  Since its first identification on the surface of Ganymede in 1995, molecular
oxygen (O2) 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
O2. Laboratory evidence suggested that solid O2 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, O2 is constantly replenished at the surface
through ion irradiation of water-containing ices. A conclusive answer on the
existence of solid O2 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 O2 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 O2 ice. Oxygen-rich ice mixtures including water (H2O) and carbon dioxide
(CO2) can reproduce observational reflectance data of the Ganymede's surface
better than pure O2 ice in the temperature range 20-35 K. Solid H2O and CO2
also provide an environment where O2 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
CO2:O2=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 N2:O2=1:2 ice mixture. The present work will support the ESA/JUICE
mission to the Jovian system.
