star uses elemental
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
(H) as a fuel source and produces
heliumHelium is the chemical element with atomic number 2, and is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table...
(He). Hydrogen is the most abundant element, and it is the basic building block for all other elements as its nucleus has only one
protonThe proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H
+...
. Gravitational pull toward the center of a star creates massive amounts of heat and pressure, which cause
nuclear fusionIn nuclear physics and nuclear chemistry, nuclear fusion is the process by which multiple like-charged atomic nuclei join together to form a heavier nucleus...
. Through this process of merging nuclear mass, heavier elements are formed.
LithiumLithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number three. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
,
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
,
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
and
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
are examples of elements that form in stellar fusion. After many stellar generations, very heavy elements are formed (e.g.
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
and
leadLead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air...
).
See also
- List of molecules in interstellar space
- Molecular astrophysics
- Interstellar medium
In astronomy, the interstellar medium is the gas and dust that pervade interstellar space: the matter that exists between the star systems within a galaxy. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
- Nucleocosmochronology
Nucleocosmochronology, also known as cosmochronology, is a relatively new technique used to determine timescales for astrophysical objects and events...
- Wikiversity:Topic:Astrochemistry
External links
- Astrochemistry division of the International Astronomical Union
The International Astronomical Union is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
- The University of Arizona Astrochemistry Group http://www.chem.arizona.edu/faculty/ziur/ziur-group.html
- Astrophysics & Astrochemistry at http://www.Astrochemistry.eu/
- The Astrochemistry Lab at NASA Ames Research Center http://www.astrochemistry.org/
- The Astrochemistry Lab at NASA Goddard Space Flight Center http://www-691.gsfc.nasa.gov/
- The University of Leiden Laboratory for Astrophysics http://www.strw.leidenuniv.nl/~lab/