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I'm working on a sci-fi Western scenario on a habitable desert planet along the lines of Tatooine from Star Wars and Arrakis from Dune.

For it to be habitable there has to be water below the surface of course and there has to be at least some precipitation to make a sustainable use of underground water reserves possible. I imagine people will additionally harvest water using atmospheric water generators also as part of their sustainability strategy. I think this is what George Lucas had in mind when he made Luke Skywalker's parents on Tatooine "moisture farmers".

Now because my narrative is a Western where people use technology available in America in the 1870s you would need fossil fuels close to the surface, coal most prominently. So the planet can't always have been covered by desert but needs to be stable in that form now.

Can you give me some ideas how I would explain that there's large deposits of fossil fuels when the planet is a desert and has been for a long time? I know that a desert can be where there were forests millions of years before when talking about Earth but I'm not sure if it's that simple when talking about about a desert planet. This gives some insight regarding the presence of coal and oil but it doesn't provide an adequate answer when these conditions are taken into account. I would need an at least semi-scientific explanation.

Regarding the questions of how men got there and why their technology doesn't evolve from what was available during the latter part of the 1800s in America see this and this.

H3R3T1K
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  • Why would it not be? Just declare it straight up: there are no conductive metals on the surface that can be extracted in a manner that is worth the effort. Do not bother trying to explain it. Just say that it is so. You have that prerogative as the author. – MichaelK Sep 04 '17 at 09:25
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    @MichaelK Sure, one can state something like that by author fiat. But it'll probably have ripple effects on other things, too. That's what we're here to do: Help people build their worlds such that they are internally consistent. Discussing what might cause a specific situation can help the author make sure that their world is internally consistent, even if it doesn't end up actually laid out within the story. Compare my discussion on desk phones and orange juice falling through desks. – user Sep 04 '17 at 09:31
  • I changed the question so as to be more precise. – H3R3T1K Sep 04 '17 at 09:36
  • Even though it's not exactly the same as what you are asking, you may still be interested in Creating a realistic world map - Fuel. – user Sep 04 '17 at 09:38
  • Both Mars and Venus had water once and they lost it. However, in both cases the transformation was much more extreme than getting a desertic climate: one froze, one burnt. – Rekesoft Sep 04 '17 at 10:00
  • How bout Pangaea. – user6760 Sep 04 '17 at 10:06
  • Classical case of Star Wars syndrome, you even mention it. A planet is huge and George Lucas tried to recreate his childhood with SW, even intellectually: "one planet = one thing". The answer is of course "no". But one can always live inside some structures. "Large" is, as always, relative. Since all oceans must have evaporated slower than on Mars for complex life to evolve, so the planet is way older than Mars and slightly larger. A comment on your time scale: For thousands of years is basically no time for oceans to evaporate. We are dealing with hundred of millions of years here – Raditz_35 Sep 04 '17 at 10:16
  • While coal can't be formed by geological processes, other petrochemicals can be. Titan (Saturn's moon) is covered in lakes and rocks that on earth would be"fossil fuels" – pojo-guy Sep 04 '17 at 10:59
  • There's gotta be water below the surface for it to be habitable. I know I'm asking for a lot. I just need to know if a planet like that is possible in theory. The time it took to become that way actually doesn't matter. – H3R3T1K Sep 04 '17 at 11:23
  • Where is the whole "conductive metals" topic coming from? Can't find it in the question... – R.. GitHub STOP HELPING ICE Sep 04 '17 at 17:13
  • You find it in the second link at the end. It's basically a requirement from another question that I didn't wanna mix up in here for fear of confusion. Amadeus answered this after answering the other question so he knew. – H3R3T1K Sep 04 '17 at 18:34
  • @H3R3T1K You can contact me via the chat room I set up; https://chat.stackexchange.com/rooms/65172/h3r3t1k-and-amadeus-chat. I won't contact you via email; I use a pseudonym for reasons I learned the hard way. Ten years ago and nothing to do with stackexchange, but now just a precaution I don't violate. – Amadeus Sep 06 '17 at 11:08

2 Answers2

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Well, to make a desert planet habitable, you will need oxygen, unless your 1870's cowboys live in domed towns, which seems incongruous. Humans need to breath oxygen! You will also need water (could be subsurface) and crops: Which means whatever factor killed off all the plants (including the forests turned coal) must no longer exist, or you couldn't grow anything new.

If you want the biological cause of the non-conductive metals being present, they will need water, too. Bacteria aren't a dry collection of sticks; they need water as their primary solvent.

Some of those problems I cannot solve in this post; perhaps I will have an idea later. The oxygen problem can be solved; however.

Until recently, it was believed oxygen would have to be a sign of life on an exo-planet; for example see this link on Signs of Life. An excerpt follows:

What would be a real biosignature? Many scientists think that abundant molecular oxygen, or its product, ozone, is a strong one because on Earth molecular oxygen is produced mostly by the photosynthesis of plants. The simultaneous detection of water, carbon dioxide, and oxygen or ozone would be a strong indicator that biological processes are occurring on a planet.

Also, the detection of large amounts of oxygen and a gas like methane or nitrous oxide would be a strong signature, because on Earth these gases are produced almost entirely by biological processes.

Also this Scientific American link: The Origin of Oxygen in Earth's Atmosphere.

It's hard to keep oxygen molecules around, despite the fact that it's the third-most abundant element in the universe, forged in the superhot, superdense core of stars. That's because oxygen wants to react; it can form compounds with nearly every other element on the periodic table. So how did Earth end up with an atmosphere made up of roughly 21 percent of the stuff?

The answer is tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen.

Which would mean a large percentage of the surface of your planet must be covered in photosynthesizing plants (algae, moss, plankton); otherwise all the highly reactive free oxygen in the atmosphere will collide with stuff (including other atmospheric gases and the ground) and be bound to it, there will be none left within a century (according to one source I read years ago but can no longer find).

However, recent studies show alternatives are possible:

The new research [by Norio Norita] shows that the interaction of titanium oxide [also called titania] with water could produce oxygen in the atmosphere of an exoplanet without the involvement of living organisms.

Fortunately for you, titanium is a low-conductivity metal; and the article goes on to say that on an earth-like planet, somewhere between 0.05% and 3.0% of the surface would need to be covered in the stuff to produce the same level of oxygen as we have on Earth (that would be 100,000 to 6,000,000 times the surface titanium dioxide found on Earth).

Titania must interact with water to produce free oxygen; I would recommend the water and titania be in deep reservoirs.

Killing the forests.

[edit]This is difficult to do on a temporary basis; I'd suggest orbital mechanics.

See this article on the formation of the Sahara: How Earth's Orbital Shift Shaped The Sahara.

The changes in the Earth’s orbital tilt and precession (or the wobbling motion) occur because of gravitational forces emanating from other bodies in the solar system. To understand exactly what happens, picture a spinning top when it is slightly disturbed. Just like a top, the Earth too wobbles slightly about its rotational axis. This tilt changes between roughly 22 and 25 degrees about every 41,000 years, while the precession varies on about a 26,000-year period. These cycles have been determined by astronomers and validated by geologists studying ocean sediment records.

So you have on Earth a 41,000 year cycle going on. This changes the insolation distribution of the Earth (yes that is spelled correctly, it is not "insulation", it is "insolation", which is a scientific term meaning the amount of solar radiation reaching the surface of a planet).

A minor change (half a degree) in orbital tilt turned a grassland into the Sahara desert by increasing the amount of sunlight to a point that plants simply couldn't handle it anymore; apparently the tipping point was about 6000 years ago (the approximate time frame you requested).

Your planet does not have to be Earth, of course; your cycle can be longer or shorter than 41,000 years: That is a peculiarity of our OTHER planets and their sizes and orbits. (you will need some other big planets to pull this off, to provide the necessary gravitational influences, but they don't have to be habitable, just like Earth's fellow planets are not habitable. I wouldn't bother trying to work out the math either, just that they exist and cause the axial tilt on a cycle: that's plausible.)

A second effect can be the presence of mercury-chloride and/or sodium-chloride (aka table salt) in large swaths of the planet, both of which are toxic enough to plants to cause desertification.

So the scenario is this: the South half of this damn planet is a natural desert due to salts. The North half can support plant life, but due to axial tilting in the orbit (due to resonances caused by some Jupiter sized gas giants in the system), on a cycle of about a hundred thousand years, the North half is exposed to extreme sunlight for a period of about ten thousand years. But that same axial tilting shifts underground water into titania deposits that react to produce oxygen; and this seeps up to sustain life on the planet. The plants have adapted, over billions of years, to produce seeds that can survive the drought. They won't sprout until the temperature has been hospitable for a few years, and the axial tilt has made the underground water available to them again. This is a cycle that has been going on for billions of years; and that is where the coal deposits come from.

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Amadeus
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  • You have me in awe again, Amadeus! Too bad I cannot contact you personally on this site to thank you. So the North would be semi-arid? People could harvest seeds in the North and grow them in the South even when the North gets "scorched" using underground water and water harvested from the air. What temperatures are we talking about in the North during the "scorching"? Could people living there move underground and survive or would they move South? It'll be fun coming up with animal species to populate the desert. I'm guessing they'll be similar to prominent desert dwelling animals on Earth. – H3R3T1K Sep 04 '17 at 12:46
  • This is a great answer,, gives me fantastic ideas (well, that's debatable, but they're ideas at least) for my universe. – Doomed Mind Sep 04 '17 at 13:56
  • @H3R3T1K No, the South is desert due to salting, nothing will grow there (unless humans remove the salt and grow crops in raised beds; that could happen, with a source of water). The North is where things can naturally grow for 90,000 out of 100,000 years: but they are currently halfway through a 10,000 year period in which daytime temp is 120F plus and all plants are gone. People and their crops must seek the shade (about 100F): Caves, buildings, etc. Simple evaporative water cooling would still work and lower temps another 15F; steam engines can provide circulation of air and water for that. – Amadeus Sep 04 '17 at 13:58
  • @H3R3T1K I *would* grow crops in the cooler south, but you can't transport just the seeds: You must transport the unsalted soil, too. Read about French Intensive Farming: https://en.wikipedia.org/wiki/French_intensive_gardening in raised beds of special soil, requires less water. There are easy (non-electrical) ways to distill water (from wells) to eliminate the salt; just steam distillation which your people should be expert in considering their high dependence on steam engines. Unlike 1870, they would capture 100% of their expelled steam as clean distilled water. – Amadeus Sep 04 '17 at 14:04
  • But underground water would be available and accessible in both north and south? So people could live in both areas if they transport the soil from the north to the south? How much cooler would the south be? – H3R3T1K Sep 04 '17 at 14:18
  • @H3R3T1K Sure, I should think so. But if the South has a lot of salt (even normal ocean salt which is our table salt), the ground water there would probably be salty; the underground aquifers could be salt water. But if you can use coal (perhaps imported) to boil and steam distill it, easily within the scope of 1870, then you have drinking water; and waste water (urine and other forms) is fine for crops. You wouldn't want to keep big open bodies of water; keep it covered or in pipes (metal or ceramic) to prevent evaporation. – Amadeus Sep 04 '17 at 15:25
  • @H3R3T1K I would also add that lenses were already well researched and known in 1870; there is no reason you cannot have glass lenses. In a desert, this offers you an alternative source of fuel: Solar power. A large lens (or a grid of smaller lenses) can be focused on a point to boil water for steam distillation; with a ready water supply, a lot of "moonshiner's stills" can be set up for a continuous, unattended way to produce hundreds of gallons of distilled well water daily. This could even power a small steam engine to pump the water up from the well. Have to clean out salt periodically. – Amadeus Sep 04 '17 at 18:48
  • Very interesting. I don't know how a machine for pumping water using steam produced with glass lenses would function in detail but I can certainly mention that people do it which will add depth. Thank you! – H3R3T1K Sep 04 '17 at 20:45
  • The lenses focus on a boiler that produces steam. The sun is the heat source. A big enough area focused on a point can melt iron; but you just want to get a lot of water to about 400F to run a steam engine. The lens array has to move like clockwork with the sun; but you have weight-driven clock tech (grandfather clocks, cuckoo clocks). The steam engine produces rotary motion which is transferred by long steel rod to a pump at the bottom of a well, to drive rotors. See the link for how the pump works; I'd suggest the 4th vane pump: 1st patent in 1874. https://www.youtube.com/watch?v=plxFeszbQD0 – Amadeus Sep 04 '17 at 21:12
  • P.S. Or rotary motion could be transferred by a chain drive, you have that tech too (like a bicycle chain on gears; but heavier duty). – Amadeus Sep 04 '17 at 21:20
  • @H3R3T1K The south could be pretty tolerable; in the 90F range I think. It is a desert because of the salt, not the temperature (and we have some cold deserts on Earth: http://www.swifty.com/destinations/4527/the-10-coldest-deserts-across-the-world/ ). We might think that is uncomfortable, but it less than normal body temperature and in the range of continuously survivable. Certainly in 1870 they soldiered through their summers. – Amadeus Sep 04 '17 at 23:16
  • What would be the day/night difference be like in the north/south? – H3R3T1K Sep 04 '17 at 23:23
  • Have to cut this short lest be we moved to chat. Look at axial tilt on Earth, for example USA vs Australia, day length in Summer/Winter, and temp differential between day/night in summer & winter. You don't have to stray from that if your story doesn't demand it. Nightly low can be 30F cooler than daily high, for example. The oblique angle of sun for the tilted-away South means less radiation hits the South, so colder, but *light* still gets there. days are shorter but not terribly shorter if not too close to the poles. See https://www.timeanddate.com/calendar/june-solstice.html for a pic. – Amadeus Sep 04 '17 at 23:50
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How hot do need the world to be? The greenhouse effect is a balancing act, if you had ancient forests that stripped the CO2 out of the atmosphere and deposited it in huge coal measures then the trees could be their own worst enemy. If you dropped CO2 far enough, early enough you could wipe out complex plants before grasses and flowers ever evolved leaving huge carbon deposits and a cold desert world with most of it's water frozen in the higher latitudes, like an ice age on steroids. There's still going to be ground water many places and basic plants, algae, on/near the surface harnessing a little sunlight and the night dew to keep the world cold and dry and highly oxygenated. Such a world, assuming it was Earthlike to start with, is going to have more oxygen than in the Carboniferous, but with a massively diminished food chain budget. Pumping bulk CO2 into the atmosphere by burning that coal is going to have effects over the very short term geologically speaking but won't do much on human timescales.

Ash
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  • Well I want the place to be pretty in a rough way as most deserts are but pretty hard on humans. It'll be Death Valley during the day (I'm guessing over 40 and up to 50 C) and quite cold during the night (minus 5 to 10 C). People will have adapted and won't move about outside much during the day and stay indoors except for early morning and evening. Their clothing will fit the climate. There'll be edible wild life that has adapted and is partially hunted for food. – H3R3T1K Sep 04 '17 at 16:24
  • Okay my technique isn't really going to work then. Much colder average temperature and especially the overnight is going to be really low. – Ash Sep 04 '17 at 17:02