As a young planet, Venus was losing hydrogen rapidly to space. The oceans boiled off, and after some period of time, perhaps 600 million years, there was no surface water.

I'm an astrobiologist, and I come from a planetary science background, so in a very broad sense, I study the evolution of planetary environments.

Earth is going to lose its oceans in the future, just as Venus did in the past. How long planets retain their oceans is a function of distance from the sun, all other things being equal.

David Grinspoon Quotes