What did Venus Express discover about Venus’s atmosphere and possible ancient oceans?

The Venus Express mission provided significant insights into the composition and history of the Venusian atmosphere. Using near-infrared and other instruments, the spacecraft studied the structure of the middle and upper atmosphere, where it observed small amounts of water and a high ratio of deuterium to hydrogen.1

These findings support the hypothesis that Venus may have possessed oceans early in its history. This theory is further reinforced by evidence of the ongoing loss of water from the planet; the interaction between the solar wind and the upper atmosphere, along with an "electric wind," causes a continuous escape of hydrogen and oxygen into space. Over time, these processes could have stripped the planet of a volume of water equivalent to a significant portion of an Earth-sized ocean. Additionally, the mission detected radio waves characteristic of lightning within the clouds of Venus.12