Abstract
The isotopic enrichment of HDO over H2O when water vapor freezes into ice at 60–170 K was studied experimentally. No such enrichment was detected (1.003–1.007 in the 95% confidence interval). Thus HDO cannot be enriched when ice is formed by freezing of water vapor. The very similar D/H ratio in the water of Comets Halley, Hyakutake, and Hale–Bopp (∼3 × 10−4) is 10–20 times larger then the D/H ratio in the solar nebula. Therefore the cometary water had to originate in a giant molecular cloud, where the HDO is enriched by ion–molecule reactions. We cannot determine whether the ice grains which agglomerated into these comets were formed in a ∼50 K warm clump in the giant molecular cloud and settled intact to the solar nebula or sublimated and refroze in the ∼50 K Uranus–Neptune region. The HDO/H2O ratio in Earth's oceans suggests that the water was delivered by both comets and rocky material formed in Earth's region of the solar nebula.
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