origin of the oceans
Some of the water in Earth's oceans came from condensation following the outgassing of water vapor from the surface of the planet, while some was delivered by impacting comets. An important question in recent years has been the relative importance of these two sources.
According to one school of thought, comets may have supplied the bulk of oceanic water during the heavy bombardment phase of the solar system, between about 4.5 and 3.8 billion years ago. If this is true, it increases the chances that organic matter, which is also found in comets, plays an important part in the origin of life generally. However, a study carried out by scientists at the California Institute of Technology, the results of which were published in March 1999,1 suggested that most of Earth's water probably did not have a cometary origin. Using Caltech's Owens Valley Radio Observatory (OVRO) Millimeter Array, cosmochemist Geoff Blake and his team found that comet Hale-Bopp contains substantial amounts of heavy water, which is rich in the hydrogen isotope deuterium. If Hale-Bopp is typical in this respect and if cometary collisions were a major source of terrestrial oceans, it suggests that Earth's ocean water should be similarly rich in deuterium, whereas in fact it is not.
1. Blake, G. A., Qi, C., Hogerheijde, M.R., Gurwell, M. A., and Muhleman, D. O. "Sublimation from Icy Jets as a Probe of the Interstellar Volatile Content of Comets," Nature, 398, 213 (1999).