Compared to all the other planets we know, ours is a very special one. While Mars and Venus are at the end stage of planetary development in a more or less dead stage of stable equilibrium, ours is somewhere in the middle and highly dynamical. Being in the narrow zone with respect to the sun where water can exist in its fluid phase, the global water cycle facilitates a number of other transport cycles. As Lovelock describes it in his book "Gaja", seen from space Earth emits light with a spectrum indicating a dynamic, a homestatic equilibrium, which can only be maintained by active life.