QUESTION:
Is it possible that Mars was similar to Earth in temperature billions
of years ago? Since there is water on Mars, could it have had an
atmosphere similar to Earth?

ANSWER from Jim Murphy on July 18, 1997:
Yes, Mars might have had similar temperatures to Earth (now?) millions of
years ago, but it would have required some extreme conditions.  Mars would
have needed a much thicker, 'green-house' producing atmosphere than it
currently has, since Mars is farther from the sun (and thus receives less
sunlight), and also because it is believed that the 'young' sun ( a feww
billion years ago) was less intense than it is now.

As for having had an atmosphere similar to Earth's now, I don't think so.
The Earth's atmospheric oxygen content (about 20 percent), is due largely
to the presence of life, i.e, plants take in carbon dioxde and release
oxygen.  Thus, without life on early Mars, there probably was not too
much oxygen.  As for the majority of Earth's atmosphere being composed of
nitrogen now, I'm not sure what would have happened to a large quantity
of nitrogen which might have been in the Martian atmosphere long ago.  Mars'
present atmosphere does contain, in addition to carbon dioxide, smaller 
amounts of other gases (oxygen, nitrogen, argon, xenon,  and other stuff)
which are also present in Earth's atmosphere, but it is the differences
in the major components which are the most important.

Jim Murphy
Mars Pathfinder ASI/MET Science Team