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Deuterium occurs in trace amounts naturally as deuterium [[gas]], written {{SimpleNuclide2|Hydrogen|2}}<sub>2</sub> or D<sub>2</sub>, but most natural occurrence in the [[universe]] is bonded with a typical {{SimpleNuclide2|Hydrogen|1}} atom, a gas called [[hydrogen deuteride]] (HD or {{SimpleNuclide2|Hydrogen|1}}{{SimpleNuclide2|Hydrogen|2}}).<ref>{{Cite journal |author=IUPAC Commission on Nomenclature of Inorganic Chemistry |title=Names for Muonium and Hydrogen Atoms and their Ions |journal=[[Pure and Applied Chemistry]] |year=2001 |volume=73 |pages=377–380 |url=http://www.iupac.org/publications/pac/2001/pdf/7302x0377.pdf |doi=10.1351/pac200173020377 |issue=2}}</ref>
 
The existence of deuterium on Earth, elsewhere in the [[solar system]] (as confirmed by planetary probes), and in the spectra of [[star]]s, is also an important datum in [[physical cosmology|cosmology]]. Gamma radiation from ordinary nuclear fusion dissociates deuterium into protons and neutrons, and there are no known natural processes other than the [[Big Bang nucleosynthesis]], which might have produced deuterium at anything close to its observed natural abundance (deuterium is produced by the rare [[cluster decay]], and occasional absorption of naturally occurring neutrons by light hydrogen, but these are trivial sources). There is thought to be little deuterium in the interior of the Sun and other stars, as at temperatures there [[nuclear fusion reaction]]s that consume deuterium happen much faster than the [[proton-proton reaction]] that creates deuterium. However, deuterium persists in the outer solar atmosphere at roughly the same concentration as in Jupiter, and this has probably been unchanged since the origin of the Solar System. The natural abundance of deuterium seems to be a very similar fraction of hydrogen, wherever hydrogen is found, unless there are obvious processes at work that concentratesconcentrate it.
 
The existence of deuterium at a low but constant primordial fraction in all hydrogen is another one of the arguments in favor of the [[Big Bang]] theory over the [[Steady State theory]] of the universe. The observed ratios of hydrogen to helium to deuterium in the universe are difficult to explain except with a Big Bang model. It is estimated that the abundances of deuterium have not evolved significantly since their production about 13.8 billion years ago.<ref>{{cite web |title=Cosmic Detectives |url=http://www.esa.int/Our_Activities/Space_Science/Cosmic_detectives |website= |publisher=The European Space Agency (ESA) |date=2 April 2013 |doi= |accessdate=2013-04-15}}</ref> Measurements of Milky Way galactic deuterium from ultraviolet spectral analysis show a ratio of as much as 23 atoms of deuterium per million hydrogen atoms in undisturbed gas clouds, which is only 15% below the [[WMAP]] estimated primordial ratio of about 27 atoms per million from the Big Bang. This has been interpreted to mean that less deuterium has been destroyed in star formation in our galaxy than expected, or perhaps deuterium has been replenished by a large in-fall of primordial hydrogen from outside the galaxy.<ref>[http://www.nasa.gov/vision/universe/starsgalaxies/fuse_stars.html NASA FUSE Satellite Solves the Case of the Missing Deuterium]. NASA</ref> In space a few hundred light years from the Sun, deuterium abundance is only 15 atoms per million, but this value is presumably influenced by differential adsorption of deuterium onto carbon dust grains in interstellar space.<ref>[http://fuse.pha.jhu.edu/wpb/sci_d2h_solved.html graph of deuterium with distance in our galactic neighborhood] {{webarchive|url=https://web.archive.org/web/20131205014518/http://fuse.pha.jhu.edu/wpb/sci_d2h_solved.html |date=5 December 2013 }} See also {{cite journal |last1=Linsky |first1=J. L. |last2=Draine |first2=B. T. |last3=Moos |first3=H. W. |last4=Jenkins |first4=E. B. |last5=Wood |first5=B. E. |last6=Oliviera |first6=C. |last7=Blair |first7=W. P. |last8=Friedman |first8=S. D. |last9=Knauth |first9=D. | last10 = Lehner | first10 = N. |last11=Redfield |first11=S. |last12=Shull |first12=J. M. |last13=Sonneborn |first13=G. |last14=Williger |first14=G. M. |year=2006 |title=What is the Total Deuterium Abundance in the Local Galactic Disk? |url= |journal=The Astrophysical Journal |volume=647 |issue= 2|pages=1106–1124 |doi=10.1086/505556 |bibcode=2006ApJ...647.1106L |arxiv=astro-ph/0608308}}</ref>