Venus clouds may be less acidic that we thought

[Colin Robinson]

I've just been looking a paper from Cornell University, written in January this year, about the quantities of SO₂ (sulfur dioxide) and H₂O (water vapor) at various levels of the atmosphere of Venus. The researchers note that considering the quantities detected near the surface, the quantities in the cloud level are anomalously low. They suggest several different hypotheses to explain this anomaly.

One hypothesis involves SO₂ being absorbed into the sulfuric acid droplets. This wouldn't work if the droplets were pure sulfuric, but would work if they also contain hydroxide salts. These salts would mean higher pH -- i.e. less extreme acidity. The paper finds (page 31) that the acidicity implied by this hypothesis is "within the range where known acidophiles can thrive". Though the acidophiles might still be prevented for thriving by the low concentration of H₂O, an issue distinct from pH.

[kzb]

If the SO2 was oxidised to SO3, it would then be absorbed by sulphuric acid. This is the Contact Process for manufacturing the acid.

Where are these alkaline salts coming from, on a planet surface exposed to sulphuric acid for millions of years?

[Colin Robinson]

If the SO2 was oxidised to SO3, it would then be absorbed by sulphuric acid. This is the Contact Process for manufacturing the acid.

Yes and it's understood that some SO₂ in the atmosphere of Venus does get oxidised to SO₃ and H₂SO₄. The limiting factor is low level of atmospheric O₂.

Where are these alkaline salts coming from, on a planet surface exposed to sulphuric acid for millions of years?

The basaltic minerals on the surface of Venus contain substantial calcium oxide (CaO), magnesium oxide (MgO), and potassium oxide (K₂O), all of which are alkaline.

Surface minerals are unlikely to be directly exposed to sulfuric acid, since high temperature near the surface breaks down the sulfuric into SO₂, O₂ and H₂O — the reverse reaction to the one you mentioned.

Even so, the co-existence of acidic sulfur compounds in the atmosphere and alkaline oxides on the surface is something of a puzzle.

One theory is that the current amount of atmospheric SO₂ and sulfuric acid is a comparatively new state of affairs, due to recent volcanic activity on a massive scale.

[cannongray]

The terrestrial bacterium Acidithiobacillus ferrooxidans lives in a very acidic environment - the Venusian clouds are composed of corrosive sulfuric acid. This persistent baby absorbs carbon dioxide and nitrogen from the air, and receives energy by oxidizing iron, hydrogen and sulfur - there is plenty of all of this in the atmosphere of Venus. Other microorganisms are also known that, in theory, would not have disappeared over the neighboring planet. Some of them are so severe that in difficult conditions they themselves produce sulfuric acid - and nothing.

[Noclevername]

Other microorganisms are also known that, in theory, would not have disappeared over the neighboring planet.

Disappeared? We have no evidence they were ever there.

[kzb]

Yes and it's understood that some SO₂ in the atmosphere of Venus does get oxidised to SO₃ and H₂SO₄. The limiting factor is low level of atmospheric O₂.
.

Is it possible for the reaction

CO2 + SO2 = SO3 + CO

to occur in the Venus atmosphere, exposed to UV and high temperature?

[Colin Robinson]

Is it possible for the reaction

CO2 + SO2 = SO3 + CO

to occur in the Venus atmosphere, exposed to UV and high temperature?

Interesting point...

I've just had another look at the paper by the Cornell University people (linked in the opening post of this thread).

They mention reactions like that — CO₂ giving up oxygen atoms, which oxidise SO₂ leading to formation of H₂SO₄. But they make the point that formation of H₂SO₄ in this way is limited by availability of water molecules.

However, formation of SO₃ (as distinct from H₂SO₄) presumably doesn't require water molecules.

To see if that's what is happening, you'd have to look at questions like the stability of SO₃ (as distinct from H₂SO₄ ) in Venus atmospheric conditions, how detectable is it, and what levels are detected?

[kzb]

Interesting point...

I've just had another look at the paper by the Cornell University people (linked in the opening post of this thread).

They mention reactions like that — CO₂ giving up oxygen atoms, which oxidise SO₂ leading to formation of H₂SO₄. But they make the point that formation of H₂SO₄ in this way is limited by availability of water molecules.

However, formation of SO₃ (as distinct from H₂SO₄) presumably doesn't require water molecules.

To see if that's what is happening, you'd have to look at questions like the stability of SO₃ (as distinct from H₂SO₄ ) in Venus atmospheric conditions, how detectable is it, and what levels are detected?

There's LOTS of CO2 available.

Also there is 17ppm of CO in the atmosphere, perhaps this is where it comes from.

SO3 is absorbed by concentrated sulphuric acid in the contact process, there is no "extra" water required in addition to what was needed to form H2SO4 in the first place. But I suppose we then have to ask can "oleum" be detected?

[Colin Robinson]

There's LOTS of CO2 available.

True...

Also there is 17ppm of CO in the atmosphere, perhaps this is where it comes from.

17 ppm of CO...

As the paper mentions, the measured concentration of SO₂ at 40 km above the surface is above 100 ppm, but 80 km above the surface it is several orders of magnitude lower — between 1 and 100 ppb (note: parts per billion, not parts per million).

So what happens to SO₂ molecules diffusing upwards?

If most of them get oxidised by the reaction CO₂ + SO₂ -> SO₃ + CO, the resulting CO concentration would logically be a lot more than 17 ppm, surely?

[kzb]

True...



17 ppm of CO...

As the paper mentions, the measured concentration of SO₂ at 40 km above the surface is above 100 ppm, but 80 km above the surface it is several orders of magnitude lower — between 1 and 100 ppb (note: parts per billion, not parts per million).

So what happens to SO₂ molecules diffusing upwards?

If most of them get oxidised by the reaction CO₂ + SO₂ -> SO₃ + CO, the resulting CO concentration would logically be a lot more than 17 ppm, surely?

Is it ppm by volume or mass? The molecular mass ratio is 68/28.

Anyhow, I was just wondering out loud if these planetary scientists have heard of the contact process.

[Colin Robinson]

Is it ppm by volume or mass?

I think it means ppm by volume, as this is the usual measure for concentrations of gases in an atmosphere.