When I look at Mars & Venus, both dry planets, I wonder if there is a ratio of the area covered by water on the surface and the area represented by dry land that determines the future status of the planet. When the newly formed planets cool (and I am going to leave alone for the time being where the water comes from) doesn't it follow rationally that a planet that is cooling & out-gassing would have a certain threshold for water retention? Mars clearly had water. If it were covered in , say 50% water, perhaps that wasn't enough to keep it there. Earth has a ratio of roughly 70% water to 30% land, I believe. I think that life may not have been as varied if the whole planet was water covered. Artificial land would require a high state of development. Also it seems that too low a percentage of land would not allow the explosion of evolution to happen quickly enough, before the planet becomes uninhabitable. Too much land would be too dry to retain any water at all, except what was in the soil or as ice caps. Both extreme ends of the ratio wouldn't allow a timely development of a varied mix of lifeforms. Sooo, I wonder if there is a narrow range where the developing planet keeps its water, dependant upon the atmospheric pressure, distance from the sun & other planets or bodies, temperature, & the water/land ratio? Your thoughts?

Absolutely.

Mars probably had water, but lost it mostly after it lost its magnetic field; the increase in high energy radiation and solar wind slowly destroyed the atmosphere after that.
Venus had water too, and developed a moist greehouse condition before it dried out and this changed to the dry runaway greehouse we see today.

In both cases the main mechanism for loss was probably photolysis, where the water is broken down by ultraviolet light into hydrogen and oxygen. In low mass planets the hydrogen escapes.

So the mass of the planet has a great influence on how much water is on the surface. A high mass Earth would probably be covered in water with no land at all.

Another influence is the impact history; Earth probably lost a lot of water during the impact that formed the Moon, and so we have a smaller ocean cover than would be the case if the Moon had never formed.

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