On the origin of molecular oxygen on the surface of Ganymede

Since its first identification on the surface of Ganymede in 1995, molecular oxygen (O 2 ) ice has been at the center of a scientific debate as the surface temperature of the Jovian moon is on average well above the freezing point of O 2 . Laboratory evidence suggested that solid O 2 may either exist in a cold (<50 K) subsurface layer of the icy surface of Ganymede, or it is in an atmospheric haze of the moon. Alternatively, O 2 is constantly replenished at the surface through ion irradiation of water-containing ices. A conclusive answer on the existence of solid O 2 on the surface of Ganymede is hampered by the lack of detailed, extensive observational datasets. We present new ground-based, high-resolution spectroscopic observations of Ganymede’s surface obtained at the Telescopio Nazionale Galileo. These are combined with dedicated laboratory measurements of ultraviolet-visible (UV-vis) photoabsorption spectra of O 2 ice, both pure and mixed with other species of potential interest for the Galilean satellites. Our study confirms that the two bands identified in the visible spectra of Ganymede’s surface are due to the (1,0) and (0,0) transition bands of O 2 ice. Oxygen-rich ice mixtures including water (H 2 O) and carbon dioxide (CO 2 ) can reproduce observational reflectance data of the Ganymede’s surface better than pure O 2 ice in the temperature range 20-35 K. Solid H 2 O and CO 2 also provide an environment where O 2 ice can be trapped at higher temperatures than its pure ice desorption under vacuum space conditions. Our experiments at different temperatures show also that the (1,0)/(0,0) ratio in case of the CO 2 :O 2 = 1:2 ice mixture at 35 K has the closest value to observations, while at 30 K the (1,0)/(0,0) ratio seems to be mixture independent with the exception of the N 2 :O 2 = 1:2 ice mixture. The present work will be part of a laboratory spectroscopic database in support of the ESA/JUICE mission to the Jovian system. ground-based two at 577.3 627.5 laboratory data are obtained from the data fit with a double sigmoidal function (Eq. (2)). Error bars highlight uncertainties in the determination of peak positions