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Author Topic: Why is the atmosphere of Venus so dense?  (Read 1539 times)

Offline chris

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Why is the atmosphere of Venus so dense?
« on: 22/01/2016 04:55:54 »
From Wikipedia: "The atmosphere of Venus is the layer of gases surrounding Venus. It is composed primarily of carbon dioxide and is much denser and hotter than that of Earth. The temperature at the surface is 740 K (467 C, 872 F), whereas the pressure is 93 bar (9.3 MPa)."

Given that Venus is almost the same size as the Earth, what is the reason for the high density and pressure of the atmosphere, compared with Earth? Is it just that the main gas species, carbon dioxide, is more massive and hence, on aggregate, the atmosphere just "weighs" more?


 

Offline Colin2B

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Re: Why is the atmosphere of Venus so dense?
« Reply #1 on: 22/01/2016 09:22:20 »
I think you are right.
Venus has a mass 82% of earth but the density of carbon dioxide is around 1.98 kg/m3, about 1.67 times that of air - that's at std temp and pressure.
Looks like they've gone and burnt all their fossil fuel  :)

 

Offline alancalverd

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Re: Why is the atmosphere of Venus so dense?
« Reply #2 on: 22/01/2016 09:28:29 »
But at 740K the density of CO2 at standard pressure would be about half that of air at 300K. 
 

Offline Colin2B

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Re: Why is the atmosphere of Venus so dense?
« Reply #3 on: 22/01/2016 09:39:39 »
But at 740K the density of CO2 at standard pressure would be about half that of air at 300K.


Ha, you beat me to it. No sooner had I hit the post button than the temp question hit me.
Is that just surface temp? Just wondering what the temp and pressure gradients would look like.
 

Offline evan_au

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Re: Why is the atmosphere of Venus so dense?
« Reply #4 on: 22/01/2016 11:27:30 »
The surface temperature of Venus is so high because of a runaway greenhouse effect. This high temperature can't escape away to space in the night (which lasts about 2 Earth months), because it is kept in by a very thick blanket of gas.

If Earth's oceans were boiled, and this mass added into our atmosphere, we would have a very high surface pressure too. The pressure at Venus's surface is equivalent to being under 900m of water.

The chemistry is clearly different - the atmosphere of Earth is 80% Nitrogen and 20% oxygen. The oxygen is maintained by life, and a lot of the Carbon is buried in Coal deposits, Carbonate rocks, and subducted into the mantle. A lot of Nitrogen is bound up in the proteins of life.

The atmosphere of Venus has 4 times as much N as Earth's atmosphere, but N only makes up 3.5% of the atmosphere, due to the greater mass of the atmosphere.
It is possible that the original water content of Venus has reacted with Carbon to produce large amounts of CO2 and with Sulfur to produce SO2.

http://en.wikipedia.org/wiki/Atmosphere_of_Venus#Composition
 
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Offline puppypower

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Re: Why is the atmosphere of Venus so dense?
« Reply #5 on: 22/01/2016 11:59:38 »
My guess is the higher pressure of Venus's atmosphere is due to Venus having less water on the surface. When various gases react with water, many will form acids. The aqueous acids, in turn, will react with the various minerals on the surface of the planet, helping to fix the atmospheric gases into nonvolatile minerals.

For example, CO2 and water will react to form H2CO2 or carbonic acid. This mild acid can react wth calcium ions in water, to form calcium carbonate. If there is no water, the reaction between CO2 and Calcium does not move forward at anywhere near the same speed.

Water is like the swiss army knife of chemicals, mediating all types of reactions, including those in life. This is why chemistry is centered on water; gold standard. Affects like pH; acid and bases, is based on the ability of water to gain or release hydrogen. The gases make acids in water, while cations like Calcium make bases in water. These will cancel each other to form minerals and restore the water for another round. Nitrogen is not reactive in water. On the earth, nitrogen became the last major gas standing.
« Last Edit: 22/01/2016 12:03:44 by puppypower »
 

Offline chris

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Re: Why is the atmosphere of Venus so dense?
« Reply #6 on: 22/01/2016 21:55:28 »
If Earth's oceans were boiled, and this mass added into our atmosphere, we would have a very high surface pressure too. The pressure at Venus's surface is equivalent to being under 900m of water.

But the atmospheric composition is chiefly carbon dioxide, so how is it that this has all ended up in the atmosphere and nothing has sequestered it back into the ground?
 

Online chiralSPO

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Re: Why is the atmosphere of Venus so dense?
« Reply #7 on: 22/01/2016 23:47:59 »
If Earth's oceans were boiled, and this mass added into our atmosphere, we would have a very high surface pressure too. The pressure at Venus's surface is equivalent to being under 900m of water.

But the atmospheric composition is chiefly carbon dioxide, so how is it that this has all ended up in the atmosphere and nothing has sequestered it back into the ground?

Metal carbonate minerals decompose at high temperatures to release carbon dioxide, and the corresponding metal oxide, for instance:

MgCO3 → MgO + CO2 (this happens at 350C in our atmosphere, though might need to be hotter given the high pressure of CO2 on Venus, but should still be quite efficient as surface temperatures are at least 100C higher)
 
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Offline chris

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Re: Why is the atmosphere of Venus so dense?
« Reply #8 on: 23/01/2016 23:26:13 »
Thanks everyone for the really insightful answers.
 

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Re: Why is the atmosphere of Venus so dense?
« Reply #8 on: 23/01/2016 23:26:13 »

 

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