Vertical distribution of ozone on Mars as measured by SPICAM/Mars Express using stellar occultations

[ 1] The ultraviolet spectrometer of the SPICAM instrument on board the European Mars Express mission has performed stellar occultations to probe the atmosphere. Vertical profiles of ozone are retrieved from inversion of transmission spectra in the altitude range 20 - 30 to 70 km. They are analyzed here as functions of latitude and season of the observations. These occultations have been monitored on the night side, from northern spring equinox (L-s = 8 degrees) to northern winter solstice (L-s = 270 degrees). The profiles show the presence of two ozone layers: ( 1) one located near the surface, the top of which is visible below 30 km altitude, and ( 2) one layer located in the altitude range 30 to 60 km, a feature that is highly variable with latitude and season. This layer is first seen after L-s = 11 degrees, and the ozone abundance at the peak tends to increase until L-s similar to 40 degrees, when it stabilizes around 6 - 8 x 10(9) cm(-3). After southern winter solstice (L-s similar to 100 degrees), the peak abundance starts decreasing again, and this ozone layer is no longer detected after L-s similar to 130 degrees. A recent model (Lefevre et al., 2004) predicted the presence of these ozone layers, the altitude one being only present at night. Though the agreement between model and observations is quite good, this nocturnal altitude layer is present in SPICAM data over a less extended period than predicted. Though a possible role of heterogeneous chemistry is not excluded, this difference is probably linked to the seasonal evolution of the vertical distribution of water vapor.