I have experimented with moonligh a while back. Because a planet, used as a moon, is no lightsource as such, all the lighting calculations can only come from Global Illumination.
What you would need to do is to create a planet (moon) that is lit by the sun, just like in the real world.
Then, you have to pump up the exposure significantly, and set the GI to values such as 3/5 or 3/6. With that, your scene will indeed be "lit" by the moon".
[EDIT] it helps to create another enviro light with ambient occlusion, but then use very small values such as 0.1 for effect on terrain and even less for the atmosphere.[/EDIT]
Another possibility is to create a moon with a sun behind it, and tell the sun not to cast shadows. That way, the sun will shine through the moon, while the "real sun", that is lighting the moon's camera facing side, ensures that you can see the moon's surface.
Regards,
Frank