Was the Earth ever totally underwater?

David Rothery from the Open University took on Goeff's question...

David - Was the Earth ever covered in water? Very definitely yes, if you’re prepared to accept water as the substance H2O. Because there have been times when the planet has been entirely covered by ice, that’s both the land masses and the oceans covered by ice from pole to equator. These are called the ice house world conditions, the most recent of which was about 635 million years ago so just before large amounts of multi-cellular life. Extreme climate events covering the Earth in solid water - yes.

提问者可能是考虑液体water. Now land masses don’t so much rise and fall as, at the moment, the continents split apart and rearrange themselves and the amount of continental material is scarcely changing over time. But if you go back about 3 billion years ago, there wasn’t much continental crust, maybe just a few ocean islands. So virtually the whole planetary surface would have been covered by liquid water at those times when it wasn’t frozen. So, very deep in the past a few islands in amongst the global oceans. That’s as close as we get to a planet we get covered in liquid water.

Can they occur elsewhere? Yes, there are certainly many worlds in this solar system - moons of Jupiter and Saturn which are ice at the surface, rock underneath, and maybe an ocean sandwiched in between. But to go to bodies covered in liquid water, well these are known as water worlds. Probably from the science fiction movie but it’s an obvious term. We think planets that are rocky but have a greater mass than the Earth (twice to ten times the mass of the Earth) are going to have so much water as well. But by the time the water’s squeezed out of the rock, it’s going to be such a deep ocean that you’ll never have land masses sticking above the surface.

So we are expecting to find water worlds among the exoplanets and we’ve got planets round 2 or 3 thousand stars now. Most stars in our galaxy we know have planets now, so some of them are going to be water worlds and we think we are identifying some of them by their size and mass.