5.2. Solar System D/H
The D/H in the ISM from which the solar system formed 4.6 Gyr ago can be deduced from the D in the solar system today, since there should be no change in D/H, except in the sun.
Measurement in the atmosphere of Jupiter will give the pre-solar D/H provided (1) most of Jupiter's mass was accreted directly from the gas phase, and not from icy planetessimals, which, like comets today, have excess D/H by fractionation, and (2) the unknown mechanisms which deplete He in Jupiter's atmosphere do not depend on mass. Mahaffy et al.  find D/H = 2.6 ± 0.7 x 10-5 from the Galileo probe mass spectrometer. Feuchtgruber et al.  used infrared spectra of the pure rotational lines of HD at 37.7 µm to measure D/H = 5.5+3.5-1.5 x 10-5 in Uranus and 6.5+2.5-1.5 x 10-5 in Neptune, which are both sensibly higher because these planets are known to be primarily composed of ices which have excess D/H.
The pre-solar D/H can also be deduced indirectly from the present solar wind, assuming that the pre-solar D was converted into 3He . The present 3He / 4He ratio is measured and corrected for (1) changes in 3He / H and 4He / H because of burning in the sun, (2) the changes in isotope ratios in the chromosphere and corona, and (3) the 3He present in the pre-solar gas. Geiss & Gloeckler  reported D/H = 2.1 ± 0.5 x 10-5, later revised to 1.94 ± 0.36 x 10-5 .
The present ISM D/H = 1.6 ± 0.1x 10-5 is lower, as expected, and consistent with Galactic chemical evolution models, which we now mention.