There are many aspects to think about when creating earth: terrain color/bump map, cloud layer casting shadows, ocean specular/color map, night city-lights map, and of course an atmosphere. But the trickiest part of creating a good looking earth is keeping the clouds white without the atmosphere tinting them blue.
I learned this technique from the two video tutorials below. What I particularly enjoy about this technique is that the planet shadow casting on the atmosphere isn’t created by a light, instead it’s controlled by a color ramp in the atmosphere shader. So you can directly control where the atmosphere falls off and imitate light scatter. In other words, it doesn’t matter where you place your light because the atmosphere is a system of its own. This means faster render times since there no volume shadows to compute.
By scaling the volume shape you can easily adjust how far out the atmosphere extends. Need to change the thickness of the atmosphere? Just adjust the edge dropoff of the shader.
When you need to have a planet revolving around a sun, well things get a little tricky since the atmosphere shadow isn’t created by a light. But the solution is to simply key the rotation to match the movement. It’s not ideal but hey, kiss it!
I’ve been quite pleased with the simplicity of using a volume primitive for an atmosphere. I’ve not experimented too much with part 2 where they replace the sphere fog shader for the volumetric fluid shader. The added wispy detail is beautiful yet it’s simply not realistic for creating a planet. (but maybe a sun?!) I’m curious to experiment with it merely out of its shadowcasting ability, but I’ve learned alot from working with fluids and textured surfaces together… flickering, odd falloff streaking, crepuscular rays, and such bugs that are so stupidly simple and yet difficult to fix.
maya scene (using mental ray) – earth with atmo, color, bump, spec, and trans