<p>No knowledge on the subject, but I'd venture from looking through that article that it could well be the same reason lighter <span class="math-container">$O_2$</span> has peak concentrations in the troposphere on Earth, while heavier <span class="math-container">$O_3$</span> has peak concentrations in the stratosphere (the troposphere and stratosphere here having fairly similar temperatures) (though the stratosphere has a much higher <span class="math-container">$\theta$</span> (<a href="https://en.wikipedia.org/wiki/Potential_temperature" rel="nofollow noreferrer">potential temperature)</a>).</p>
<p>From that <a href="https://en.wikipedia.org/wiki/Atmosphere_of_Uranus" rel="nofollow noreferrer">Wikipedia article</a>:</p>
<blockquote>
<p>Infrared spectroscopy, including measurements with Spitzer Space
Telescope (SST), and UV occultation observations, found trace
amounts of complex hydrocarbons in the stratosphere of Uranus, which
are thought to be produced from methane by <strong>photolysis induced by solar
UV radiation.</strong> They include ethane (<span class="math-container">$C_2H_6$</span>), acetylene
(<span class="math-container">$C_2H_2$</span>), methylacetylene (<span class="math-container">$CH_3C_2H$</span>), diacetylene (<span class="math-container">$C_2HC_2H$</span>).</p>
</blockquote>
<p>And the ozone layer being in the stratosphere on Earth has the same basic <a href="https://physics.stackexchange.com/questions/681967/why-ozone-is-found-so-high-up-in-atomosphere">explanation</a>.</p>
<p>Now what that doesn't explain is why the heavier hydrocarbons don't wind up concentrating quite a bit in the troposphere as well -- precipitation/mixing/mass stratification all would seem to help increase concentration below.  Ozone here is unstable so that its highest concentration stays in the stratosphere by far... perhaps the complex hydrocarbons likewise react away before they can accumulate in major concentrations below??</p>

CC BY-SA 4.0
2023-09-26T03:42:03.683Z
Good answer. For the first paragraph, [here is an answer I have written in Chem.SE](https://chemistry.stackexchange.com/a/162607/17368). Perhaps, you can research on this topic and expand your third paragraph. "why the heavier hydrocarbons don't wind up concentrating quite a bit in the troposphere" --> this is a really good question and I am curious.
Nilay Ghosh
CC BY-SA 4.0
2023-09-25T01:28:24.483Z
2023-09-25T01:28:24.483Z
JeopardyTempest
<p>Methane is more abundant in the thermosphere <em>and</em> the stratosphere.</p>
<p>On the stratosphere, the Wikipedia article you cite says</p>
<blockquote>
<p>The most abundant stratospheric hydrocarbons after methane...</p>
</blockquote>
<p>Also note that having condensed clouds of a material at one height does <em>not</em> mean that this material is confined to that height.</p>
<p>A further point is that the atmosphere of Uranus is turbulent and mixed. Any vertical stratification in composition is unlikely to be determined just by molecular mass.</p>

CC BY-SA 4.0
2023-09-25T01:33:03.490Z
Having consistently condensed clouds at that height does suggest that there is greater $relative$ concentrations at that level at least, doesn't it?  (Since all saturation values are related to the temperature similarly?)  So they're not confined to the stratosphere... but still relatively higher concentrations there than in the troposphere?
JeopardyTempest
CC BY-SA 4.0
2023-09-25T06:47:35.920Z
@JeopardyTempest yes, I think you are correct. The heavier hydrocarbons are produced there through the action of photolysis.
ProfRob
CC BY-SA 4.0
2023-09-24T09:15:14.570Z
2023-09-24T09:15:14.570Z
ProfRob
<p>I recently answered <a href="https://astronomy.stackexchange.com/q/54739/35206">this question</a>. While researching for the answer, I found one point quite counter-intuitive. The heavier hydrocarbons resides in stratosphere while methane, the lightest hydrocarbon is more abundant in troposphere which is considered the densest layer. <a href="https://en.wikipedia.org/wiki/Atmosphere_of_Uranus#Structure" rel="nofollow noreferrer">Wikipedia</a> says that heavier hydrocarbons like ethane, methane, diacetylene all condense in a narrow layer between 160 and 320 km in altitude which is the colder lower part of stratosphere forming haze layers while methane which is the most abundant hydrocarbon in Uranus reside in the troposphere. Shouldn't it be the other way around - methane in the upper atmosphere while the heavier and denser HCs all condensing in the lower part of atmosphere?</p>

<planetary-atmosphere><uranus>
Why are heavier hydrocarbons not present in lower part of Uranian atmosphere?
Nilay Ghosh
# Why are heavier hydrocarbons not present in lower part of Uranian atmosphere?

(Asked by: username_0 on 2023-09-24)

I recently answered [this question](https://astronomy.stackexchange.com/q/54739/35206). While researching for the answer, I found one point quite counter-intuitive. The heavier hydrocarbons resides in stratosphere while methane, the lightest hydrocarbon is more abundant in troposphere which is considered the densest layer. [Wikipedia](https://en.wikipedia.org/wiki/Atmosphere_of_Uranus#Structure) says that heavier hydrocarbons like ethane, methane, diacetylene all condense in a narrow layer between 160 and 320 km in altitude which is the colder lower part of stratosphere forming haze layers while methane which is the most abundant hydrocarbon in Uranus reside in the troposphere. Shouldn't it be the other way around - methane in the upper atmosphere while the heavier and denser HCs all condensing in the lower part of atmosphere?

**Tags:** planetary-atmosphere, uranus

---

## 2 answers

### Answer by username_2 on 2023-09-24. Score: 5 | Accepted answer

Methane is more abundant in the thermosphere *and* the stratosphere.

On the stratosphere, the Wikipedia article you cite says

> The most abundant stratospheric hydrocarbons after methane...

Also note that having condensed clouds of a material at one height does *not* mean that this material is confined to that height.

A further point is that the atmosphere of Uranus is turbulent and mixed. Any vertical stratification in composition is unlikely to be determined just by molecular mass.

> **Comments:**
> **username_1**: Having consistently condensed clouds at that height does suggest that there is greater $relative$ concentrations at that level at least, doesn't it?  (Since all saturation values are related to the temperature similarly?)  So they're not confined to the stratosphere... but still relatively higher concentrations there than in the troposphere?  
> **username_2**: @JeopardyTempest yes, I think you are correct. The heavier hydrocarbons are produced there through the action of photolysis.  

---

### Answer by username_1 on 2023-09-25. Score: 3

No knowledge on the subject, but I'd venture from looking through that article that it could well be the same reason lighter $O\_2$ has peak concentrations in the troposphere on Earth, while heavier $O\_3$ has peak concentrations in the stratosphere (the troposphere and stratosphere here having fairly similar temperatures) (though the stratosphere has a much higher $\theta$ ([potential temperature)](https://en.wikipedia.org/wiki/Potential_temperature)).

From that [Wikipedia article](https://en.wikipedia.org/wiki/Atmosphere_of_Uranus):

> Infrared spectroscopy, including measurements with Spitzer Space
> Telescope (SST), and UV occultation observations, found trace
> amounts of complex hydrocarbons in the stratosphere of Uranus, which
> are thought to be produced from methane by **photolysis induced by solar
> UV radiation.** They include ethane ($C\_2H\_6$), acetylene
> ($C\_2H\_2$), methylacetylene ($CH\_3C\_2H$), diacetylene ($C\_2HC\_2H$).

And the ozone layer being in the stratosphere on Earth has the same basic [explanation](https://physics.stackexchange.com/questions/681967/why-ozone-is-found-so-high-up-in-atomosphere).

Now what that doesn't explain is why the heavier hydrocarbons don't wind up concentrating quite a bit in the troposphere as well -- precipitation/mixing/mass stratification all would seem to help increase concentration below. Ozone here is unstable so that its highest concentration stays in the stratosphere by far... perhaps the complex hydrocarbons likewise react away before they can accumulate in major concentrations below??

> **Comments:**
> **username_0**: Good answer. For the first paragraph, [here is an answer I have written in Chem.SE](https://chemistry.stackexchange.com/a/162607/17368). Perhaps, you can research on this topic and expand your third paragraph. "why the heavier hydrocarbons don't wind up concentrating quite a bit in the troposphere" --> this is a really good question and I am curious.  

---

