9457457 Griffith The origin and evolution of planetary atmospheres will be investigated by focusing initially on two processes: cometary impacts and the greenhouse effect. Collisions of comets and asteroids with planets have recently been found to significantly shape the nature of the solar system. These impacts provide the best explanation for the existence of the moon, Uranus' tilt and the KT extinction. Pioneering work by Zahnle et al (1992) suggests that cometary impact explains the reason why Titan has an atmosphere and the Galilean satellites have none. The idea is that, because of Jupiter's greater mass, impact velocities are larger on the Galilean satellites than on Titan, and thus more efficient at eroding the atmospheres. Monte Carlo calculations that address the stochastic nature of the problem will be used to investigate this effect more rigorously. This technique will also be used to investigate the capability of Mars to retain volatiles. The ultimate goal is to determine the effect that planetary rubble has on a planetary atmosphere as a function of a planet's size and its context within a solar system. This research will address the following question: for a given planetary system, on which bodies might one expect to find an atmosphere? Dr. Griffith will pursue complementary work, at NASA's Infrared Telescope Facility, as a member of the science team that will observe comet Shoemaker-Levy's collision with Jupiter. The aim here is, in part, to investigate how the impact energy is partitioned into chemical energy and kinetic energy to the atmosphere. The partitioning into kinetic energy determines the efficiency with which comets erode planetary atmsopheres. The temperature at Venus' surface is 7500 K, almost 4000 K warmer than the Earth. The great efficiency with which the greenhouse effect warms Venus'atmosphere suggests that warm atmospheres would exist at large distances from the Sun, if greenhouse gases were present. This ra ises an interesting possibility for the ecosphere of the Solar System, that is, the region where liquid water exists, a necessary ingredient for life. Greenhouse gases, if present on a planetary body, may extend the ecosphere out to large distances from the Sun. This possibility will be investigated by focusing on Titan's organic-rich atmosphere. Titan's atmosphere may have originally consisted of ammonia (NH3 ultra violet radiation, a strong greenhouse gas) and subsequently been dissociated by solar on the nitrogen gas (N2) we presently observe. In collaboration with Chris McKay, Kevin Zahnle and Jim Pollack (NASA Ames Research Center), the possibility that Titan's atmosphere was warm enough to have had liquid water in the past will be investigated. This effort will be extended to consider the greenhouse efficiency of the highly abundant volatiles in the solar system (of which, governed by cosmic abundances, there are few). The larger aim of this project is to understand the thermal properties of atmospheres as a function of their composition. This award is to recognize an outstanding young faculty member in science and engineering. The award will enhance the career of the faculty member by providing flexible support for research and educational activities. Cooperation with industry and institutions that support research and education is encouraged.
