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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.

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    $\begingroup$ Well, Venera didn’t need to support humans inside, which changes the engineering requirements greatly… $\endgroup$
    – Jon Custer
    Mar 28, 2023 at 1:48
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    $\begingroup$ The key detail here is the amount. There is a lot more atmosphere on venus. $\endgroup$
    – Topcode
    Mar 28, 2023 at 2:04
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    $\begingroup$ This is a duplicate of space.stackexchange.com/questions/22856/… $\endgroup$ Mar 28, 2023 at 2:10
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    $\begingroup$ "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. $\endgroup$ Mar 28, 2023 at 21:26
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    $\begingroup$ Iron is only about 4x as dense as sand. How can a barbell weigh so much more than a grain of sand? $\endgroup$ Mar 30, 2023 at 13:34

2 Answers 2

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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.

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    $\begingroup$ @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. $\endgroup$ Mar 28, 2023 at 18:02
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    $\begingroup$ @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. $\endgroup$ Mar 28, 2023 at 20:36
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    $\begingroup$ @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. $\endgroup$
    – Kaia
    Mar 28, 2023 at 22:07
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    $\begingroup$ 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 $\endgroup$
    – Kaia
    Mar 28, 2023 at 22:10
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    $\begingroup$ @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. $\endgroup$ Mar 28, 2023 at 22:30
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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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