I watched an episode of the new Cosmos series and in it the astronomer mentioned that Venus receives 30% more solar radiation than the Earth but that was not the cause of the out of control global warming on Venus. Further he said that Venus had oceans similar to Earth but lost them early. I wondered why Venus lost its' oceans? If the atmosphere of early Venus was as thick as that on Earth then surely this would provide some protection for light elements such as hydrogen from being stripped from Venus. I know that Venus does not have a magnetic field generated internally (although it has a magentotail). Is that the reason Venus lost its' oceans? Further, I know that Venus does not have plate tectonics currently. I know that water is believed to be a lubricant allowing plate tectonics to exist. But is there any data to indicate whether Venus had plate tectonics formerly when it had oceans - or was the lack of plate tectonics responsible partially for the warming and volcanic activity seen on Venus currently - perhaps helping to boil away the oceans of Venus?