QUESTION:
Could careless terraforming cause the orbit of Mars to change significantly,
affecting the orbits of other planets?  If so, what do you think is the
probability? Or do you think that Mars is too massive in order to let
terraforming affect its orbit significantly?

ANSWER from Bruce Jakosky on September 3, 1997:
The idea of terraforming Mars is a popular one, but I'm not convinced that
it's even feasible technically (let alone whether it's an appropriate thing
to do).  The basic concept would involve changing the atmosphere in a way
that would raise the surface temperature, allow liquid water to exist, and
allow terrestrial (or human) life to survive at the surface.  The warming
that is required, some 50 K or more in temperature, would require the
addition to the atmosphere of a thick gas that would act as a greenhouse. If
the gas were CO2, which is an excellent greenhouse gas, somewhere around 1
atmosphere of gas would be required.  It is not clear to me that this much
CO2 is available and/or readily accessible on Mars.

Even if it is, the presence of a thick greenhouse atmosphere and liquid
water on the surface would not be stable.  The CO2 would dissolve in the
water and form carbonate minerals, removing the CO2 from the atmosphere. 
Some ill-defined procedure would need to be followed to continually put the
CO2 back into the atmosphere.  [On the Earth, this process is plate
tectonics, which takes the CO2 minerals that form this way, primarily
limestone, and shoves them under the surface.  There, they are heated, the
CO2 is released, and it comes back into the atmosphere as volcanic gases. 
Because there is no plate tectonics on Mars, such a CO2 cycle would not be
closed and the CO2 would be removed from the atmosphere.]

However, your question actually is much simpler than that.  No matter what
you do to the Martian atmosphere, whether the CO2 is present in the
atmosphere or as minerals in the crust, you can't change the orbit of Mars
or of the other planets.  In order to change the orbit, you must exert some
additional force on the planet from outside of the planet itself.  Thus,
Mars' orbit is changing due to the gravitational pulls from the other
planets, but it can't change on its own.  And, the mass of the atmosphere is
so small that it wouldn't affect the gravitational interactions between Mars
and any of the other planets.

Let me pose a question to you, however:  Even if we could change the Martian
atmosphere and make it suitable for terrestrial life, should we?  Which
would be of more value to us, a terraformed Mars or a planet in its current,
pristine state?