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The pressure on Venus is 1334 PSI or 92 times the pressure at sea level on Earth, which is 14.5 PSI. Now, in our oceans the pressure increases by 14.5 PSI every 33 feet. So, we're told that the atmospheric pressure on Venus is equal to the pressure on Earth at 3036 feet under the ocean? Do I have this right? If so what is causing that much pressure in a gas?

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    $\begingroup$ The pressure is caused by the gravity of Venus and the amount of gas around venus. Gravity of Venus is 90 % of Earth. The mass of the whole atmosphere of Venus is about 93 times the mass of the Earth atmosphere, see en.wikipedia.org/wiki/Venus#Atmosphere_and_climate $\endgroup$ – Uwe Aug 31 '17 at 21:20
  • $\begingroup$ Yeah, but I'm going to find out where all that gas comes from. Maybe it's the nearness to the sun. $\endgroup$ – Richard Wales Aug 31 '17 at 21:34
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    $\begingroup$ Given that the atmosphere on Venus is 95% CO2, the CO2 most likely originates from the volcanic activity on Venus, the same way the Earth's early atmosphere was mostly CO2 because of early volcanic activity. If it wasn't for ancient microbes using the carbon in CO2 & releasing the oxygen back into the atmosphere, Earth's atmosphere would still largely be composed of CO2. $\endgroup$ – Fred Sep 1 '17 at 12:41
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    $\begingroup$ Sorry, but most of Earth's original CO2 was converted to various calcites (CaCo3, mostly limestone) long before photosynthesis started. Water mixed with CO2 to form carbonic acid, which reacted with calcium minerals to form calcites. $\endgroup$ – BillDOe Sep 1 '17 at 20:09
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    $\begingroup$ @RichardWales Please clarify the title and text of your queston to properly reflect that you are indeed interested in how venus acquired such a heavy/dense atmosphere. Currently, your question (What causes so much pressure) and what you ask in your comment are two different things. $\endgroup$ – Polygnome Sep 2 '17 at 11:15
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The pressure exerted on a surface area under the ideal gas law is $P = \frac{\rho k_B T}{\mu}$ with the following notation:

  • $\rho$ being the volume mass density (or simply density)
  • $k_B$ is the Boltzmann constant
  • $T$ is the temperature in Kelvin
  • $\mu$ is the mean molecular mass in g/mol

On Earth $\mu_E \approx 29$, while on Venus it is $\mu_V \approx 44$. Furthermore $T_E \approx 300$ and $T_V \approx 750$, so the factor of $\frac{T}{\mu}$ accounts for only a factor of $1.76$ between those pressures.
The main difference here comes from the density: $\rho_E = 1.2 kg/m^3$ while $\rho_V=67 kg/m^3$.
Together you get $P_V = P_E \cdot 1.76 \cdot 55.8 \approx 98 \cdot P_E$. The difference from this calculation compared with the factor of $92$ that is actually true, come from the fact that Venus atmosphere on the surface mostly is in a state of supercriticality, which means it is somewhere between the state of gas and fluid. This makes it necessary to apply corrections to the ideal gas law which I've not used above, for simplicity.

So the main difference in pressure comes from the higher density of Venus atmosphere. What does that mean?
There is simply so much more 'stuff' in Venus' atmosphere. Earth's total atmosphere has a mass of around $5 \cdot 10^{18}$kg, while the Venusian one has $5 \cdot 10^{20}$kg, so this is 100 times more than what Earth has.

How could this happen? Or you could ask "How can a planet nearly the same mass as Earth have an atmosphere 100 times more massive?". This is one of the big mysteries of Planet formation & evolution. There are two halfway compelling scenarios that come to my head now, both involve Earth and Venus having similar atmospheres at a relatively young age of $\sim 100$ Million years.

  1. Venus might have gotten less accretional heat in the phase as the solar system formed. Together with it's smaller size, it was able to radiate this heat away even faster than Earth was. Also smaller size implies less radiogenic heat production throughout the ages.
    All those factors might have conspired to make plate tectonics much weaker/nonexistent on Venus. This prevents burying of the atmosphere, as it happens in Earth's mantle. But it is hard / impossible to estimate how strong this effect was in the past.
  2. Venus went early into a stage of runaway greenhouse effect. We know this, because we find it today in this state. Somehow this protected the atmosphere from escaping (I don't want to specify any mechanism, as there are many possibilities), while Earth lost a significant portion of its atmosphere over time.

If you're more interested in this topic I recommend as introductory read Imke de Pater and Jack Lissauer's book "Planetary Science".

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    $\begingroup$ +1 This is a very helpful answer as it addresses what I think the OP is actually asking about, and what I think many people would like to know; How could this happen? $\endgroup$ – uhoh Sep 2 '17 at 3:12
  • $\begingroup$ In a constant volume, increasing temperature increases pressure. But the atmosphere of a planet is not a constant volume. If the temperature rises, the atmosphere gets higher, but the mass of the atmosphere over a given area does not change and pressure does not change. There are areas of lower and higher pressure on earth, but this is caused by temperature differences. But we assume here a constant temperature at the surface of the planet. $\endgroup$ – Uwe Sep 3 '17 at 23:07
  • $\begingroup$ @Uwe: What are you trying to say? Argue math and physics, not semantics. Also weather on Earth is not the same as the average state of the atmosphere on Venus. $\endgroup$ – AtmosphericPrisonEscape Sep 3 '17 at 23:32
  • $\begingroup$ @AtmosphericPrisonEscape What I want to say: Are you sure that the formula for the pressure you wrote in the first line is applicable to the atmosphere of Venus or Earth? It seems to be the ideal gas law for a constant volume, but an atmosphere of a planet is not a constant volume. Temperature does not change the gravity of the planet and the mass of its atmosphere, so temperature can not change the mean pressure at the surface of the planet. By the way, I did argue mathematical and physical. $\endgroup$ – Uwe Sep 4 '17 at 11:26
  • $\begingroup$ @Uwe: You're right in saying that this is the ideal gas law for a constant volume. But you have to check which volume you're talking about. The way the gas law is derived it is locally valid for all individual test volumes on which the density $\rho$ is defined, thus it is valid everywhere in the atmosphere. Only when phase-changes become involved it's not fully correct anymore, but I've pointed that out in my answer. $\endgroup$ – AtmosphericPrisonEscape Sep 4 '17 at 11:38
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The question you did ask:
Why is the atmospheric pressure on Venus so high?

The answer is that the mass of the atmosphere of Venus is very high compared to that of the Earth, by almost a factor of 100. On the other hand, the two planets have similar surface areas and similar gravitational accelerations at the surface. Since surface pressure is approximately the weight (mass times gravitational acceleration) of the atmosphere divided by the surface area, this means that surface pressure on Venus is very high compared to that of the Earth, by almost a factor of 100.


This leads to the question you didn't ask:
Why is the atmosphere of Venus so massive (and why is it almost all CO2)?

The answer to this question is that the Earth has water on the surface, plate tectonics, and abundant life. Venus has none of these.

The combination of the above means that Earth has a vigorous carbonate-silicate cycle that sequesters vast amounts of carbon underground. This cycle does not exist on Venus. Venus has instead expelled a large amount of its crustal carbon into its atmosphere, mostly in the form of carbon dioxide.

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    $\begingroup$ Interesting answer. Does the argument apply to Mars as well? Mars does not have a crushing CO2 atmosphere because it (once) had a carbonate-silicate cycle? Let's give this hypothesis a test drive. $\endgroup$ – uhoh Sep 2 '17 at 11:56
  • $\begingroup$ Carbonate cycle is all well and good, but is this mechanism able to bury 91 atmospheres of material? Would also be interested in references for this. $\endgroup$ – AtmosphericPrisonEscape Sep 2 '17 at 15:50
  • $\begingroup$ @uhoh - Mars is too small to hold onto a substantial atmosphere. $\endgroup$ – David Hammen Sep 2 '17 at 16:04
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    $\begingroup$ It's pretty amazing that Venus can have 100 x Earth atmosphere but Mars only 0.01 x Earth atmosphere just because of the size difference. Also I thought Mars' problem was the lack of magnetic field? $\endgroup$ – uhoh Sep 2 '17 at 16:11
  • $\begingroup$ On Earth there is not only the carbonate-silicate cycle, there is also the en.wikipedia.org/wiki/Carbon_cycle, both are removing carbon dioxide from the atmosphere. But both cycles are not possible on Venus due to the high temperature and the lack of photosynthesis. On Earth, a lot of oxygen removed by photosynthesis from carbon dioxide was bound in metall oxides long ago, for instance iron ore. $\endgroup$ – Uwe Sep 3 '17 at 14:25
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Atmospheric pressure (or water pressure, for that matter) is simply the weight of the atmosphere that one square inch (or any square unit) supports. The atmospheric pressure on earth is, for the sake of argument, 14.5 PSI. That simply means that there is 14.5 pounds of atmosphere above one square inch of surface at sea level on Earth. (To calculate the pressure at a given depth in the ocean, you only need to calculate the weight of the ocean above that depth.) As @Uwe pointed out in comments, the mass of the atmosphere on Venus is 93 times that of Earth's, so one would expect that the surface pressure would be that much greater, as well.

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Yes you are right. There is a lot more and heavier gas on Venus than on Earth, and since Venus is smaller than Earth, the pressure that the gas exerts per square inch is much higher.

Here is a good comparison between the two planets. Notice that according to Dr Soper's table, there are 66 molecules per square meter on Venus, versus only 1 on Earth.

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    $\begingroup$ "since Venus is smaller than Earth, the pressure that the gas exerts per square inch is much higher" What does that mean? $\endgroup$ – Organic Marble Sep 1 '17 at 16:57
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    $\begingroup$ There are much more molecules per square meter than only 66. Carefully read the explanation of Dr. Soper's table. $\endgroup$ – Uwe Sep 1 '17 at 21:04

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