Jonathan Silverlight  <jsilver@merseia.fsnet.co.uk> wrote:

I'd guess it's been discarded in favour of the idea that 
Venus has a thick atmosphere because it's closer to the Sun and hence 
hotter.

     Actually, "hotter" -- under identical conditions -- would yield a 
*thinner* atmosphere.  As others have noted, Earth lost lots of its 
atmosphere through chemical interactions with the crust.
     A hotter atmosphere, though, would mean that more gas molecules can 
achieve escape velocity (since the mean kinetic energy of the molecules 
increases with temperature) -- and leave the planet forever.
--
                             -- With Best Regards,
                                Matthew Funke (mff@hopper.unh.edu)