Mona L. Delitsky

Ice chemistry on the Galilean satellites

Jupiters icy satellites Europa, Ganymede, and Callisto are subject to energy fluxes from electrons, photons, and magnetospheric plasma ion bombardment. As water ice and CO2 ice are thought to be present on their surfaces, the radiolysis of these materials over time should produce more complicated CHO-containing molecules. These may include CH3OH, H2CO, CH2CO, C3O2, HCOOH, CH3COOH, H2CO3, HCOOCH, (CH3)2CO, CH3CH2OH, HOCH2CH2OH, polymeric C3O2, and polymeric H2CO. The water fragmentation products OH, O2, HO2, H2O2, and O3 should also be produced. The molecules formed should be detectable with ground-based intruments because of their many active infrared bands. Another product produced is CO, which will have a high vapor pressure over a Galilean satellite surface at typical temperatures. The vapor pressure of CO at the nighttime temperature of 70 K could be as high as 150 mbar. Ganymedes unique dipolar magnetic field should induce more chemistry in its polar regions due to the focusing of radiation to higher latitudes. The observed lack of leading/trailing asymmetry in its SO2 absorption correlates with this redirection of plasma ions toward the poles. The observance of O atom emissions at high latitudes by Hall et al. [1998] is also consistent with this picture. The ratio of plasma energies directed to the poles to those directed to the equatorial regions is ∼4.

High-phase-angle observations of Neptune at 2650 and 7500 Å: Haze structure and particle properties

Abstract Spatially resolved Voyager 2 PPS photometric and polarimetric observations of Neptune at 2650 and 7500 A are presented and analyzed. A vertically homogeneous model with Rayleigh scattering and absorption fit the 2650 A at 137° phase and below, but not the 159° phase data. The 159° phase data imply stratospheric particles of mean radius 0.20 ± 0.05 μm, with a best-fit imaginary refractive index at 2650 A of 0.03. The optical depth of the stratospheric haze in the 5- to 100-mbar region is 0.19 ± 0.08 at 2650 A. The 7500 A data at latitude - 25° constrain the stratospheric haze mean radius to be less than 0.4 μm, with a best-fit value of 0.25 μm. Stratospheric haze optical depth at 7500 A is 0.05 ± 0.02. A continuum absorber is abundant in the troposphere. If it is confined to the region below the methane haze layer, the optical depth of the methane haze can be no larger than about 0.8 (too much limb brightening is produced for larger optical depths). The methane haze optical depth at 7500 A must be greater than 0.1. Models with this optical depth as large as 3 produced too much limb darkening if the continuum absorber is mixed in both the methane haze and deeper cloud. Tropospheric aerosols have phase functions with shalloow backward lobes, described by a double Henyey-Greenstein parameterization with g2 (the asymmetry parameter for the backward lobe) in the range −0.07 to −0.22. The planetary phase integral (the ratio of the spherical albedo to the geometric albedo) at 7500 A is 1.4 ± 0.1; at 2650 A it is 1.26 ± 0.05.