Clearly, we need to increase the average temperature, decrease
day/night temperature variations, increase pressure, and introduce
O2 and N2 into the atmosphere. Possible
starting points include:
Increase Greenhouse Gases
We can't rely on CO2 alone. Mars being further out
than Earth, and working alone, requires a toxic level (for humans,
NOT plants) for adequate warming. To do it with CFCs would
require a trillion tons per year, which is a million times more
than the current million tons per year being produced on Earth!
Ammonia, NH3, is an efficient greenhouse gas, but
it would need to be created on Mars, or perhaps brought on an
icy outer planetary body. Both methods would be extremely
difficult.
Push an Instability
Presently, the very low atmospheric pressure means heat is quickly
lost to space and the ice caps get REALLY REALLY cold, condensing
out still more of the atmosphere. If we increased the pressure
just a few 10s of mbars, the polar regions would
retain more heat, leaving more CO2 to warm
the atmosphere. More CO2
would be released from the polar ice caps, increasing
pressure and retaining more heat. A new equilibrium of
100 to 1000 mbar (like Earth) might be reached.
This requires that there is enough CO2 to be released! The Carbon may be locked up in rocks, which would need to be broken up or heated to release the CO2. Even if this did work (and it would make the atmosphere TOXIC for humans), the very high concentrations of CO2 would cause it to absorb into any liquid water on the surface.. removing it as a greenhouse gas, in the form of carbonates.
Is Terraforming Mars Feasible?
We don't know how much CO2 or H2O is in the
surface ices and regolith. We don't understand the
complex instabilities inherent in the Martian climate system.
However, Mars did at one time have a dense atmosphere, when
geological activity was high and impacts were often, heating
the surface and keeping the atmosphere full of CO2
and H2O. These molecules were lost to space or went into
the crust.