Home > Clement's Game, Fermi Problems, Worldbuilding, Your Turn > Your Turn, Round 2-6: A Little Rapid World-Building – Tāwhiri

Your Turn, Round 2-6: A Little Rapid World-Building – Tāwhiri

During a discussion I participated in elsewhere , science-fiction world building spontaneously appeared in a discussion of another subject when a couple of people brought up the example of floating forests / floating islands.  These have shown up in fiction before, and also in real life.  I was inspired to add them to the Fermi Problems setting.  When I had a notepad and a little time, I designed a planet to put them on and named it Tāwhiri.

What Is A Floating Forest?

When I say “floating forest” here, I mean a large mass of vegetation floating in a body of liquid.  Perhaps there is too much similarity here to the micro-gravity asteroid-surrounding forests and floating aerogel-clumps that already feature in the setting, but I will include it anyway.  It is relatively straightforward to produce a planet where such islands occur to a much greater extent than they do on Earth:

Consider an ocean gyre like the Sargasso Sea.  Have masses of floating seaweed-equivalent that grow densely enough and stack thick enough to give a portion of the mass that is persistently well above the water line – avoids the photosynthesizing bits getting shadowed by or eaten by other stuff growing in the water.  Then the clumps that are strong enough to avoid being disrupted by frequent waves/wind have a survival advantage, because they keep their energy supply more reliably.  To get something that looks like a terrestrial forest, rather than a lumpy-mat-berg-plant-thing, invoke selection for traits that keep the clump in the gyre and so at a good latitude for growing.  Growth dependence based on sunlight illumination patterns can do that, and progressively give “trees” with trunks and sail-leaves that adjust themselves unintelligently to take advantage of prevailing winds and stay in the gyre.

Some clumps will still escape, and either freeze in cold water or die in too-warm water or end up in another gyre.  Clumps that get too big will fragment during storms, some will have die-offs due to competition between the various species that accumulate to form the colonies, some may capsize and persist as dead-or-dying rafts that can be colonized by new growth, and so on.

Tāwhiri – A Waterworld

But how to make a planet where these floating forest-mats are the tallest-standing lifeforms?  We need to avoid having much land-based life to compete with.  We need a waterworld.

There is a problem with making a world that is entirely covered in water: Carbon dioxide emitted from the mantle during volcanism enters the atmosphere/ocean, and acts as a powerful greenhouse gas.  Unless it goes into making limestone or something similar, biosynthesis of carbon compounds only keeps so much of the carbon out of the atmosphere and only for so long.  On the Earth, weathering processes on rocks exposed to the air create carbonate rocks, which eventually get subducted and return the CO2 to the mantle.  If there is no exposed land at all, this process doesn’t work and the climate tends to run away into a steamhouse atmosphere, which is not conducive to abundant life.  So I will do a little fine-tuning on the volatile content.  The planet we are considering will have just enough water to cover all but 1% of the surface – we have various small exposed, largely volcanic, pieces of land; and larger relatively shallow areas of ocean that we would call submerged continental fragments if they were on Earth.  This is enough to give a stable climate, albeit one with more CO2 in the atmosphere than we humans can breathe:

  • Mass: 0.95 Earth masses
  • Insolation: 340 W/m^2 average.
  • Mean surface temperature: 300 K
  • Atmospheric pressure: ~1.1 atm, 0.01 atm CO2

That surface temperature is a few degrees hotter than Earth was during the Permian, and is such that there are no ice caps at the poles.  Some additional properties follow in part from the above and in part by arbitrary decision:

  • Host Star: Kiwi – 0.85 solar masses, 0.43 solar luminosity
  • a = 0.7 AU, year = 0.635 yr (291.4 local days)
  • Mean solar day: 19h6m.
  • Two satellites, each between 30% and 50% the mass of the Moon.

The relatively-shallow regions of the ocean also serve a useful setting function: they give a pattern of gyres and overall surface currents that is more-or-less fixed relative to the seafloor, rather than drifting around the planet.  Keeping the maps straight will still be annoying, but at least a given gyre will stay over the same geology.


I am also pleased to assume people living in/on these floating forest-mats.  Their cultures, history, anatomy, and evolution are not yet specified – this is a rapid once-through, after all.

But consider this technical problem: where do you get metal if you live on this world?  There are several possible sources: those few areas of dry land that do exist, sulfide or hydroxide deposits on the sea floor, and some manner of catalytic process that separates out the metals dissolved in the sea water.  The last has a terrestrial precedent: a ~3-cm snail called the scaly-foot gastropod that hangs out near some deep-sea vents:

Scaly-Foot Gastropod.  That shell is coated in iron sulfide.  via https://www.jamstec.go.jp/e/about/press_release/20091130/

Scaly-Foot Gastropod. That shell is coated in iron sulfide. via https://www.jamstec.go.jp/e/about/press_release/20091130/

So there are ways to start developing the hardware necessary to get off-planet and start expanding through space, although it would be somewhat more difficult than doing so on Earth.  I will have the inhabitants of  Tāwhiri in the Fermi Problems setting not have done this yet, in keeping with the large gaps in the setting between the number of places where life has appeared, the number of places where cultures have appeared, and the number of places where cultures have spread across large volumes.

  1. 2014/07/17 at 3:19 pm

    Interesting idea. I’ll have to think about it some. As a planetologist, my concept of a water world is about 5 Earth masses, maybe 500 K, and an atmosphere of (for example) 50% CO2, 30% H2, 10% H2O, and 10% O2, with most of the life being in the high-pressure sub-surface environment. Something close to Earth-like, like Tawahiri, would be more improbable, though not (heh) astronomically so.

    • michaelbusch
      2014/07/17 at 3:52 pm

      I did have to invoke some fine-tuning here. As long as there is some land surface exposed, the geologic carbon cycle is self-adjusting (less land -> more CO2; more CO2 -> more carbonate-producing weathering per unit area), provided that the CO2 concentration doesn’t get so high as to push the entire planet into a run-away steamhouse environment. Tāwhiri has about 2x the H2O content per unit mass as the Earth; going any higher would have covered the land completely, shutting down the geologic carbon-sink entirely and giving the steamhouse runaway.

      The same problem applies to what you’ve described (and the somewhat similar planet Hal Clement assumed in Close To Critical). It’s extremely hard to balance the climates of planets with a lot of volatiles at anything between iced-over and steamhouse.

      Taking your example: on a 5 Earth-mass object with the same volatile content as Earth you’d have the equivalent of global oceans ~5 km deep – lots of volatiles that can boil off into the atmosphere. At 500 K, the equilibrium vapor pressure of H2O is about 25 atm – less than 250 m of ocean-equivalent, given the surface gravity. But 25 atm of H2O plus the CO2 and other greenhouse gases would give you a greenhouse effect of +400 K or so, even in the absence of geothermal heat. Except at extremely great distances from a star and with extremely low geothermal heating, that 25 atm atmosphere pushes the surface temperature well above 500 K – pushing even more water vapor into the atmosphere and warming the planet still further. The end result is Venus, if the water inventory is destroyed by thermal disassociation / photochemistry, or a world above critical point where there is no sharp transition between the extremely thick steam atmosphere and the ocean beneath, if the water inventory is not destroyed.

      • 2014/07/17 at 7:46 pm

        Ah, didn’t think to check the vapor pressure. Obviously, the numbers need to be adjusted. My point was that you could reasonably have double-digit percentages of free hydrogen–something else that shows up in Clement (and is implied by some recent planet discoveries).

        I actually don’t think the fine-tuning is that bad. If, say 1.5-2.5 times Earth’s volatile content would produce a world like Tawahiri, that’s a range about 0.2 dex wide–uncommon, but not that rare. It’s the formation mode that you have to watch out for to get hundreds of times as much (to the extent that we understand the formation).

        The bistability (tristability?) issue is an interesting one. For the right parameters, you can get Earth’s “warm” climate, a snowball world, and a runaway greenhouse as stable states, without much in between, but then if the planet is very dry, or tidally locked, or…well, the combinatorial explosion is precisely my problem at the moment, and I’m not even working on Earth-like planets.

      • michaelbusch
        2014/07/17 at 10:32 pm

        There are indeed a very large number of parameters I haven’t even considered here.

        For another example: Tāwhiri’s chemical composition is nearly identical to Earth’s, just with more water. We know that real exoplanets have a range of compositions, and nobody really knows how many of the possible variations will change their structures. There’s been some speculation about what planetary structures you’d get if you started with more carbon than oxygen, rather than the other way around, but nobody’s yet done the lab work to learn the actual high-pressure and -temperature phase diagrams of the relevant materials.

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