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In A Very Short Introduction: Earth System Science by Tim Lenton, the author discusses the composition of atmosphere:

the first observations from land-based telescopes showed that Mars had an atmosphere dominated by carbon dioxide, just as would be expected in the absence of life. So too does Venus. But the Earth has a remarkable atmosphere, containing a chemical cocktail of highly reactive gases, sustained by life.

Oxygen is the prime anomaly—at just over a fifth of Earth’s atmosphere it is essential for our existence as mobile, thinking animals, but without photosynthesis to create it, oxygen would be a very rare trace gas. Mixed in with oxygen are gases like methane that react eagerly with it—so much so that they are on the verge of combusting together.

Why is oxygen an anomaly in this context?

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    $\begingroup$ I’m voting to close this question because it's both hardly on topic and pretty much already answered in citation in the question itself. $\endgroup$
    – Mithoron
    Jul 4 at 12:49

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Because oxygen is so reactive it won't be present in a planetary atmosphere unless there is a source continuously producing it

In the context of things found in planetary atmospheres, oxygen is a major anomaly (or, to put this another way, the earth has a very unusual atmosphere).

The reason is simple. Oxygen is so reactive that an atmosphere can only contain a significant amount if there is something continuously producing large amounts of it. On earth, that is life or more specifically living things that photosynthesise.

If photosynthesis stopped on earth it is estimated that there would be about enough oxygen to last only 20,000 years (based on the estimated rate of photosynthesis which is broadly in balance with the rate of consumption). Even if all the air-breathing animals died the oxygen would get used up pretty quickly by other processes like burning, rusting and geological weathering.

The atmospheres of planets typically depend on geological processes. But though some of these can produce some oxygen, there are many, faster, processes that use it up. So we don't expect to see free oxygen in a planetary atmosphere where the planet has no life.

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  • $\begingroup$ The conclusion may be true on "planets", but not on some other bodies. See my remark about Ganymede. $\endgroup$ Jul 4 at 16:46
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    $\begingroup$ The OP was reading the citation literally and maybe thinking that oxygen was the anomaly. The real anomaly is the atmosphere of the Earth among those of other planets. Oxygen is simply the indicator of this anomaly. I don't get what the citation says about methane being in almost explosive proportions to oxygen in our atmosphere. In fact the proportion of methane in air is 1.7 ppm. Surely a much bigger proportion is needed for combustion ? $\endgroup$
    – Trunk
    Jul 4 at 17:11
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    $\begingroup$ @OscarLanzi That is a fair point. OTOH the "atmosphere" of ganymede is so tenuous it was once thought to be nonexistent. So it is something of a edge case: a body with enough water and surface radiation to yield oxygen by photochemical splitting. But in very, very small amounts. $\endgroup$
    – matt_black
    Jul 4 at 17:17
  • $\begingroup$ @Trunk An atmosphere with a lot of oxygen is the anomaly (so i'm not sure the phrasing of the quote is wrong). But notable amounts of methane is also an anomaly (doubly so in an oxygen rich atmosphere). Earth methane is nowhere near the explosive limit but there are plenty of reaction pathways that leave it with a very short atmospheric lifetime. Notably, methane on Mars is an anomaly (if measurements are accurate) as it is destroyed rapidly even in the absence of oxygen and most known processes making it are biological. $\endgroup$
    – matt_black
    Jul 4 at 17:23
  • $\begingroup$ @matt_black Titan's atmosphere is made mostly of methane - it's not so unusual. $\endgroup$
    – Mithoron
    Jul 21 at 22:34
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For the reasons mentioned in the quotation, oxygen -- unlike other major atmospheric components like nitrogen, argon or carbon dioxide -- cannot survive from primordial times and must be created by some active, ongoing process.

On Earth that process is, of course, life. But on some other worlds there are alternatives, namely spalling of oxygen atoms off rock or ice by ultraviolet radiation. Jupiter's moon Ganymede, with its water-ice surface, has a thin oxygen-bearing atmosphere.

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Oxygen is not an anomaly when talking about "reactive gases" in the atmosphere as the author clearly mentions that oxygen is also highly reactive. In fact there are less reactive gases (nitrogen) present. I think the author means to say that oxygen is the anomaly when comparing atmospheric composition of planets. Other planets will contain all kinds of gases (but not oxygen). Here, it says that planetary atmospheres consist of many gases including carbon dioxide, methane, ammonia, and hydrogen and helium on distant planets. The Earth has a unique atmosphere of oxygen which helps sustain life.

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