Identifying the source of Earths water is central to understanding the origins of life-fostering environments and to assessing the prevalence of such environments in space. Water throughout the solar system exhibits deuterium-to-hydrogen enrichments, a fossil relic of low-temperature, ion-derived chemistry within either (i) the parent molecular cloud or (ii) the solar nebula protoplanetary disk. Utilizing a comprehensive treatment of disk ionization, we find that ion-driven deuterium pathways are inefficient, curtailing the disks deuterated water formation and its viability as the sole source for the solar systems water. This finding implies that if the solar systems formation was typical, abundant interstellar ices are available to all nascent planetary systems.
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