Ozone Vertical Distribution on Mars from SPICAM/MEX UV Occultations

The SPICAM/MEX spectrometer probed the Martian atmosphere with ultraviolet stellar and solar occultations from 2004 until 2011, during four Martian Years with a good spatial and seasonal coverage. We have derived from these occultations a climatology of the vertical distribution of ozone, a key chemical species. Ozone has received considerable interest since its discovery on Mars: it is readily detectable, and due to the tight coupling of O3 and HOx chemistries, O3 can be considered as a sensitive tracer of the HOx chemistry that regulates the composition of the CO2 atmosphere of Mars. SPICAM stellar occultations probe ozone on the nightside atmosphere, whereas the solar occultations are acquired at the terminator (sunrise or sunset), allowing the study of the sunset/sunrise-night transition of this photochemically active species. We have compared the occultation retrievals with the LMD Mars global climate model (GCM). The observations are overall qualitatively in agreement with the GCM, but significant quantitative differences are found in certain regions. In particular, SPICAM is detecting ozone above 20 km at low and midlatitudes in the latter half of the Martian year, which is not predicted by models. The low- and midlatitude ozone layer forming in northern spring is mapped throughout both hemispheres and its terminator-night variations are observed. The occultations probe the southern polar winter vortex and its mid-altitude ozone layer (peaking at 50 km) that is well predicted by the model. The ozone concentrations of the polar vortex in the lower atmosphere (20-30 km) are larger than the model by a factor of 5-10, hinting to a potentially larger total ozone column than predicted by modeling. During the northern polar spring, SPICAM observes the lower atmosphere ozone layer, reaching from the surface up to 30 km, showing higher O₃concentrations than expected from modeling. The full dataset will be available upon article publication.