In this Letter we present, for the first time, VUV (109-220 nm) photoabsorption spectra of high density amorphous and crystalline forms of O-3 relevant to both planetary/satellite and InterStellar Medium (ISM) conditions. Absorption in the Hartley region was observed to be very weak or absent when pure O-3 ice was formed in the crystalline phase. Our results suggest that pure solid O-3 effectively absorbs in the Hartley band but only at temperatures <70 K. Such results suggest the need for in-situ techniques to detect the presence of O-3, which otherwise is a minor constituent in the lunar exosphere. (C) 2014 Elsevier B.V. All rights reserved.