More water worlds than we thought might support life
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This is interesting but raises some questions:
1. If water isn't in its molecular form (para 3), then is it actually water? Or is the implication that it's water coordinated in complexes e.g. to iron or silicates as mentioned later?
2. Even if there is high-pressure ice at the interface of the water and rocky core/surface, isn't there the possibility of mineral exchange through volcanic activity or hydrothermal vents?
1. If water isn't in its molecular form (para 3), then is it actually water? Or is the implication that it's water coordinated in complexes e.g. to iron or silicates as mentioned later?
2. Even if there is high-pressure ice at the interface of the water and rocky core/surface, isn't there the possibility of mineral exchange through volcanic activity or hydrothermal vents?
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I wonder if exoplanets with conditions difficult for life to survive in might lead to organisms whose genetic material is inherently more stable and thus doesn't mutate, thus preventing Darwinian evolution. However, they might still evolve via a Lamarckian mechanism. I suspect this would make it difficult to evolve multi-cellular life, however (as the Lamarckian mechanism would need to propagate changes to every cell's genetic material, or at least a germ cell's).
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Sounds like a whole new field of planetary geodynamics opening up. For example, is the amount of water bound up in molten nickel/iron different than that in crystalline nickel/iron? As the molten outer core slowly solidifies onto the inner core, does that release water or absorb it? How might that affect magnetic fields? Or cooling rates? The questions are endless.
Remember when someone once said that all the hard questions had already been answered?
Remember when someone once said that all the hard questions had already been answered?
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Do you ever wonder if the big filter on spacefaring civilization is having enough water for life, but not so much water that it stops you from doing any tasks that can only be performed in a dry environment?
Maybe someone else has better answers than I do, but so much of our technology is harder to work with in the ocean. I don't know if it's possible to kickstart metalworking without dry land, and good luck with electrical circuits when the medium you exist in is conductive; basketweaving might be easier, but I don't think you'll be making a rocket by weaving kelp together.
Maybe someone else has better answers than I do, but so much of our technology is harder to work with in the ocean. I don't know if it's possible to kickstart metalworking without dry land, and good luck with electrical circuits when the medium you exist in is conductive; basketweaving might be easier, but I don't think you'll be making a rocket by weaving kelp together.
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This reminds me of a meta study I saw about atmospheres on potentially habitable planets. The error bars are generally so large that the "liquid water 75 degree sunny planet" was just as likely to have no atmosphere or be like Venus.
Its possible to construct any possible situation on a planet you want. However that doesn't mean its likely.
Its possible to construct any possible situation on a planet you want. However that doesn't mean its likely.
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Octopus are evidently pretty intelligent, but they are by no means dominating the oceans. Intelligence is not necessarily an evolutionary advantage. Sharks have been around a long time, still dumb. Whales might be smart, but haven't developed any level of technology.
I could imagine a water based civilization using biochemistry to construct large structures. But electronics and water don't mix well. Could one replicate that with biologic based neural networks? Without electronics, hard to comprehend how one would develop the advanced technologies needed to leave the planet.
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I came to ask about the volcanism/thermal vents too. Also, what about the effect of topography? It does seem like you could have a planet where a substantial portion of the ocean is deep and high pressure, with pressure-ice at the bottom, but seems like there would be underwater mountains sticking up, and much like mountains in our atmosphere will have an ice line above a certain elevation, it seems like there would be an ice line below a certain elevation within the ocean, and the bits of the mountains that rose up above that line due to either tectonic forces or volcanism, would be ice-free and be exposed to the water and thus be subject to mineral exchange and erosion from the water - and of course, it would seem like thermal vents and active volcanoes would cause direct mineral deposition into the surrounding water?
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It depends on how it isn't in its molecular form. For example, in gypsum (CaSO4·2H2O), the water is bound to the sulfate via the van der walls force and so isn't truly in its molecular form. But it doesn't take much energy to return it to being just plain water. But in limonite (FeO(OH)·nH2O), there is enough of a bond that it is very hard to extract the water.
Yes. In addition, the top of the rock is unlikely to be perfectly smooth; there will be "mountains" and "valleys". The tops of the highest "mountains" might stick out of the ice and the ice might be melted by the heat at the bottom of the deepest "valleys".
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This hardly moves the needle at all.
Why? What is not mentioned is that Earth has only 0.02% water by mass. The blue marble notwithstanding. Doubling that by doubling the mass of mantel water, gets us up to 0.04%. Most putative water worlds discovered so far have water by mass percentages in the range of 10-30%. If half is embedded in rock, that still leaves massive high pressure ice layers. Earth would have exotic ice sea floor if it had 20-30x more water. That would still be "only" 1%-2% water by mass.
A high pressure ice layer multiple(or hundreds) km thick will drastically encumber the potential oceanic nutrient content. Let alone the development of life. Such a layer is easily attained at multiples of Earth's water content which is still little in the context of most water worlds.
Earth is more like Mars hydrologically. Luckily. Earth sits on a hydrological knife edge. It has a precious mix of land and ocean and sea bed liquid water/rock interface. There is a wide range of posibilities that are either too dry or too wet to be habitable. 0.02% sounds almost dry. But it's what gets us the Goldy Locks state we live in.
Why? What is not mentioned is that Earth has only 0.02% water by mass. The blue marble notwithstanding. Doubling that by doubling the mass of mantel water, gets us up to 0.04%. Most putative water worlds discovered so far have water by mass percentages in the range of 10-30%. If half is embedded in rock, that still leaves massive high pressure ice layers. Earth would have exotic ice sea floor if it had 20-30x more water. That would still be "only" 1%-2% water by mass.
A high pressure ice layer multiple(or hundreds) km thick will drastically encumber the potential oceanic nutrient content. Let alone the development of life. Such a layer is easily attained at multiples of Earth's water content which is still little in the context of most water worlds.
Earth is more like Mars hydrologically. Luckily. Earth sits on a hydrological knife edge. It has a precious mix of land and ocean and sea bed liquid water/rock interface. There is a wide range of posibilities that are either too dry or too wet to be habitable. 0.02% sounds almost dry. But it's what gets us the Goldy Locks state we live in.
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Although in the planetary example given, K2-18b, at 8.6 times Earth's mass, gravity would constitute a serious suppressor of mountain heights. On the other hand, I'm not sure how much erosion would take place under those conditions either.
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Topography might not help much unless the planet is right on the boundary of forming exotic ices. On an Earth-like planet you'd get exotic ices forming on the bottom of an ocean that's on the order of 100 km deep. Yet the tallest mountains Earth can sustain are on the order of 10 km high (Mauna Kea) because mountains sink into the crust due to their added weight.
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Wouldn't that same increase in gravity, though, increase the force of tectonic lift, canceling out the effect of extra weight of the rock being uplifted?
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Hey! Note the quotation marks, please!
And can you imagine the mechanical erosion due to rivers on a planet with 8.6 Me?
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I wonder... A watery planet with a thick, non-permeable layer of ice at its floor might not allow minerals to move upwards through it, but it might collect a fairly respectable rain of asteroidal and cosmic dust particles on top of it. Some of that material would take the form of dissolved salts, some would remain solid. I wouldn't give up hope just yet.
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And the dissolved salts would be entrained by the solids as the solids sank down into the ice. Billions of little test tubes, each ready to create life!
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Hey! Note the quotation marks, please!
They were there, but I suppressed them with the weight of my thoughts.
Depends on the gradient, doesn't it?
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Would the material raining down from space just sit on top of the ice, or gradually migrate down through the ice due to weight and molecular interactions? Although I agree that before that happened, it seems like some of the material coming down from space, through the liquid water at the top of the ocean, would dissolve before it settled at the bottom? Is there any reason it wouldn't?
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Followup question: The depth of the ocean doesn't matter - the depth of the ice is what matters. What would be the depth of the ice layer? For example, if it's 1km of exotic ice and a mountain goes up 3km, you potentially have 2km of mountain above the ice, and 97km of liquid water above that?
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I think octopi are largely held back by their semelparous reproduction and solitary nature. They get six months to live and no generational knowledge from their parents. By the time an octopus reaches a peaceful end to its life, most other creatures with notable intelligence are still being babied by their parents.
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You might be thinking of isostatic uplift, and you might have a point. A massive planet with a water-saturated mantle might hold a hefty volume of radioactive isotopes leading to lots of internal heating, and a vigorous amount of convective churning. That would seem to encourage lots of mountain building. However, the weight of the ice (the ice-ostatic forces!) coupled with warmer and wetter rock, which would be more plastic and more deformable, might mitigate against extreme surface relief. We might see lower, broader zones of uplift instead.
Hotter and more widespread magma might also discourage the kinds of chemical differentiation we see on Earth, the distinction between basalt and granite, which is so important to the surface layout of the continental Earth, could be a different thing under non-Earthlike conditions. We might not even see the development of the plated-tectonic style segmentation of crustal rocks.
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Ignoring the ice VII problematic for the moment, the sequestration of a large mass of water in the mantle (1%, 2%, 5% or more) could also spell doom for plate tectonics. It would soften the mantle and plate tectonics would be much less efficient or stop entirely. The geodynamical and geochemical knock on effects for habitability would be catastrophic.
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A planet 8.6 times the Earth's mass, but less dense, would not necessarily have that much higher surface (sub-surface) gravity. (Saturn is an extreme example. It has the mass of 95 Earths, but a gravitational force only a little more than one Earth gravity.)
High pressure ice is also dense ice. It is about 30% more dense than water. The hypothetical mountains would displace that ice and effectively weigh much less. (Think of the weight of a person floating in water.)
The result could be enormously tall mountains and volcanoes pushing up through the high pressure ice into the ocean above.
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I’m confused how we can tell simultaneously that a planet is water-rich and thus at risk of forming high pressure ices and iron-rich and thus actually it’s fine because the iron sequestered the water, just from the bulk density.
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Wouldn’t a more lubricated mantle mean you have more convection rather than less?
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Water doesn't lubricate the mantle, per se. Instead, it weakens the rocks so that convection becomes easier. (Yeah, that's a pedant's quibble. So sue me.)
The question is "does the amount of weakening due to water reach a maximum and plateau, or does it keep going up with water content, or does it reach a peak and then drop back down?" (Followed closely by "what effect does composition have?") Given that it can take up to twenty years to run a strain experiment, the answer to this question is still somewhat up for debate.
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No. More like ammonia ice and nitrogen ice and CO2 ice.
In geology, "ice" is a generic for "a solid that you would normally find as a liquid under STP".
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All good points, and I once had a handy formula that could give me the surface gravity of any given mass/density or mass/diameter combination. It's been a while, but maybe I could dig it up.
We're dealing with a wide raft of potential variables here, so anything you or I could say would be at best plausibly speculative, at worst wild supernatural fantasy. To your point about water and mountain density, though, granitic rock is about 2.7 times the density of liquid water. Basaltic rock averages about 2.9 times the density of water. Increasing the water density via ice would not be that big a difference. Also, if there is a thick layer of high-pressure ice overlying the rock, then you are not likely to have the near-surface magma zoning regime that you have in Earth's lithosphere, and the rocks are more likely to be more similar to mantle rocks, which are 4.5 times as dense as water.
Also, density is not directly related to plasticity. There's some indication that high-pressure high-temperature ice structures are relatively plastic, and mantle rocks at mantle temperatures are themselves quite plastic. Even under an Earthlike gravity, materials of high plasticity would not form tall mountain ranges. In fact, the plasticity, and susceptibility to deformation of underlying mantle rocks, is one of the main controls of mountain heights on Earth.
Generally, a rocky, hot planet will have a density proportional to its mass in the same range of Earth's density/mass relationship, and that of other terrestrial planets. You could add a heck of a lot of water without appreciably changing that range. If, as the article suggests, the planet in question has a water abundance of 50%, then there's no way to even think about molten rocky cores and mantles and mountains as we know them on Earth. If the ice layer is thin enough to allow you to entertain the idea of mountains breaking through it, then the conditions are not all that different from Earthlike.
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It is, but the Ice-9 artifice is one of those "suspend your disbelief" plot devices akin to the storm on Mars that sets up "The Martian."
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More convection doesn't mean more plate tectonics. The other required ingredient is a rigid and relatively brittle lithosphere associated with the crust. In the ice-laden world we're envisioning, I see no guarantee that a lithosphere would exist.
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I caught the reference, but arrived to the thread too late to give a proper notification of understanding with a dash of foma.
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In his defense, unlike Weir, Vonnegut was not trying to write science fiction. I would pigeon-hole his work as science-fantasy.
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I've always thought of it as "social commentary", but then, a lot of science fiction fills that same purpose.
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