THE MARTIAN OZONE LAYER AS SEEN BY SPICAM/MARS-EXPRESS. F. Lefèvre 1 , J.-L. Bertaux 1 , S. Perrier 1 , S. Lebonnois 2 , O. Korablev 3 , A. Fedorova 3 , F. Montmessin 1 , and F. Forget 2 . 1 Service d’Aéronomie, Institut Pierre-Simon Laplace, France (UPMC, box 102, 75252 Paris cedex 05, franck.lefevre@aero.jussieu.fr), 2 Laboratoire de Météorologie Dynamique, France (UPMC, box 99, 75252 Paris cedex 05), 3 Space Research Insti- tute, Russia (IKI, 84/32 Profsovuznava, 117810 Moscow, Russia). Introduction: Ozone is one of the most important, and one of the most reactive species in the Martian atmosphere. It controls the UV flux that reaches the ground, thus the habitability of the planet. The abun- dance of ozone on Mars is directly controlled by the local concentration of HO x radicals (H, OH, and HO 2 ). HO x are also responsible for the apparent stability of the Mars atmosphere, by recycling the CO 2 molecules photodissociated in the upper atmosphere. HO x have never been directly observed, but ozone can be used as an efficient tracer of these species. Thus, the distribu- tion and variability of ozone is related to fundamental problems in the Martian atmosphere. The comparison between ozone measurements and model predictions can highly improve our understanding of the whole photochemistry and stability of the Martian atmos- phere. We will present in this paper an overview of the ozone measurements performed from the SPICAM instrument on board Mars-Express. These are the first continuous observations of ozone on Mars since the Mariner missions in the early 1970s [1,2]. The wealth of data obtained by SPICAM has considerably broad- ened the pool of available O 3 data with which to vali- date the photochemical models. We will use a General Circulation Model (GCM) with chemistry to analyze them and to evaluate our progress towards a quantita- tive understanding of the Martian photochemistry. SPICAM Instrument Overview: SPICAM is a 4.7 kg UV-IR dual spectrometer which has been op- erational on board the European Mars-Express mission since January 2004 [3]. The orbiter has an elliptical polar orbit with a period of about 7 hours, a pericenter altitude of ~300 km and apocenter altitude of ~10000 km. SPICAM is mainly dedicated to the determination of atmospheric characteristics of Mars from the ground up to 160 km altitude, and ozone mapping is one of its major objectives. The spectral range of the UV imag- ing spectrometer (118-320 nm, resolution ~1.5 nm, intensified CCD detector) allows to cover the strong UV absorption of CO 2 (λ < 200 nm) and the Hartley ozone absorption band between 220 and 280 nm. For the purpose of ozone measurements, the Mars-Express spacecraft can be oriented in two different observation modes: Nadir viewing. In nadir mode, SPICAM observes Mars through its atmosphere with a line of sight per- pendicular to the ground. For each orbit, spectra are averaged over 50 seconds and divided by a data refer- ence spectrum taken over Olympus Mons. A full radia- tive transfer forward model is then used in an iterative loop to fit the data with four parameters: the surface albedo at 210 nm and 300 nm, the dust opacity, and the daytime vertical ozone column, with a typical un- certainty of 10-15% [4]. Stellar occultations. In stellar occultation mode, the SPICAM UV field of view is pointed toward a star, and its detector records the stellar spectrum as the spacecraft drifts on its orbit and the star rises or sets behind the atmosphere of Mars. A reference spectrum of the star is measured outside the atmosphere, than all spectra during the occultation are ratioed to this refer- ence stellar spectrum, to get atmospheric transmission spectra as a function of altitude. The O 3 slant column is then retrieved from the transmission obtained in the Hartley band. Finally, the nighttime vertical profile of ozone is inverted with an ‘onion peeling’ procedure and a vertical resolution of about 5 km [5]. Ozone Observations: Figure 1 displays the sea- sonal evolution of the O 3 vertical column measured by SPICAM (nadir mode) during the first Martian year of observation (MY27). This is the first O 3 mapping ever obtained on Mars, although the coverage in season and latitude is not as complete as desirable. In particular, few data are available at high latitudes. This is due both to the large solar zenith angles, and to the need to share observing time with other instruments. More- over, there is a lack of data between L s = 70° and L s = 90°, related to the period of solar eclipses that decrease power resources. However, the coverage of the SPICAM O 3 measurements is sufficient to describe the distribution and seasonal evolution of the Martian ozone layer with unprecedented detail. The ozone col- umn is found to be highly variable with season, geog- raphy, and meteorology. It varies from essentially 0 to a maximum of about 30 μm-atm. The Martian ozone layer is very thin by Earth standards, corresponding to a maximum of 3 Dobson (one Dobson unit = 10 μm- atm), to be compared to a usual Earth’s value of 300 Dobson units. As a result, the actinic flux reaching the ground of Mars in the UV is extremely high. At high latitudes of both hemispheres, the ozone column is maximum in winter, when condensation on polar caps suppresses most of the atmospheric water vapor. The Seventh International Conference on Mars 3137.pdf