But there was always one obvious exception to this generality: Venus. Venus too has no magnetic field but easily possesses the densest atmosphere of the inner worlds, much denser than Earth’s. Part of this is probably due to the carbon dioxide composition of Venus’ atmosphere; CO2 is a large, heavy molecule that is no doubt easier to retain than Earth-like gasses of oxygen, nitrogen, argon, and water vapor. Furthermore, the active (though periodic) heavy volcanism on Venus means that this gas is being regularly generated and introduced into the planet’s atmosphere (the early loss of heavy volcanism is probably why the same hasn’t occurred on Mars), perhaps as fast or even faster than the solar wind can strip it away.
But I wonder if there is another reason. Venus is topped by a thick layer of sulfuric acid clouds, which largely (I think) overlays the carbon dioxide nearer the surface. This layer could act as a shield against incoming electrons and protons from the sun. Sulfuric acid is a very heavy molecule which would probably fracture into smaller radicals and ions when struck by such particles. Possible reactions could be:
H2SO4 + e- ® OH- + HSO3·
H2SO4 + p+ ® H2O2 + SO22+
If OH- ions are formed they are probably to weak to be helped by Venus’ gravity, but they might recombine with other radicals/ions first. The other species will probably also hang around long enough to recreate sulfuric acid, while scattering the electrons and protons at lower energies. Bear in mind, this is probably a very small sampling of the kind of chemistry that occurs at Venus’ cloud levels. If it is happening then Venus protects its atmosphere with chemistry instead of magnetism. Similar chemistry could be going on at Saturn’s moon Titan’s cloud tops, although the solar wind is much weaker there.