Why are the atmospheres of Venus and Titan so much thicker than Earth?
Volcanoes on Venus and cryovolacnoes on Titan. Somehow both are geologically active which replenishes their atmosphere of gases to make them thick enough, thick enough to be denser than that of Earth. Both bodies have no significant magnetic field to keep their atmospheres. At least for Venus, it is massive enough for gravity to hold onto most of its atmosphere and the geological activity is a lot faster in replacing the gas stripped by the solar wind on the cloud tops. As for Titan, aside from being larger than Mercury and the fact that it replenishes its atmosphere just like Venus does, it is also far enough to be cold and frigid. It also hides from the solar wind using the very large veil of Saturn’s magnetic field. http://en.wikipedia.org/wiki/Venus http://en.wikipedia.org/wiki/Titan_moon Clear skies!
