By contrast, the temperature on Venus, our nearest neighbor sunward, is about 900 degrees Fahrenheit. On Mars, the planet just beyond us, it barely gets above freezing even in midsummer at the equator. A substantial amount of water is needed to support life, though a few organisms have techniques for living in arid conditions. But this is all concentrated at the surface, so that our globe is two-thirds covered by water to an average depth of three miles. The water on Venus and Mars is infinitesimal by contrast.
Earth has the right atmosphere.
At a few per cent less oxygen, animals would not have enough to breathe. A few per cent more, and plant life would burn up. Mars and Venus have virtually no free oxygen, so necessary to most kinds of life. Earth s gravity is just right. If the earth were only one-fourth as massive, the atmospheric pressure would be too small for life.