The earth has a thiner atmosphere because of all the carbonates that are tied up in earth's chrust through precipitation and subduction.  Venus is so hot that carbonates are not stable hence the CO2 blanket.  As for plate techtonics, the latest radar scans of the Venus surface seem to indicate not to distant plate activity.

Gary

Jonathan Silverlight wrote:

In message <llsmp09lk7.fsf@adams.patriot.net>, Joseph Lazio <jlazio@adams.patriot.net> writes

Well, no.  A huge difference between Earth and Venus, in addition to
the presence of a moon, is the thickness of their atmospheres.  I've
certainly never heard of the Moon's tides as being responsible for
keeping the Earth's interior molten.  That is ascribed to the presence
of radionuclides in its interior.  Venus is different, not only
because it has lost its oceans, but because its atmosphere keeps the
surface so hot.

In his story "Wrong Way Street" Larry Niven wrote that the Moon helped there, by stripping off most of the Earth's atmosphere. Was that ever a serious theory? 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.
And would the Earth still have a thick atmosphere if it hadn't been blown off by the collision that formed the Moon?