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Looking only at the calculations given above, Venus should have retained water vapor because the thermal velocity for water vapor is only a few percent of the escape velocity for the planet’s exosphere. However, Venus lacks water vapor due to another reason - the average temperature on Venus is 461.85 degrees Celsius, and since water boils at 100 degrees Celsius, water cannot exists on the surface. In the clouds and atmosphere, very little water vapor exists due to a stream of hydrogen and oxygen atoms blasted from the planet by the Sun’s solar winds. The Earth avoids this problem due to our magnetosphere, generated by the convection of material within the planet and the large temperature difference between the inner and outer cores. Venus lacks this high temperature gradient, so inner convection cannot currently occur, leaving it without a planetary magnetosphere to shield from solar winds which deplete the planet’s water vapor.
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