Consequences of the day-to-night variation of the exospheric temperature of Venus
Résumé
DURING the exploration of the planet Venus by the Soviet space probe Venera 9, we measured the quantity of hydrogen in the upper atmosphere and its temperature through the optical observation of far UV atmospheric emission. The temperature at the altitude of 250 km (the upper boundary of the atmosphere) was found to vary from 500 K on the dayside to 200 K on the nightside. This variation is compatible with our current knowledge of the heating of upper atmosphere by the solar radiation. It is shown here that this variation, which is larger than on Earth, induces an exospheric transport (out of the atmosphere) of hydrogen from the dayside to the nightside. As an indirect consequence, this transport allows the existence of a large quantity of atomic oxygen in the upper atmosphere, a feature which was still controversial.