Mars in its current condition
Mars after terraforming
This is the shading network for the surface of the planet
This is for the clouds. I used a clouds 3d texture for the colour which I also plugged into the transparency. It needed a reverse node so that the black was cut out rather than the white. I animated the scale of the 3d placement node so that the clouds 'swim' giving the appearance of them moving and changing.
For the atmosphere I used a sample info node to pick out the facing ratio and then used a ramp for the colour and another ramp for the transparency.