1 The third planet
Why does life exist on Earth but not, apparently, on our neighbouring planets?
The Earth is the third planet from the Sun. Its orbit lies between that of Venus, which is closer to the Sun, and that of Mars, which is further away. All three planets are composed largely of rock and metals, and have similar sizes, although Mars is smaller than the other two. In these two respects, they should all be capable of supporting life. Figure 1 shows the famous view of Earth from space, sometimes referred to as the ‘blue marble’, taken by the Apollo 17 astronauts in 1972.
It shows a blue planet that is mostly covered by oceans and decorated with complex cloud patterns indicating our ever-changing weather. Mars and Venus, in contrast, are like barren deserts: there is no liquid water on their surfaces and they are apparently lifeless. The Earth teems with life, but the other two planets do not. Why is there such a dramatic difference?
The oceans have played a critical role in supporting life on Earth. Life began in the oceans, and for much of the Earth’s history existed only in the oceans. The presence of water, then, appears to be a requirement for life to survive and thrive, at least in the forms found on Earth. The history of the Earth also indicates that global temperatures, while varying considerably, have virtually always stayed within a range suitable for life and to maintain oceans. What factors have helped maintain the Earth’s mean temperature and make it habitable?