# How far can you see on the surface of Venus?

As from the images by Venera probes, it seems to me that on Venus you can't see as far as on Earth, e.g. you can't see the horizon, because it's so misty there. Venera 9 and 10 shots.Venera 14 shot. Is the surface always covered in the sulfuric fog so that you can't see farther than a quarter mile at most, or is it possible that you'd be able to see on Venus very far too, like this?

• A furlong? Ok, definitely +1 for the most casual use of the word furlong in a serious question! For those who are unfamiliar, a furlong is equal to ten chains. :-) – SusanW Jul 7 '20 at 7:03
• A furlong is an eighth of a (statute) mile. – LoveForChrist Jul 7 '20 at 7:15
• Absolutely! I've just never heard anyone use it before outside of horse-racing (and round here, those folks still use guineas) - I feel I've dropped into Lord of the Rings. Would it be worth clarifying the estimate as "200 metres", or maybe adding a link to Furlong? – SusanW Jul 7 '20 at 9:57
• @SusanW You know, I use feet rather for altitude while for length on land I rather use yards (strides). I could say 1000 ft but that's not well expressible in yards. Therefore I used the furlong which is an eighth of a mile. But the images show you can look even farther, I'll change it. – LoveForChrist Jul 7 '20 at 10:31

Apparently you can't see very far, but not because of mist.

The visibility was a pleasant surprise to the scientists who, after reviewing Venera 8, had predicted a dark, murky and dusty atmosphere in which only the near field would be available for inspection.

The indistinctness and apparent nearness of the horizon in all of the images from the Venera landers were due to the high refractivity of the dense atmosphere which made Venus appear to be a small-diameter spherical body with a horizon much less than 1 km away. The phenomenon is similar to a terrestrial mirage and is probably a function of the observer's height above the ground.

Huntress and Marov, Soviet Robots in the Solar System, p. 307 (emphasis mine) (selections from the book, including this page, are available at Google Books here)

There is no sulfuric acid fog on the surface of Venus. Sulfuric acid clouds, formed in the upper atmosphere when dissociated oxygen reacts with water and sulfur dioxide, rains down towards the surface but evaporates in the increasing heat as it falls until the air becomes clear again. The evaporation is accompanied by decomposition of the acid as this is reported to begin at 300°C.

Sulfuric acid is produced in the upper atmosphere by the Sun's photochemical action on carbon dioxide, sulfur dioxide, and water vapour.[1] Ultraviolet photons of wavelengths less than 169 nm can photodissociate carbon dioxide into carbon monoxide and monatomic oxygen. Monatomic oxygen is highly reactive; when it reacts with sulfur dioxide, a trace component of the Venusian atmosphere, the result is sulfur trioxide, which can combine with water vapour, another trace component of Venus's atmosphere, to yield sulfuric acid.[2]

CO2 → CO + O SO2 + O → SO3 2SO3 + 4H2O → 2H2SO4·H2O

Surface level humidity is less than 0.1%.[3] Venus's sulfuric acid rain never reaches the ground, but is evaporated by the heat before reaching the surface in a phenomenon known as virga.[4] It is theorized that early volcanic activity released sulfur into the atmosphere and the high temperatures prevented it from being trapped into solid compounds on the surface as it was on the Earth.[5]

Cited references:

1. "VenusExpress: Acid clouds and lightning". European Space Agency (ESA). Retrieved 2016-09-08.

2. Krasnopolsky, V. A.; Parshev, V. A. (1981). "Chemical composition of the atmosphere of Venus". Nature. 292 (5824): 610–613.

3. Koehler, H. W. (1982). "Results of the Venus sondes Venera 13 and 14". Sterne und Weltraum. 21: 282. Bibcode:1982S&W....21..282K.

4. "Planet Venus: Earth's 'evil twin'". BBC News. 7 November 2005.

5. "The Environment of Venus". hyperphysics.phy-astr.gsu.edu. Retrieved 2014-04-06.

Similar acid-forming processes also occur in thunderstorms in the atmosphere of Earth, with $$\text{O}_2$$ as the oxygen source and both nitrogen and sulfur compounds reacting to produce "acid rain" (and even some nitric acid from atmospheric nitrogen in unpolluted air).

• And so what is that foggy haze on the horizon of those images? – LoveForChrist Jul 7 '20 at 10:34
• And to "How far can you see on the surface of Venus?" is it possible to estimate an answer or at least a lower limit and/or cite a source? – uhoh Jul 7 '20 at 12:27
• This is helpful, but isn't a complete answer to the question. The photos do clearly show hazy conditions on the surface, so the question is why. – Ben Crowell Jul 7 '20 at 15:48