# Is Earth unique in its fairly clear atmosphere?

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?

Mars

Venus

Titan (Thanks LocalFluff!)

• 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 Nov 17 '15 at 19:37
• @localfluff The second one was from the Russian venera 13, launched in 1982. – Sidney Nov 17 '15 at 20:18
• An artist’s conception of Titan’s sand dunes. – Digital Trauma Nov 18 '15 at 0:38
• The earth is pretty dusty, too... – Michael Hampton Nov 18 '15 at 9:20
• 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 Nov 18 '15 at 11:21

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

Image from Wikipedia, 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 $10\mu m$, eyes wouldn't work as they would be swamped by their own glow. However this window is used by thermal imaging devices.

• 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 Nov 17 '15 at 23:43
• Not really clear how this answers the question. Has the OP been edited? – Rob Jeffries Nov 18 '15 at 1:15
• 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 Nov 18 '15 at 2:37
• 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 Nov 18 '15 at 8:34
• @Nit The question very clearly asks about the amount of dust in the atmosphere, not its chemical composition. – David Richerby Nov 18 '15 at 9:30

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; Lemmon et al. 2014). 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".