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Arrakis (also known as "Dune") is an entirely desert planet that has had a sizable population of Fremen who have lived there for generations, in addition to numerous offworlders. Breathing of course converts oxygen to carbon dioxide, and plants are the primary way that carbon dioxide is converted back into oxygen. Yet vegetation is virtually absent on Arrakis.

How is it that Arrakis maintains a breathable atmosphere without vegetation?

Credit to the web video Frank Herbert's Dune - The Dom Reviews for raising this question.

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    It's not canon, so probably not worthy of an answer in itself, but The Dune Encyclopedia attempted to answer this: en.wikipedia.org/wiki/Arrakis#The_Dune_Encyclopedia – tobiasvl Mar 11 '17 at 20:46
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    @tobiasvl - I though of adding it in. There's a whole bunch about the oxygen cycle on Arrakis but it largely boils down to "the worms". – Valorum Mar 11 '17 at 21:43
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Pardot Kynes, the Imperial Planetologist noticed that too.

“Water overshadows the other problems,” Kynes said. “This planet has much oxygen without its usual concomitants—widespread plant life and large sources of free carbon dioxide from such phenomena as volcanoes. There are unusual chemical interchanges over large surface areas here.”

Dune

And many visitors wondered where all the free oxygen comes from.

“The Arrakeen environment built itself into the evolutionary pattern of native life forms,” his father said. “How strange that so few people ever looked up from the spice long enough to wonder at the near-ideal nitrogen-oxygen-CO2 balance being maintained here in the absence of large areas of plant cover. The energy sphere of the planet is there to see and understand—a relentless process, but a process nonetheless. There is a gap in it? Then something occupies that gap. Science is made up of so many things that appear obvious after they are explained. I knew the little maker was there, deep in the sand, long before I ever saw it.”

Dune

Without realising that it was coming from the same place as the spice, from the giant sandworms of Arrakis.

But his inner digestive “factory,” with its enormous concentrations of aldehydes and acids, was a giant source of oxygen. A medium worm (about 200 meters long) discharged into the atmosphere as much oxygen as ten square kilometers of green-growing photosynthesis surface.

Dune


Additionally, there are some plants in the lower latitudes, where the air is (relatively) moistest.

“Are there any plants down there?” Paul asked.
“Some,” Kynes said. “This latitude’s life-zone has mostly what we call minor water stealers—adapted to raiding each other for moisture, gobbling up the trace-dew. Some parts of the desert teem with life. But all of it has learned how to survive under these rigors. If you get caught down there, you imitate that life or you die.”

Dune

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    I guess Kynes eventually found out where oxygen came from first hand. – Kimberly W Mar 11 '17 at 22:39
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    @KimberlyW - There's nothing like an object lesson. – Valorum Mar 11 '17 at 22:43
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    @JanusBahsJacquet: The characteristic smell of old laser printers was caused by the high energies in the printing mechanism producing radical O1 some of which then fuses with O2 to O3. I think. Something like that. Anyway, it's ozone smell. – Jörg W Mittag Mar 12 '17 at 9:39
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    @JörgWMittag - And the smell before it rains. – Valorum Mar 12 '17 at 9:53
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    @Valorum: … and after lightning. Come to think of it, there are quite a number of ozone sources both natural and technological. – Jörg W Mittag Mar 12 '17 at 10:00
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Thought I'd throw in another (non-canonical) explanation for a planet supporting a breathable atmosphere with little vegetation without resorting to giant worms. Check out The Life and Death of Planet Earth: How the New Science of Astrobiology Charts the Ultimate Fate of Our World by Peter Ward and Donald Brownlee for more details.

It boils down to Dune being a planet with an ecosystem on its last legs due to its aging sun getting steadily brighter. Conceivably, Arrakis once supported a climate like the Earth. Assuming Arrakis's sun is a G-type star similar in size to our own sun (not Canopus as described in Dune, which is a giant star likely too young for life to have evolved on Arrakis), over billions of years its luminosity gradually increased as it evolved off of the main sequence. As temperatures rose on Arrakis's surface, weathering scrubbed more CO2 from Arrakis's atmosphere, maintaining a hospitable environment on the surface even as its sun grows brighter. However, at some point the CO2 buffer in Arrakis's atmosphere will be reduced to near zero levels, barely enough to support plants.

This would leave us with a hot desert planet with a breathable oxygen/nitrogen atmosphere, no or very little CO2, and few plants. How long free oxygen would remain in the atmosphere is a good question. If the surface is already completely oxidized and there are no volcanoes producing gases like hydrogen and methane which react strongly with oxygen, an oxygen atmosphere could be stable for millions of years. This scenario doesn't explain what happened to Dune's oceans, unless the planet had very little water to being with.

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