<p>Rainbows occur when sunlight shines through rain. This is rare in the solar system. Rain (of sulphuric acid) might be common enough under Venus's clouds, but there is no sun. Conversely, there is plenty of sun in Mars, but no rain, and only very rare clouds.</p>

<p>It rains on Titan: methane rain. Methane has a lower refractive index than water (1.27 instead of 1.33), which would make the rainbows slightly larger (though not by much 42->52). However the atmosphere of Titan is hazy, and while there is some light on the surface, the sun's disc is not visible.</p>

<p>There is rain in some layers of the gas giants, but again not on the outer layers where the sun is visible.</p>

<p>It is likely that the Earth is the only place in the solar system where rainbows are a common phenomenon.</p>

CC BY-SA 4.0
2019-02-21T21:07:46.590Z
Maybe they are there but we can't see them because the sun, planets outside Earth's orbit and observer is never around that 40degree angle needed to produce a rainbow from the Sun of the atmosphere.
Muze
CC BY-SA 4.0
2019-02-22T02:56:43.113Z
Yes. Earth should be the only place where rainbows are vulgar. Other celestrial bodies should be also able to support rainbows where there is mist or vapor of some chemical, and enough sunlight, but those criteria are rarely met.
Max0815
CC BY-SA 4.0
2019-02-22T16:38:39.307Z
It's not the refractive index which leads to rainbows but rather the *dispersion* (variation of $n$ with wavelength).
Carl Witthoft
CC BY-SA 4.0
2019-02-23T00:14:01.633Z
@CarlWitthoft When dispersion is low (or the spreading is otherwise confounded) there will still be a rainbow, but it will be less colorful; it may stop *dispersing* but it doesn't stop *refracting!* See [What actually happens to reduce the perceived color in a 'white rainbow“ or ”fog-bow"?](https://earthscience.stackexchange.com/q/9180/6031)
uhoh
CC BY-SA 4.0
2021-05-17T05:22:20.190Z
What do you mean by "but there is no sun (in Venus)"?
Nilay Ghosh
CC BY-SA 4.0
2023-04-02T21:04:33.220Z
I would not entirely dismiss Mars. [It snows there](https://www.sfchronicle.com/weather/article/mars-snow-photos-earth-17755425.php), and the snow could possibly disperse light. It may, however, be too sparse and fine to make visible bows.
Oscar Lanzi
CC BY-SA 4.0
2023-04-02T21:16:30.970Z
@NilayGhosh it would be more accurate there is no direct sun on Venus. It's all dispersed. A collimated, linearly traveling light beam, meaning direct instead of dispersed sunlight, is needed to generate a rainbow.
Oscar Lanzi
CC BY-SA 4.0
2019-02-21T19:18:58.297Z
2019-02-22T07:29:06.827Z
2019-02-22T07:29:06.827Z
James K
<p>Take a look at these charts.  The methane one is the best I could find on a quick search, but it suggests the dispersion over the visible wavelength band is a fraction of the value for water.  </p>

<p><a href="https://i.sstatic.net/6waQo.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/6waQo.gif" alt="refractive index"></a></p>

<p><a href="https://i.sstatic.net/ss4AL.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ss4AL.jpg" alt="refractive index"></a></p>

<p>Since the existence of a rainbow depends on the ability of the substance to 'bend' different wavelengths different amounts, you can see that methane, at least, would produce a rather unsatisfying rainbow. And even that assumes that you had an atmosphere which supported methane droplets of an appropriate size to achieve a prismatic effect.  </p>

<p>Roughly speaking, you would want the methane droplets to be larger than the water droplets which produce rainbows on Earth by the ratio of their dispersions.  This is because the angular output spread depends in part on the length of path thru the droplets. </p>

CC BY-SA 4.0
2019-02-22T19:43:23.703Z
Any differences in the range of color in the rainbow? Keep in mind not only the form of rain can produce rainbow. Clouds of Jupiter and other planets can as well.
Muze
CC BY-SA 4.0
2019-02-22T19:44:40.440Z
@Muze  Unless the molecule in question (water, methane or other) has a severely sharp absorption edge, the color range is limited only by our retinal ability to discriminate wavelengths.
Carl Witthoft
CC BY-SA 4.0
2019-02-22T19:46:57.860Z
Yes but doesn't most transparent liquids refract light?
Muze
CC BY-SA 4.0
2019-02-22T23:58:55.767Z
@Muze there's two things here that often get lumped together, and they shouldn't be. While *refract* just means bend, *disperse* means bend different colors differently. If you had rain droplets (or prisms) with *low dispersion* you would still get a rainbow, but it would be white. [What actually happens to reduce the perceived color in a 'white rainbow“ or ”fog-bow"?](https://earthscience.stackexchange.com/q/9180/6031) Carl and many others might be "unsatisfied" by it, but it would still be there, narrower and more concentrated but less colorful.
uhoh
CC BY-SA 4.0
2019-02-23T01:22:01.747Z
@CarlWitthoft I don't not see how droplets *larger than* those on Earth would have any effect on a rainbow's appearance. I think spheres of 1mm and 1cm would perform equally, the ray optics [shown here](https://astronomy.stackexchange.com/a/29692/7982) doesn't require any scale. As long as you are well above the size of a wavelength, it doesn't seem to matter. Can you *cite an authoritative source* explaining your position?
uhoh
CC BY-SA 4.0
2019-02-23T01:22:24.503Z
Also can you cite the sources for those images? Can you show a comparison to water? Check the update to my answer; I've verified JamesK's value of ~1.27 so I don't know how to interpret your data, one at ~1.34 and the other at ~1.45.
uhoh
CC BY-SA 4.0
2019-02-23T11:43:57.630Z
I think both the statements about dispersion and the need to change droplet size are incorrect.
uhoh
CC BY-SA 4.0
2019-02-25T12:30:26.390Z
@uhoh yeah you're partly right - the angular output (not just the translation) depends on the entrance and exit angles more than the droplet size.
Carl Witthoft
CC BY-SA 4.0
2019-02-22T16:49:32.253Z
2019-02-23T02:23:46.060Z
2019-02-23T02:23:46.060Z
Carl Witthoft
<p>Somewhat outside the box is the Jovian moon Io. Direct sunlight us rwadily availabke, albeit only about 4% as ibtense at it would be at Earth's distance. Instead of rain, Io produces sulfur-bearung volcanic gases that condense on the cold environment of the space surrounding Io.</p>
<p><a href="https://i.sstatic.net/g8Ltj.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/g8Ltj.jpg" alt="enter image description here" /></a></p>
<p>That the sulfurous compounds can disperse light is suggested by [Maurizio and Aschauer <a href="https://i.sstatic.net/g8Ltj.jpg" rel="nofollow noreferrer">1</a>](<a href="https://doi.org/10.1088/0957-0233/11/12/310" rel="nofollow noreferrer">https://doi.org/10.1088/0957-0233/11/12/310</a>), who show that sulfur dioxide is about 2% more refractive to violet light than to orange light. The chief limitation appears to be whether the density of the material would be sufficient to make a visible bow.</p>
<p>Source: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. Retrieved from <a href="https://www.space.com/20528-jupiter-moon-io-volcanoes-location.html" rel="nofollow noreferrer">space.com</a>.</p>
<p><strong>Reference</strong></p>
<ol>
<li>Musso, Maurizio &amp; Aschauer, R &amp; Asenbaum, A. &amp; Vasi, Cirino &amp; Wilhelm, Emmerich. (2000). &quot;Interferometric determination of the refractive index of liquid sulphur dioxide&quot;. <em>Measurement Science and Technology</em>. <strong>11</strong>. 1714. 10.1088/0957-0233/11/12/310.</li>
</ol>

CC BY-SA 4.0
2023-04-02T21:45:21.960Z
2023-04-02T21:45:21.960Z
Oscar Lanzi
<p><strong>note 1:</strong> I've verified @JamesK's <a href="https://astronomy.stackexchange.com/a/29688/7982">answer</a>'s index of refraction of 1.27 (since no source was cited), at least for a temperature of 111K, yay! On a colder day, say 90K, the index goes up and the rainbow will shrink a few degrees, close to the size of that on Earth.</p>

<p><strong>Source for methane:</strong> </p>

<ul>
<li><a href="https://refractiveindex.info/?shelf=organic&amp;book=methane&amp;page=Martonchik-liquid-111K" rel="noreferrer">https://refractiveindex.info/?shelf=organic&amp;book=methane&amp;page=Martonchik-liquid-111K</a> which comes from </li>
<li><a href="https://www.osapublishing.org/ao/abstract.cfm?uri=ao-33-36-8306" rel="noreferrer">Optical constants of liquid and solid methane</a> John V. Martonchik and Glenn S. Orton, Applied Optics Vol. 33, Issue 36, pp. 8306-8317 (1994) <a href="https://doi.org/10.1364/AO.33.008306" rel="noreferrer">https://doi.org/10.1364/AO.33.008306</a> </li>
<li>available at <a href="https://www.researchgate.net/publication/47510751_Optical_constants_of_liquid_and_solid_methane" rel="noreferrer">ResearchGate</a> </li>
<li>and <a href="https://trs.jpl.nasa.gov/handle/2014/34442" rel="noreferrer">through NASA</a> thanks to @mistertribs's <a href="https://astronomy.stackexchange.com/questions/29687/how-would-rainbows-appear-on-other-planets/29692?noredirect=1#comment53778_29692">comment</a></li>
</ul>

<p><strong>Source for water:</strong> </p>

<ul>
<li><a href="https://www.osapublishing.org/ao/abstract.cfm?URI=ao-12-3-555" rel="noreferrer">https://www.osapublishing.org/ao/abstract.cfm?URI=ao-12-3-555</a></li>
<li>Optical Constants of Water in the 200-nm to 200-Mm Wavelength Region</li>
<li>George M. Hale and Marvin R. Querry March 1973 / Vol. 12, No. 3 / APPLIED OPTICS 555</li>
</ul>

<p>Now @CarlWitthoft <a href="https://astronomy.stackexchange.com/a/29705/7982">shows two unlabeled plots</a> with no sources cited and very different values for <span class="math-container">$n$</span>.</p>

<p><strong>note 2:</strong> @CarlWitthoft's <a href="https://astronomy.stackexchange.com/a/29705/7982">unsourced claim</a> that methane has a significantly lower dispersion than water in visible light appears to be without merit. I've plotted both materials on the same axis and they are comparable. The rainbows will have slightly different spreading of colors, but I do not thing the rainbow will disappoint!</p>

<p><a href="https://i.sstatic.net/PXLoV.png" rel="noreferrer"><img src="https://i.sstatic.net/PXLoV.png" alt="enter image description here"></a></p>

<hr>

<p>@JamesK's <a href="https://astronomy.stackexchange.com/a/29688/7982">answer</a> mentions that Titan could see rainbows from liquid methane rain.</p>

<p>Using math from <a href="http://www.trishock.com/academic/rainbows.shtml" rel="noreferrer">1</a>, <a href="https://www.physics.harvard.edu/uploads/files/undergrad/probweek/sol81.pdf" rel="noreferrer">2</a>, <a href="https://www.stewartcalculus.com/data/ESSENTIAL%20CALCULUS%202e/upfiles/instructor/eclt_wp_0301_inst.pdf" rel="noreferrer">3</a>:</p>

<p><span class="math-container">$$k = \frac{n_{droplet}}{n_{atmosphere}}$$</span></p>

<p><span class="math-container">$$\alpha = \arcsin\left(\sqrt{ \frac{r-k^2}{3} } \right) $$</span>
<span class="math-container">$$\beta = \arcsin\left( \frac{\sin\alpha}{k} \right)  $$</span>
<span class="math-container">$$\theta = 2\phi = 4\beta - 2\arcsin(k \sin \beta)$$</span></p>

<p><a href="https://i.sstatic.net/RTpfT.png" rel="noreferrer"><img src="https://i.sstatic.net/RTpfT.png" alt="enter image description here"></a> <a href="https://i.sstatic.net/ivQre.png" rel="noreferrer"><img src="https://i.sstatic.net/ivQre.png" alt="enter image description here"></a></p>

<p>Actually, lower index makes the rainbow <em>larger</em>. Remember that red is on the <em>outside</em>. With <span class="math-container">$k=4/3\approx1.33$</span> the rainbow is at ~42°, for <span class="math-container">$k=1.27$</span> it blows up to ~52°. </p>

<p>All else equal it would be a little brighter as well; with a larger incident angle at the back of the drop, the fresnel reflection will be a bit stronger.</p>

<p><a href="https://i.sstatic.net/Fz1y2.png" rel="noreferrer"><img src="https://i.sstatic.net/Fz1y2.png" alt="enter image description here"></a></p>

<p><a href="https://www.stewartcalculus.com/data/ESSENTIAL%20CALCULUS%202e/upfiles/instructor/eclt_wp_0301_inst.pdf" rel="noreferrer">Source</a></p>

<pre><code># https://www.stewartcalculus.com/data/ESSENTIAL%20CALCULUS%202e/upfiles/instructor/eclt_wp_0301_inst.pdf
# https://www.physics.harvard.edu/uploads/files/undergrad/probweek/sol81.pdf
# nice math http://www.trishock.com/academic/rainbows.shtml

import numpy as np
import matplotlib.pyplot as plt

halfpi, pi, twopi = [f*np.pi for f in (0.5, 1, 2)]
degs, rads = 180/pi, pi/180

k = np.linspace(1.2, 1.5, 31)

alpha = np.arcsin(np.sqrt((4.-k**2)/3.))
beta  = np.arcsin(np.sin(alpha)/k)
phi   = 2*beta - np.arcsin(k*np.sin(beta))
theta = 2 * phi

things = (alpha, beta, theta)
names  = ('alpha', 'beta', 'theta = 2phi')
if True:
    plt.figure()
    for i, (thing, name) in enumerate(zip(things, names)):
        plt.subplot(3, 1, i+1)
        plt.plot(k, degs*thing)
        plt.title(name, fontsize=16)
        plt.plot(k[7],  degs*thing[7],  'ok')
        plt.plot(k[13], degs*thing[13], 'ok')
    plt.show()
</code></pre>

CC BY-SA 4.0
2019-02-22T16:40:02.663Z
I can't tell but I think you've missed the important part: water is dispersive (delta $n$ with $\lambda$ ; if methane is not then all wavelengths enter and exit at the same angle, and no rainbow.
Carl Witthoft
CC BY-SA 4.0
2019-02-22T23:04:28.387Z
@CarlWitthoft "...if methane is not (dispersive)..." can you name even one dielectric that isn't? Dispersion in visible wavelengths comes from absorption in the UV and is a pretty universal attribute of collections of atoms. I think you mean "substantially less dispersive than water"
uhoh
CC BY-SA 4.0
2019-02-23T11:41:42.003Z
@mistertribs thank you very much; I've incorporated that into my answer.
uhoh
CC BY-SA 4.0
2019-02-21T23:01:32.570Z
2019-02-23T02:21:30.837Z
2019-02-23T02:21:30.837Z
uhoh
<p>Are other planets capable of producing rainbows? How would those rainbows appear? Can rain, clouds or ice from elements other than water produce rainbows?</p>

<p>Related: <a href="https://space.stackexchange.com/questions/34357/rainbow-space-probe">https://space.stackexchange.com/questions/34357/rainbow-space-probe</a></p>

CC BY-SA 4.0
2023-04-02T23:36:21.227Z
Great question—and great answers! It would be awesome if someone could come up with a graphic simulation so we could actually see how those rainbows would look!
Pierre Paquette
<planet><atmospheric-effects>
How would rainbows appear on other planets?
Muze
# How would rainbows appear on other planets?

(Asked by: username_0 on 2019-02-21)

Are other planets capable of producing rainbows? How would those rainbows appear? Can rain, clouds or ice from elements other than water produce rainbows?

Related: <https://space.stackexchange.com/questions/34357/rainbow-space-probe>

**Tags:** planet, atmospheric-effects

> **Comments:**
> **username_1**: Great question—and great answers! It would be awesome if someone could come up with a graphic simulation so we could actually see how those rainbows would look!  

---

## 4 answers

### Answer by username_5 on 2019-02-21. Score: 11 | Accepted answer

note 1: I've verified @JamesK's answer's index of refraction of 1.27 (since no source was cited), at least for a temperature of 111K, yay! On a colder day, say 90K, the index goes up and the rainbow will shrink a few degrees, close to the size of that on Earth.
Source for methane: 

https://refractiveindex.info/?shelf=organic&book=methane&page=Martonchik-liquid-111K which comes from 
Optical constants of liquid and solid methane John V. Martonchik and Glenn S. Orton, Applied Optics Vol. 33, Issue 36, pp. 8306-8317 (1994) https://doi.org/10.1364/AO.33.008306 
available at ResearchGate 
and through NASA thanks to @mistertribs's comment

Source for water: 

https://www.osapublishing.org/ao/abstract.cfm?URI=ao-12-3-555
Optical Constants of Water in the 200-nm to 200-Mm Wavelength Region
George M. Hale and Marvin R. Querry March 1973 / Vol. 12, No. 3 / APPLIED OPTICS 555

Now @CarlWitthoft shows two unlabeled plots with no sources cited and very different values for $n$.
note 2: @CarlWitthoft's unsourced claim that methane has a significantly lower dispersion than water in visible light appears to be without merit. I've plotted both materials on the same axis and they are comparable. The rainbows will have slightly different spreading of colors, but I do not thing the rainbow will disappoint!


@JamesK's answer mentions that Titan could see rainbows from liquid methane rain.
Using math from 1, 2, 3:
$$k = \frac{n_{droplet}}{n_{atmosphere}}$$
$$\alpha = \arcsin\left(\sqrt{ \frac{r-k^2}{3} } \right) $$
$$\beta = \arcsin\left( \frac{\sin\alpha}{k} \right)  $$
$$\theta = 2\phi = 4\beta - 2\arcsin(k \sin \beta)$$
 
Actually, lower index makes the rainbow larger. Remember that red is on the outside. With $k=4/3\approx1.33$ the rainbow is at ~42°, for $k=1.27$ it blows up to ~52°. 
All else equal it would be a little brighter as well; with a larger incident angle at the back of the drop, the fresnel reflection will be a bit stronger.

Source
# https://www.stewartcalculus.com/data/ESSENTIAL%20CALCULUS%202e/upfiles/instructor/eclt_wp_0301_inst.pdf
# https://www.physics.harvard.edu/uploads/files/undergrad/probweek/sol81.pdf
# nice math http://www.trishock.com/academic/rainbows.shtml

import numpy as np
import matplotlib.pyplot as plt

halfpi, pi, twopi = [f*np.pi for f in (0.5, 1, 2)]
degs, rads = 180/pi, pi/180

k = np.linspace(1.2, 1.5, 31)

alpha = np.arcsin(np.sqrt((4.-k**2)/3.))
beta  = np.arcsin(np.sin(alpha)/k)
phi   = 2*beta - np.arcsin(k*np.sin(beta))
theta = 2 * phi

things = (alpha, beta, theta)
names  = ('alpha', 'beta', 'theta = 2phi')
if True:
    plt.figure()
    for i, (thing, name) in enumerate(zip(things, names)):
        plt.subplot(3, 1, i+1)
        plt.plot(k, degs*thing)
        plt.title(name, fontsize=16)
        plt.plot(k[7],  degs*thing[7],  'ok')
        plt.plot(k[13], degs*thing[13], 'ok')
    plt.show()

> **Comments:**
> **username_4**: I can't tell but I think you've missed the important part: water is dispersive (delta $n$ with $\lambda$ ; if methane is not then all wavelengths enter and exit at the same angle, and no rainbow.  
> **username_5**: @CarlWitthoft "...if methane is not (dispersive)..." can you name even one dielectric that isn't? Dispersion in visible wavelengths comes from absorption in the UV and is a pretty universal attribute of collections of atoms. I think you mean "substantially less dispersive than water"  
> **username_5**: @mistertribs thank you very much; I've incorporated that into my answer.  

---

### Answer by username_2 on 2019-02-21. Score: 9

Rainbows occur when sunlight shines through rain. This is rare in the solar system. Rain (of sulphuric acid) might be common enough under Venus's clouds, but there is no sun. Conversely, there is plenty of sun in Mars, but no rain, and only very rare clouds.

It rains on Titan: methane rain. Methane has a lower refractive index than water (1.27 instead of 1.33), which would make the rainbows slightly larger (though not by much 42->52). However the atmosphere of Titan is hazy, and while there is some light on the surface, the sun's disc is not visible.

There is rain in some layers of the gas giants, but again not on the outer layers where the sun is visible.

It is likely that the Earth is the only place in the solar system where rainbows are a common phenomenon.

> **Comments:**
> **username_0**: Maybe they are there but we can't see them because the sun, planets outside Earth's orbit and observer is never around that 40degree angle needed to produce a rainbow from the Sun of the atmosphere.  
> **username_3**: Yes. Earth should be the only place where rainbows are vulgar. Other celestrial bodies should be also able to support rainbows where there is mist or vapor of some chemical, and enough sunlight, but those criteria are rarely met.  
> **username_4**: It's not the refractive index which leads to rainbows but rather the *dispersion* (variation of $n$ with wavelength).  
> **username_5**: @CarlWitthoft When dispersion is low (or the spreading is otherwise confounded) there will still be a rainbow, but it will be less colorful; it may stop *dispersing* but it doesn't stop *refracting!* See [What actually happens to reduce the perceived color in a 'white rainbow“ or ”fog-bow"?](https://earthscience.stackexchange.com/q/9180/6031)  
> **username_6**: What do you mean by "but there is no sun (in Venus)"?  
> **username_7**: I would not entirely dismiss Mars. [It snows there](https://www.sfchronicle.com/weather/article/mars-snow-photos-earth-17755425.php), and the snow could possibly disperse light. It may, however, be too sparse and fine to make visible bows.  
> **username_7**: @NilayGhosh it would be more accurate there is no direct sun on Venus. It's all dispersed. A collimated, linearly traveling light beam, meaning direct instead of dispersed sunlight, is needed to generate a rainbow.  

---

### Answer by username_4 on 2019-02-22. Score: 1

Take a look at these charts. The methane one is the best I could find on a quick search, but it suggests the dispersion over the visible wavelength band is a fraction of the value for water.

[![refractive index](https://i.sstatic.net/6waQo.gif)](https://i.sstatic.net/6waQo.gif)

[![refractive index](https://i.sstatic.net/ss4AL.jpg)](https://i.sstatic.net/ss4AL.jpg)

Since the existence of a rainbow depends on the ability of the substance to 'bend' different wavelengths different amounts, you can see that methane, at least, would produce a rather unsatisfying rainbow. And even that assumes that you had an atmosphere which supported methane droplets of an appropriate size to achieve a prismatic effect.

Roughly speaking, you would want the methane droplets to be larger than the water droplets which produce rainbows on Earth by the ratio of their dispersions. This is because the angular output spread depends in part on the length of path thru the droplets.

> **Comments:**
> **username_0**: Any differences in the range of color in the rainbow? Keep in mind not only the form of rain can produce rainbow. Clouds of Jupiter and other planets can as well.  
> **username_4**: @username_0  Unless the molecule in question (water, methane or other) has a severely sharp absorption edge, the color range is limited only by our retinal ability to discriminate wavelengths.  
> **username_0**: Yes but doesn't most transparent liquids refract light?  
> **username_5**: @username_0 there's two things here that often get lumped together, and they shouldn't be. While *refract* just means bend, *disperse* means bend different colors differently. If you had rain droplets (or prisms) with *low dispersion* you would still get a rainbow, but it would be white. [What actually happens to reduce the perceived color in a 'white rainbow“ or ”fog-bow"?](https://earthscience.stackexchange.com/q/9180/6031) Carl and many others might be "unsatisfied" by it, but it would still be there, narrower and more concentrated but less colorful.  
> **username_5**: @CarlWitthoft I don't not see how droplets *larger than* those on Earth would have any effect on a rainbow's appearance. I think spheres of 1mm and 1cm would perform equally, the ray optics [shown here](https://astronomy.stackexchange.com/a/29692/7982) doesn't require any scale. As long as you are well above the size of a wavelength, it doesn't seem to matter. Can you *cite an authoritative source* explaining your position?  
> **username_5**: Also can you cite the sources for those images? Can you show a comparison to water? Check the update to my answer; I've verified JamesK's value of ~1.27 so I don't know how to interpret your data, one at ~1.34 and the other at ~1.45.  
> **username_5**: I think both the statements about dispersion and the need to change droplet size are incorrect.  
> **username_4**: @username_5 yeah you're partly right - the angular output (not just the translation) depends on the entrance and exit angles more than the droplet size.  

---

### Answer by username_7 on 2023-04-02. Score: 0

Somewhat outside the box is the Jovian moon Io. Direct sunlight us rwadily availabke, albeit only about 4% as ibtense at it would be at Earth's distance. Instead of rain, Io produces sulfur-bearung volcanic gases that condense on the cold environment of the space surrounding Io.

[![enter image description here](https://i.sstatic.net/g8Ltj.jpg)](https://i.sstatic.net/g8Ltj.jpg)

That the sulfurous compounds can disperse light is suggested by [Maurizio and Aschauer [1](https://i.sstatic.net/g8Ltj.jpg)](<https://doi.org/10.1088/0957-0233/11/12/310>), who show that sulfur dioxide is about 2% more refractive to violet light than to orange light. The chief limitation appears to be whether the density of the material would be sufficient to make a visible bow.

Source: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. Retrieved from [space.com](https://www.space.com/20528-jupiter-moon-io-volcanoes-location.html).

**Reference**

1. Musso, Maurizio & Aschauer, R & Asenbaum, A. & Vasi, Cirino & Wilhelm, Emmerich. (2000). "Interferometric determination of the refractive index of liquid sulphur dioxide". *Measurement Science and Technology*. **11**. 1714. 10.1088/0957-0233/11/12/310.


---

