COMETS, IMPACTS, AND ATMOSPHERES

We are proposing a model for the delivery of volatiles to the inner pl anets by icy planetesimals (comets). Laboratory studies of the trappin g of gases in ice forming at low temperatures simulate the formation o f comet nuclei at various distances from the Sun in the solar nebula. The total gas content as well as the relative proportions of gases tra pped in the ice are strong functions of temperature. As they trap N-2 inefficiently, all planetesimals formed interior to Neptune are defici ent in nitrogen, acquiring values of C/N resembling those found in the inner planet volatile inventories. A mixture of three basic types of comets appears capable of accounting for the observed volatile invento ries on Venus, Earth, and Mars, with the caveat that impact erosion is necessary to explain the present condition of the martian atmosphere. The model includes the possibility of several epochs of clement condi tions on early Mars. Some tests of these ideas are suggested, includin g measurements in Jupiter's atmosphere by the Galileo probe. (C) 1995 Academic Press, Inc.