The deuterium enrichment observed in recent comets is consistent with the cometary origin of seawater

The deuterium enrichment observed in the water molecules of comets Halley, Hyakutake and Hale–Bopp is about twice as large as that of seawater. Their orbital features suggest that they probably originated in the zones of the outer planets; this implies that their deuterium enrichment was not modified from its interstellar origin. In contrast, most of the seawater was likely brought to Earth by an early bombardment of comets, that originated mostly in Jupiters zone, where steam from the inner Solar System condensed onto icy interstellar grains before they accreted into larger bodies, explaining why Jupiters comets were of a different kind. The deuterium enrichment of seawater is quantitatively consistent with its origin from a cometary bombardment of the primitive Earth.