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Sulphur dioxide (SO 2) is known to be present on Callisto. In this paper, we studied the chemical evolution of SO 2 astrochemical ice induced by irradiation using 9 eV (137.7 nm) vacuum ultraviolet light produced from the high-flux beamline of the storage ring at the National Synchrotron Radiation Research Centre in Taiwan. Ultraviolet absorption spectra of the irradiated ice recorded in the 240–320 nm region reveal a broad absorption band indicating the formation of ozone. By comparing these laboratory spectra with Hubble Space Telescope data of the surface of Callisto in this spectral region, we find compelling evidence to consider the presence of ozone on Callisto. Furthermore, there is an additional band peaking at 309 nm whose assignment is as yet unclear but is quite similar to a band observed on Ganymede, thus leading us to suspect a common molecular source. • 9 eV photo irradiation of Sulphur dioxide (SO 2) under astrochemical icy condition. • Vacuum ultraviolet spectra of pre- and post-irradiation Sulphur dioxide (SO 2) at 9 K. • Additional absorption and shift in the peak position (281 nm to 265 nm) ascribed to Hartley absorption band of ozone (O 3). • Shift in peak position when warmed to 100 K is due to sublimation of O 3 except the trapped O 3 in SO 2 ice. • Comparison of the UV spectra from HST and present experiment provides concrete evidence for presence of Ozone on Callisto. [ABSTRACT FROM AUTHOR]