Nonthermal Production of Atomic and Molecular Oxygen and Hydrogen in Saturn's Ring Atmosphere
Exciting data from the Cassini mission indicates that a tenuous atmosphere consisting primarily of atomic and molecular oxygen exists within Saturn's ring system. The large yields of O+ and O2+ were unexpected and thus solar ultraviolet decomposition of icy ring particles and photoionization of the gas phase neutral products have been implicated as the primary sources. The presence of such a large number density of molecular oxygen necessary to support the photoionization hypothesis also implies the production and release of significant amounts of atomic and molecular hydrogen. Indeed, the presence of H+ and H2+, as well as water-group ions, such as OH+, H2O+ and H3O+ were also detected. However, these ions as well as N+ and N2+, are not dominant and are conspicuously missing in the regions dominated by the oxygen ions. To date, the 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 producing and releasing atomic and molecular oxygen and hydrogen from icy surfaces. The values presently used have been extracted from photostimulated desorption studies on pristine samples of thick ices where absolute cross sections were not determined. Thus, much of the needed information is simply not available. This program addresses this by 1) determining the absolute threshold and absolute cross section for producing and releasing molecular oxygen and molecular hydrogen from the surfaces of silicates and water covered ring particle analogs, 2) examining the role of co-adsorbed carbon containing species in the formation of neutral and ionic forms of atomic and molecular oxygen and hydrogen, and 3) investigating the role of charging and energy transfer format the ring particle:adsorbate interface in the formation of oxygen and hydrogen. These issues have not been addressed in any other studies and the information that will be obtained in this program is absolutely necessary for understanding some of the new and exciting Cassini mission data.
We usually think of planetary atmospheres pertaining only to large planets such as Earth, Venus, Mars, or the Gas Giants. However, many smaller bodies also possess atmospheres that can be at least as dense as Earths, although many are significantly thinner. Saturn's largest moon Titan for instance, has a surface pressure 1.5 times that of Earth, and is covered by a dense haze of aerosols. Other planetary bodies that are covered more by ice than by rock can also have thin atmospheres. These exist because they are generated at a rate equal to the rate that it gets blown off into space. Jupiter's moon Europa lives in a fairly intense radiation field, due to the huge magnetosphere of it's parent body. This radiation causes chemical degradation of the ice surface and production of gases. In fact, Europa's atmosphere is primarily made of molecular oxygen! Long thought of as a potential biomarker (indicator of life on another planet), it is now known that this oxygen comes from radiolysis of ice. The precise mechanisms of radiation damage and chemical processing of ice is our main interest, and has applications to many planets, including Earth.
The Galilean satellites of Jupiter are deeply embedded within an enormous planetary magnetosphere. High energy radiation is channeled onto the surfaces of these moons which are also mainly ice covered. Electronic sputtering processes on the surface produce chemical transformations of the ice surface and create tenuous atmospheres about these low mass objects. The signature of condensed O2 has been reported in optical reflectance measurements of the Jovian moon Ganymede, and a tenuous oxygen atmosphere has been observed surrounding Europa. The surfaces of these moons contain large amounts of water ice, and it is thought that O2 is formed by sputtering of ice by energetic particles from the Jovian magnetosphere. Knowledge of how O2 is produced in low-temperature ice is crucial for accurate theoretical and experimental simulations of the surfaces and atmospheres of icy solar system bodies. However, the role of electronic excitations and the mechanistic details are poorly understood.
Using ultrahigh vacuum techniques, we can deposit ice onto a sample surface that is cooled by a closed cycle helium cryostat. Making sure the ice film is thick enough to avoid interactions with the sample substrate, the ice can be irradiated by low energy electrons or UV photons, while desorbed products are monitored by Time-of-Flight or Quadrupole mass spectrometry. Neutral products can also be analyzed using Resonance Enhanced Multiphoton Photo Ionization Spectroscopy, yielding important information about the internal state distributions.
Quantitative measurement of the production of molecular oxygen as a function of incident energy, radiation flux and total dose, the chemical kinetics can be determined. Kinetic analysis reveals that a rate-determining step involves a metastable reactive precursor. This study proved that oxygen is not produced by the diffusion of radicals through ice. At low temperatures, the precursor is stable and remains in the ice as a remnant of radiation damage. The damaged areas show a higher yield of O2 because there is already an accumulation of precursor in the damaged areas. Further measurements have shown that molecular hydrogen is strongly correlated to oxygen production. This suggests certain possibilities for the dynamics involved in creation of the precursor, and for subsequent destruction of it.
Tenuous atmospheres of minor planetary bodies exist in dynamic equilibrium, the rate of loss to vacuum equals the rate of production from stimulated desorption. The nature of the radiation environment the body experiences is the direct source, but observation of the atmosphere, combined with knowledge of the production mechanism, provides a measurement of the radiation itself. The composition of outer solar system bodies carries important information about the formation conditions of our solar system. The extreme space weathering dramatically effects their evolution, and consequently must be taken into consideration when extrapolating backward in time.
References
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- Johnson, R.E., Cooper, P.D., Quickenden, T.I., Grieves, G.A., Orlando, T.M. Journal of Chemical Physics. 2005. 123, 184715.
- Sieger, M.T.; Orlando, T.M. Nature. 1998. 394, 554.
- Orlando, T.M.; Sieger, M.T. Surface Science . 2003. 528, 1.
Members on Project
Gregory Grieves
Funding
NASA