There’s plenty of carbon dioxide and radiation throughout the solar system, why is it a phenomena only associated with the outer planets?
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1$\begingroup$ French Wikipedia page says tholins only form in reduced conditions, which is why they are not formed on Earth. But there is no references to this claim, and it is absent from the English version, so take it with a pinch of salt... $\endgroup$– Jean-Marie PrivalCommented Sep 30, 2022 at 7:53
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2$\begingroup$ More is involved than requiring reducing conditions. You need methane to make tholins, and for reasons that vary among different bodies not much is there in the inner solar system. $\endgroup$– Oscar LanziCommented Sep 30, 2022 at 20:41
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$\begingroup$ Also, see the top answer, by @Andrew_Jon_Dodds to the question Are fossil fuels really formed from fossils?, from 2017. $\endgroup$– FredCommented Oct 25, 2022 at 18:01
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1 Answer
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Tholins are primarily polymerized hydrocarbons, and the key raw material for forming them is methane, not carbon dioxide. For a combination of physical and chemical reasons noted below, there is just not much methane to be had on the major orbiting bodies of the inner Solar Syatem.
Too hot to handle
This chart shows the combination of escape velocities and temperatures at the 1-bar pressure level on various Solar System bodies, along with bands showing the lightest gases that can be retained.
Among the five major orbiting bodies in the inner Solar System, our Moon and Mercury appear far down the chart where only a gas as heavy as xenon (or possibly sulfur dioxide, which is not shown) may be retained; methane would he light enough to escape given the limited gravity and relatively warm temperature of these bodies. Mars, which is a bit more massive, comes closer, able to retain nitrogen (which is found as a minor component of its atmosphere); given the likely uncertainty in the chart maybe it could retain methane. However, the methane spikes seen on Mars are just that -- temporary, localized spikes that do not add up to a significant long-term global presence. Either the methane is escaping or it is being consumed promptly, which brings up the second barrier against methane presence and tholin formation in the inner Solar System.
Burning up
Earth and Venus have enough gravitational heft to retain methane at their 1-bar levels, and Mars is close to the borderline ... if the local chemistry is favorable. However, methane is prone to oxidation, and there is evidence that the methane is consumed by this reaction on all three bodies that might otherwise retain it.
Going back to Mars, discussed above, that planet has little or no elemental oxygen in its atmosphere. However, mineral evidence indicates oxidizing conditions on Mars. This includes most of the iron present in these minerals being oxidized to the +3 oxidation state, as in the iron oxide that reddens its surface and other iron(III) minerals found by rovers on Mars. In addition, nitrogen has been found on an oxidized state as nitrates in Martian rock.
Earth is more straightforward. Even without burning as in a flame, methane is destroyed over time by oxygen in the atmosphere, its carbon ending as carbon dioxide.
Which leaves Venus. Sulfuric acid and associated sulfur oxides also oxidize methane in the atmosphere; again most of the carbon is converted to carbon dioxide (there is a little carbon monoxide) and methane is not reported even at parts per billion levels. Sulfate salts can also be oxidizing to organic matter and are actually used in this way by some anaerobic bacteria on Earth.
