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[1] This paper is intended as an introduction to several companion papers describing the results obtained by the SPICAM instrument on board Mars Express orbiter. SPICAM is a lightweight (4.7 kg) UV-IR dual spectrometer dedicated primarily to the study of the atmosphere of Mars. The SPICAM IR spectrometer and its results are described in another companion paper. SPICAM is the first instrument to perform stellar occultations at Mars, and its UV imaging spectrometer (118-320 nm, resolution $1.5 nm, intensified CCD detector) was designed primarily to obtain atmospheric vertical profiles by stellar occultation. The wavelength range was dictated by the strong UV absorption of CO 2 (l < 200 nm) and the strong Hartley ozone absorption (220-280 nm). The UV spectrometer is described in some detail. The capacity to orient the spacecraft allows a great versatility of observation modes: nadir and limb viewing (both day and night) and solar and stellar occultations, which are briefly described. The absolute calibration is derived from the observation of UV-rich stars. An overview of a number of scientific results is presented, already published or found in more detail as companion papers in this special section. SPICAM UV findings are relevant to CO 2 , ozone, dust, cloud vertical profiles, the ozone column, dayglow, and nightglow. This paper is particularly intended to provide the incentive for SPICAM data exploitation, available to the whole scientific community in the ESA data archive, and to help the SPICAM data users to better understand the instrument and the various data collection modes, for an optimized scientific return.

Hydrogen radicals are produced in the martian atmosphere by the photolysis of water vapour and subsequently initiate catalytic cycles that recycle carbon dioxide from its photolysis product carbon monoxide. These processes provide a qualitative explanation for the stability of the atmosphere of Mars, which contains 95 per cent carbon dioxide. Balancing carbon dioxide production and loss based on our current understanding of the gas-phase chemistry in the martian atmosphere has, however, proven to be difficult. Interactions between gaseous chemical species and ice cloud particles have been shown to be key factors in the loss of polar ozone observed in the Earth's stratosphere, and may significantly perturb the chemistry of the Earth's upper troposphere. Water-ice clouds are also commonly observed in the atmosphere of Mars and it has been suggested previously that heterogeneous chemistry could have an important impact on the composition of the martian atmosphere. Here we use a state-of-the-art general circulation model together with new observations of the martian ozone layer to show that model simulations that include chemical reactions occurring on ice clouds lead to much improved quantitative agreement with observed martian ozone levels in comparison with model simulations based on gas-phase chemistry alone. Ozone is readily destroyed by hydrogen radicals and is therefore a sensitive tracer of the chemistry that regulates the atmosphere of Mars. Our results suggest that heterogeneous chemistry on ice clouds plays an important role in controlling the stability and composition of the martian atmosphere.