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:
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.