Fig. 2. Maps of Ganymede’s 3.5 μm H₂O₂ absorption compared to those of the 3.1 μm Fresnel peaks of water ice and corresponding projections of the U.S. Geological Survey Voyager-Galileo imaging mosaic.

H₂O₂ appears constrained to the upper latitudes, particularly on the leading hemisphere, which exhibits sharp boundaries at approximately ±30° to 35° latitude. These boundaries are roughly coincident with the onset of Ganymede’s polar frost caps (17, 18) and with the latitudes at which most of the impinging Jovian magnetospheric particles can access the surface (13, 18). Maps of the Fresnel reflection peak of water ice, which generally track the distribution of ice deduced from shorter-wavelength water bands (28, 29), also show the areas of greatest H₂O₂ on the leading hemisphere to be enriched in water ice. The trailing hemisphere shows comparatively weak Fresnel reflections and, overall, less-icy spectra. This hemispheric dichotomy in water ice may help explain the leading/trailing contrast in H₂O₂, while the overall polar H₂O₂ distribution may reflect a combination of precursor water availability and temperature and/or radiation intensity effects. The approximate average boundary between open and closed field lines from (18) are included as red dashed lines. The 60°S, 30°S, 0°N, 30°N, and 60°N parallels are also included in gray for both hemispheres. The leading-hemisphere map includes the 45°W, 90°W, and 135°W meridians, while the trailing-hemisphere map shows those for 225°W, 270°W, and 315°W. The Voyager-Galileo mosaic used can be found at https://astrogeology.usgs.gov/search/map/Ganymede/Voyager-Galileo/Ganymede_Voyager_GalileoSSI_global_mosaic_1km.