QUESTION: 
Is terraforming Mars really possible?

ANSWER from Jack Farmer on November 20, 1996:
For those who don't know, terraforming means to make changes in Mars'
climate to make it Earth-like. We believe it would be possible to terraform
Mars.  But if we began doing this, it would probably take hundreds of years 
to complete.

ANSWER from Ted Roush on March 6, 1997:
Terraforming is not my area of expertise. However, you might check out
the chapter titled "Planetary Engineering" by Jim Pollack and Carl Sagan
in the book entitled "Resources of Near-Earth Space", edited by J. Lewis,
M.S. Matthews, and M.L. Guerrieri, from the University of Arizona Press,
located in Tucson, Arizona.

ANSWER from Mike Mellon on July 27, 1997:
Certianly terraforming Mars has a romantic appeal similar to early explorers
setting out across the ocean in search of new lands and riches. The reality
is that terraforming is a technological and scientific puzzle of enormous
complexity. We hardly understand the Earth's climate and what effects human
activity will have on the future. To modify the Martian climate to make it
earth-like could have unknowns consequences. For example, increasing the
amount of CO2 in the atmosphere would indeed increase the greenhouse effect
and warm the surface. But the polar deposits do not contain adequately
significant quantities of CO2 to make a difference. In addition, scientists
studying Mars' early climate have recently come to realize that too much CO2
will cause CO2 clouds to form blocking the sunlight before the surface can
warm to earth-like temperatures.  In general many complex problems regarding
resources and climate feedback effects would need to be solved before we can
consider such an undertaking.

I think a bigger question than "can we do it?" is "should we do it?" While
there might be economic appeal to terraforming Mars, there is aesthetic
appeal to leaving it pristine and perhaps colonizing Mars by working with
the existing Martian nature. We wouldn't want to bulldoze in the grand
canyon to make room for condominiums, or turn Yellowstone Park into
farmland. Eventually, I believe we will colonize Mars, but perhaps the the
most appealing and least costly way will be to work with Martian nature
rather than attempt to change it.

ANSWER from Bruce Jakosky on November 14, 1997:
If we wanted to allow liquid water to be stable at the surface, which is
what would be required in order to have plant life to survive (and grow and
multiply), the temperature would need to be raised by about 75 degrees F.
Realistically, this could only be done by adding a "greenhouse" gas to the
atmosphere, and the best such gas is carbon dioxide, CO2.  It would take
about as much gas as is in the Earth's atmosphere, but all of it being CO2,
to raise the temperature enough to allow liquid water to be stable. There is
lots of CO2 on Mars, but it's all locked up either in the crust or in the
polar deposits.  If something could trigger the release of this CO2 into the
atmosphere, then the temperature would warm up.

The major uncertainties concern whether there is enough CO2 available and
how one can get it into the atmosphere.  There is no universal agreement as
to whether enough CO2 exists.  Also, it is not clear that there is any
mechanism that would cause it to be released into the atmosphere.  And, even
if it could be released into the atmosphere and the temperatures warmed up,
the CO2 would rapidly be removed into the crust again, so that it would be
difficult to sustain a warmer environment.

Of course, there also are philosophical and moral issues connected with
changing the planet's environment.