No. 68: Mar-Apr 1990
Where did the earth's oceans come from? For decades the stock answer has been: from the condensation of vapors escaping from the planet's cooling crust; that is, "outgassing." The possibility that some terrestrial water might have ar rived from extraterrestrial sources after the earth's formation has been discounted. The major reason behind this neglect was the expectation that the erosive effects of large-scale impacts of water-carrying comets and asteroids would preclude any net accumulation of volatiles, and could even reduce any existing inventories of surface water.
C.F. Chyba has recently reexamined this question of cometary water influx vs. impact-caused water losses using the latest estimates of comet/asteroid fluxes during the period between 4.5 and 3.5 billion years ago, when bombardment of the inner solar system was thought to be especially severe. Rather than the expected net loss, Chyba computes that the earth would really have gained more than 0.2 - 0.7 ocean masses in that billion-year period. Venus would have fared equally well, but Mars, more sensitive to impact erosion, would have accreted "only" a layer of water 10-100 meters deep over the whole planet! (This Martian water is now mostly below the surface supposedly.)
(Chyba, Christopher F.; "Impact Delivery and Erosion of Planetary Oceans in the Early Inner Solar System," Nature, 343:129, 1990.)
Comment. Not mentioned in this paper is what might have happened after 3.5 billion years ago. The comet/asteroid flux did not drop suddenly to zero. In fact, there may still be some net influx of cometary extraterrestrial water, as suggested by L.A. Frank. Incidently, the work of Frank et al is not mentioned at all in Chyba's article. Too contro versial?