Contributions of icy planetesimals to the Earth's early atmosphere

Laboratory experiments on the trapping of gases by ice forming at low tempe ratures implicate comets as major carriers of the heavy noble gases to the inner planets. These icy planetesimals may also have brought the nitrogen c ompounds that ultimately produced atmospheric N-2. However, if the sample o f three comets analyzed so far is typical, the Earth's oceans cannot have b een produced by comets alone, they require an additional source of water wi th low D/H. The highly fractionated neon in the Earth's atmosphere may also indicate the importance of non-icy carriers of volatiles. The most importa nt additional carrier is probably the rocky material comprising the bulk of the mass of these planets. Venus may require a contribution from icy plane tesimals formed at the low temperatures characteristic of the Kuiper Belt.