the dense atmosphere of venus

[Barabino]

venus has almost no magnetic field, and yet, the planet manages to retain a dense atmosphere: how comes that the powerful solar wind doesn't swipe it away?

[Hornblower]

venus has almost no magnetic field, and yet, the planet manages to retain a dense atmosphere: how comes that the powerful solar wind doesn't swipe it away?

I would imagine that the solar wind is swiping it away. It just takes a lot longer when there is more to begin with than on other planets such as Mars. Imaging two pans of water side by side, with the same surface area but with much more water in one than in the other. With the same evaporation rate the one with more to start with will take longer to empty.

[Swift]

The solar wind has a very strong impact; it removes most of the light gases; that's why it has almost no water.

wikipedia

Unlike Earth, Venus lacks a magnetic field. Its ionosphere separates the atmosphere from outer space and the solar wind. This ionised layer excludes the solar magnetic field, giving Venus a distinct magnetic environment. This is considered Venus's induced magnetosphere. Lighter gases, including water vapour, are continuously blown away by the solar wind through the induced magnetotail.[3] It is speculated that the atmosphere of Venus up to around 4 billion years ago was more like that of the Earth with liquid water on the surface. A runaway greenhouse effect may have been caused by the evaporation of the surface water and subsequent rise of the levels of other greenhouse gases.[7][8]

[Spacedude]

This has been a puzzle for me too, I chalked it up to a continuous replenishing of the atmosphere via billions of year of ongoing volcanism. So far it's not possible to determine how many of Venus's volcanoes are active but the map below indicates that they blanket the surface of the planet.

http://volcano.oregonstate.edu/oldro...nus/intro.html

[Swift]

I suspect the lack of water contributes to the carbon dioxide atmosphere on Venus. On Earth, a lot of CO2 is absorbed into the oceans and is eventually precipitated out as carbonate minerals, which can get incorporated into the crust. Some of that eventually gets recycled back into the atmosphere by volcanoes, but at any given time carbonate rocks are a big sink for CO2. Without water, I suspect none of this happens on Venus.

[Van Rijn]

venus has almost no magnetic field, and yet, the planet manages to retain a dense atmosphere: how comes that the powerful solar wind doesn't swipe it away?

The effect of solar wind in popular press tends to be a tad overstated. Venus is the second most massive terrestrial planet in the system, so there is a decent escape velocity, and co2 and nitrogen molecules have decent mass, so it is harder to accelerate these molecules to escape velocity. Hydrogen, on the other hand, can be stripped from h2o efficiently by ultraviolet light, and hydrogen can fairly easily escape Venus. So Venus lost hydrogen, but not so much of the molecules with more mass. Solar wind, in part, affected the evolution of the atmosphere.

Mars and Mercury have much less mass and would be affected much more by solar wind as well as other loss mechanisms.

A couple of other points, the solar wind is far less intense than it was for the early sun, so the loss rate due to this now is also much lower. Also, the upper atmosphere protects the deeper atmosphere from solar wind, slowing loss.

[Eclogite]

I suspect the lack of water contributes to the carbon dioxide atmosphere on Venus. On Earth, a lot of CO2 is absorbed into the oceans and is eventually precipitated out as carbonate minerals, which can get incorporated into the crust. Some of that eventually gets recycled back into the atmosphere by volcanoes, but at any given time carbonate rocks are a big sink for CO2. Without water, I suspect none of this happens on Venus.

Correct. The carbon dioxide in Venus’ atmosphere yields approximately 90 bars partial pressure. The mass of CO2 responsible for this is a close order of magnitude to the CO2 locked into carbonates in the Earth's crust. See, for example, Lammer et al "Origin and evolution of the atmospheres of early Venus, Earth and Mars" 2018 The Astronomy and Astrophysics Review.