Mapping spherical density?

[Morten Bartholdy]

I have a sphere I want to render with the built in particle volumetric shader. Now I need to map the density so it is spherical - imagine the earths atmosphere for instance where I might want to control the density with a gradient from the centre of the sphere and out to the circumference where the density fades out, and likewise for coloring.

I have been through the various permutations with gradients but fail to get a really three dimensional mapping of spherical density.

Am I missing something simple here or is it not easily done?

Morten