How to Cause a Hydrosphere Apocalypse Part II The Great Flood

Question

This is part two of a two part question. You can find part one here.(How to Cause a Hydrosphere Apocalypse Part I The Great Drought)

One of the more interesting movies I watched growing up was Waterworld. I never understood why no one ever talked about it, I always thought it was cool, the music, the action the endless ocean, what more could you want? Admittedly, as I grew up I understood that there wasn’t enough water frozen in the ice caps to make that future happen, only enough to put New York and a lot of other coastal places under 60 ft of water.

But what if there was a world where there was a planetary drowning?

So let’s imagine another interstellar expedition that has discovered a habitable water world. There are no icecaps at the poles, the water is clear and shallow, and there is an absolute abundance of marine life. But no dry land, not even a small set of islands.

But the more astronauts investigate this planet the more anomalies they come across. Dozens of species that come to the ocean surface to breath like whales or dolphins, several species that still have clearly defined arms and legs, and in the shallower parts of the global ocean there are clear trace fossils of foot prints as well as fossils of animals that clearly spent their entire lives on dry land.

The evidence is clear, this world was once not so different from earth until it experienced a Noah’s Ark level flood, only this time there was no dry land left to go to.

So where did all that water come from?

What scientifically feasible scenario could happen that would cause a life supporting planet to flood and loose most of its dry land?

Answer 1

A rain of ice from a dying moon. Saturn's rings drop "each second ... perhaps thousands of pounds of water ice" per second onto the planet. Now true, there's an issue, namely if we compare 1000 kg/s = 1000 L/s = 1 m^3/s to 43.14(6350 km)^2 [the surface area of Earth] x maybe 1 km rise in ocean level = 506 x 10E6 km^3 = 506 x 10E15 m^3, well, it's gonna take a bit more than 40 days and 40 nights, like 16 billion years more.

Nonetheless, we can't deny that a moon can be largely covered in ice, and have sufficient volume. So we need a big moon of mostly water that is 3/4 (506 x 10E6 km^3)^(1/3) = 600 km radius, and it needs to stray within the Roche limit. Over some time, it converts to debris that falls into the atmosphere and raises the planet's ocean a kilometer.

Now, doing this quickly without smashing or roasting the planet would be quite a feat. I wonder if you could roast the moon a little, convert much of that ice to water vapor that drift with relatively little disruption into the atmosphere. The heating from all the potential energy released might have been partly offset by covering the planet in thick white clouds. The slower the process could be made the less outrageously optimistic you need to be with your explanation.

Answer 2

The one idea that comes to my mind is an icy, millenia-long meteor shower. suppose a vast number of icy meteorites crash into the planet and melt. I know that there's a lot of loose ice in the solar system, but I don't know how likely it is that, should a massive storm of meteors hit the planet, most of them are made of ice rather than rock.

The storm in question would ideally be composed of billions of tiny ice asteroids rather than a few big ones, because we don't know of any major marine amniote groups that survived the chixclub event. surviving multiple such events would be even more difficult. However, if the impacts are smaller than the K.T. extinction impact, and there exists a species of small, adaptable former land-dwellers, they might live to see the new waterworld.

I can't see a way this works via melting ice caps. If there is enough water to cover all the land on the planet, and the planet is warm enough to allow earth-like conditions, then it should be flooded. It might be possible for it to be just warm enough for a very narrow strip of liquid ocean on the equator, which might expose some land, but I don't think this is sustainable for the hundreds of millions of years required for land life to evolve and develop. (I don't know a lot about snowball earth, but not only was it too cold for this to work, but a brief wikipedia search suggests it did not last long enough for the conditions specified.)