The presence of greenhouse gases has already been stressed. In particular, CO2, may be the most common. Carbon and Oxygen are the most abundant elements in the universe after H, He, and it creates a hardy, stable molecule.
Carbon Cycle
Earth has a complex
cycle of carbon, cycling it from atmosphere to ocean, settling out as
carbonates, then active plate tectonics sub-ducting the carbonates and then
re-releasing them through volcanoes.
Without liquid water and active volcanism, there is no way to monitor
atmospheric CO2 levels.
Active volcanism is as important at temperature!
Venus' broken
C-Cycle
The heat at its
surface is sufficient to keep (nearly) all the CO2 in its
atmosphere, and it has no oceans to absorb it back into the rock.
Mars' broken C-Cycle
Early Mars appears to
have had active volcanism and liquid water. There may have been an active c-cycle.
Also, impacts may have helped in heating the
surface, despite the Sun's diminished luminosity. However, Mars' small size
meant its internal heat was more quickly lost, cooling its mantle.
Even if Mars was moved to Earth's distance
(within the HZ), without active volcanism, there is no recycling of the CO2
back into the atmosphere.
Lack of a Magnetic Field
The Earth is protected
from fast moving, high energy particles originating in the Sun. These particles
are charged (+ or -) and can not pass through our magnetic field.
Mars' tenuous atmosphere is instead
bombarded by these particles, sputtering off a significant fraction of the
atmosphere. Venus' atmosphere is so
dense, the sputtering has an unimportant effect.
UV Protection &
Impacts
Without an ozone layer,
molecules in the Mars and Venus atmosphere can be broken up (water in
particular) and the constituents lost to space.
For a small planet like Mars, impacts easily provided velocities
sufficient to lose material to space.
Important Characteristics: Active geology, magnetic field, mass, ozone layer.