ABSTRACT
Over the last two decades, scientific debate about life on other planets has recognised that something we usually take for granted is in fact remark-able. This is that conditions on Earth are ‘just right’ for living things. This is all the more surprising when we compare the Earth with Mars and Venus. All three planets are formed from the same materials and started with atmospheres consisting largely of carbon dioxide. Today Mars has virtually no atmosphere, so temperatures on the surface fluctuate enormously between day and night and average – 60 °C. Venus, on the contrary, has a very dense atmosphere of carbon dioxide and surface temperatures average 460 °C. By contrast, large areas of the Earth’s surface have fairly equable temperature regimes as well as moderate variations in wind and water availability. Just as Goldilocks found some things in the three bears’ cottage too hot or too cold, too hard or too soft, but others ‘just right’. so the Earth is remarkably well suited to complex life forms. The key to these conditions is the nature of the Earth’s atmosphere, because without it surface tem-peratures would, like the moon, average -18 °C.
