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Imagine a desertic planet, large dune seas and barren rocky wastes ... Mountains and the like suggesting a past geological activity and even water once exhisted on the surface.

Now though the planet is fully desertic, it has a human breatheable or partially breatheable atmosphere (for brief periods before having to rely on the use of masks breathers).

My question is the following: how could such a planet justify the atmosphere?

My idea was for example large underground water caves, may be some sort of fungi ? Lichen or else producing a sort of reaction to make it breatheable?

Volcanism and thermal activity could be acceptable eventually. The planet possibly should also have a warm or hotter climate at least on the central bands.

Thanks for any answer :).

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    $\begingroup$ It does not look that desert precludes an atmosphere. But you want a breathable atmosphere, right? $\endgroup$
    – Alexander
    Jun 30, 2017 at 17:08
  • $\begingroup$ Breatheable or partially breatheable wich means that humans could stay on surface without a spacesuit , eventually breathing a bit but then would need after a while a breather either for a long term poisonous atmosphere or pherhaps by beeing very rarefact like on high quotes on earth ... $\endgroup$
    – Naima
    Jun 30, 2017 at 17:11
  • $\begingroup$ Mars would quite possibly have been like this at some point in its history. $\endgroup$
    – jamesqf
    Jun 30, 2017 at 17:23
  • $\begingroup$ This link might be useful: Breathable atmosphere on Arrakis $\endgroup$
    – Alexander
    Jun 30, 2017 at 17:46
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    $\begingroup$ Is your question more along the lines of "How can a planet have a breathable atmosphere for humans, without plant life?" $\endgroup$
    – House
    Jun 30, 2017 at 17:59

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There's two ways to do this. First is to explain why the desert planet has a stable atmosphere suitable for humans. Second is to simply say humans happened to show up at a time in the planet's history when it happens to have an atmosphere stable for humans. Depends on what kind of story you're writing.

If It Didn't, There Would Be No Story

If this is a planet that humans chose to colonize, then it wasn't chosen by chance. It was chosen because it happens to have the right circumstances right now to be suitable for human life. And there's enough planets out there that you're going to find at least one of even unlikely configurations.

The atmosphere doesn't have to be stable. While over geological time it might become unsuitable for humans, that's still time for thousands of years of civilization. And humans have proven very good at altering their atmosphere even accidentally.

So, for the purposes of your story, you can lean on the weak anthropic principle which basically says if the environment were not suitable for human life, we wouldn't be there to complain about it. You don't have to explain why it has a suitable atmosphere, the colonists might not even know, it just does. If it didn't, then they would have chosen some other planet with a suitable atmosphere. Even if they didn't choose the planet, even if they crash landed, if it didn't have a suitable atmosphere there would be no story.

With that bit of hand-waving out of the way, let's go into the issues with keeping an atmosphere suitable for humans.

Atmospheric Oxygen Likes To Party

Contrary to the sci-fi trope, you're more likely to find abundant water than you are to find abundant oxygen. Water is a byproduct of many, many exothermic reactions and relatively stable. If you oxidize (ie. burn) many things, you get water and lose O2.

The problem with a nitrogen-oxygen atmosphere is oxygen likes to react with everything; that's why we call it "oxidizing". If your planet, for whatever reason, developed a nitrogen-oxygen atmosphere suitable for humans it would be difficult to maintain it. Over geological time the oxygen would react with any number of things to produce other substances (such as water) and the atmosphere would become unbreathable.

In order to maintain a level of oxygen comfortable to humans, you need some mechanism that continuously replenishes atmospheric oxygen. Or you need humans to show up at just the right geological time, another application of the weak anthropic principle.

On Earth the mechanism for sustaining the atmospheric mix is life. Life is continuously maintaining the atmosphere's careful balance to sustain itself, while also evolving to match long term changes in the atmosphere. Photosynthetic organisms add oxygen, aerobic life removes it. IF you decide your desert planet has life, it would also have to need a nitrogen-oxygen atmosphere.

Making atmospheric oxygen by geological means is a problem. Because it is SO reactive it requires complex chemistry and energy to make it. This is why, for example, it's a by-product of photosynthesis. Plants get free energy from the Sun, use it to get the carbon they need from CO2, and O2 is the byproduct.

Toxins

Humans can withstand large variations in the partial pressure of oxygen, but not of other trace gases. Carbon dioxide, for example. At 0.5% it will cause headaches and fatigue. At 1% you get drowsy. At 7% you die. Since carbon dioxide is a byproduct of many chemical processes, you need something removing it from the atmosphere.

On Earth, this is plant life and other photosynthetic organisms. Photosynthetic organisms remove CO2 (they need the carbon), aerobic organisms add it.

There are non-biological ways to remove CO2. For example, quicklime will react with carbon dioxide to form limestone. Large areas of the necessary minerals could be exposed on the surface absorbing CO2. Some mechanism, perhaps wind & grit perhaps geological uplift, would continuously scrub the surface exposing more fresh material to react with the atmosphere.

CO2 is just one problem. You don't want, for example, too much arsenic in the atmosphere. Or even naturally occurring radioactive elements. Accounting for why each and every one of them isn't present at toxic levels gets a little tiresome.

In sum, unless this is a key part of your story, take the well trod sci-fi road and just don't explain it. There's enough planets in the solar system that humanity will find one desert planet at just the right time in its geological history to happen to have a nitrogen-oxygen atmosphere where the levels of toxic gases aren't too high.

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    $\begingroup$ +1 I like the way you explained the "if not, there would be no story" concept. I feel like that's a struggle in a lot of worldbuilding questions, and of course it's nice to avoid handwavery when possible, but it makes a lot of sense the way you put it. $\endgroup$
    – Jan
    Jun 30, 2017 at 22:21
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Water is not necessary for a planet to have an atmosphere. Planets like Mars have atmospheres without there being water on the planet.

(It is worth noting that Mars' atmosphere is about 100x thinner than Earth's, but that is due to its lack of magnetic field rather than any lack of water. In fact, the article just linked states that its thin atmosphere is the reason all the surface water on Mars dried up.)

Even if you need a human-breathable atmosphere, there is no need for water vapor. As explained in this answer, the atmospheric pressure and relative amounts of different gasses in the atmosphere will have a much greater effect on an atmosphere's breathability than the presence or absence of water will.

As for humans needing drinking water to survive on the planet in the first place, you could have it shipped in from other planets if space travel is available, or have moisture farmers (sort of Luke's aunt and uncle on Tatooine) that mine or gather water from the sparse underground sources you mentioned.

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  • $\begingroup$ A relatively think atmosphere with free oxygen and no hydrogen (in form of water vapor, methane, ammonia etc.) is hard to imagine. $\endgroup$
    – Alexander
    Jun 30, 2017 at 17:49
  • $\begingroup$ @Alexander hard to imagine does not equal impossible. Still, free oxygen without biosphere is probably impossible - at least was never found as far as I know. $\endgroup$
    – Mołot
    Jun 30, 2017 at 17:53
  • $\begingroup$ @Mołot - agree, but I am still looking for a plausible scientific explanation. $\endgroup$
    – Alexander
    Jun 30, 2017 at 17:57
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    $\begingroup$ Note that water is exceedingly easy to make, it's a byproduct of many, many, many chemical reactions. The ingredients are available just about everywhere, hydrogen and oxygen are the most abundant elements in the universe, it just requires energy. On Earth, with abundant fresh water, it often isn't even worthwhile to distill seawater. On a desert planet, it's a lot less energy than shipping it in from off planet, or trying to wring it out of the dry air. $\endgroup$
    – Schwern
    Jun 30, 2017 at 18:05
  • $\begingroup$ @Schwern "it just requires energy" - no. If you have hydrogen and oxygen in mixture, having them separate requires energy. Water creation releases energy, all you need is spark, but on geological scale you don't even need that. $\endgroup$
    – Mołot
    Jun 30, 2017 at 18:09
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High levels of UV radiation from the star around which the planet orbits can de facto sterylize the surface, killing any organism which dares to venture on it.

Life can still prosper in the sea, where few meters of water are enough to shield the dangerous radiation.

Active volcanism, filling the atmosphere with ozone killing molecules, prevent the formation of an effective ozone layer.

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So, you need to create a bunch of oxygen without much surface water. Free oxygen is pretty rare without biology to constantly regenerate it, due to its extreme reactivity, but there are purely geochemical ways to get it. Europa, for example, has a thin atmosphere of mostly oxygen and ozone, due to dissociation of water molecules in Jupiter's radiation environment, and escape of hydrogen in the low gravity.

So, one option would be to posit that the world had some surface water in the past, but it was all photo-dissociated over time with hydrogen lost to space, and oxygen left behind. With a terrestrial planet, rather than an ice moon, however, that raises the question of why, even if there is a breathable excess of oxygen left over after it's reacted with everything there is to react with, there isn't a crushing, Venus-like amount of carbon dioxide, too! But, as long as that was explicitly acknowledged as an unsolved mystery, I'd be fine with it in a story.

A second option, of course, is that you do have some kind of life regenerating atmospheric oxygen despite the lack of liquid water on the surface. Life in water-filled underground caves isn't going to cut it, because where does that life get the energy to generate oxygen from if it's cut off from sunlight? But if you're willing to have water underground, and water vapor in the atmosphere, we can work with that! You could, for example, have plants with extremely deep tap roots that manage to connect these deep underground water supplies to photosynthetic organs (i.e., leaves) on the surface. I'd expect those to be symbiotic with something like fungi that can help with burrowing through the intervening rock, powered by chemical energy from breaking down said rock. You could also have surface organisms with extremely hygroscopic, water-gathering organs that concentrate water vapor out of the air, allowing for the existence of the occasional macroscopic plant and a thin gruel of "sand plankton scattered over the whole surface.

Of course, a third option is just to declare the whole thing a super-interesting unsolved mystery in planetary science. Combined with the previous option, that gives you practically a whole plot all by itself.

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  • $\begingroup$ How abouth thermal energy in underearth water caves? $\endgroup$
    – Naima
    Jul 2, 2017 at 11:02
  • $\begingroup$ Unless the world is so extremely geologically active as to be uninhabitable to humans for that reason alone, I would find it highly implausible that an ecosystem based entirely on thermosynthesis / chemosynthesis could generate enough free oxygen to support human life. $\endgroup$
    – Logan R. Kearsley
    Jul 2, 2017 at 16:13
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Perhaps it could be explained by exotic silicon-based lifeforms, which are able to photosynthesize and release the oxygen without using water.

Or they have actually absorbed all water up and bound it into their rock/crystal bodies. Even on Earth, many "dry" crystals actually contain water, for example opals (consisting fom silica and water) can contain up to 21% of water by mass.

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On earth, the generation of oxygen is dependent on the presence of surface water. In a planet without this abundant water, you will need a different chemical reaction to provide this.

https://en.wikipedia.org/wiki/Chemical_oxygen_generator

Sodium or sotassium chorate mixed with iron will burn and generate oxygen. So a geology on this planet that spews chlorates over red iron rich sands, and creates thousands of square miles of shouldering wastelands at the base of mountain rages is one option.

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