QUESTION:
Why does Venus have so much gas in its atmosphere?

ANSWER from Ted Dunham on May 9:
Venus' atmosphere has LOTS of carbon dioxide in it - it is 90
times thicker than Earth's atmosphere.  One way to understand the difference
is to ask the question in reverse - why doesn't the Earth have that much
carbon dioxide in its atmosphere?  

The answer to that question is that the
Earth's carbon dioxide is all tied up in rocks like limestone.  It got there
over billions of years because lots of little animals in the ocean make shells
which contain calcium carbonate.  One molecule of carbon dioxide is used up
to make each molecule of calcium carbonate.  After a while, each little animal
dies and its shell sinks to the bottom of the ocean.  These shells are what
make up limestone.  If you figure out the amount of limestone in the world,
there is about as much carbon dioxide tied up there as there is in Venus'
atmosphere.  

If we didn't have an ocean with shell-making animals in it,
we'd be fried by the greenhouse effect just like Venus!