<p>No, the clarity of the Earth's atmosphere cannot be considered unique. We don't have to speculate about exoplanets.</p>

<p>You could argue the answer is no, because both the Moon and Mercury have (very, very) thin atmospheres, and these are obviously "clear".</p>

<p>If you regard that argument as tricksy, then we can turn to Mars. Yes Mars has occasional dust storms. In normal conditions, the optical depth of the Martian atmosphere is usually somewhere between 0.5 and 1 per airmass. (<a href="http://www.sciencedirect.com/science/article/pii/S0032063311002960">Petrova et al. 2012</a>; <a href="http://arxiv.org/abs/1403.4234">Lemmon et al. 2014</a>). Most of this extinction is caused by dust and is nearly wavelength independent. i.e. between 60% and 37% of light would travel through it's atmosphere from outside. This compares with typical extinctions of about  0.2-0.4 <em>magnitudes</em> of visual extinction per airmass on Earth (0.1 mag at the best astronomical sites in the world), corresponding to 80% to 69% of light passing through the Earth's atmosphere from outside (to sea level). Most of this extinction is due to dust, though there is some absorption by water and other aerosols).</p>

<p>Thus, though Mars is dustier than Earth on average, it is not outrageously so. It would be stretching the use of the word unique to say that the clarity of the Earth's atmosphere was "unique".</p>

CC BY-SA 3.0
2015-11-18T09:49:05.883Z
2015-11-18T09:49:05.883Z
ProfRob
<p>Our atmosphere is only transparent to visible light, In most other wavelengths, some or all of the light is absorbed </p>

<p><a href="https://i.sstatic.net/wvref.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/wvref.png" alt="Image from wikipedia"></a> 
<a href="https://en.wikipedia.org/wiki/Optical_window#/media/File:Atmospheric_electromagnetic_opacity.svg" rel="nofollow noreferrer">Image from Wikipedia</a>, adapted from image by NASA</p>

<p>Our eyes have evolved to take advantage of the transparency at these wavelengths. If we had evolved in an atmosphere with a very different mix of gases. One in which visible light was absorbed, we would have evolved eyes that see different wavelengths.</p>

<p>There is a notable "window" at about $10\mu m$ in the diagram above. And you might wonder if any animal has evolved to see in this window. However, <a href="https://en.wikipedia.org/wiki/Black-body_radiation#Human%20body%20emission" rel="nofollow noreferrer">our own bodies emit thermal radiation at about $10\mu m$</a>, eyes wouldn't work as they would be swamped by their own glow. However this window is used by thermal imaging devices.</p>

CC BY-SA 3.0
2015-11-17T23:43:19.477Z
A very nice answer.   I might add, more intuitively and anything else that Earth's rain keeps it's soil moist and helps keep large dust clouds from forming, though they still do happen from time to time in dry locations.     Trees and plants obviously help keep the Earth from being "dusty" too.   A planet with liquid water and water rain that reaches the surface is interesting to us for many reasons.   Earth is the only one like that in our solar-system.  Venus has sulfuric acid rain, but it doesn't reach it's surface.
userLTK
CC BY-SA 3.0
2015-11-18T01:15:55.253Z
Not really clear how this answers the question. Has the OP been edited?
ProfRob
CC BY-SA 3.0
2015-11-18T01:24:06.643Z
@RobJeffries I think this answers the question very clearly, what's the issue?
Etheryte
CC BY-SA 3.0
2015-11-18T02:37:44.307Z
It could be spelt out more explicitly that our eyes have not evolved for the purpose of seeing through the low opacity part of the spectrum for other worlds, whatever that night be. What would be interesting is whether the absorption spectrum plots for worlds other than Earth are available.
Keith
CC BY-SA 3.0
2015-11-18T08:34:16.860Z
One should mention that the only reason why we call those specific wavelengths "visible light" and the others "invisible" is because our eyes have evolved to see in that spectrum. IOW: "visible light" is not a property of the light, it's a property of our vision. Had we evolved on a different planet with a different atmosphere and a different absorption spectrum, our eyes would be sensitive to different wavelengths, and we would call *that* spectrum "visible".
Jörg W Mittag
CC BY-SA 3.0
2015-11-18T09:30:09.917Z
@Nit The question very clearly asks about the amount of dust in the atmosphere, not its chemical composition.
David Richerby
CC BY-SA 3.0
2015-11-18T11:42:24.327Z
Please add a source attribution far thot image.
gerrit
CC BY-SA 3.0
2015-11-18T12:42:06.070Z
@DavidRicherby Personally I wouldn't say so. As I read it, the question asks why the pictures _appear_ to be very dusty. This doesn't necessarily mean that the phenomenon has anything to do with actual dust. Of course, you can interpret the question differently.
Etheryte
CC BY-SA 3.0
2015-11-18T16:35:32.427Z
@Nit Dust absorption is *grey* and it is what is responsible for most of the absorption on Mars. Therefore it wouldn't matter what wavelengths our eyes are tuned to. The "absorption" you can see in those pictures is of an entirely different nature to that presented in the picture in this answer.
ProfRob
CC BY-SA 3.0
2015-11-18T17:29:49.077Z
@RobJeffries If the answer is wrong why don't you write one that addresses that issue? As someone who isn't familiar with the topic, this answer sounds like a good explanation of the issue.
Etheryte
CC BY-SA 3.0
2015-11-18T17:49:48.800Z
I thought Rob's answer was good... @gerrit, source attribution added
James K
CC BY-SA 3.0
2015-11-18T21:04:59.553Z
@Nit I don't say the answer is wrong, there are no inaccuracies contained therein. Perhaps you hadn't noticed that I have answered the question...
ProfRob
CC BY-SA 3.0
2015-11-18T21:18:20.227Z
@RobJeffries I've seen your answer but it doesn't address the concerns you bring out here.
Etheryte
CC BY-SA 3.0
2015-11-17T20:34:05.060Z
2015-11-18T17:51:06.160Z
2015-11-18T17:51:06.160Z
James K
<p>So, we have surface pictures from two alien planets, Venus (captured by the Venera 13), and Mars (captured by the rover).</p>

<p>Both of these pictures appear to be very dusty. For Venus we see strong storms; that makes sense. However, the Martian air also appears very dusty. Is Earth relatively unique in its clear atmosphere?</p>

<p><a href="https://i.sstatic.net/BYQw2.jpg" rel="noreferrer"><img src="https://i.sstatic.net/BYQw2.jpg" alt="Martian Landscape"></a>
<br>
Mars</p>

<p><a href="https://i.sstatic.net/dIjXZ.jpg" rel="noreferrer"><img src="https://i.sstatic.net/dIjXZ.jpg" alt="Venusian Landscape"></a>
<br>
Venus</p>

<p><a href="https://i.sstatic.net/kllDJ.jpg" rel="noreferrer"><img src="https://i.sstatic.net/kllDJ.jpg" alt="Titan Landscape"></a>
<br>
Titan (Thanks LocalFluff!)</p>

CC BY-SA 3.0
2015-11-17T19:37:22.670Z
And a third one! Titan, moon of Saturn! You can pretty easily find online images of its surface both from the landings site of Huygens, the lander which Cassini brought, and during its descent. So you could compare visibility from any altitude for Titan. Titan's atmosphere is less than twice as thick than that of Earth, I gather. But much less clear. I have no answer to your question. I just want to compound it a bit.
LocalFluff
CC BY-SA 3.0
2015-11-17T20:15:51.510Z
Isn't the second Venus image "artistic"? Do you have a source for it? Because I'm doubtful that formation was actually captured by any instrument. I think you've been fooled by a fraudster! Watch out, so you don't do it again!
LocalFluff
CC BY-SA 3.0
2015-11-17T20:18:34.200Z
@localfluff The second one was from the Russian venera 13, launched in  1982.
Sidney
CC BY-SA 3.0
2015-11-18T00:38:43.190Z
[An ***artist’s conception*** of Titan’s sand dunes.](http://beyondearthlyskies.blogspot.in/2012/03/titan-airplane-mission-concept.html)
Digital Trauma
CC BY-SA 3.0
2015-11-18T09:20:13.860Z
The earth is pretty dusty, too...
Michael Hampton
CC BY-SA 3.0
2015-11-18T11:21:36.937Z
Very nice images, and probably somewhat realistic scenaria too. But it should be clearly pointed out that they are artists' impressions. The camera of Venera 13 pointed down on the ground and never imaged the horizon.
LocalFluff
CC BY-SA 3.0
2015-11-18T16:11:30.467Z
@Sidney: [No horizon in Venera imagery](http://www.donaldedavis.com/2003NEW/NEWSTUFF/DDVENUS.html). No floating dust, either, for what it's worth.
DevSolar
<earth><planetary-atmosphere><atmospheric-effects>
Is Earth unique in its fairly clear atmosphere?
Sidney
# Is Earth unique in its fairly clear atmosphere?

(Asked by: username_0 on 2015-11-17)

So, we have surface pictures from two alien planets, Venus (captured by the Venera 13), and Mars (captured by the rover).

Both of these pictures appear to be very dusty. For Venus we see strong storms; that makes sense. However, the Martian air also appears very dusty. Is Earth relatively unique in its clear atmosphere?

[![Martian Landscape](https://i.sstatic.net/BYQw2.jpg)](https://i.sstatic.net/BYQw2.jpg)
  
Mars

[![Venusian Landscape](https://i.sstatic.net/dIjXZ.jpg)](https://i.sstatic.net/dIjXZ.jpg)
  
Venus

[![Titan Landscape](https://i.sstatic.net/kllDJ.jpg)](https://i.sstatic.net/kllDJ.jpg)
  
Titan (Thanks LocalFluff!)

**Tags:** earth, planetary-atmosphere, atmospheric-effects
> **Comments:**
> **username_1**: And a third one! Titan, moon of Saturn! You can pretty easily find online images of its surface both from the landings site of Huygens, the lander which Cassini brought, and during its descent. So you could compare visibility from any altitude for Titan. Titan's atmosphere is less than twice as thick than that of Earth, I gather. But much less clear. I have no answer to your question. I just want to compound it a bit.  
> **username_1**: Isn't the second Venus image "artistic"? Do you have a source for it? Because I'm doubtful that formation was actually captured by any instrument. I think you've been fooled by a fraudster! Watch out, so you don't do it again!  
> **username_0**: @localfluff The second one was from the Russian venera 13, launched in  1982.  
> **username_2**: [An ***artist’s conception*** of Titan’s sand dunes.](http://beyondearthlyskies.blogspot.in/2012/03/titan-airplane-mission-concept.html)  
> **username_3**: The earth is pretty dusty, too...  
> **username_1**: Very nice images, and probably somewhat realistic scenaria too. But it should be clearly pointed out that they are artists' impressions. The camera of Venera 13 pointed down on the ground and never imaged the horizon.  
> **username_4**: @Sidney: [No horizon in Venera imagery](http://www.donaldedavis.com/2003NEW/NEWSTUFF/DDVENUS.html). No floating dust, either, for what it's worth.  

---

## 2 answers

### Answer by username_6 on 2015-11-17. Score: 21 | Accepted answer

Our atmosphere is only transparent to visible light, In most other wavelengths, some or all of the light is absorbed

[![Image from wikipedia](https://i.sstatic.net/wvref.png)](https://i.sstatic.net/wvref.png)
[Image from Wikipedia](https://en.wikipedia.org/wiki/Optical_window#/media/File:Atmospheric_electromagnetic_opacity.svg), adapted from image by NASA

Our eyes have evolved to take advantage of the transparency at these wavelengths. If we had evolved in an atmosphere with a very different mix of gases. One in which visible light was absorbed, we would have evolved eyes that see different wavelengths.

There is a notable "window" at about $10\mu m$ in the diagram above. And you might wonder if any animal has evolved to see in this window. However, [our own bodies emit thermal radiation at about $10\mu m$](https://en.wikipedia.org/wiki/Black-body_radiation#Human%20body%20emission), eyes wouldn't work as they would be swamped by their own glow. However this window is used by thermal imaging devices.

> **Comments:**
> **username_7**: A very nice answer.   I might add, more intuitively and anything else that Earth's rain keeps it's soil moist and helps keep large dust clouds from forming, though they still do happen from time to time in dry locations.     Trees and plants obviously help keep the Earth from being "dusty" too.   A planet with liquid water and water rain that reaches the surface is interesting to us for many reasons.   Earth is the only one like that in our solar-system.  Venus has sulfuric acid rain, but it doesn't reach it's surface.  
> **username_5**: Not really clear how this answers the question. Has the OP been edited?  
> **username_8**: @RobJeffries I think this answers the question very clearly, what's the issue?  
> **username_9**: It could be spelt out more explicitly that our eyes have not evolved for the purpose of seeing through the low opacity part of the spectrum for other worlds, whatever that night be. What would be interesting is whether the absorption spectrum plots for worlds other than Earth are available.  
> **username_10**: One should mention that the only reason why we call those specific wavelengths "visible light" and the others "invisible" is because our eyes have evolved to see in that spectrum. IOW: "visible light" is not a property of the light, it's a property of our vision. Had we evolved on a different planet with a different atmosphere and a different absorption spectrum, our eyes would be sensitive to different wavelengths, and we would call *that* spectrum "visible".  
> **username_11**: @Nit The question very clearly asks about the amount of dust in the atmosphere, not its chemical composition.  
> **username_12**: Please add a source attribution far thot image.  
> **username_8**: @DavidRicherby Personally I wouldn't say so. As I read it, the question asks why the pictures _appear_ to be very dusty. This doesn't necessarily mean that the phenomenon has anything to do with actual dust. Of course, you can interpret the question differently.  
> **username_5**: @Nit Dust absorption is *grey* and it is what is responsible for most of the absorption on Mars. Therefore it wouldn't matter what wavelengths our eyes are tuned to. The "absorption" you can see in those pictures is of an entirely different nature to that presented in the picture in this answer.  
> **username_8**: @RobJeffries If the answer is wrong why don't you write one that addresses that issue? As someone who isn't familiar with the topic, this answer sounds like a good explanation of the issue.  
> **username_6**: I thought Rob's answer was good... @gerrit, source attribution added  
> **username_5**: @Nit I don't say the answer is wrong, there are no inaccuracies contained therein. Perhaps you hadn't noticed that I have answered the question...  
> **username_8**: @RobJeffries I've seen your answer but it doesn't address the concerns you bring out here.  

---

### Answer by username_5 on 2015-11-18. Score: 8

No, the clarity of the Earth's atmosphere cannot be considered unique. We don't have to speculate about exoplanets.

You could argue the answer is no, because both the Moon and Mercury have (very, very) thin atmospheres, and these are obviously "clear".

If you regard that argument as tricksy, then we can turn to Mars. Yes Mars has occasional dust storms. In normal conditions, the optical depth of the Martian atmosphere is usually somewhere between 0.5 and 1 per airmass. ([Petrova et al. 2012](http://www.sciencedirect.com/science/article/pii/S0032063311002960); [Lemmon et al. 2014](http://arxiv.org/abs/1403.4234)). Most of this extinction is caused by dust and is nearly wavelength independent. i.e. between 60% and 37% of light would travel through it's atmosphere from outside. This compares with typical extinctions of about 0.2-0.4 *magnitudes* of visual extinction per airmass on Earth (0.1 mag at the best astronomical sites in the world), corresponding to 80% to 69% of light passing through the Earth's atmosphere from outside (to sea level). Most of this extinction is due to dust, though there is some absorption by water and other aerosols).

Thus, though Mars is dustier than Earth on average, it is not outrageously so. It would be stretching the use of the word unique to say that the clarity of the Earth's atmosphere was "unique".


---

