This is the follow-up of this chem.SE question.

According to Wikipedia, water vapor on Venus is present in trace amount (20 ppm). There are multiple reasons why Venus has very low water content:

  • The rich carbon dioxide atmosphere generates the strongest runaway greenhouse effect in the Solar System reaching temperatures at least 735 K, hot enough to boil most of the water vapors.
  • Free hydrogen and oxygen atoms has been swept into interplanetary space by the solar wind because of the lack of a planetary magnetic field
  • Water vapors are continuously blown away by the solar wind through the induced magnetotail.
  • UV and photodissociation of water vapor creating hydrogen and oxygen atoms and radicals
  • High D/H ratio in Venus due to hydrogen atoms escaping into space since it is light thus unable to form water
  • The atmosphere is rich in sulfuric acid clouds which is a powerful dehydrating agents. Most of the water reacts with H2SO4 to form hydronium (H3O+) and bisulfate (HSO4-) ions. As a result, the concentrations of “free” H2O in the acid solution and in the vapor over the acid are extremely low.

So, the question is "since water vapor is always blown away or reacting in some way and there is only a trace amount of stationery and unreacted water vapor available, why/how is it able to create chemical compounds like phosphoric acid and sulfuric acid?"

Background information

A 1986 paper1 concluded that P4O6 is the main phosphorus bearing gas on Venus from the Vega mission results. It was also reported that some phosphorus bearing particles were present which was found to be phosphoric acid, H3PO4.

I am pretty sure phosphoric acid/phosphorous acid is the result of reaction between phosphorous anhydride and water vapor in sulfuric acid environment (which is later speculated to be the pathway of phosphine formation). But "is water vapor present on Venus sufficient enough to form phosphoric acid? What is the concentration of phosphoric acid on Venusian atmosphere (which would later contribute to formation of phosphine)?"


  1. Krasnopol'skii, V A. Vega Mission results and chemical composition of Venusian clouds. United States: N. p., 1989. Web. doi:10.1016/0019-1035(89)90168-1.
  2. https://www.vanderbilt.edu/AnS/physics/astrocourses/AST101/readings/water_on_venus.html
  • $\begingroup$ Why do you compare the water being blown away with the water that stays in the atmosphere ? It's a tiny. tiny amount of the water that stays in the atmosphere, after one million years that amount of water will practically be the same, maybe 0.1 % less or so. $\endgroup$ – Cornelis in space Sep 17 '20 at 16:21
  • $\begingroup$ I've calculated that 3. 10^6 kg of water escapes into space every year from Venus, whereas there is about 9.6 x 10^15 kg of water in its atmosphere. space.stackexchange.com/questions/8002/… $\endgroup$ – Cornelis in space Sep 18 '20 at 8:53
  • $\begingroup$ @Cornelisinspace Not just water escaping, there are several other factors acting which I had listed. Granted, only a little amount of water vapor is blown into space, there are other factors which plays a role in the depletion of water vapor in atmosphere There is more to just water escaping and evaporating. The answerer stated that 1. there is an equilibrium maintaining between water lost and water retained and reacting. How is that equilibrium maintained? 2. Sulfuric acid acts as reservoir for hydrogen (but it dissociates into acidic ions^) 3. The ending note is left open to interpretation. $\endgroup$ – Nilay Ghosh Sep 20 '20 at 3:59
  • $\begingroup$ @Cornelisinspace In that question you linked, you came to the conclusion that " there is much less sulfuric acid than water !". But according to nasa.gov, "... and thick clouds of sulfuric acid completely cover the planet." If there is less sulfuric acid than water (which is already present in trace), how did it manage to completely cover the planet? (It seems to be a different question, might post as a new question, if it seems interesting.) $\endgroup$ – Nilay Ghosh Sep 20 '20 at 11:47
  • $\begingroup$ @Cornelisinspace I posted a new question based on my above comment. $\endgroup$ – Nilay Ghosh Sep 21 '20 at 12:26

Think about the water constantly being both added to and removed from the atmosphere.

Adding water:

  1. Water migrating from deeper layers of the planet crust and mantle into the atmosphere.
  2. Water being formed from sun wind hydrogen and the available oxygen in the atmosphere (the high D/H ratio hints that the primordial water dominates on Venus)
  3. Water being formed from hydrogen and oxygen produced by decomposition of other substances in the atmosphere.

Removing water:

  1. Photolysis - water decomposed to hydrogen and oxygen, both of them engaged in other chemical processes
  2. Escaping hydrogen and possibly whole water molecules into the space.

There is always some equilibrium between the available water and hydrogen and also other hydrogen containing substances like sulphuric acid (that's what Venus clouds are made of) and the widely-discussed today phosphine.

On Earth, we have stratospheric and mesospheric clouds made of water, sulphuric acid and nitric acid in atmospheric layers that are otherwise notoriously dry. On Venus, the atmosphere has little free oxygen so it is tolerant to much more of a chemical diversity. (E.g. phosphine would self-ignite in Earth's oxygen-rich atmosphere)

  • $\begingroup$ Good answer. Can you back your answer with some references? Or provide some data/numbers? $\endgroup$ – Nilay Ghosh Sep 18 '20 at 13:29
  • $\begingroup$ @NilayGhosh Why should there be an equilibrium when Venus gradually lost most of its water over billions of years ? $\endgroup$ – Cornelis in space Sep 20 '20 at 10:41
  • $\begingroup$ @Cornelisinspace That is also my question. The answerer stated that water is constantly being added and removed from the atmosphere and there is some sort of an equilibrium (between the available water and other chemicals). What is this equilibrium? How is that equilibrium maintained? Is he talking about amount of water being constant in the atmosphere? If that is so, then it is contradicting the fact that most of the water has been lost and that there is sufficient water available for reaction or is he talking about some sort of chemical equilibrium between water and other chemicals present? $\endgroup$ – Nilay Ghosh Sep 20 '20 at 11:38
  • $\begingroup$ ... if he explains the "equilibrium" a little bit more and/or add some reference or some data about the abundance of those compounds, I will accept the answer. $\endgroup$ – Nilay Ghosh Sep 20 '20 at 11:49
  • $\begingroup$ @NilayGhosh Yes, so there is some sort of chemical equilibrium between water and sulfuroxides and phosphorusoxides that greatly depends on temperature and pressure., that's why that happens only in the upper and middle atmosphere. But more than 90% of the water is free steam in the lower, high temperature and pressure region. $\endgroup$ – Cornelis in space Sep 20 '20 at 13:26

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