Earth as a Habitable Planet
Earth is the only planet we know where water exists widely in its three phases as a solid (as snow and ice), a fluid (in the ocean and in lakes, rivers and groundwater) and as a gas (in the air, as a vapor). Why does Earth have an ocean, while the Moon, Mars, and Venus do not? A satisfying answer to this deceptively simple question can only come from a careful consideration of the evolution of each planet. For now, a crude explanation is that the moon is too small, Mars is too cold, and Venus is too hot.
To illustrate this let us make a simple thought experiment. First, we reduce gravity on Earth to that of the moon. In this situation, the Earth would have a problem holding on to its atmosphere. The air would start to get thinner as the gas molecules of the atmosphere leave the planet. The ocean would release its own gas content, mainly carbon dioxide, and then it too would slowly evaporate in an attempt to replenish the atmosphere. These atmospheric molecules would escape into space until the ocean would be used up.
In another mental experiment, we keep Earth's gravity as it is but move Earth into the orbit of Mars. We are now a good deal further from the sun (1.5 times to be exact), and much less energy would arrive from the Sun, with the solar constant being reduced by a factor of 1.5 squared. Earth would immediately enter an ice age worse than anything seen in geologic history. The oceans would completely freeze over, down to great depth. Volcanic ashes and wind-blown dust would by and by cover this ocean of ice, and it would disappear from view. Because of the low temperature, water content in the atmosphere would be very small. The composition of the atmosphere would change drastically, since its composition is currently maintained by life? s activities. The end result might not be much different from what we see on Mars.
Finally, let us move Earth into the orbit of Venus. Immediately, it would become unpleasantly hot, since we are now much closer to the sun (about 1.4 times closer). The entire atmosphere would rapidly turn into a steam bath. Thick clouds would form in the upper atmosphere where the vapor would condense. The Earth would turn white and thus reflect much of the incoming sunlight. However, the steamy atmosphere would also act like a thick thermal blanket. The sunlight that does reach the surface would keep heating it, evaporating ever more water and making the atmosphere ever less transparent to outgoing heat radiation. The temperature would rise until rocks called limestones, made of calcium carbonate (CaCO3), would disintegrate and give off carbon dioxide. Large amounts of this greenhouse gas would now be added to the atmosphere. The result would be something like a wet Venus, a state perhaps not unlike a former state of that planet.