FYI -- Below is a response I got back from an MIT earth sciences prof to my speculations on atmospheric loss due to K-T. I'm not convinced; we also used to think the continents didn't move, so why wouldn't the atmospheric composition be subject to changes? This prof is also ignoring the loss of a huge fraction of nitrogen on Earth relative to Venus. Re K-T event: we still aren't sure if it was one impact or a large number of impacts. If it was a large number of impacts of which the Yucatan crater is only the largest (and maybe not even the largest, if the largest crater were hidden in one of our oceans), then there could still be a lot of atmospheric stripping. "I think there is very little (no?) evidence that Earth's atmospheric pressure was higher in the last 100 Myr than it is today, dinosaurs notwithstanding. There is nothing but disagreement over atmospheric chemistry over Earth time. Venus' atmosphere is more massive because it is made of carbon compounds which are too massive to easily strip from the upper atmosphere, while Earth's carbon is sequestered as limestone rock thanks to our oceans. If all of Earth's limestone were vaporized we would be much like Venus." "Very large impacts like the Moon-forming impact are thought to be able to strip up to half the atmospheric mass of a dry planet, and more if the planet has oceans -- the ocean can deform far more and thus transmit forces to the atmosphere to accelerate it to escape. The K-T impact probably did not have anything like that effect, based on hydrocode impact modeling. It was just not big enough to create a big stripping event. there are a lot of people working on the effects of impacts, notably Sarah Stewart at Harvard, and this is not my main area of expertise."