Waterworlds: A Rarity Among Terrestrial Planets?

[bostonnew]

If we only consider the terrestrial planets and moons, do you think the amount of water on Earth is in the top 10%? Or are we perhaps average here?

Could it be that the norm for terrestrial planets is to be completely covered with water? And that the Earth is rare in its lack of water? (perhaps contrary to the normal rare Earth hypothesis). I mean, we only have to go to Jupiter's Europa to find a place with a 100% water surface (although it is frozen).

Do you know if any astronomer has tried to predict the prevalence of waterworlds based on available exoplanetary evidence?

If perhaps it is rare for terrestrial planets to have continents we can understand why intelligent life is not all over the place in our neighborhood. It's hard to built fancy technologies if you're swimming, no matter how smart you are.

 

[Nik_2213]

A couple of years ago, you might have got confident guesstimates, but there's now such a zoo of exoplanets, with dozens more from each tranche of transit data, that all bets are currently off...

IMHO, we need to get a grip both on the transit sample of exoplanets *and* the growing local population of 'brown dwarfs' before we dare tackle this question...

Uh, one possible 'gotcha' for water worlds might be 'floating islands', with extensive rafts of super-sized bubble-wrack sea-weed analogs supporting a sub-aerial eco-system in their equivalent of the Sargasso...

 

[Chronos]

The principle of mediocrity suggests water worlds are not unusual. We lack, however, any observational support.

 

[Kerellen]

Very few, I think, because most would rapidly become snowball worlds, or if their temperature was higher, would end up like venus.

The snowball Earth episodes of our own planet are believed to have happened when continents were clustered near the equator. When ice forms on open ocean below 30 degrees latitude, sufficient energy is radiated to space that cooling is irreversible, ending in sea ice hundreds of meters thick even at the equator and an equatorial temperature of -50C.

Earth is thought to have escaped from the snowball condition after several million years because volcanoes on the continents emitted CO2 which built up in the atmosphere until warming was sufficient to melt the ice. This would not work on a snowball world as there would be no continents above sea level, so the CO2 would dissolve in the seawater and not be emitted into the atmosphere. Snowball forever.

 

[Nik_2213]

I tried finding such an ocean's capacity for CO2 but that depends on the depth, gravity and temperature. If the 'waterworld' ocean is deep enough, you could get some weird phenomena: According to wiki (YMMV) http://en.wikipedia.org/wiki/Carbon_dioxide
"The Champagne hydrothermal vent, found at the Northwest Eifuku volcano at Marianas Trench Marine National Monument, produces almost pure liquid carbon dioxide, one of only two known sites in the world.[43]"

 

[qraal]

Very few, I think, because most would rapidly become snowball worlds, or if their temperature was higher, would end up like venus.

The snowball Earth episodes of our own planet are believed to have happened when continents were clustered near the equator. When ice forms on open ocean below 30 degrees latitude, sufficient energy is radiated to space that cooling is irreversible, ending in sea ice hundreds of meters thick even at the equator and an equatorial temperature of -50C.

Earth is thought to have escaped from the snowball condition after several million years because volcanoes on the continents emitted CO2 which built up in the atmosphere until warming was sufficient to melt the ice. This would not work on a snowball world as there would be no continents above sea level, so the CO2 would dissolve in the seawater and not be emitted into the atmosphere. Snowball forever.

I think the Snowball might be harder to initiate on a Water World because cloud formation would be suppressed and oceans are low albedo. Also carbon dioxide would build up easier because there's much, much less weathering to cause CO2 absorbing reactions. Carbonates can be formed in the ocean, but also dissolved.

So my suspicion is that Snowballs would really require continents, unless the insolation is below about 0.5 current levels, depending on the greenhouse gases available. More nitrogen would mean a stronger greenhouse effect too, thanks to pressure spreading of the IR absorption spectra of CO2 etc.

 

[Gliese123]

Interesting theard! Well, consider that waterplanets should be quite common throughout the universe. Planets and especially moons in this solarsystem has considerable amounts of either ice, water or water vapour. Why not totally covered planets and moons with the right conditions? Such planets and moons would probably contain ice VII: http://en.wikipedia.org/wiki/Ice_VII . However whould they be huge or small? (I'm referring to, for example: http://en.wikipedia.org/wiki/Gliese_436_b

 

[bostonnew]

To my mind, any planet covered completely in water is a water world, regardless of the state of that water (icy or not). So I count Europa as a water world.

My point is that maybe life in the galaxy is predominantly marine life. Wouldn't that make complete sense given what we know about evolution here on Earth?

Extraterrestrial life is often envisioned as being land-dwelling, but maybe Europa is a much more common life-supporting planet than Earth is. Maybe we are slightly rare. Not in having unusually great conditions for life, but in having continents that are not completely submerged in oceans.

Would love to get some reactions to this. Why does the Earth on Earth peak out of its oceans while the Earth on Europa is completely covered by a hundred miles deep ocean? Is it largely a question of CO2?

 

[qraal]

To my mind, any planet covered completely in water is a water world, regardless of the state of that water (icy or not). So I count Europa as a water world.

My point is that maybe life in the galaxy is predominantly marine life. Wouldn't that make complete sense given what we know about evolution here on Earth?

Extraterrestrial life is often envisioned as being land-dwelling, but maybe Europa is a much more common life-supporting planet than Earth is. Maybe we are slightly rare. Not in having unusually great conditions for life, but in having continents that are not completely submerged in oceans.

Would love to get some reactions to this. Why does the Earth on Earth peak out of its oceans while the Earth on Europa is completely covered by a hundred miles deep ocean? Is it largely a question of CO2?

According to planet formation simulations, the amount of water on the Inner Planets, inside of the Snow Line (about where Jupiter is), is due to stochastic late-stage accretion events. If the asteroids/planetoids that accreted in the late stage were wet, then so was Earth, and contrariwise if they were dry. Earth is very dry compared to the typical 3% water in the asteroids, so in that respect it isn't a water-world. Much of Earth's initial ocean might have subducted since our earliest era, also. The average elevation of Earth's surface is just ~800 metres, so a bit more water would mean a largely flooded planet - thus the vast epicontinental seas in epochs past.