Several years ago on math-fun, we had a spirited discussion about the possibility of smoke-stacks tall enough to reach orbit -- a la space elevators. During this discussion, I came to the conclusion that the Earth's atmosphere was far from equilibrium -- i.e., the Earth's current atmosphere is far too thin to have outgassed into space during its lifetime. Proof: Venus, which has a far denser atmosphere & due to its closeness to the Sun, should have outgassed much more of its atmosphere during this time than the Earth. So what happened to the Earth's atmosphere? The violent collision(s) that produced the Earth's Moon probably took much of the atmosphere away, but probably not enough to produce the current thin atmosphere. I contend that the K-T event (65.5 million years ago) stripped away a significant amount of the Earth's atmosphere & probably some amount of ocean. I don't think that large asteroids hitting the Earth simply get absorbed into the crust; I think that there are rebound splashes that take some amount of the Earth's material into space, including rock & ocean & atmosphere. The analogy to throwing a rock into a pool of water isn't exact, but has some of these features. It should also be noted that with the amount of life on Earth at the time, a huge amount of organic material would have been splattered into space, including a lot of plant & dinosaur DNA. So if we find organic material on the Moon or on Mars, asteroid impacts on Earth may have been one mechanism for getting it there. We know that there were flying dinosaurs prior to 65 million years ago that are too big to fly today. If the Earth's atmosphere was _denser_ prior to 65 million years ago, then it makes it much easier for these dinosaurs to fly. The higher oxygen content (30-35%) prior to K-T would help: oxygen molecules are 14% heavier than nitrogen molecules, but I guess that the atmospheric density would have to have been even greater than the O2/N2 ratio to allow these big birds to fly. I'm not an aerospace engineer, so without knowing the scaling laws for flight, I can't estimate the appropriate density. The higher oxygen content helps animals of large mass quite significantly: anyone who has attempted to exercise at high altitudes will quickly understand this issue. This higher oxygen partial pressure translates into relatively smaller lungs, arteries, etc., for a given size animal, thus allowing the huge dinosaurs to prosper. If the atmospheric pressure were significantly higher as well as having higher oxygen content, the partial pressure of oxygen could have been 2.5-3x current levels pre-K-T. But simply having more efficient circulatory systems & muscles still wouldn't have allowed these enormous birds to fly unless the atmosphere were also significantly denser. I wonder if this atmospheric pressure/density issue has been considered before? Perhaps asteroid collisions cause a lot more damage than we previously considered? -------- Below are some interesting tidbits from Wikipedia. http://en.wikipedia.org/wiki/K%E2%80%93T_boundary "The KÂT boundary is a geological signature, usually a thin band, dated to (65.5 ± 0.3) Ma (megaannum, or million years ago).[1] K is the traditional abbreviation for the Cretaceous period, and T is the abbreviation for the Tertiary period. The boundary marks the end of the Mesozoic era and the beginning of the Cenozoic era, and is associated with the CretaceousÂTertiary extinction event, a mass extinction" "the Alvarez team went on to calculate the size of the asteroid. The answer was about 10 km (6.2 mi) in diameter, about the size of Manhattan" "suggest that the oxygen content of the atmosphere was very high (30Â35%) during the late Cretaceous" "the possibility of near simultaneous multiple impacts, perhaps from a fragmented asteroidal object" "The potential Shiva crater, 450Â600 km (280Â370 mi) in diameter, would substantially exceed Chicxulub in size and has also been dated at about 65 mya, an age consistent with the KÂT boundary" http://en.wikipedia.org/wiki/Atmosphere_of_Venus "The temperature at the surface is 740 K (467°C, 872°F), while the pressure is 93 bar" "The main atmospheric gases are carbon dioxide and nitrogen" http://en.wikipedia.org/wiki/Pterosaur http://en.wikipedia.org/wiki/Pterosaur_size "Very large animals are defined as megafauna. Speculation exists about the reasons for megafauna development in animals of different epochs. The gigantic sizes reached by some species of pterosaurs have puzzled many scientists most specifically, because of their capacity for flight. Although some pterosaurs were as small as a sparrow, the largest had wingspans that exceeded thirty feet and some are estimated to have reached a weight of five hundred and fifty pounds. The albatross has the largest wingspan of modern birds, spanning eleven feet, yet it weights less than twenty pounds so the ratio is quite different for analyzing flight mechanisms. Factors such as the warmer climate of the Mesozoic or higher levels of oxygen existing in the atmosphere back then have been proposed, but it's now generally agreed that even the largest pterosaurs could have flown in the modern skies. Partially, this is due to the presence of air sacs in their wing membranes [1], as well as the assessment that some pterosaurs may have launched into flight with their front limbs in a quadrupedal stance, perhaps as bats do (birds are bipedal). [2] The mechanism for launching their flight remains uncertain." http://en.wikipedia.org/wiki/Quetzalcoatlus "Quetzalcoatlus was a pterodactyloid pterosaur known from the Late Cretaceous of North America (Maastrichtian stage, 70Â65.5 ma)" "More recent estimates based on greater knowledge of azhdarchid proportions place its wingspan at 10Â11 meters (33Â36 ft)." "A 2002 study suggested a body mass of 90Â120 kilograms (200Â260 lb) for Quetzalcoatlus, considerably lower than most other recent estimates.[5] Higher estimates tend toward 200Â250 kilograms (440Â550 lb)." "In 2010, Donald Henderson argued that the mass of Q. northropi has been underestimated, even the highest estimates, and that it was too massive to have achieved powered flight. Henderson argued that it may have been flightless.[13] Paul MacCready undertook an aerodynamics experiment testing the flight of Quetzalcoatlus in 1984. He constructed a model flying machine or ornithopter with a simple computer functioning as an autopilot. The model successfully flew with a combination of soaring and wing flapping;[14] however, the model was half scale based on a then-current weight estimate of around 80 kg, far lower than more modern estimates of over 200 kg." http://en.wikipedia.org/wiki/Hatzegopteryx "Hatzegopteryx hails from the Densu-Ciula Formation of western Romania, which has been dated to the late Maastrichtian stage of the late Cretaceous Period, around 65 million years ago." "The skull of Hatzegopteryx was also unique in its heavy, robust construction. Most pterosaur skulls are made up of very lightweight plates and struts. In Hatzegopteryx, the skull bones are stout and robust, with large-ridged muscle insertion areas. In their 2002 description, Buffetaut and colleagues suggested that in order to fly, the skull weight of this pterosaur must have been reduced in some unconventional way (while they allowed that it could have been flightless, they found this unlikely due to the similarity of its wing bones to flying pterosaurs). The authors theorized that the necessary weight reduction was accomplished by the internal structure of the skull bones, which were full of small pits and hollows (alveoli) up to 10 mm long, separated by a matrix of incredibly thin bony struts (trabeculae), a feature also found in some parts of Hatzegopteryx wing bones. The authors pointed out that this unusual construction, which differed significantly from the irregular intern al structure of other pterosaur skulls, resembles the structure of expanded polystyrene, the substance used to make Styrofoam. They noted that this would allow a sturdy, stress-resistant construction while remaining lightweight, and would have allowed the huge-headed animal to fly" "In 2003 they moderated the estimates to a close to twelve metres wing span and an over 2.5 metres skull length.[3] In 2010 Mark Witton e.a. stated that any appearance that the Hatzegopteryx humerus was bigger than TMM 41450-3 had been caused by a distortion of the bone after deposition and that the species thus likely had no larger wingspan than Quetzalcoatlus, today generally estimated at ten to eleven metres."