Hydrogen peroxide at the poles of Ganymede

Abstract

Ganymede is the only satellite in the solar system known to have an intrinsic magnetic field. Interactions between this field and the Jovian magnetosphere are expected to funnel most of the associated impinging charged particles, which radiolytically alter surface chemistry across the Jupiter system, to Ganymede's polar regions. Using observations obtained with JWST as part of the Early Release Science program exploring the Jupiter system, we report the discovery of hydrogen peroxide, a radiolysis product of water ice, specifically constrained to the high latitudes. This detection directly implies radiolytic modification of the polar caps by precipitation of Jovian charged particles along partially open field lines within Ganymede's magnetosphere. Stark contrasts between the spatial distribution of this polar hydrogen peroxide, those of Ganymede's other radiolytic oxidants, and that of hydrogen peroxide on neighboring Europa have important implications for understanding water-ice radiolysis throughout the solar system.

Fig. 1.. Average JWST spectra of Ganymede for different latitude bins across the leading and trailing hemispheres.

(A) Average spectra representing 30° latitude bins for the leading-hemisphere observation. The bins decrease in latitude from 60°N to 90°N for the top spectrum to 60°S to 90°S for the bottom spectrum. All of the spectra have been normalized at 3.6 μm and then spaced vertically for clarity. The spectra are each offset relative to the second spectrum from the bottom in increments of 0.22 units. Red dashed lines superimposed on the spectra indicate third-order polynomial continuum fits. The H₂O₂ band can be seen clearly at 3.5 μm in the most poleward spectra. (B) Corresponding continuum-removed H₂O₂ bands for the leading-hemisphere latitude bins. Red dashed lines indicate the continuum level. Fits of Europa’s leading-hemisphere H₂O₂ absorption (21) to the strongest Ganymede bands are shown in blue, where Europa’s band has been scaled to ~69% of its strength for both the top and bottom bins. Here, we fit the Europa continuum using an identical approach to that which we take for Ganymede. The excellent match between the confirmed H₂O₂ band on Europa and Ganymede’s 3.5 μm feature definitively identifies the presence of H₂O₂ on Ganymede. (C) Average spectra for the same 30° latitude bins on the trailing hemisphere, where the H₂O₂ feature is only weakly seen, even in the polar-most averages. The spectra are each offset relative to the third spectrum from the bottom in increments of 0.22 units. (D) Corresponding continuum-removed spectra for the trailing-hemisphere latitude bins. For all panels (A) to (D), the latitudinal averages contain all pixels on the disk with centers within the given latitude ranges and range from averages of ~5 pixels for the highest-latitude bins to ~68 pixels for the lowest-latitude bins.

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.