Was the total amount of water on earth ever significantly different?
Is it plausible to assume that the total amount of water (edit: water molecules) on planet earth was, in different periods of earth's history, significantly lower or significantly higher, or rather, the amount of water is pretty much (99%?) stable since the dawn of earth?
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Kind of depends on whether you mean water as in H2O molecules or water as in state of matter = liquid. The amount of liquid water will naturally vary a lot depending on seasons and warmth. We know that the temperature on earth has varied a lot in different time periods. In warmer periods, more water will be bound as steam in the atmosphere and in colder periods more water will be bound as ice at the poles.
Also, the amount of water versus carbon dioxide depends on the amount of biomass on the planet and on the amount of vegetation versus the amount of living creatures. Plants use photosynthesis to turn carbon dioxide and water into sugar and oxygen. Whereas living creatures uses the opposite reaction called Cellular respiration to turn sugar and oxygen into carbon dioxide and water.
So it is likely that periods where lots of water has been bound by plants had less water in the atmosphere and in seas/lakes, though I have no idea how much of an impact the photosynthesis has on the total amount.
At any rate we can at least assume that no water leaves the planet in natural ways, since almost all molecules no matter state of matter are pulled to the planet by gravity. Perhaps with the exception of the lightest gasses like hydrogen and helium that might leave the atmosphere during some conditions - atmospheric escape is apparently a big topic of its own.
It seems that what is now the earth started as an aggregation of rocky material. Large amounts of water were then added by bombardment of comet-like bodies. These contain a lot of ice.
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