<p>This is an intriguing question, with a solution suggested by the Irwin et al. (2022; <a href="https://arxiv.org/abs/2201.04516" rel="nofollow noreferrer">arXiv:2201.04516</a>) paper that inspired the YouTube, NewScientist, ScienceAlert, <a href="https://phys.org/news/2022-02-explanation-difference-blue-hues-uranus.html" rel="nofollow noreferrer">Phys.org</a>, etc. coverage of the past couple days. Still, I would wait a few years to see if others have alternate ideas before calling this case closed. Also, the paper needs to go through peer review.</p>
<p>Here is the top half of Figure 1 from the Irwin et al. paper:
<a href="https://i.sstatic.net/LEaBD.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/LEaBD.png" alt="enter image description here" /></a></p>
<p>&quot;Human eye&quot; curves are compared with Hubble Space Telescope spectra of the two planets. A couple things to note: Neptune is slightly more reflective where human blue optical cones peak, while Uranus is slightly more reflective where the red cones peak. So this is the color difference that the OP was asking and answering about. These Hubble observations of <a href="https://doi.org/10.1016/j.icarus.2009.02.010" rel="nofollow noreferrer">Uranus</a> and <a href="https://doi.org/10.1016/j.icarus.2010.08.013" rel="nofollow noreferrer">Neptune</a> were already published in 2009 and 2011 based on 2002 and 2003 observations.</p>
<p>What is new from the Irwin et al. study is a model, or theoretical description, of the haze and cloud layers present in both Uranus and Neptune. The paper actually argues for a simple structure of 3 haze/cloud layers on Uranus (plus one additional layer on Neptune), mostly similar between the two planets. Earlier papers assumed all the layers were either hazes (like photochemical smog) or ices (of methane or hydrogen sulfide), but this new model has hazes mixed into all the layers (except for that extra methane-cirrus layer on Neptune).</p>
<p>The &quot;Aerosol-2&quot; layer is the one claimed to account for color differences. In this new model, the Aerosol-2 layer is a little bit absorbing, especially at UV and blue wavelengths, which is a pretty standard assumption for haze layers. But it is located deeper in the atmosphere, where previous studies had called for a methane-ice condensation layer. Other scientists may use different haze/cloud models to describe Uranus and Neptune data. So it would be a good idea to wait and see whether the new Irwin et al. model gets challenged by others. Also it would be great to actually send probes to these planets and see whether any of these models are correct.</p>
<p><strong>In the new Irwin et al. model, the main factors affecting the visible color of the two planets---the differences between the pink and black lines above---are more gaseous methane on Neptune (making Neptune less reflective than Uranus at red wavelengths), and more haze particles in one of Uranus' layers (making Uranus less reflective than Neptune at blue wavelengths).</strong></p>

CC BY-SA 4.0
2024-01-05T07:13:49.090Z
This just in; Phys.org's January 4, 2024 [New images reveal what Neptune and Uranus really look like](https://phys.org/news/2024-01-images-reveal-neptune-uranus.html)
uhoh
CC BY-SA 4.0
2022-02-03T02:17:59.640Z
2022-02-07T17:57:52.920Z
2022-02-07T17:57:52.920Z
giardia
<p><strong>Tl:DR</strong>: According to a team led by planetary physicist Patrick Irwin of the University of Oxford in the UK, an extended layer of haze dilutes the hue of Uranus, resulting in a paler color compared to Neptune</p>
<p><strong>Long answer</strong></p>
<p>Uranus and Neptune has very similar structural and atmospheric composition (A small, rocky core is surrounded by a mantle of water, ammonia, and methane ices and a gaseous atmosphere consisting primarily of hydrogen, helium and methane and a cloud top in upper atmosphere. The atmosphere isn't homogeneous but rather layered). Irwin and colleagues analyzed visible and near-infrared observations of the two planets to generate new models of the atmospheric layers which explained the storms and the dark spots of the planets very well.</p>
<p>In their models, both planets have a layer of photochemical haze. This occurs when ultraviolet radiation from the Sun breaks down aerosol particles in the atmosphere, producing haze particles. They called this layer &quot;Aerosol-2 layer&quot;, and on both planets it seems to be a source of the cloud seeds that condense into methane ice at the lower boundary and snows deeper into the atmosphere but on Uranus, this layer seems to be twice as opaque as it is on Neptune and this is why the two planets look different. According to the paper<sup>2</sup>:</p>
<blockquote>
<p>Since these particles are found to be UV-absorbing, this explains Uranus's lower observed UV reflectivity and also explains why Uranus appears to have a paler blue color to the human eye than Neptune, since these particles are found to have a roughly white visible reflectivity spectrum.</p>
<p>The lower opacity of Neptune's Aerosol-2 layer also explains why dark spots … are easier to observe in Neptune's atmosphere than in Uranus's.</p>
</blockquote>
<p>Below Aerosol-2, there is another layer called &quot;Aerosol-1 layer&quot; which is a deeper haze layer, where the methane re-evaporates and redeposits the haze particles. These haze particles then condense into sub-micron crystals of hydrogen sulfide. The Aerosol-1 is concentrated at certain places which is considered to be the dark spots of Neptune.</p>
<p>Methane concentration also plays a role here. In Uranus, there is 2% methane while in Neptune, there is 1.5% methane in atmosphere and thus more methane absorbs infrared light and reflect blue light.</p>
<p><strong>References</strong></p>
<ol>
<li><a href="https://www.sciencealert.com/we-might-finally-know-why-uranus-and-neptune-are-different-colors/amp" rel="noreferrer">https://www.sciencealert.com/we-might-finally-know-why-uranus-and-neptune-are-different-colors/amp</a></li>
<li>Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark Spots by Patrick G.J. Irwin et.al (arXiv:<a href="https://arxiv.org/abs/2201.04516" rel="noreferrer">2201.04516</a>)</li>
<li><a href="https://www.news18.com/news/buzz/why-is-neptunes-blue-hue-different-from-uranus-scientists-may-finally-have-an-answer-4725674.html" rel="noreferrer">Why is Neptune's Blue Hue Different from Uranus?</a></li>
</ol>

CC BY-SA 4.0
2022-02-03T05:36:53.793Z
Yes, but check Table 1 of that paper. Most of the aerosol layers differ, plus the methane concentration is different by > factor of 2. I think this affects the red part of the spectrum where methane absorption becomes large. This contributes to how "blue" each planet looks.
giardia
CC BY-SA 4.0
2022-02-03T04:52:45.263Z
2022-02-03T08:24:38.313Z
2022-02-03T08:24:38.313Z
Nilay Ghosh
<p>According to <a href="https://phys.org/news/2022-02-explanation-difference-blue-hues-uranus.html" rel="nofollow noreferrer">https://phys.org/news/2022-02-explanation-difference-blue-hues-uranus.html</a> it's because Uranus has a thicker atmosphere than Neptune.</p>

CC BY-SA 4.0
2022-02-03T00:38:00.537Z
see also https://www.newscientist.com/article/2306445-we-may-now-know-why-uranus-and-neptune-are-different-shades-of-blue/
sno
CC BY-SA 4.0
2022-02-03T00:44:23.340Z
and https://www.youtube.com/watch?v=3-RZ9T0ZEZ0
sno
CC BY-SA 4.0
2022-02-03T01:57:04.710Z
Doesn't Jupiter have "a thicker atmosphere" than both, yet it is not bluer? I will post a more detailed summary of the research that phys.org is based on.
giardia
CC BY-SA 4.0
2022-02-11T13:45:50.087Z
This can better be in the comment section
Kavin Ishwaran
CC BY-SA 4.0
2022-02-02T22:08:21.400Z
2022-02-02T22:08:21.400Z
sno
<p>According to a new study the images of Neptune that we usually see are colour-enhanced to pick out features and that actually Uranus and Neptune are pretty much the same colour.</p>
<p><a href="https://phys.org/news/2024-01-images-reveal-neptune-uranus.html" rel="nofollow noreferrer">https://phys.org/news/2024-01-images-reveal-neptune-uranus.html</a></p>

CC BY-SA 4.0
2024-01-05T17:02:46.393Z
2024-01-05T17:02:46.393Z
sno
<p>If Neptune and Uranus have similar composition then why is Neptune a darker blue than Uranus?</p>

CC BY-SA 4.0
2022-02-02T13:58:58.070Z
I imagine temperature matters.
ProfRob
CC BY-SA 4.0
2022-02-02T14:00:09.437Z
Seems to be a duplicate of https://astronomy.stackexchange.com/questions/8778/why-are-gas-giants-colored-the-way-they-are but htis at least invites a more specific answer.
ProfRob
CC BY-SA 4.0
2022-02-02T14:42:39.713Z
Looking at the spectra, it is clearly because Neptune is closer to a Rayleigh-scattering regime in the blue, and has deeper absorption in the red/green than Uranus. But the reason for that is unclear to me.
AtmosphericPrisonEscape
<neptune><uranus><color>
Why is Neptune a darker blue than Uranus?
sno
# Why is Neptune a darker blue than Uranus?

(Asked by: username_0 on 2022-02-02)

If Neptune and Uranus have similar composition then why is Neptune a darker blue than Uranus?

**Tags:** neptune, uranus, color
> **Comments:**
> **username_1**: I imagine temperature matters.  
> **username_1**: Seems to be a duplicate of https://astronomy.stackexchange.com/questions/8778/why-are-gas-giants-colored-the-way-they-are but htis at least invites a more specific answer.  
> **username_2**: Looking at the spectra, it is clearly because Neptune is closer to a Rayleigh-scattering regime in the blue, and has deeper absorption in the red/green than Uranus. But the reason for that is unclear to me.  

---

## 4 answers

### Answer by username_0 on 2024-01-05. Score: 4 | Accepted answer

According to a new study the images of Neptune that we usually see are colour-enhanced to pick out features and that actually Uranus and Neptune are pretty much the same colour.

<https://phys.org/news/2024-01-images-reveal-neptune-uranus.html>


---

### Answer by username_3 on 2022-02-03. Score: 28

This is an intriguing question, with a solution suggested by the Irwin et al. (2022; [arXiv:2201.04516](https://arxiv.org/abs/2201.04516)) paper that inspired the YouTube, NewScientist, ScienceAlert, [Phys.org](https://phys.org/news/2022-02-explanation-difference-blue-hues-uranus.html), etc. coverage of the past couple days. Still, I would wait a few years to see if others have alternate ideas before calling this case closed. Also, the paper needs to go through peer review.

Here is the top half of Figure 1 from the Irwin et al. paper:
[![enter image description here](https://i.sstatic.net/LEaBD.png)](https://i.sstatic.net/LEaBD.png)

"Human eye" curves are compared with Hubble Space Telescope spectra of the two planets. A couple things to note: Neptune is slightly more reflective where human blue optical cones peak, while Uranus is slightly more reflective where the red cones peak. So this is the color difference that the OP was asking and answering about. These Hubble observations of [Uranus](https://doi.org/10.1016/j.icarus.2009.02.010) and [Neptune](https://doi.org/10.1016/j.icarus.2010.08.013) were already published in 2009 and 2011 based on 2002 and 2003 observations.

What is new from the Irwin et al. study is a model, or theoretical description, of the haze and cloud layers present in both Uranus and Neptune. The paper actually argues for a simple structure of 3 haze/cloud layers on Uranus (plus one additional layer on Neptune), mostly similar between the two planets. Earlier papers assumed all the layers were either hazes (like photochemical smog) or ices (of methane or hydrogen sulfide), but this new model has hazes mixed into all the layers (except for that extra methane-cirrus layer on Neptune).

The "Aerosol-2" layer is the one claimed to account for color differences. In this new model, the Aerosol-2 layer is a little bit absorbing, especially at UV and blue wavelengths, which is a pretty standard assumption for haze layers. But it is located deeper in the atmosphere, where previous studies had called for a methane-ice condensation layer. Other scientists may use different haze/cloud models to describe Uranus and Neptune data. So it would be a good idea to wait and see whether the new Irwin et al. model gets challenged by others. Also it would be great to actually send probes to these planets and see whether any of these models are correct.

**In the new Irwin et al. model, the main factors affecting the visible color of the two planets---the differences between the pink and black lines above---are more gaseous methane on Neptune (making Neptune less reflective than Uranus at red wavelengths), and more haze particles in one of Uranus' layers (making Uranus less reflective than Neptune at blue wavelengths).**

> **Comments:**
> **username_4**: This just in; Phys.org's January 4, 2024 [New images reveal what Neptune and Uranus really look like](https://phys.org/news/2024-01-images-reveal-neptune-uranus.html)  

---

### Answer by username_5 on 2022-02-03. Score: 10

**Tl:DR**: According to a team led by planetary physicist Patrick Irwin of the University of Oxford in the UK, an extended layer of haze dilutes the hue of Uranus, resulting in a paler color compared to Neptune

**Long answer**

Uranus and Neptune has very similar structural and atmospheric composition (A small, rocky core is surrounded by a mantle of water, ammonia, and methane ices and a gaseous atmosphere consisting primarily of hydrogen, helium and methane and a cloud top in upper atmosphere. The atmosphere isn't homogeneous but rather layered). Irwin and colleagues analyzed visible and near-infrared observations of the two planets to generate new models of the atmospheric layers which explained the storms and the dark spots of the planets very well.

In their models, both planets have a layer of photochemical haze. This occurs when ultraviolet radiation from the Sun breaks down aerosol particles in the atmosphere, producing haze particles. They called this layer "Aerosol-2 layer", and on both planets it seems to be a source of the cloud seeds that condense into methane ice at the lower boundary and snows deeper into the atmosphere but on Uranus, this layer seems to be twice as opaque as it is on Neptune and this is why the two planets look different. According to the paper2:

> Since these particles are found to be UV-absorbing, this explains Uranus's lower observed UV reflectivity and also explains why Uranus appears to have a paler blue color to the human eye than Neptune, since these particles are found to have a roughly white visible reflectivity spectrum.
>
> The lower opacity of Neptune's Aerosol-2 layer also explains why dark spots … are easier to observe in Neptune's atmosphere than in Uranus's.

Below Aerosol-2, there is another layer called "Aerosol-1 layer" which is a deeper haze layer, where the methane re-evaporates and redeposits the haze particles. These haze particles then condense into sub-micron crystals of hydrogen sulfide. The Aerosol-1 is concentrated at certain places which is considered to be the dark spots of Neptune.

Methane concentration also plays a role here. In Uranus, there is 2% methane while in Neptune, there is 1.5% methane in atmosphere and thus more methane absorbs infrared light and reflect blue light.

**References**

1. <https://www.sciencealert.com/we-might-finally-know-why-uranus-and-neptune-are-different-colors/amp>
2. Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark Spots by Patrick G.J. Irwin et.al (arXiv:[2201.04516](https://arxiv.org/abs/2201.04516))
3. [Why is Neptune's Blue Hue Different from Uranus?](https://www.news18.com/news/buzz/why-is-neptunes-blue-hue-different-from-uranus-scientists-may-finally-have-an-answer-4725674.html)

> **Comments:**
> **username_3**: Yes, but check Table 1 of that paper. Most of the aerosol layers differ, plus the methane concentration is different by > factor of 2. I think this affects the red part of the spectrum where methane absorption becomes large. This contributes to how "blue" each planet looks.  

---

### Answer by username_0 on 2022-02-02. Score: -1

According to <https://phys.org/news/2022-02-explanation-difference-blue-hues-uranus.html> it's because Uranus has a thicker atmosphere than Neptune.

> **Comments:**
> **username_0**: see also https://www.newscientist.com/article/2306445-we-may-now-know-why-uranus-and-neptune-are-different-shades-of-blue/  
> **username_0**: and https://www.youtube.com/watch?v=3-RZ9T0ZEZ0  
> **username_3**: Doesn't Jupiter have "a thicker atmosphere" than both, yet it is not bluer? I will post a more detailed summary of the research that phys.org is based on.  
> **username_6**: This can better be in the comment section  

---

