Venus
There is some water vapour in its atmosphere, but if you were to condense it, the layer of water on the planet would only be approximately 3 cm thick. By comparison, all the water on Earth would make a layer ~ 3 km deep. However, early in its history Venus likely had much more water, which has gradually been lost. The water has been lost by sunlight breaking up water molecules in the atmosphere. The resulting hydrogen atoms and oxygen atoms then escape into space. The recent Venus Express mission has documented that twice as many hydrogen atoms as oxygen atoms are escaping into space, confirming the breakdown of water. Exactly how much water was present on early Venus remains an open question. As this Venus Express release points out:
In short it remains an open question if there was once oceans on Venus. But if you go there now, there will be No water for you.“Everything points to there being large amounts of water on Venus in the past,” says Colin Wilson, Oxford University, UK. But that does not necessarily mean there were oceans on the planet’s surface.
Eric Chassefière, Université Paris-Sud, France, has developed a computer model that suggests the water was largely atmospheric and existed only during the very earliest times, when the surface of the planet was completely molten. As the water molecules were broken into atoms by sunlight and escaped into space, the subsequent drop in temperature probably triggered the solidification of the surface. In other words: no oceans.
