Altitude profiles of O2 on Mars from SPICAM stellar occultations

We determine the first altitude profiles of O2 in the important photochemical region below 120 km in the atmosphere of Mars by analyzing Mars Express/SPICAM ultraviolet observations of six occultations of stars by the atmosphere. Over the range of 90-130 km the altitude-averaged mixing ratio of O2 relative to the major constituent CO2 varies in space and time in the range of 3.1×10-3-5.8×10-3, with a mean value of 4.0×10-3. This mean value exceeds by a factor of 3-4 those reported earlier for the lower atmosphere. However, some of the O2 abundance and mixing ratio profiles determined here are similar to those measured by Viking in 1976 in the upper atmosphere.