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It is a well-known fact that an opaque layer of highly reflective clouds of sulfuric acid completely shrouds Venus preventing its surface from being seen from space in visible light. This is mentioned in the official NASA site and also in some papers:

  1. The atmosphere of Venus is made up mainly of carbon dioxide, and thick clouds of sulfuric acid completely cover the planet. (nasa.gov)

  2. The clouds that completely shroud the planet are located in the altitude range of approximately 50–70 km with upper and lower haze extending up to about 100 km and 30 km correspondingly, thus occupying the upper troposphere and the entire mesosphere.

    Titov, D.V., Ignatiev, N.I., McGouldrick, K. et al. Clouds and Hazes of Venus. Space Sci Rev 214, 126 (2018). https://doi.org/10.1007/s11214-018-0552-z

  3. The sky of Venus is fully covered by thick clouds of sulfuric acid that are located at a height of 45-70 km, making it hard to observe the planet's surface from Earth-based telescopes and orbiters circling Venus. (eurekalert.org)

But in this question, user @Cornelisinspace came to the conclusion that "there is much less sulfuric acid than water !". So, if there is less sulfuric acid than water (which is already present in trace amount):

  1. how is it able to "completely spread over" and "fully cover the planet"?

  2. why is the cloud cover so thick (so thick that it reflects most of the solar radiation (except in some dark patches where it gets absorbed) making the planet opaque)?

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    $\begingroup$ I am not convinced that the referenced answer by @Cornelinspace is correct. I am also not sure it is incorrect, so I didn't downvote that answer. The key problem is that Cornelinspace takes the mass of the water in Venus's atmosphere based on a very poorly sourced number from a Wikipedia article as a given. That is a very dubious thing to do. The number is not just poorly sourced but extremely poorly sourced. $\endgroup$ – David Hammen Sep 22 at 12:03
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    $\begingroup$ That there may be more water in Venus's atmosphere than sulfuric acid is irrelevant because the cloud layers start at 35-40 km above the surface as a sulfuric acid haze, become thick at about 45-55 km above the surface, and top out at 75-100 km above the surface. The bulk of Venus's atmosphere, in terms of mass, is in the lowest part of its atmosphere where sulfuric acid is not present. $\endgroup$ – David Hammen Sep 23 at 8:38
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The Venusian cloud deck is highly acidic. How much water there is in Venus's atmosphere above and below the cloud deck is irrelevant. What matters is how much sulfuric acid there is within that cloud deck.

And it is a lot, with the cloud deck droplets being between 75% to nearly 100% sulfuric acid by weight. Since the atomic weight of sulfuric acid is about 5.44 times that of water, this weight range corresponds to between 35.5% to nearly 100% sulfuric acid by volume, or by molecule. It's only in the polar regions where the sulfuric acid content of the cloud droplets is at the low value of 75% by weight.

The top of Venus's thick cloud deck is marked by where incoming solar radiation can dissociate the sulfuric acid in the cloud deck. The bottom of Venus's thick cloud deck is where the high temperature (temperature increases with increasing depth in the cloud deck) dissociates sulfuric acid. Sulfuric acid is stable within these bounds.

The dissociation at the top and bottom of the cloud deck means there has to be a source of sulfuric acid to counterbalance the dissociation. The source is the dissociation of other molecules at the top of the cloud deck, plus the large amounts of sulfur dioxide ("large" compared to other trace components) in Venus's atmosphere. The dissociation above the cloud deck creates atomic oxygen. This atomic oxygen is very volatile and readily combines with sulfur dioxide to form sulfur trioxide. This in turn readily combines with water to form sulfuric acid.

In short, conditions just above the cloud deck replenish the sulfuric acid in the cloud deck.


References:

Barstow, J. K., et al. "Models of the global cloud structure on Venus derived from Venus Express observations." Icarus 217.2 (2012): 542-560.

Lee, Yeon Joo, et al. "Vertical structure of the Venus cloud top from the VeRa and VIRTIS observations onboard Venus Express." Icarus 217.2 (2012): 599-609.

Young, A. T. "Are the clouds of Venus sulfuric acid?." Icarus 18.4 (1973): 564-582.

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