[MUSIC] So looking at our three Goldilocks planets, Venus, Earth, and Mars. Venus, suffered a runaway greenhouse effect, you already know. And so the water in the oceans boiled, goes into the upper atmosphere where the ultraviolet light breaks it apart. You lose the hydrogen to space, the water's gone, that's it forever. Well now the nail in the coffin is, if you have no water, you can't have any weathering. And so, you have broken the thermostat. All of the carbon gets cooked and put into the atmosphere, that's why the atmosphere of Venus has 70 bars of CO2. That's why all of Venus's carbon is in the atmosphere, rather than on Earth where almost all of our carbon is in calcium carbonate or other sedimentary rocks. On Earth, the thermostat will eventually clean up the fossil fuel CO2, but it takes a very long time. And the other thing you should know is that there are ways in which the geometry of the geology can change to sort of alter the set point of the thermostat. The thermostat regulates the temperature the way a thermostat of a house can regulate the furnace and regulate a house's temperature. But it takes a long time to get there. But you can also slowly change the dial on the thermostat by changing the geometry of where the land and ocean are and mountain building and how fast the CO2 is coming out of the Earth by degassing and things like that. So Earth has tended to drift back and forth between sort of hothouse climates where there's basically no ice on Earth at all. And then climates like today. We call this an Ice Age climate even though. It's not as big an Ice Age as it was 20,000 years ago, at the last glacial maximum. But there still are major ice sheets on Earth, in Antarctica, and in Greenland. So geologists would say that we are still in an Ice Age. If you go to a natural history museum, and go see the dinosaur dioramas, they always looks very tropical. That was the dinosaurs existed during a sort of hothouse period of Earth's climate. And so this going back and forth between these two major kinds of climate is driven by setting the thermostat, by changing the way that the CO2 is de gassing from the Earth, or the way that it's weathering or something like that. And then Mars, is a smaller planet than Venus or Earth, which meant that it cooled down more quickly. So actually, the biggest volcano in the Solar System is on Mars, Olympus Mons it's called, but it's tectonically dead. The interior of Mars has cooled down enough that it doesn't do this trick of metamorphosis, cooking the calcium carbonate to push the CO2 out to the atmosphere. So, the thermostat is broken on Mars because it's tectonically dead, it's broken on Venus because weathering can't happen but on Earth it's still functioning, in a very beautiful way. [MUSIC]
