How to do an effect like "gas" inside a sphere ?

lets say i have a sphere, and i want the player to see a gas inside of this sphere

i like to play with existing particle effect but i dont know if smoke particle or something else would be ok for this case?

can a particle effect be countained inside a sphere?

like this thing, but with gas inside it :o

or another solution could be a material on the sphere, and then if player enter inside, i add a green effect on screen ?