First Detection of Ozone in the Middle Atmosphere of Mars from Solar Occultation Measurements

Abstract

Solar occultations performed with an ultraviolet spectrometer on board the Soviet spacecraft Phobos 2 (Blamont et al. 1989, Planet and Space Sci. 39, 175-187) provided data on the vertical profile of the ozone density in the martian atmosphere above ≈35 km altitude in the equatorial region (0°-20° N latitude) near the northern spring equinox ( L s = 0°-20°). Measurements were made close to the evening terminator. Due to instrumental problems, only six occultations may be analyzed in the near UV wing of the Hartley continuum (≈270-283 nm). Ozone is detected at one time with a high degree of confidence below ≈55 km and above ≈37 km altitude. Its number density shows a maximum of ≈10 8 cm -3 around 42-45 km. Its scale height above 45 km is 6 ± 2 km. Ozone, present in two other cases with a similar vertical profile, is absent, not in excess of ≈2 × 10 7 cm -3, for the three other observations. An analysis based on a one-dimensional steady-state photochemical model indicates that, when ozone is present, the eddy diffusion coefficient K is ≈2 × 10 6 cm 2 sec -1 around 40-45 km altitude within a factor of ≈2 on either side. Below 50 km. the water vapor mixing ratio is lower than ≈2 × 10 -5. Such a low ratio is consistent with a temperature decreasing with increasing altitude, with a typical temperature of ≈ 150 K at ≈50 km altitude. Above 50 km, the H 2O mixing ratio is definitely lower than 10 -5, strongly suggesting saturation at high altitude. The agreement between the modeled and observed profiles of ozone is significantly improved by assuming a sharp increase of K over a typical scale of one atmospheric scale height around 40-45 km altitude, from ≈5 × 10 5 cm 2 sec -1 below 40 km up to ≈10 7 cm 2 sec -1 above 45 km. A possible explanation is turbulence induced by breaking gravity waves. The absence of ozone found in at least one-half of cases may be simply reproduced by using the thermal structure and humidity conditions derived from the analysis of the clouds (Chassefière et al., 1992, Icarus 97, 46-69), i.e., a temperature and a H 2O mixing ratio of respectively ≈165 K and ≈10 -4 at 50 km altitude.