<p>It is the sheer amount of carbon dioxide in the atmosphere that makes the difference. Now the next very natural question is something like: The planet Venus has about the same mass as Earth, so why is there that much carbon dioxide in the atmosphere? A partial answer in the Venus–Earth comparison is that on Earth most of the carbon dioxide is bound in carbonates in the geosphere (chalc and dolomite being the dominant minerals forming impressive geological deposits).</p>

CC BY-SA 4.0
2023-03-30T10:17:54.167Z
2023-03-30T10:17:54.167Z
Sir Cornflakes
<p>Venus's atmosphere is very dense at the surface because Venus's atmosphere is very massive. The composition is nearly irrelevant. The pressure at Venus's surface is proportional to the mass of Venus's atmosphere and inversely proportional to Venus's surface area. The constant of proportionality is Venus's surface gravitational acceleration: <span class="math-container">$P \approx g_\text{Venus} \frac{m}{A}$</span>. (I used <span class="math-container">$\approx$</span> rather than = because corrections are needed for Venus's uneven surface and to account for Venus being more or less spherical as opposed to a flat plane.)</p>
<p>That said, <span class="math-container">$P \approx g_\text{Venus} \frac{m}{A}$</span>, or <span class="math-container">$m \approx \frac {PA}{g_\text{Venus}}$</span> is a very good approximation. What this means based on observations of Venus's surface pressure, surface area, and surface gravity is that Venus's atmosphere is very massive compared to that of the Earth.</p>
<p>Regarding the Venera and Vega landers not being crushed, it was the high surface temperature rather than the high surface pressure that did them in. People have built vessels that have descended to the bottom of the Challenger Deep, where the pressure is 1000 atmospheres. Compare that with the 93 atmospheres Venus's surface. Nuclear submarines operate at one atmosphere so as to be able to rise to the surface quickly, which means they can indeed be crushed by pressure at a sufficient depth. The Venera and Vega landers didn't operate under those constraints.</p>

CC BY-SA 4.0
2023-03-28T14:54:25.060Z
..thank you so much for such an outstanding answer. Now the light bulb went off in my head. Total mass. By the that was a very interesting insight you gave with Challenger Deep and submarines are designed for a different purpose and need to get to the surface more easily.
Sedumjoy
CC BY-SA 4.0
2023-03-28T18:02:11.300Z
@Sedumjoy I think "ease of getting to the surface" is a bit distracting. The reason nuclear subs can't go deeper than 700 m or whatever is simply that they're not designed for it, apparently because it wouldn't serve any military purpose. At least not enough to justify the increased mass of the stronger walls, higher cost, and/or reduced usable space inside, but with enough money it would also be possible to increase the depth capability. The actual point however is that it's much easier if the interior is also pressurized to match the outside. Which would crush any humans inside, though.
leftaroundabout
CC BY-SA 4.0
2023-03-28T20:36:32.263Z
@leftaroundabout Going deeper than 700 meters (or whatever) would require a different breathing atmosphere and might require coming up slower, with stops. Keeping the air inside a submarine at one atmosphere obviates the need for that slow rise, but also precludes going deep to prevent the submarine pressurized at a mere one atmosphere pressure from collapsing in on itself.
David Hammen
CC BY-SA 4.0
2023-03-28T20:41:48.877Z
@leftaroundabout Deep sea vessels do not operate at one atmosphere. There's no problem per se with breathing "air" at much greater than one atmosphere so long as the "air" doesn't have nitrogen (which can result in the bends on rising) and so long as it doesn't contain all that much oxygen (which also can be toxic).
David Hammen
CC BY-SA 4.0
2023-03-28T22:07:02.830Z
@DavidHammen, I believe most DSVs (eg, Trieste or w/e) maintain ~1atm, don't they? At the very least, the depths of challenger deep etc are *far* too deep for any diving mixture we have, they don't ascend slowly enough to be doing decompression stops, and I don't see any mention of breathing mixtures online.
Kaia
CC BY-SA 4.0
2023-03-28T22:10:47.053Z
That doesn't change the answer though, a Venus probe doesn't have to be a pressure vessel the way a crewed submarine does, and so there's not anything to crumple under that pressure
Kaia
CC BY-SA 4.0
2023-03-28T22:30:26.610Z
@Kaia exactly. Humans are able to survive up to a few dozen atmospheres given the right breathing mixture, but that's still only a small fraction of deep sea pressures. And if you want to benefit from interior pressure to reduce structural requirements, it needs to be at least something like half of the external pressure. In case of the Mariana Trench, it doesn't really make a difference in terms of structural loads whether the pressure inside is 1 bar or 100 bar, so there's no point using anything more than one atmosphere in the capsule.
leftaroundabout
CC BY-SA 4.0
2023-03-29T02:55:08.930Z
Thank you. I learned much by listening to this interesting discussion about pressure.
Sedumjoy
CC BY-SA 4.0
2023-03-30T17:02:03.790Z
If a SF novel was factual in this area, at significantly higher pressures there is also concern about regulation of body heat; the denser atmosphere is a better conductor of heat.  Something like staying in water for days.
Technophile
CC BY-SA 4.0
2023-03-31T07:33:21.973Z
There’s also the fact that the higher surface temperature on Venus makes the pressure roughly twice what it would be if it had Earth’s surface temperature.
Peter Erwin
CC BY-SA 4.0
2023-03-31T13:03:02.373Z
@Technophile Temperatures hot enough to melt lead aren't conductible to electronics, let alone life as we know it.
David Hammen
CC BY-SA 4.0
2023-03-31T13:33:45.800Z
@PeterErwin Venus's tropopause is somewhere between 60 to 80 km above the surface, depending on who one reads. The lapse rate is fairly constant from the one bar level (50 km above the surface) down to the surface, consistent with an adiabatic non-ideal gas. This results in a very high atmosphere temperature at the surface. Venus's surface is hot because the atmosphere is hot, not the other way around.
David Hammen
CC BY-SA 4.0
2023-03-31T17:56:46.523Z
@DavidHammen  I was referring to an additional design constraint for the internal habitable portion.  I agree that the temperature range required for human habitability is below the melting point of lead.
Technophile
CC BY-SA 4.0
2023-03-31T18:22:58.570Z
@DavidHammen I was attempting to refer to “the temperature of the atmosphere at or just above ground level” in a compact way, but obviously that was a bit ambiguous. The point is that the high “surface atmospheric pressure” — the basis of the original question — is also due to the high atmospheric temperature there, though the high density is obviously the dominant factor.
Peter Erwin
CC BY-SA 4.0
2023-03-28T10:35:49.283Z
2023-03-28T10:35:49.283Z
David Hammen
<p>Obviously I have forgotten by basic college chemistry. I am getting carbon dioxide at 1.87kg per cubic meter and compared it to nitrogen and oxygen but source says carbon dioxide is 1.5 times heavier than air. Let's go with that and since the earth is about the same size as Venus we should be able to use weight instead of density I assume.</p>
<p>Venus's surface atmospheric pressure is 75 times that of earth. Some sources say 90. The atmosphere is mostly carbon dioxide. The numbers just seem not to work out here. 90 times is much more than 1 and 1/2 times which is how much heavier CO<sub>2</sub> is relative to air.</p>
<p>P.S.
Next paragraph is not part of the question but just and interesting observation. If Venus's atmosphere really is 90 times that of earth the hull of a the most advanced nuclear sub would be crushed like a pancake. 90 ATM should be 750 meters and crush depth is 730 meters for nuclear sub. I mention this because it is so interesting Venera survived in order to take the measurements at the surface.</p>

CC BY-SA 4.0
2023-03-28T01:48:28.967Z
Well, Venera didn’t need to support humans inside, which changes the engineering requirements greatly…
Jon Custer
CC BY-SA 4.0
2023-03-28T02:04:09.790Z
The key detail here is the amount. There is a lot more atmosphere on venus.
Topcode
CC BY-SA 4.0
2023-03-28T02:10:28.520Z
This is a duplicate of https://space.stackexchange.com/questions/22856/why-is-the-atmospheric-pressure-on-venus-so-high/22868#22868
AtmosphericPrisonEscape
CC BY-SA 4.0
2023-03-28T21:26:42.827Z
"source says carbon dioxide is 1.5 times heavier than air" Does that source have a link? That's a rather imprecise statement. A more precise statement would be that $CO_2$ has a molar mass 1.5 times the average molar mass of air.
Acccumulation
CC BY-SA 4.0
2023-03-30T13:34:17.857Z
Iron is only about 4x as dense as sand.  How can a barbell weigh so much more than a grain of sand?
Jason Goemaat
<planetary-atmosphere><venus>
Why is Venus's atmospheric pressure 75 times that of earth when carbon dioxide is only 1.5 times heavier than air?
Sedumjoy
# Why is Venus's atmospheric pressure 75 times that of earth when carbon dioxide is only 1.5 times heavier than air?

(Asked by: username_0 on 2023-03-28)

Obviously I have forgotten by basic college chemistry. I am getting carbon dioxide at 1.87kg per cubic meter and compared it to nitrogen and oxygen but source says carbon dioxide is 1.5 times heavier than air. Let's go with that and since the earth is about the same size as Venus we should be able to use weight instead of density I assume.

Venus's surface atmospheric pressure is 75 times that of earth. Some sources say 90. The atmosphere is mostly carbon dioxide. The numbers just seem not to work out here. 90 times is much more than 1 and 1/2 times which is how much heavier CO2 is relative to air.

P.S.
Next paragraph is not part of the question but just and interesting observation. If Venus's atmosphere really is 90 times that of earth the hull of a the most advanced nuclear sub would be crushed like a pancake. 90 ATM should be 750 meters and crush depth is 730 meters for nuclear sub. I mention this because it is so interesting Venera survived in order to take the measurements at the surface.

**Tags:** planetary-atmosphere, venus
> **Comments:**
> **username_1**: Well, Venera didn’t need to support humans inside, which changes the engineering requirements greatly…  
> **username_2**: The key detail here is the amount. There is a lot more atmosphere on venus.  
> **username_3**: This is a duplicate of https://space.stackexchange.com/questions/22856/why-is-the-atmospheric-pressure-on-venus-so-high/22868#22868  
> **username_4**: "source says carbon dioxide is 1.5 times heavier than air" Does that source have a link? That's a rather imprecise statement. A more precise statement would be that $CO_2$ has a molar mass 1.5 times the average molar mass of air.  
> **username_5**: Iron is only about 4x as dense as sand.  How can a barbell weigh so much more than a grain of sand?  

---

## 2 answers

### Answer by username_7 on 2023-03-28. Score: 32 | Accepted answer

Venus's atmosphere is very dense at the surface because Venus's atmosphere is very massive. The composition is nearly irrelevant. The pressure at Venus's surface is proportional to the mass of Venus's atmosphere and inversely proportional to Venus's surface area. The constant of proportionality is Venus's surface gravitational acceleration: $P \approx g\_\text{Venus} \frac{m}{A}$. (I used $\approx$ rather than = because corrections are needed for Venus's uneven surface and to account for Venus being more or less spherical as opposed to a flat plane.)

That said, $P \approx g\_\text{Venus} \frac{m}{A}$, or $m \approx \frac {PA}{g\_\text{Venus}}$ is a very good approximation. What this means based on observations of Venus's surface pressure, surface area, and surface gravity is that Venus's atmosphere is very massive compared to that of the Earth.

Regarding the Venera and Vega landers not being crushed, it was the high surface temperature rather than the high surface pressure that did them in. People have built vessels that have descended to the bottom of the Challenger Deep, where the pressure is 1000 atmospheres. Compare that with the 93 atmospheres Venus's surface. Nuclear submarines operate at one atmosphere so as to be able to rise to the surface quickly, which means they can indeed be crushed by pressure at a sufficient depth. The Venera and Vega landers didn't operate under those constraints.

> **Comments:**
> **username_0**: ..thank you so much for such an outstanding answer. Now the light bulb went off in my head. Total mass. By the that was a very interesting insight you gave with Challenger Deep and submarines are designed for a different purpose and need to get to the surface more easily.  
> **username_8**: @Sedumjoy I think "ease of getting to the surface" is a bit distracting. The reason nuclear subs can't go deeper than 700 m or whatever is simply that they're not designed for it, apparently because it wouldn't serve any military purpose. At least not enough to justify the increased mass of the stronger walls, higher cost, and/or reduced usable space inside, but with enough money it would also be possible to increase the depth capability. The actual point however is that it's much easier if the interior is also pressurized to match the outside. Which would crush any humans inside, though.  
> **username_7**: @leftaroundabout Going deeper than 700 meters (or whatever) would require a different breathing atmosphere and might require coming up slower, with stops. Keeping the air inside a submarine at one atmosphere obviates the need for that slow rise, but also precludes going deep to prevent the submarine pressurized at a mere one atmosphere pressure from collapsing in on itself.  
> **username_7**: @leftaroundabout Deep sea vessels do not operate at one atmosphere. There's no problem per se with breathing "air" at much greater than one atmosphere so long as the "air" doesn't have nitrogen (which can result in the bends on rising) and so long as it doesn't contain all that much oxygen (which also can be toxic).  
> **username_9**: @DavidHammen, I believe most DSVs (eg, Trieste or w/e) maintain ~1atm, don't they? At the very least, the depths of challenger deep etc are *far* too deep for any diving mixture we have, they don't ascend slowly enough to be doing decompression stops, and I don't see any mention of breathing mixtures online.  
> **username_9**: That doesn't change the answer though, a Venus probe doesn't have to be a pressure vessel the way a crewed submarine does, and so there's not anything to crumple under that pressure  
> **username_8**: @Kaia exactly. Humans are able to survive up to a few dozen atmospheres given the right breathing mixture, but that's still only a small fraction of deep sea pressures. And if you want to benefit from interior pressure to reduce structural requirements, it needs to be at least something like half of the external pressure. In case of the Mariana Trench, it doesn't really make a difference in terms of structural loads whether the pressure inside is 1 bar or 100 bar, so there's no point using anything more than one atmosphere in the capsule.  
> **username_0**: Thank you. I learned much by listening to this interesting discussion about pressure.  
> **username_10**: If a SF novel was factual in this area, at significantly higher pressures there is also concern about regulation of body heat; the denser atmosphere is a better conductor of heat.  Something like staying in water for days.  
> **username_11**: There’s also the fact that the higher surface temperature on Venus makes the pressure roughly twice what it would be if it had Earth’s surface temperature.  
> **username_7**: @Technophile Temperatures hot enough to melt lead aren't conductible to electronics, let alone life as we know it.  
> **username_7**: @PeterErwin Venus's tropopause is somewhere between 60 to 80 km above the surface, depending on who one reads. The lapse rate is fairly constant from the one bar level (50 km above the surface) down to the surface, consistent with an adiabatic non-ideal gas. This results in a very high atmosphere temperature at the surface. Venus's surface is hot because the atmosphere is hot, not the other way around.  
> **username_10**: @DavidHammen  I was referring to an additional design constraint for the internal habitable portion.  I agree that the temperature range required for human habitability is below the melting point of lead.  
> **username_11**: @DavidHammen I was attempting to refer to “the temperature of the atmosphere at or just above ground level” in a compact way, but obviously that was a bit ambiguous. The point is that the high “surface atmospheric pressure” — the basis of the original question — is also due to the high atmospheric temperature there, though the high density is obviously the dominant factor.  

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### Answer by username_6 on 2023-03-30. Score: 5

It is the sheer amount of carbon dioxide in the atmosphere that makes the difference. Now the next very natural question is something like: The planet Venus has about the same mass as Earth, so why is there that much carbon dioxide in the atmosphere? A partial answer in the Venus–Earth comparison is that on Earth most of the carbon dioxide is bound in carbonates in the geosphere (chalc and dolomite being the dominant minerals forming impressive geological deposits).


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