two atmospheres make up the main planets atmoshere but the blast wave was done in photoshop As the idea only came to me after i had finished rendering though it could easily be done in terragen i think.
the actual cloud explosion was done with a mini planet set underground i then added a thick cumulus layer 4000 metres thick and got it to cover most of the mini planet i then set the altitude to about 12,000 meters to get this orb like cloud formation