Can oceans be too deep for life?

[lpetrich]

Exoplanet hunters rethink search for alien life : Nature News & Comment

“We have this stereotype that if we have oceans, we have life,” says Tessa Fisher, a microbial ecologist at Arizona State. But her recent work contradicts this idea. Fisher and her colleagues studied what would happen on an “aqua planet” with a surface that is almost or completely covered by enough water to fill Earth’s oceans five times.

On Earth, rainwater hitting rocks washes phosphorus and other nutrients into the oceans. But without any exposed land, there is no way for phosphorus to enrich water on an aqua planet over time, Fisher reported at the Laramie meeting. There would be no ocean organisms, such as plankton, to build up oxygen in the planet’s atmosphere, she says — making this type of world a terrible place to find life.

Wet blanket

The wettest planets would run into a different sort of trouble, says Cayman Unterborn, a geologist at Arizona State who analysed the planet-wide effects of having as much as 50 Earth oceans’ worth of water. The sheer weight of all that liquid would exert so much pressure on the sea floor that the planet’s interior would not melt at all, Unterborn found.

Planets need at least some internal melting to sustain geological activity, such as plate tectonics, and to provide the right geochemical environment for life. In this case, Unterborn says, “too much water is too much of a good thing.”

The average depth of our planet's oceans is 3.7 km, and spread out over all the surface is 2.6 km. Multiplying by 5 gives 13 km, more than the height of Mauna Loa and Mauna Kea above the surrounding seabed, about 10 km.

But circulation through hydrothermal vents might nevertheless supply some nutrients.

Multiplying by 50 gives 130 km, and if it suppresses volcanic activity, then it suppresses hydrothermal vents. Meaning that that big ocean is likely to be freshwater instead of saltwater.


So our planet has neither too little nor too much water.

I couldn't find anything in the literature on what happens when a planet forms with very little water. It likely forms with very little of other volatiles, like N2 and CO2, meaning that it will have a very thin atmosphere. Thus, SF planets Arrakis and Tatooine, desert planets with breathable atmospheres, are likely very implausible.

 

[Jokodo]

Exoplanet hunters rethink search for alien life : Nature News & Comment

“We have this stereotype that if we have oceans, we have life,” says Tessa Fisher, a microbial ecologist at Arizona State. But her recent work contradicts this idea. Fisher and her colleagues studied what would happen on an “aqua planet” with a surface that is almost or completely covered by enough water to fill Earth’s oceans five times.

On Earth, rainwater hitting rocks washes phosphorus and other nutrients into the oceans. But without any exposed land, there is no way for phosphorus to enrich water on an aqua planet over time, Fisher reported at the Laramie meeting. There would be no ocean organisms, such as plankton, to build up oxygen in the planet’s atmosphere, she says — making this type of world a terrible place to find life.

Wet blanket

The wettest planets would run into a different sort of trouble, says Cayman Unterborn, a geologist at Arizona State who analysed the planet-wide effects of having as much as 50 Earth oceans’ worth of water. The sheer weight of all that liquid would exert so much pressure on the sea floor that the planet’s interior would not melt at all, Unterborn found.

Planets need at least some internal melting to sustain geological activity, such as plate tectonics, and to provide the right geochemical environment for life. In this case, Unterborn says, “too much water is too much of a good thing.”

The average depth of our planet's oceans is 3.7 km, and spread out over all the surface is 2.6 km. Multiplying by 5 gives 13 km, more than the height of Mauna Loa and Mauna Kea above the surrounding seabed, about 10 km.

But circulation through hydrothermal vents might nevertheless supply some nutrients.

Multiplying by 50 gives 130 km, and if it suppresses volcanic activity, then it suppresses hydrothermal vents. Meaning that that big ocean is likely to be freshwater instead of saltwater.


So our planet has neither too little nor too much water.

I couldn't find anything in the literature on what happens when a planet forms with very little water. It likely forms with very little of other volatiles, like N2 and CO2, meaning that it will have a very thin atmosphere. Thus, SF planets Arrakis and Tatooine, desert planets with breathable atmospheres, are likely very implausible.

I suppose it's not entirely impossible as water is much lighter than eg CO2. So if the planet remains hot enough for most of its water to persist in a gaseous state for long enough that much of it is lost to the solar wind before it cools down to liquid-water temperatures, it might still be possible to get a water-poor planet with a dense atmosphere.

 

[Kharakov]

ehhhh.....

The sheer weight of all that liquid would exert so much pressure on the sea floor that the planet’s interior would not melt at all, Unterborn found.

ehhh. It appears the sheer weight of gravel, dirt, rocks, and other more dense substances wouldn't exert pressure on the planets interior. Here I thought the moon was responsible for a little bit of the activity we see....


How does a (non rogue) planet form without something close to the average cosmological ratio of the elements (with the exception of H2 and He) in its vicinity at the time its star system forms?

To further what Loren mentioned: there are some theories that life started near hydrothermal vents. If there is a close source of tidal energy (like Jupiter for a moon, or the Moon for Earth), there could be a lot of free nutrients without erosion. Not to mention the various compounds that ended up in the ocean during the planet's formation....

Not too sure about that article.

 

[lpetrich]

I couldn't find anything in the literature on what happens when a planet forms with very little water. It likely forms with very little of other volatiles, like N2 and CO2, meaning that it will have a very thin atmosphere. Thus, SF planets Arrakis and Tatooine, desert planets with breathable atmospheres, are likely very implausible.

I suppose it's not entirely impossible as water is much lighter than eg CO2. So if the planet remains hot enough for most of its water to persist in a gaseous state for long enough that much of it is lost to the solar wind before it cools down to liquid-water temperatures, it might still be possible to get a water-poor planet with a dense atmosphere.

I must concede that. Water has an additional problem in that it can be split into hydrogen and oxygen, and hydrogen is much lighter than carbon, nitrogen, or oxygen.

There is a planet that has lost most of its surface water while continuing to have a thick atmosphere: Venus. So that scenario is not fundamentally impossible.

 

[fromderinside]

Aren't most of the ions in galactic space hydrogen? If so, wouldn't expectation of atmospheric absorption be as high as loss at some point? Also in planetary space of sol aren't there enough water containing meteors striking earth to counter loss of water by hydrogen disassociation and atmospheric leakage perhaps defeating that process in our planetary orbit location.

 

[lpetrich]

Aren't most of the ions in galactic space hydrogen? If so, wouldn't expectation of atmospheric absorption be as high as loss at some point? Also in planetary space of sol aren't there enough water containing meteors striking earth to counter loss of water by hydrogen disassociation and atmospheric leakage perhaps defeating that process in our planetary orbit location.

But that does not seem to be enough for Venus or Mars.

Interstellar hydrogen would get blown out of the Solar System by the solar wind. Most of the Solar System's water condensed outside of the asteroid belt, and there don't seem to be enough comets to supply that much water.

 

[Elixir]

Aren't most of the ions in galactic space hydrogen? If so, wouldn't expectation of atmospheric absorption be as high as loss at some point? Also in planetary space of sol aren't there enough water containing meteors striking earth to counter loss of water by hydrogen disassociation and atmospheric leakage perhaps defeating that process in our planetary orbit location.

But that does not seem to be enough for Venus or Mars.

Interstellar hydrogen would get blown out of the Solar System by the solar wind. Most of the Solar System's water condensed outside of the asteroid belt, and there don't seem to be enough comets to supply that much water.

What about accretion prior to the sun's ignition? Once it's water, it's not so easy to blow away in the nascent solar wind.

 

[lpetrich]

All that means is starting of with a lot of water.

My dry-state scenario can come about from (1) losing a lot of water or (2) starting off with very little water. Scenario (1) is what happened to Venus. Most of its water is in its clouds, as concentrated sulfuric acid. Its hydrogen is heavily enriched in deuterium, something like 1% D as opposed to the Earth's 0.015% D. This points to a history of water loss.

I was more interested in scenario (2), where a planet would start off with very little water. I suspect that if it did so, it would also be low on other volatiles, like nitrogen and carbon dioxide. The Moon is a good example of this -- its rocks are very low in volatiles.

So (1) would make an atmosphere with lots of CO2 and N2, and (2) would make a very thin atmosphere. Neither is likely to be breathable by us.

 

[steve_bank]

Water is thought to have come from comets.

Life evolved around black smokers, deep sea volcanic vents. Life exists on chemical energy. I believe they were one of the major discoveries made by the submersible Alvin.

https://en.wikipedia.org/wiki/Hydrothermal_vent

 

[bilby]

If an ocean is deep enough, the bottom of the water column will freeze even at high temperatures, which could be a problem - if you posit a planet made of almost pure water, with a core of ice, or planet with a rocky core and very deep oceans, such that ice (likely ice VII) seals the liquid phase of the oceans off from the other minerals in the core, it's hard to see where sufficient other nutrients for the development or support of life would come from. But planets are big, relative to animals and plants; so perhaps such a planet could have a small amount of life, with intense competition for the traces of non-aqueous resources.