No. 83: Sep-Oct 1992
That the earth is continuously bombarded by icy minicomets is unpopular in the Court of Science. Even less acceptable is the notion that over the eons these house-sized chunks of ice contributed substantially to our planet's inventory of water.
In what will surely be hailed as the death knell of the icy comet theory is the discovery by K. Muehlenbachs, of the University of Alberta, and F. Robert and M Javoy, from the University of Paris, that the water contained in the earth's rocks, both ancient and recent, is isotopically different from the water found in meteorites. Meteoric water is assumed to be isotopically the same as cometary water. Conclusion: comets could not have contributed substantially to our planet's water inventory in the geological past.
(Anonymous; "Earth's Water Did Not Come from Comets," New Scientist, p. 19, June 20, 1992.)
Comment. Of course, the isotpic measurements have to be weighed against all the data supporting the icy comet theory Icy comets could, after all, be a recent phenomenon. Also, no one has ever actually analyzed a piece of cometary ice; it is simply assumed that it would be similar to meteoric water.