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Venus famously has clouds of concentrated sulfuric acid in its upper atmosphere. Frequently I will see posts here citing the acidity of the atmosphere as a challenge to probes on the surface (such as "What material properties would be necessary to shield a lander from the environment of the Venusian surface?"). But would there be appreciable quantities of acid at the surface? Is sulfuric acid stable at those temperatures?

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  • $\begingroup$ This is a great question! this answer suggests that there's not much left of the landers on Venus: "(Aluminum)... will eventually succumb to the sulfuric acid. Iron will have dissolved in any sulfuric acid that makes it to the surface." but that's the author's authoritative-sounding hypothesis and is, as yet, unsourced. $\endgroup$
    – uhoh
    Sep 23, 2020 at 2:59
  • $\begingroup$ The boiling point of pure sulfuric acid is 337C. The surface temperature of Venus is somewhere around 460C in the sun, less at night. If water vapor is present it readily decomposes, without water it would seem to convert to various SOx molecules. So, on the day side, it likely decomposes, but would return to stability on the night side, with a chance of raining down on the surface. Fun fun. $\endgroup$
    – Jon Custer
    Sep 23, 2020 at 13:12
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    $\begingroup$ @JonCuster the surface temperature of Venus basically only varies by altitude. It never gets cool enough for sulfuric acid to be stable, let alone liquid. (see chemistry.stackexchange.com/a/60711/93851) $\endgroup$
    – Christopher James Huff
    Sep 23, 2020 at 15:47
  • $\begingroup$ @ChristopherJamesHuff - Fair enough, although nature.com/articles/s41598-018-38117-x would indicate there may be regions where the liquid is stable. $\endgroup$
    – Jon Custer
    Sep 23, 2020 at 15:53
  • $\begingroup$ @NilayGhosh - That doesn't quite answer the question (which is perhaps why you just gave it as a comment). After all, the troposphere of Earth's atmosphere is almost entirely O2 and N2, but there is still plenty of water vapor to cause all kinds of issues. I suspect that, by contrast, sulfuric acid is basically not present in the lower troposphere of Venus, so that there would not be much corrosion. But perhaps this is not known. $\endgroup$
    – Mark Foskey
    Oct 6, 2020 at 15:24

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We don't need to invoke sulfuric acid or sulfur oxides. Even at relatively low partial pressures and temperatures close to those found on the surface of Venus, carbon dioxide alone can oxidize iron. Thus we need a metal more robust than common steel to avoid being corroded on Venus. See for example Ref. 1, which studies the impact of carbon dioxide on iron catalysts.

Reference

1. Ewa Ekiert and Walerian Arabczyk, "Passivation versus Oxidation of Iron Catalyst with Carbon Dioxide", J. Phys. Chem. C 2015, 119, 8, 4000–4008

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Recently, I answered this question. I came to know that descent probes also provided some evidence for thin aerosol layers near the surface. It is written that:

A recent reanalysis of Venera-13, -14 descent probe spectrophotometer data found a sharp decrease of light levels at 1–2 km altitude, interpreted as indicating a detached layer of aerosols of unknown nature at this level. The authors point out that its altitude is similar to that at which radar-bright deposits attributed to metallic condensate on the mountain tops have been found . The aerosol layer could also be associated with volcanic ash or dust lifted by wind or near surface sulfuric acid haze. Further investigation of such low-altitude hazes may be possible by radar investigations or near-infrared spectroscopy on the nightside.

So there might be a minute amount of sulfuric acid just above the surface but that should not be our concern. If we want to protect our spaceship or lander from being corroding, we should concentrate on the vast amount of sulfuric amount present on the clouds.

Reference

  1. Titov, D.V., Ignatiev, N.I., McGouldrick, K. et al. Clouds and Hazes of Venus. Space Sci Rev 214, 126 (2018). DOI: 10.1007/s11214-018-0552-z
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  • $\begingroup$ Near the surface (462⁾ C) and up to 20 km above it there can be no sulfuric acid. It gradually decomposes above 300⁾ C into water and SO3. en.wikipedia.org/wiki/Sulfuric_acid $\endgroup$
    – Cornelis
    Oct 10, 2020 at 8:43
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    $\begingroup$ @Cornelisinspace I've been thinking about this; chemical reactions can be pressure sensitive; something that can't exist at 10^5 bar might still exist at 10^7 bar. I don't think it's enough to make a several hundred K difference, but it should probably be checked first. Also maybe H2O and SO3 can still be corrosive even when it can't be called sulphuric acid. I just don't know. Different (but related) example is that iron will not rust unless both O2 and H2O are present. The O2 doesn't have to be dissolved in the H2O. $\endgroup$
    – uhoh
    Oct 10, 2020 at 10:16
  • $\begingroup$ @uhoh Yes, we have to rely on scientific information. One example is Figure 7 from this article: agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2006JE002794 $\endgroup$
    – Cornelis
    Oct 10, 2020 at 11:41
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    $\begingroup$ @uhoh I should also have said that SO3 becomes SO2 and O, and i've read that the O is picked up by CO coming from the surface. $\endgroup$
    – Cornelis
    Oct 10, 2020 at 13:45
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    $\begingroup$ This article that has a link in your reference mentions PbS or Bi2S3 but no sulfuric acid haze researchgate.net/publication/… $\endgroup$
    – Cornelis
    Oct 10, 2020 at 16:45

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