| Summary: | Cross-hemispheric circulation of the atmosphere is a major feature of the Martian troposphere and Venusian thermosphere. On Mars, it is driven by the latitudinal gradient of insolation at the surface, which generates a global summer-to-winter Hadley cell reversing orientation during equinoxes and maximizing intensity at solstices. On Venus, it is driven by a longitudinal gradient between the dayside and the nightside and takes place above the super-rotating mesosphere and troposphere. This subsolar-to-antisolar (SSAS) circulation is known to induce major observational features, such as the vast O2 and NO emission zones observed close to midnight. Recently, SPICAV onboard Venus Express has detected for the first time the presence of ozone on Venus, accounting for a thin thermospheric layer at about 100 km. We suggest that this ozone layer forms as a result of O atoms carried from the dayside and recombining in the subsiding branch of the SSAS where the O2 singlet delta emission is also observed. Interestingly, a similar feature was recently identified on Mars with SPICAM, with the presence of a >10 km thick ozone layer in the southern winter hemisphere near the pole. Contrarily to the ozone layer otherwise observed on Mars which extends from the surface and is controlled by reaction with HOx radicals, this layer is only related to O atoms produced in the summer hemisphere and carried via global circulation towards the polar night where they can recombine, yielding the newly detected O2 emission feature evidenced by OMEGA.
|
|---|