I thought the Venus theory was runaway greenhouse driven initially by water vapor. Going off memory, H2O is roughly 10 times as effective a greenhouse gas as CO2, with Venus being closer to the sun A larger percentage of water ended up in vapor form, leading to a feed back loop where the increased heat pushes more water to vapor leading to more heat, eventually liberating the co2 from the rock, making everything worse, ending up with the current situation where venus has way too much atmosphere.
Which is the long way to say, I think there is a lot of water on venus.
