Outer Solar System

Saturn's Rings

Exciting results from the Cassini mission indicated that a tenuous atmosphere consisting primarily of atomic (O) and molecular oxygen (O2) exists within Saturn's ring system. The initial primary supposition was that decomposition and photoionization of gas-phase water and icy particles by solar ultraviolet radiation are the dominant O sources. The O2 also likely forms from solar ultraviolet photon-induced decomposition of water-ice coated grains. Therefore models used to describe the dynamic magnetosphere and the formation of a toroidal oxygen atmosphere in the Saturn ring system rely upon accurate yields and cross sections for the production and release of O and O2 from both gas-phase water and icy ring particle surfaces. Very little experimental work has been carried out regarding nonthermal production of O and O2 under conditions typical of Saturn's magnetosphere. Thus, this project aims to 1) determine the absolute thresholds and cross sections for UV/VUV production and release of O and O2 from water covered ring particle analogs, 2) examine the role of water "surface states" and the grain interface electronic structure on photostimulated production of O and O2, and 3) incorporate the cross section and formation rate data into an ongoing modeling program that treats ion-neutral interaction into a plasma dynamic framework.


Comets are frequently considered time capsules which hold important clues regarding the formation and early evolution of the outer planets 4 to 4.5 billion years ago. This is a time when the presolar dense molecular cloud was condensing to form the protoplanetary disk. Consequently there has been much interest in the chemical transformations that may skew any extrapolation backward to this time. An important, but often-overlooked process is radiation-induced isotope enrichment. In our studies of electron-beam induced processing of ice surfaces, we have measured large isotope effects leading to enrichment of deuterium in the condensed phase. This effect can be enhanced due to pores and the presence of co-adsorbed species. These phenomena may be potentially relevant to understanding the deuterium/hydrogen ratios observed in comets Halley, Hyakutake and Hale-Bopp (and in future comets). Since comets are related to early planet formation models, we plan to study the radiation processing, reactions and release of gas from mixed ices as surrogates of comet cores.

Several missions (e.g. Deep Impact, Star Dust, CHANDRA, Hubble, ROSAT, EUVE, IUE, ISO and FUSE) and ground-based IR studies have or will provide a wealth of information concerning the composition of comets, and the cosmic ray, far ultraviolet and x-ray fluxes permeating the solar system. This project seeks to provide further information on the physics and chemistry involved in the radiation processing and stimulated chemical reactions within pre-cometary ices and in the outer layer of current comet nuclei. The approach is a laboratory and observational-based program.

The lab work uses state-of-the-art experimental capabilities recently developed at the Electron- and Photon-Induced Chemistry on Surfaces (EPICS) laboratory at the Georgia Institute of Technology (GIT) and GIT beam-time at the Advanced Photon Source at Argonne National Laboratory. The techniques we use involve, but are not limited to; pulsed electron beam irradiation, tunable x-ray irradiation, pulsed laser resonance enhanced multiphoton-ionization (REMPI) detection of neutrals, time-of-flight detection of ions, Fourier Transform Infrared (FTIR) spectroscopy of surfaces, interfaces and desorbates and ultrahigh vacuum (< 10-10 Torr) cryogenically cooled sample preparation and characterization chambers.