Waterworld or Iceworld?

I have read that Earth underwent complete freezing of its oceans several
hundred million years ago - the sea ice at the equator was a mile thick, and
equatorial daytime temperatures were below -50 C. This was due to the fact
that if ice forms below 35th latitude, enough sunlight is reflected to space
that runaway cooling occurs, since at Earth's orbital distance the
temperature of a non-greenhouse body averages about -60 C.

Earth only escaped from that condition because volcanoes on continents
released CO2 linto the atmosphere and the Earth was so cold there was no
rainfall to wash it out. After 20 million years there was enough CO2 to
raise the temperature and melt the ice - and then caused a runaway
greenhouse for a few thousand years with temperatures at the equator of +50
C.

So if a terrestrial planet has more water than Earth, such that no
continents appear above sea level, will that planet be destined to end up
sooner or later as a permanent iceworld, or is there some other method of
escape?

If the planet is close enough to it's sun that water would not freeze in the
absence of greenhouse gases, would it inevitably end up like Venus?

In summary, can a waterworld even exist?

"Roger Stokes" wrote:

Earth only escaped from [an Icehouse] condition because volcanoes
on continents released CO2 into the atmosphere...

Probably true, although understand that there is still some debate
is to if the Earth went into an Icehouse state (probably) and exactly
what happened going in & out of it. Also note that while deep-water
volcanoes might not directly release CO2 to the atmosphere, they would
still release CO2 into solution in the oceans, which if they reached
saturation would de-gas to the atmosphere, producing the same result.
With no continents (and thus no calcium input to the oceans), the seas
could (possibly) reach this "carbonated" state.
Geochemistry is fun. Complex, but fun.

So if a terrestrial planet has more water than Earth, such that no
continents appear above sea level, will that planet be destined to end up
sooner or later as a permanent iceworld, or is there some other method of
escape?

First, as a star ages and grows warmer, you could (I think;
somebody check me on this) have the star get warm enough to melt the
Iceworld. Second, a true waterworld will have difficulty makign
icecaps that large - forming snow & ice on land (or in land-locked
oceans) is much easier than doing it on an all water world, where
circulation within the oceans will both transport heat poleward much
more efficiently, as well as break up the icecap.

If the planet is close enough to it's sun that water would not freeze in the
absence of greenhouse gases, would it inevitably end up like Venus?

Depends (I hate that answer ass well, but...). Pushing a planet
through a moist or wet greenhouse state into a "hothouse" like Venus
depends on a couple of things, mostly solar insolation but also 2nd
order effects like cold traps at the tropopause or above.
As a counterexample, Earth has been ice-free on more than one
occassion (even at the poles), and we've not entered the hothouse
state (yet...).

--
Brian Davis