Home > Chott, Clement's Game, Worldbuilding, Your Turn > Your Turn, Round 1-1: Chott

Your Turn, Round 1-1: Chott


This time, we’re going to do things somewhat differently.  I will describe a fictional planet that I created for a role-playing game scenario, which Rachel and a bunch of players on alternatehistory.com ran through on different occasions.  Then you get to point out anything and everything that I got wrong.

Despite my repeated poking at the economics and sociology of other works, here we’ll just start with the description of the planet and let you all tell me the right way to do planet formation, geophysics, geology, and meterology.  The people and cultures that have lived on it and the plot of the scenario can wait for another post.

Ready?  Here we go.

Chott

Chott is a planet with a mass of about 0.5 Earth masses, formed out of material with a similar ratio of carbon to oxygen.  A shortage of oxygen would have meant a mineralogy dominated by carbides or some other more exotic materials rather than oxides of silicon.  While such mineralogies likely occur on some exoplanets, our knowledge of the geophysics and geochemistry for such places is very limited, so I’ve left Chott with near-solar C:O and changed some other things instead.

Chott has somewhat less water and other volatiles per unit mass than Earth or Venus, although it has retained more than Mars.  There is enough water in the crust to keep plate tectonics operating roughly as it does on Earth, with differences in arc volcanism because there isn’t quite as much water-rich subducted ocean sediment.  Mid-ocean ridges do exist, but the rate of spreading is less because there is less geothermal heat to drive the convection.  The lower ocean depth exposes the ridges above the surface of the water, adding to already larger areas of land with relatively little vertical relief.  The rate of orogeny is slower than on Earth, so the balance between erosion and mountain-building is reached at much lower heights.

Climate

In part because of the insolation from its host star and in part because of the weaker greenhouse effect from a thinner atmosphere, Chott is colder than Earth.  The polar regions have permanent ice caps, and relatively little water evaporates from the oceans to be rained out on the land.  While there is some groundwater, most streams and lake beds are transient, with the lakes getting water during only part of the year and being salt pans the rest of the time.  This is where I got the name for the place, although I admit to using the word “chott” incorrectly – the rainfall is seasonal, but not necessarily concentrated in the winter.

Over geologic time, Chott’s oceans have become hypersaline relative to Earth.  The lakes are also hypersaline, and so is much of the groundwater.  Depending on the location, the water reservoirs can also be acidic or basic or have interesting concentrations of other ions, but almost everything moist on the planet is very salty.  The exceptions are clouds, rainwater, and areas where the soil has been washed free of salt.  The salinity does help keep the lakes and ocean at equatorial and mid-latitudes from freezing at night and during the winter.  Chott has ~30 degree obliquity with large long-term oscillations  and ~26 hour rotation, so nights can get quite cold.

Mid-morning in a dry lakebed on Chott.  The high ground on the horizon is about 20 km away.  There is still some surface water in the basin, feeding a multi-colored brew of microbes.  But that is 2 meters downhill and ~50 km behind you.  This was the first place the characters in the role-playing scenario encountered.

Basically, the nicest-to-humans areas of Chott are something like the Mojave.  Other areas are like the Atacama , with a bit more precipitation but lower temperatures.  At the poles, it gets cold enough to freeze out CO2, but there isn’t that much in the atmosphere.  As you can tell, I designed this place to be somewhere where humans could just barely survive without lots of sophisticated technology.

Biology

There is life on Chott, based on nucleic acids and proteins and using several different sources of energy.  There are sufficent oxygen-producing organisms that the atmosphere has enough oxygen for humans to breath (partial pressure ~0.2 bars), and enough consumers of CO2 that the partial pressure of that is not so high as to be toxic to humans, although there is still far more of it than on Earth.  But how did the Chott biosphere evolve?

Halophile microbes and larger organisms on Earth evolved into those environments, and almost all survive by expending lots of energy to keep their insides relatively free of salt as compared to their surroundings.  This is necessary because many normal (for life on Earth) proteins are not soluble in salt water.  Increase the salt concentration too much, and they come out of solution – salting out.  It has been far easier for Earth microbes to adapt cell membranes to keep out the high concentration of salt than it has been to use an entirely different set of proteins with a greater propensity to form hydrogen bonds with water molecules.

Life on Chott works differently.  The background of simple proteins accessible in whatever equivalent there was of primordial soup were ones that on Earth could have been too soluble for their jobs.  The structural materials used by early Chott microbes were solid/gels/membranes in salt-rich environments, but would dissolve or disintegrate in low-salinity places.

This causes an obvious problem for larger organisms outside of the oceans, lakes, and groundwater: they will dissolve in pure rainwater.  Combined with the high value of water on much of Chott’s surface, this means that the plants and animals on the land will all be armored with shells of things that don’t dissolve in pure water – be those proteins or non-polar hydrocarbons or some long-chain organic polymer.  For the plants, only areas above ground need to be armored, since water below the surface will have mixed with the soil and picked up enough salt to be usable.  At the risk of being insufficiently creative and of overselling convergent evolution, I designed a few Chott plants inspired by real succulents.

Chott plants.  The shape and surface texture were inspired by Euphorbia obsesa, but the color is different because Chott plants don’t use chlorophyll.

The need for protection against rainfall extends to the animals.  Some may be exoskeletal, others simply have thick hides.  Something like a thorny devil would work, since any means of collecting water is useful as long as that water can be combined with a little salt-rich dirt before being drunk (or as long as whatever equivalent the animal has of a stomach has a water-insoluble lining).
Both the animals and plants differ as much from place to place on Chott as on they do on Earth.  There is no such thing as a single-biome planet.  I have not detailed the entire biosphere, because that would mean generating a consistent ecosystem of hundreds of thousands of different species.  But I have considered various interesting local variations.
In areas with relatively high rainfall, the density of plants becomes high enough that there is an advantage to being taller than the plants around you.  That leads to things not entirely unlike trees.  One difference is what they use for support.  Woody plants on Earth use cellulose and lignin.  But tall Chott plants use salt crystals surrounded by organic membranes.  The areas that could be called forests are underlaid by large deposits of rock salts – both halite and salts other than NaCl.  Groundwater brings in some salt from areas uphill, and the insoluble membranes prevent the dead trunks from dissolving away too quickly.  These forests can’t grow up onto ridgelines, since there is no supply of salt there once the local soil has been depleted.

What Have I Missed?

So that is Chott, in terms of planetary science, geophysics, geology, meteorology, biochemistry, and biology.  What have I missed or gotten wrong?

Your Turn Round 1-2 will go into the history and cultures and economics of Chott.  And you all can thank Rachel for the pictures.

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  1. 2012/11/23 at 3:41 am

    I’m not a geologist, but my concern is whether a 0.5 Earth-mass planet would have enough internal heat to drive plate tectonics at all. Also, note that mid-ocean ridges take on a different look above the waterline. The classic examples are Iceland and the Great Rift Valley in Africa.

    • michaelbusch
      2012/11/23 at 5:59 am

      I gave Chott a slightly higher density than Earth, with relatively more iron and siderophile elements in the core to compensate for there not being as much internal pressure. Given that, its radius ended up being 0.76 Earth radii. Given an equal fraction of radioactives, the geothermal heat per unit area would be about 70% that on Earth (accounting for the lower contribution from the planet’s gravitational binding energy). I defer to those with more training in geophysics than me, but given enough water in the subducted crust I think that is enough for plate tectonics to work, just with a slower spreading rate. If it isn’t, I can handwave by saying that Chott is a bit younger than Earth is or started with a bit more uranium and thorium.

      Re. the rift valley structure:

      Iceland isn’t the best of examples, because it is a mid-ocean ridge intersecting with a mantle plume. The hot spot from the plume is what pushes the volcanism to the point that Iceland ends up above the surface.

      The East African Rift is a better example, but again it isn’t a perfect analogy. There the rift zone is different pieces of continental crust coming apart, rather than relatively dense oceanic crust. Exposing the mid-ocean ridges to the surface of the oceans does change how ocean crust is deposited, but it’s not like anything that happens on Earth. That said, I did lift the East African Rift topography for Chott. In the next post, I’ll be showing a map of a section of Chott’s south temperate zone. There is a volcanic province there, which has some similarities to East Africa.

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