ABSTRACT
Are Earth-like planets with water oceans rare or common among planetary systems? Although we do not yet have an answer to this question from direct observations, the study of the origin of water on Earth and Mars can provide important insights. Current evidence points toward a variety of mechanisms for the delivery of water to Earth and Mars. Most, if not all, of these delivery mechanisms are expected to be common to the formation of other planetary systems. Hence, the presence of water on Earth, and its role in the origin of life, does not appear to be unique.
