Rayleigh number

scatterbrain8

I have a quick question about the definition of the Rayleigh number:

In my homework, I am asked to compute the Rayleigh numbers for both Earth and Mars. I am given the formula:

((density)*(g)*(thermal expansivity)*(Temperature difference)*d^3)/(viscosity*thermal diffusivity)

However, I am confused as to what d is. My homework says that d is the depth of layer, does this mean the depth of the planet (i.e radius)? Does it mean the distance from the surface to the core of the planet?