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The traditional explanation for rocky inner planets and large gaseous outer planets is that the outer planets are farther from the Sun, and therefore can hold onto large atmospheres that don't get eroded by the solar wind.

Okay so let's look at Venus. 30% closer to the Sun and has a much weaker magnetic field than Earth. So we would expect much less of an atmosphere, if any at all. But Venus' atmosphere is 92 times thicker than ours. How?

Now I know the Venusian atmosphere is 96% carbon dioxide, which is heavier than O2 or N2, so it's a little more "stalwart" against the solar wind. But honestly I don't see how that's enough. CO2 molecular weight compared to O2 is 44/32 = 1.375 times heavier. But again, Venus is 30% closer to the Sun and has an extremely weak magnetic field. Let's also not forget how darn hot Venus is, which means its air molecules are bouncing around much harder, making it easier for the solar wind particles to come along and knock out that CO2 into space.

Let's also not forget Mars, which also has an atmosphere 96% CO2 (very interesting why they are both 96%). But Mars' atmosphere is extremely thin, just a few hundred pascals. If the solar wind can do that all the way out to Mars, which is 2.1 times farther than Venus...then what is Venus doing with all that air?

Some things I tried to think of that make Venus different from Earth or Mars: Venus rotates extremely slowly. Venus rotates in the opposite direction. Venus has no moon(s). Venus has no water, no tectonic or volcanic activity, and no people. I don't see how any of this would help hold its atmosphere against the solar wind.

(Any joke comments on an extinct venusian people making foolish use of carbon fuels, hence the runaway greenhouse effect, 96% CO2, and cooked planet, will be met with skepticism.)

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    $\begingroup$ It has more mass, therefore more gravity. $\endgroup$ – gerrit Feb 1 '16 at 11:15
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    $\begingroup$ Good question, especially since Ganymede has a magnetic field but no atmosphere, and Titan has a thick atmosphere but no magnetic field. But each of those are much less substantial than those of Mars. $\endgroup$ – LocalFluff Feb 1 '16 at 13:24
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    $\begingroup$ @gerrit Venus has less mass than Earth. Surface gravity on Venus is about 90% of Earth. $\endgroup$ – DrZ214 Feb 1 '16 at 23:18
  • $\begingroup$ @LocalFluff Nice, I didn't even think of those two although I know I've heard of that before. It is interesting to compare outer planets with inner planets, because according to the traditional explanation, all those outer planet moons could easily accumulate atmospheres too. $\endgroup$ – DrZ214 Feb 1 '16 at 23:21
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    $\begingroup$ Side Note: I think Venus does have volcanic activity... $\endgroup$ – honeste_vivere Feb 4 '16 at 23:41
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The mass of Mars is so small, and its magnetic field so weak that it cannot hold onto carbon for a long period of time, making almost all of it to escape.

Venus Earth and Mars
(Venus and Earth are close to identical in size, Mars is much smaller)

Venus does not really leak that much of its atmosphere, the only noticeable exception is that all hydrogen compounds are virtually absent.

Why is the Venus atmosphere so much denser, compared to Earth? Surely we can resist the solar wind even better than Venus can. Both Venus and Earth are massive enough, and has both has significant magnetic fields, to resist the solar wind, and hold a significant atmosphere (Mars can not, it is just too small, and the magnetic field is too weak). So why the difference?

The mass of the Venus atmosphere is $4.8 \cdot 10^{20}kg$, or about $1.2 \cdot 10^{20}kg$ of carbon, as it is mainly carbon dioxide. Where is all that carbon on Earth? Our atmosphere does only contain $8.1 \cdot 10^{14}kg$. A lot of carbon is also dissolved in the oceans, approximately $3.6 \cdot 10^{16}kg$, but that is still over three orders of magnitude less. Adding all coal, oil, etc. we are still below $5 \cdot 10^{16}kg$, so a Venus civilization burning all their fossil fuels can not be the explanation.

Where is that carbon on Earth?

A lot of that missing carbon on Earth is in limestone. The total amount of carbon in limestone in the crust is about $4 \cdot 10^{19}kg$. Now we are talking! Limestone is just 10% of the sedimentary rock, so there are probably a lot more carbon in the crust. Such carbon containing compounds releases carbon at high temperatures, thus they are already decomposed on Venus.

Earth has a thick atmosphere too, just like Venus. But it is currently bound to the crust

As strange as it may sound, the higher temperature of Venus is the cause of the dense atmosphere.

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    $\begingroup$ The planet's mass is not as much of a factor as its induced ionosphere, which creates a similar magnetic bubble to that of Earth's magnetosphere. Once ionized, particles in the upper atmosphere can easily escape the planet since the pickup velocity is several hundred km/s (i.e., much much larger than the planetary escape speed). $\endgroup$ – honeste_vivere Feb 4 '16 at 23:43
  • $\begingroup$ @honeste_vivere Are you sure that the magnetic field of Venus is more important than its mass? The field is 300 times weaker than the Earth's. Thanks anyway, I may edit my answer a little. $\endgroup$ – Hohmannfan Feb 4 '16 at 23:49
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    $\begingroup$ Yes the induced ionospheric fields are weak, but as you know gravity is something 40 orders of magnitude weaker than electromagnetic forces. The pickup particle velocity, namely the speed to which a newly ionized particle will be accelerated by the convective electric field of the solar wind, is much much faster than the escape speed for any of the planets in our solar system (even Jupiter). In the absence of any shielding magnetic fields (i.e., those from a magnetosphere), a newly ionized particle will be picked up by the solar wind, thus the name pickup ions... $\endgroup$ – honeste_vivere Feb 5 '16 at 13:45
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    $\begingroup$ There was a recent paper by the MAVEN mission team that discussed why Mars lost its atmosphere and they suggest that the solar wind was the primary cause due to pickup effects, among other things. The size of the planet matters as well at that radial distance because the gyroradii of some of the pickup ions can be larger than Mars. Whereas at Venus, the same particle will generally have a much smaller gyroradii owing to the larger solar wind magnetic field... $\endgroup$ – honeste_vivere Feb 5 '16 at 13:48
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    $\begingroup$ @Hohmannfan: What is your point? You're talking about the mass of the atmosphere, while the question is about the mass loss-rate. A thick atmosphere should even easier leak out to space (the self-gravity of Venus' atmosphere is still negligible vs. planetary mass) as it fills up the gravity well easier and reaches the jeans radius/sonic point easier as well. Thus the question how Venus retained its atmosphere is still open. $\endgroup$ – AtmosphericPrisonEscape Feb 7 '16 at 22:38
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This looks like a good read if your really interested in the subject but the simple answer appears to be that the ionosphere of Venus is strong enough to sufficiently deflect the solar wind, without the need of a strong magnetic field. Although this doesn't completely protect the planet, Venus still looses some of it's atmosphere into space.

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