Editing Helium-3 (section)

=== Solar nebula (primordial) abundance ===
One early estimate of the primordial ratio of <sup>3</sup>He to <sup>4</sup>He in the solar nebula has been the measurement of their ratio in the atmosphere of Jupiter, measured by the mass spectrometer of the Galileo atmospheric entry probe. This ratio is about 1:10,000,<ref>{{Cite journal | bibcode = 1996Sci...272..846N | title = The Galileo Probe Mass Spectrometer: Composition of Jupiter's Atmosphere | last1 = Niemann | first1 = Hasso B. | last2 = Atreya | first2 = Sushil K. | last3 = Carignan | first3 = George R. | last4 = Donahue | first4 = Thomas M. | last5 = Haberman | first5 = John A. | last6 = Harpold | first6 = Dan N. | last7 = Hartle | first7 = Richard E. | last8 = Hunten | first8 = Donald M. | last9 = Kasprzak | first9 = Wayne T. | display-authors = 8| volume = 272 | date = 1996 | pages = 846–9 | journal = Science | doi = 10.1126/science.272.5263.846 | pmid = 8629016 | issue = 5263| s2cid = 3242002 }}</ref> or 100 parts of <sup>3</sup>He per million parts of <sup>4</sup>He. This is roughly the same ratio of the isotopes as in [[lunar regolith]], which contains 28&nbsp;ppm helium-4 and 2.8&nbsp;ppb helium-3 (which is at the lower end of actual sample measurements, which vary from about 1.4 to 15&nbsp;ppb). Terrestrial ratios of the isotopes are lower by a factor of 100, mainly due to enrichment of helium-4 stocks in the mantle by billions of years of [[alpha decay]] from [[uranium]], [[thorium]] as well as their [[decay product]]s and [[extinct radionuclide]]s.