VERTICAL TRANSPORT OF WATER ICE AT LOW LATITUDES IN THE MARTIAN ATMOSPHERE

Solar occultation measurements made from the Phobos 2 spacecraft show that two condensation levels for H2O are generally present at respectively approximately 15 km and approximately 50 km altitude at low latitudes. Above approximately 15 km, a regular decrease of the water vapor mixing ratio up to approximately 35 km is observed (Krasnopolsky et at, 1991) and it is interpreted as the signature of a saturated atmosphere. Around approximately 50 km, thin clouds, very likely formed of water ice, are detected (Blamont et al, 1991). This means that above approximately 35 km exists a region where the atmosphere is not saturated in H2O, containing enough water vapor to give rise to the formation of clouds. We show that upwelling in the ascending branch of the Hadley cell is able to supply water ice, across the cold saturated region (approximately 15-35 km), to the non-saturated layer below the clouds, where ice may sublimate and condense again at cloud level (approximately 50 km). Different constraints may be introduced relating to the coagulation probability during the upward transport phase, the minimum amount of H2O which must be deposited at the sublimation level and the infrared optical thickness of ice particles. It is shown that these constraints are jointly satisfied for reasonable values of parameters, indicating that a quantity (zonally averaged) of approximately 0.01 pr. mu-m of water ice in the shape of small particles of radius 0.1-0.2-mu-m might be present in the middle atmosphere (15-35 km) above equatorial regions.