Deuterium-to-hydrogen ratio
The deuterium-to-hydrogen ratio compares heavy hydrogen to ordinary hydrogen and is used to trace the source of water.
Hydrogen has a heavy stable isotope, deuterium, whose nucleus contains a neutron as well as a proton. The deuterium-to-hydrogen ratio, or D/H, varies between water sources in the Solar System, which makes it a powerful tracer of where water came from.
Earth's oceans have a D/H of about 156 parts per million. Water in CI and CM carbonaceous chondrites is broadly similar, which has made carbonaceous asteroids the favourite candidates for delivering much of Earth's water. Many comets measured from Earth or by spacecraft have higher ratios. The Rosetta mission found comet 67P/Churyumov–Gerasimenko's water to have about three times Earth's value, while comet Hartley 2 has a ratio close to the oceans.
Samples returned from asteroid Ryugu by Hayabusa2 also contain water-bearing minerals with D/H values similar to carbonaceous chondrites. Together these results support asteroids as a major source of Earth's water, while leaving room for some contribution from comets and the solar nebula.
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