Venus may once have had liquid water

The measurements from Venus Express seem to validate this theory. They show hydrogen and oxygen being leaked into space from the planet's atmosphere at a rate of about 2:1, hydrogen to oxygen. It also showed a heavy isotope of hydrogen called deuterium in the atmosphere. This heavier molecule is less likely to escape into the boundless depths of space than oxygen due to its weight.

While this doesn't prove that the planet had a watery past, and it is believed that due to the lifecycle of the planet and its atmosphere that it never would have had oceans like Earth, it does further advance the theory that Venus may have previously had liquid water on its surface. Liquid water is believed to be one of the base requirements for carbon-based life to form, and so coming close to proving its existence helps scientists in their search for the possibility of life on other planets.