Ice chemistry on the Galilean satellites

Jupiter's 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. Ganymede's 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.