Why is the Earth blue?

Seen from space, the Earth is blue. The Earth has been blue for over 4 billion years because of the liquid water on its surface. How has the Earth managed to sustain liquid water on its surface for such a long time?

The four most promising worlds for alien life in the solar system

The Earth's biosphere contains all the known ingredients necessary for life as we know it. Broadly speaking these are: liquid water, at least one source of energy, and an inventory of biologically useful elements and molecules.

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