About three years ago Catherine de Berg and I published a short article in Nature in which we attempted to explain why the chemistry of the atmosphere of the Earth is today so completely different from that of our two neighbor ing planets, Mars and Venus. Our atmosphere is composed mainly of N2 and O with traces of A, H0, CO, 0, etc., while the atmospheres of both 2 2 2 3 Mars and Venus are almost entirely made up of CO, Also, the Earth appears 2 to be the only one ofthe three planets which has oceans ofliquid water on the surface. Since the presence of liquid water on Earth is probably an...