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I'm working on a planet containing an atmosphere composed mainly of sulfur dioxide, with, notably, only trace amounts (if any) of oxygen.

Is such a planet one that could realistically pop up in our universe?

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    $\begingroup$ Hey there, and welcome to WB.SE; That's a nice question you've got there and the idea of having a sulfur dioxide atmosphere is actually intriguing. Sadly your question in its current form does not really fit the QA format on this page, a possible solution would be to split it up into multiple questions, each going more into detail about one aspect. Personally I would start with a question about the feasability of a planet maintaining a sulfur dioxide atmosphere and then create follow-ups after having gotten a sufficient answer :) $\endgroup$ – dot_Sp0T Oct 18 '16 at 9:49
  • $\begingroup$ Oh, sorry! I'll edit out all my follow-ups and save them for later, then :) edit: question has been amended $\endgroup$ – Keira Aro Oct 18 '16 at 9:55
  • $\begingroup$ Please be more specific. What amount of it would be sufficient for you? Why Venus is not what you want in your story? Of course it's not mainly of sulfur dioxide, but amount of it is substantial. $\endgroup$ – Mołot Oct 18 '16 at 10:04
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    $\begingroup$ I suggest to edit your question to include these specifications also! Is it really humans you need to walk he surface? Mutant humans maybe? Also include Biology tag, as this is majorly involving biosphere topics. $\endgroup$ – Sonic Oct 18 '16 at 10:34
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    $\begingroup$ Welcome to the site, Keira. Note that you can reply to another user by using the @<username> syntax, but only one user per comment. $\endgroup$ – Frostfyre Oct 18 '16 at 12:25
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Factors that affect Sulfur occurance

The first factor we have to account for is the relative abundance of elements. First off, in the solar system (pg 24-26) Sulfur is the 10th most common element, so that is a good start. In the earth, the picture isn't as rosy. Sulfur is the 5th most common element, but most of it is in the core and it isn't listed in the most common materials in the crust. So the first challenge is a planet where sulfur is relatively common, and not all locked in the core.

Secondly, various molecules form from the elements due to the relative abundance of other elements. For example, if you had a lot of sulfur on an oxygen poor world, you wouldn't get much sulfur dioxide, but lots of hydrogen sulfide instead. Oxygen is required for sulfur dioxide as well, so your oxygen can't be locked up in other molecules like carbon dioxide and water.

High relative Sulfur concentration

To solve the first problem, we need a mechanism for concentrating sulfur and preventing it from being locked underneath the crust of our planet. Being close to the sun is a good start to removing volatiles. The ice line of our sun being at ~4 AU, things beyond have much more water, carbon dioxide, and ammonia ice than the things nearer. The heat and solar wind of the early sun helped carry those lighter elements outward. While sulfur dioxide is also a volatile, most sulfur is in the form of sulfide and sulfate minerals and would not be burned off in the inner solar system the way that carbon (CO$_2$) or nitrogen (NH$_3$) would be. Fortunately, oxygen (the other contituent of SO$_2$) is also common in various minerals and won't be driven off by a hot young sun.

Sulfur recycling

Now we need a mechanism for keeping sulfur on the surface and not in the core or mantle. Volcanism will do the trick. A volcanically active world would do the trick. Io has surface volcanoes that spew sulfur and sulfur dioxide (although most are basalt). Mercury seems to have some high sulfur regions on the surface deposited by ancient volcanoes.

Life and the Sulfur Cycle

The Earth's atmosphere is regulated by life. Various life forms add and remove oxygen and carbon dioxide to support metabolic pathways.

Life on earth does not generally use Sulfur Dioxide, it uses sulfate and sulfite instead. Sulfate ions (SO$_4^{2-}$) are used by plants and animals for various processes, and sulfate can also be used in sulfate-reducing bacteria to make hydrogen sulfate H$_2$S. However, earth's life formed on a water rich world and we are supposing that this world is low in water (and also low in carbon and nitrogen).

Therefore, you could posit a life form that operates on a sulfur-based life cycle, with different forms of microbial life moving sulfur from elemental or mineralized sulfur to sulfur dioxide and free oxygen, to sulfate ion, and back to minerals. I don't know nearly enough about biochemistry to suggest some metabolic pathways, but I think that you could get some energy storage by swapping minerals back and forth from sulfates to oxides. Micro-organisms could use sunlight energy to convert from Iron (II) Sulfate to Iron (II) Oxide, perhaps.

As result of the above, to support any sort of life cycle, it seems that the other major constituent of the atmosphere would have to be free oxygen.

Conclusion

A volcanically active planet close to the sun would be relatively sulfur rich and carbon and nitrogen poor. If this planet also had life with sulfur-based metabolic pathways, it might end up with a sulfur dioxide-oxygen atmosphere.

Such a planet could be formed as a large (Mercury sized or larger) moon of a 'hot jupiter,' a huge gas giant orbiting close to a parent sun. Tidal squeezing between the planet and star would drive heavy volcanic activity.

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